Analytical Chemistry of Synthetic Colorants

Advances in Color Chemistry Series - Volume 2

Analytical Chemistry of Synthetic Colorants

edited by

A.T. PETERS Chemistry & Chemical Technology,

University of Bradford, Bradford

UK

and

H.S. FREEMAN Dept. of Textile Engineering, Chemistry & Science,

North Carolina State University, Raleigh

USA

SPRINGER SCIENCE+BUSINESS MEDIA, LLC

© 1995 Springer Science+Business Media New York Originally published by Chapman & Hali in 1995 Softcover reprint of the hardcover 1 st edition 1995

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The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made.

A catalogue record for this book is available from the British Library

Library of Congress Catalog Card N umber: 94-71949

ISBN 978-94-010-4593-3 ISBN 978-94-011-1358-8 (eBook) DOI 10.1007/978-94-011-1358-8

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Preface

More than one and a half decades have passed since the last book was published describing developments in the analytical chemistry of synthetic colorants. In the intervening period, the scope and technical capabilities of instrumentation for analysing dyes and pigments has significantly expanded. It is now possible to rapidly resolve a number of problems whose solutions were previously either unattainable or very difficult to achieve. For instance, the unambiguous assignment of all the signals in the proton NMR spectrum of a trisazo direct dye, and the confirmation of the molecular weight of involatile, and, in particular, sulphonated dyes, without derivatisation, are now routine analytical techniques in many laboratories today. In addition, it is now possible to record the NMR spectrum of a dye molecule on less than 1 mg of material, and we are no longer limited to solution spectra, since solid samples can now be routinely analysed in NMR experiments.

Whilst not attempting to be all encompassing, this volume is intended to bridge the gap between what was covered in the earlier work edited by Professor Venkataraman and the developments which have since ensued in some key areas. It provides important updates in X-ray crystallography, proton NMR, IR spectroscopy and mass spectrometry, and additionally covers topics such as ESR, micro spectrophotometry and emission spectroscopy.

The X-ray chapter provides a critical analysis of reports of new crystal forms of various organic pigments and summarises some of the hazards connected with the characterisation of a proposed new form. The NMR chapter contains a review of the fundamental principles of solid state NMR and some examples of the types of problems which can be solved using this technique. The IR chapter focuses on the use of MO techniques to predict the NIR spectrum of a dye molecule not yet synthesised, and includes specific examples of dyes useful for laser printers, optical recording media, and other non-textile areas. The chapters on ESR and'microspectrophotometry cover, amongst other matters, the utility of these analytical tools in characterising the diffusion, distribution, and molecular environment of dyes in a polymer matrix. A further chapter illustrates the application of emission spectroscopy to the evaluation of optical brightening agents.

The editors noted in their Preface to the first volume of this series that colour chemistry was very much alive, and expanding into realms totally unenvisaged in the not too distant past. This new volume exemplifies the concurrent developments which have taken place in analytical techniques and structural characterisation. It contains a blend of fundamental concepts

VI PREFACE

and practical applications germane to the topics covered, thus rendering it of interest to scientists involved in teaching and research areas and to practising analytical chemists interested in organic colorants.

The editors wish to thank all the contributors to the various chapters; without their expertise and commitment, this volume would not have become a reality.

A.T.P. H.S.F.

Contributors

R.D. Bereman

M. DaRocha

U.S. Freeman

K.P. Ghiggino

J. Jirman

Y.K. Kamath

A. Lycka

M. Matsuoka

C. Nicolaou

A.T. Peters

S.B. Reutsch

J. Straka

North Carolina State University, Department of Chemistry, Box 8204, Raleigh, NC 27695, USA

Sun Chemical Corporation, 441 Tompkins Avenue, Rosebank, Staten Island, NY 10305, USA

North Carolina State University, Department of Textile Engineering, Chemistry and Science, College of Textiles, Box 8301, Raleigh, NC 27695, USA

University of Melbourne, Department of Physical Chemistry, Victoria 3052, Australia

Research Institute of Organic Syntheses, 532 18 Pardubice-Rybitvi, Czech Republic

Textile Research Institute, PO Box 625, Princeton, NJ 08542, USA

Research Institute df Organic Syntheses, 532 18, Pardubice-Rybitvi, Czech Republic

Kyoto Women's University, 35 Kitahiyoshi Imakumano, Higashiyama-ku, Kyoto 605, Japan

Sun Chemical Corporation, 441 Tompkins Avenue, Rosebank, Staten Island, NY 10305, USA

Reader in Colour Chemistry, Chemistry & Chemical Technology, University of Bradford, Bradford, West Yorkshire BD7 IDP, UK

Textile Research Institute, PO Box 625, Princeton, NJ 10502, USA

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague, Czech Republic

R.B. Van Breemen University of Illinois at Chicago, Dept. of Medicinal Chemistry and Pharmocognosy, College of Pharmacy, 833 South Wood Street, Chicago, IL 60612-7273, USA

Vlll CONTRIBUTORS

H.-D. Weigmann Textile Research Institute, PO Box 625, Princeton, NJ 08542, USA

A. Whitaker Brunei University, Department of Physics, Uxbridge, Middlesex UB8 3PH, UK

Contents

1 X-ray powder diffraction of synthetic organic colorants A. WHITAKER

1.1 Introduction 1.1.1 Diffraction of X-rays from crystals 1.1.2 Advantages and disadvantages of the powder technique 1.1.3 Reliability of an X-ray pattern 1.1.4 Presentation of X-ray powder data

1.2 X-ray powder diffraction data 1.2.1 Nitroso and nitro compounds 1.2.2 Monoazo compounds 1.2.3 Disazo compounds 1.2.4 Azoic compounds 1.2.5 Stilbene optical brighteners 1.2.6 Quinacridones 1.2.7 Dioxazines 1.2.8 Amino ketone compounds 1.2.9 Anthraquinone compounds 1.2.10 Perylene compounds 1.2.11 Indigoids 1.2.12 Phthalocyanines 1.2.13 Miscellaneous compounds

Acknowledgement References

2 Solid state NMR spectroscopy of synthetic dyes A. LYCKA, J. JIRMAN and J. STRAKA 2.1 Introduction 2.2 Basic principles of cross polarization/magic angle spinning measurements

2.2.1 Dipolar interactions 2.2.2 Chemical shift anisotropy 2.2.3 Cross polarization 2.2.4 Cross-polarization/magic angle spinning method

2.3 Examples of solid state NMR spectra of dyes 2.4 Survey of published chemical shift data on the solid state

2.4.1 "C NMR data 2.4.2 I5N NMR data

2.5 Conclusions Acknowledgement References

3 Near IR spectroscopy M. MATSUOKA 3.1 Introduction 3.2 Chromophoric systems ofNIR dyes

3.2.1 Intramolecular charge-transfer chromophores

1

I I 2 4 5 5 6 6

19 22 22 24 27 28 28 31 32 32 38 40 40

49

49 50 50 52 53 55 56 66 66 72 73 73 74

75

75 76 77

x

3.3 NIR spectra of dyes 3.3.1 Cyanine dyes 3.3.2 Quinone dyes 3.3.3 Metal complex dyes

CONTENTS

3.3.4 Phthalocyanine and naphthalocyanine dyes 3.3.5 Azo dyes 3.3.6 Miscellaneous chromophoric systems

3.4 Conclusion References

78 79 85 87 89 91 92 94 94

4 Mass spectroscopy 96 R.B. VAN BREEMEN 4.1 Introduction 96 4.2 Ionization methods 98

4.2.1 Electron impact 98 4.2.2 Chemical ionization 99 4.2.3 Desorption EI and CI 100 4.2.4 Field desorption 102 4.2.5 Fast atom bombardment and liquid secondary ion mass spectrometry 103 4.2.6 Laser desorption 107 4.2.7 Plasma desorption 108 4.2.8 Thermospray 110 4.2.9 Electrospray and ion spray 112

4.3 Conclusion 114 References 115

5 Electron spin resonance spectroscopy

H.S. FREEMAN and R.D. BEREMAN 5.1 Introduction 5.2 Basic principles

5.2.1 Spin relaxation and line broadening 5.2.2 The g-value 5.2.3 Hyperfine coupling 5.2.4 Anisotropic effects

5.3 Applications 5.3.1 Sensitizing and desensitizing dyes 5.3.2 Azo dyes 5.3.3 Triarylmethane dyes 5.3.4 Acridine dyes 5.3.5 Miscellaneous dyes

5.4 Conclusion References

117

117 117 118 120 120 121 122 122 124 125 126 127 131 132

6 Microspectrophotometry 133 H.-D. WEIGMANN, Y.K. KAMATH and S.B. RUETSCH 6.1 Introduction 6.2 Microdensitometry and microspectrophotometry

6.2.1 Instrumentation 6.2.2 Analysis of in situ dye spectra 6.2.3 Dye concentration profiles in fibres and films 6.2.4 Applications

6.3 Microfluorometry 6.3.1 Background 6.3.2 Methods of measurement

133 133 133 135 137 144 156 156 159

6.3.3 Applications 6.4 Conclusion References

7 Emission spectroscopy K.P. GHIGGINO 7.1 Introduction 7.2 Principles 7.3 Techniques

7.3.1 Steady state measurements 7.3.2 Time-resolved measurements

CONTENTS

7.4 Applications to fluorescent whitening 'dyes' Acknowledgements References

8 Identification and analysis of diarylide pigments by spectroscopic

xi

162 168 168

171

171 171 175 175 178 181 184 185

and chemical methods 186 C. NICOLAOU and M. DA ROCHA 8.1 Introduction

8.1.1 Historical background 8.1.2 Method of manufacture

8.2 Analytical methodology of diarylide pigments 8.3 Analysis of diarylide pigments by infra-red spectroscopy

8.3.1 Sample preparation 8.3.2 Characteristic bands of the IR spectra of diarylide pigments

8.4 Analysis of diarylide pigments by visible spectroscopy 8.5 Thin layer chromatography in diarylide pigments

8.5.1 Identification of diarylide pigments 8.5.2 Determination ofunreacted coupler in diarylide pigments 8.5.3 Determination of aromatic amines in diary Ii de pigments 8.5.4 Analysis offatty diamines and triamines in diarylide pigments

8.6 General scheme for the analysis in diarylide pigments 8.7 Identification of diarylide pigments by reduction 8.8 High performance liquid chromatography of aromatic amines

in diarylide pigments 8.8.1 Determination of 3,3' -dichlorobenzidine in diarylide pigments 8.8,2 Determination of 2,5-dimethoxy-4-chloroaniline (DMCA) in

c.1. Pigment Yellow 83 8.9 Gas chromatography of 3,3' -dichlorobiphenyl in diarylide pigments Acknowledgements References

Index

186 186 187 189 192 192 193 195 197 198 199 199 200 200 204

204 205

205 206 207 207

208

1 X-ray powder diffraction of synthetic organic colorants A. WHITAKER

1.1 Introduction

Some years ago the author contributed a chapter to a book! in which the X-ray powder diffraction data of synthetic dyes and pigments were collected and reviewed. Information therein is now somewhat dated and this present review gives an update of subsequently published data. However, this chapter should not be considered to be a replacement for the earlier one but a sequel, and readers requiring a complete review are advised to refer to both. Additionally, earlier data are discussed in this present article in cases where subsequent data have become available for comparison and omissions, fortunately few, in the earlier work have been rectified. Suitable entries for this review may also have been overlooked mainly because the starting point for any work of this nature is Chemical Abstracts, and not all abstracters mention whether an article contains X-ray diffraction data.

I t is very noticeable that there has been a considerable change of emphasis in the period between the two reviews, at least from the point of view of using X-ray powder diffraction data. In the earlier review most of the data were on the polymorphs of copper phthalocyanine and of substituted and unsubstituted linear trans-quinacridones. In this review the emphasis has changed to monoazo dyes (usually disperse dyes) and compounds for photoreceptors.

J. J. J Diffraction of X-rays from crystals

It is not proposed to deal in detail with the theory and practice of X-ray powder diffraction; this is covered in many books2- 5 and interested readers are advised to refer to them. However, there appears to be some common misconceptions found in the literature and these are considered together with a form of presenting results that will be most useful to other investigators.

Basically, if an X-ray beam strikes a crystal it may be diffracted; for this to occur there must be a relationship between the interplanar spacing, d, of the atomic planes within the crystal, the glancing angle of incidence, 8, of the X-ray beam to the relevant atomic planes and the wavelength, -t, of the X-rays used. This relationship is known as Bragg's law:

2d sin 8 =-t

2 ANAL YTICAL CHEMISTR Y OF SYNTHETIC COLORANTS

and X-ray diffraction is based on this relationship. This equation applies to crystalline materials and is not applicable to amorphous ones. The distinction is important. A crystal may be defined as a solid within which there is a threedimensional array of atoms (or molecules) and it is this three-dimensional array (analogous to a three-dimensional diffraction grating) that gives rise to the diffraction pattern. An amorphous material lacks this three-dimensional periodicity and therefore will not give an X-ray diffraction pattern; however, it will give one, or sometimes two, broad peaks at low scattering angle «(}~ 5-10°, eu Ka radiation) but these are not due to diffraction but to lowangle scattering. These peaks are generally very broad, as the author has found peak breadths of A(} ~ 6° (eu Ka radiation) at the level of the background radiation. It is the position and breadth of these peaks that identify them, and a broad peak in this position should not be confused with a single diffraction peak due to a disordered phase!; this will be much sharper and is usually at a diffraction angle corresponding to an interplanar spacing of about 3.4 A «(}~ 13°, eu Ka radiation).

1.1.2 Advantages and disadvantages of the powder technique

As mentioned in the earlier review!, the main advantage of the technique is that the combination of interplanar spacings, d, and associated intensities, I, depends upon the crystal structure (the three-dimensional array). This means that the powder pattern is, to all intents and purposes, unique not only for the element or compound, but also for polymorphs of the same chemical composition and for solid solutions or mixed crystals. It is excellent for characterizing both these substances and it is noticeable that in patent literature the technique is frequently used for characterizing new polymorphs and mixed crystals with improved properties. For this it is unequalled. Although some patents class the amorphous state as a polymorph, it is not. Polymorphism is defined as 'the property of crystallizing in two or more forms ... '6.

The operative word is 'crystallizing', and it means that an amorphous form cannot be a polymorph. However, the misconception is fairly common.

In many instances, the preparation of a colorant results in an amorphous form which the manufacturer then converts into a more useful polycrystalline state. The manufacturer refers to both forms as polymorphs. To complicate matters further, some investigators call the amorphous form a, others {3. In this review the amorphous form will not be considered as a polymorph.

Of course physical properties such as density, solubility, melting point, absorption and reflectance of light (colour) in the solid state are directly dependent upon the crystal structure. Because of this, two polymorphs should have different properties (although sometimes the difference may be too small to detect) and this can give rise to problems. The following are two illustrations of typical problems which illustrate the major contribution of the X-ray powder technique.

X-RAY POWDER DIFFRACTION 3

In the case ofC.I. Disperse Yellow 42, there are two polymorphs (a andfJ) which are produced simultaneously during manufacturing7• A mixture of the two polymorphs is not suitable as a disperse dye, as the dyeings produced are un level and in many cases they are spotted and not sufficiently fast. The conversion of the mixture of the polymorphs to either the a or the fJ form has been described7• It is claimed that dyeing with either form produces a color that is fast, level and of full depth. Only X-ray diffraction shows whether a given sample is a or fJ, or a mixture of both.

The second illustration has been reported by Biedermann8 who discussed the dyeing behaviour of amino pyrazole yellow. He pointed out that it existed in one amorphous and four crystalline forms, and found that crystal modifications were important in applying disperse dyes to cellulose acetate and polyester because of their influence on colour yield and the stability of the dispersion. In addition, Biedermann pointed out that only one modification (a) is stable and all the others tend to be transformed to this form in the dye bath. Since the a form has the lowest saturation value on cellulose acetate, the dyer would probably wish to use one of the metastable forms. Such forms produce deeper colours but also tend to be unstable in the dye bath, producing the a form.

Biedermann summarized the advantage of the X-ray powder technique in the following way:

Although commercial dyes often exist in metastable forms, the recrystallization of disperse dyes by slow cooling results preferentially in stable crystal modifications. If physico-chemical measurements (saturation value and solubility in water) are carried out on such pure stable products, the results cannot be applied to the corresponding commercial products unless the X-ray spectra are identical. Certain discrepancies between the conclusions of various workers probably have arisen because this requirement has been overlooked, and many other inconsistencies in the field of disperse dyes might be due to modification problems.

Like all analytical techniques, the X-ray powder technique also has dis-advantages; the major one is that it is impossible to predict the X-ray powder pattern of a phase unless the single crystal data are available and this is rare. Therefore, a large data base of standard data is required and the technique is no better than the quantity and quality of these standards. The major ~ource of these standard data is the X-ray Powder Data File (PDF) (published by the International Centre for Diffraction Data (ICDD), 12 Campus Boulevard, Newtown Square, PA 19073-3273, USA). This file contains an enormous amount of X-ray powder data, some of which is abstracted from scientific journals, some of which is commissioned directly and some is from direct submission to the file. It is divided into two sections (a) inorganic and (b) organic and organo-metallic compounds. Although there is no official figure as to the number of entries, the author estimates that up to the end of 1991 (set 41) the organic section contains some 16000 patterns, most of which are of no interest to the colour chemist. However, some are, and these are

4 ANAL YTICAL CHEMISTRY OF SYNTHETIC COLORANTS

included in this review with the prefix PDF to the file number. If the data have been abstracted from literature, the original reference of this is also given after the PDF number as it may contain further useful information.

Because of the large number of entries in the PDF, the ICDD has started to produce sub-files of interest to specialists in various disciplines. One of these sub-files is of forensic materials, consisting of approximately 1500 inorganic and 1700 organic patterns. The decision has now been made to include pigments and dyes in this group. Unfortunately, this decision was made after the publication ofthe present edition ofthe sub-file (1983) and so we shall have to wait for the next edition to take advantage of these changes. A separate subfile for colorants and intermediates is the long-term goal.

At the moment much X-ray powder data of colorants is published in the form of patents and these are not abstracted by the ICDD. Data from these would only be entered if directly submitted and patentees are urged to do this.

Users of X-ray powder diffraction may be misled by an effect which involves the assessment of intensities. If one has crystals of small particle size, (0.1 J..Lm or so) or particles in which the crystal array has been strained and distorted, then the diffracted beams are broadened and the peak heights reduced (theory shows that the total energy in each diffraction line is independent of this effect, but one measures peak height for convenience). This reduction of height is dependent upon (}, being proportional to cos (} for small particle size and tan (} for strain. In either case (or both together) the effect is to cause a reduction in relative peak heights for higher values of (}. Thus relative intensities may not be the same for different specimens. In fact, the ranking of the lines may change. An example of this effect in c.1. Pigment Violet 19 (C.1. 46500) has been reported9• In addition as peaks broaden they may merge into each other. The combination of these effects may give the impression of a different phase.

1.1.3 Reliability of an X-ray pattern

Probably the most reliable powder patterns are those which have been successfully indexed from the unit cell dimensions, i.e. the diffraction peaks are assigned to the various diffraction planes. If this cannot be successfully accomplished then the powder probably contains one or more impurities. The ICDD prefers indexed patterns, but these can only be produced if the unit cell dimensions are known, and this is fairly rare.

The alternative is to examine the reproducibility of a pattern using different specimens, preferably ones made under different conditions. Thus reports confirming an existing pattern are not without merit; the more confirming patterns there are, the greater the reliability. For this reason all confirming patterns are included in the present review, although the author often expresses his opinion as to which is the preferred pattern.

X-RA Y POWDER DIFFRACTION

1.1.4 Presentation of X-ray powder data

5

This is an important matter, as the utility of X-ray powder diffraction stands or falls on the quality and quantity of the standard X-ray patterns.

The first point to stress is that diffractometer or photometer traces are not good enough by themselves, as it is difficult to obtain accurate values of (J from them (although one can compare traces). They are useful in determining line broadening effects (section 1.1.2) but a listing of the interplanar openings, d, or Bragg angle (Jor 2(J, together with the associated intensities, I, should also be given. If Bragg angles are given then A. should also be included. However, it is not always provided, and users should be wary of lists of (J or 2(J when the wavelength has been omitted.

It is preferable to include all diffraction peaks that can be measured, but there may be a risk that some of the weak ones may be the result of an impurity. The alternative is to list only the very Strongest diffraction lines, but this leaves a subsequent user with extra peaks in his pattern; these mayor may not be caused by a second phase. Despite this risk, the former method is better on the grounds that ifthe weak peaks cannot be found subsequently, it is easier to ignore them rather than to try to guess whether the extra lines arise from a second phase.

Also, since organic phases are often rich in diffraction lines, if too few are included in the published pattern, this may not be sufficient to uniquely characterize the phase. Some published patterns only include three or four peak positions without any intensities, and it is surprising that Patent Examiners accept such poor quality data as sufficient evidence of a new phase.

Therefore, for reporting powder data, the recommendations are: (i) to list interplanar spacings, d, and intensities, I, measured above background; (ii) to index the pattern (if possible); and (iii) to include a diffractometer or photometer trace (if available).

1.2 X-ray powder diffraction data

As mentioned in section 1.1.2, the major disadvantage of the powder technique is the amount of background knowledge of standard data required. The remainder of this chapter reviews the available powder data. The order of presentation approximates to that in the Colour Index lO • As in the previous review l , powder data from election diffraction have been ignored because of the unreliability of the intensity measurements, and where polymorphs have been X-rayed the nomenclature used is that of the original reference. Although many so-called azo-colorants are actually hydrazones in the solid state, the formulae quoted are those from the references cited.


1.2.1 Nitroso and nitro compounds

There appear to be data on one nitroso and four nitro compounds: CI. Pigment Green 8 (CI. 10006) [PDF 36-1873]11; picric acid (CI. 10305) [PDF 9-789, PDF 30-1877]12 for which the second pattern is probably the more reliable; and CI. Disperse Yellow 42 (CI. 10338r13•14. The last is dimorphic and the earliest patterns7 are the most comprehensive. There is also a pattern available for CI. Pigment Brown 2215. 2-Nitro-I,I'-diphenylamine-4-sulphamide has been obtained in three polymorphic forms: a and P by recrystallization from various organic solvents, while r was obtained by hightemperature treatment in an aqueous solution of a dispersing agent l6. X-ray traces of all three are given.

1.2.2 Monoazo compounds

Although a reasonable proportion ofmonoazo colorants studied have polymorphs, there appears to be only one case in which there has been a systematic attempt to investigate these changes using X-ray powder techniques.

1.2.2.1 Investigation of Carmine 6B (CI. Pigment Red 57:1, CI. No. 15850.1). Some interesting work has been carried out on Brilliant Carmine 6W7•18. However, the nomenclature used is somewhat confusing. The pigment is prepared by a chemical reaction in water to give what the authors term the a form. This is transformed to the P form by heating above 80°C and then to another form (unnamed) at about 200°C The P form can be converted into the a form in water. X-ray traces are given of both a and P forms and the authors refer to these forms as polymorphs. However, inspection of the accompanying thermogravimetric curves indicates approximately 8% weight loss at about 80°C and 4% loss at about 200°C. However, the latter is not a single-stage process: 8% weight loss corresponds to the loss of two molecules of water and 4% loss to one. Therefore, the most probable explanation is that the a form is a trihydrate and the P form is a monohydrate, an anhydrous form occurring at about 200°C This means that a and P forms are not polymorphs and that neither is a polymorph of the anhydrous form which is called CI. Pigment Red 57:1 (CI. No. 15850:1).

One of the papers also reports the X-ray traces of Carmine 6B crystals, produced in ethylene glycol, propylene glycol and glycerine, that differ from each other and the a and P forms l8 . Chemical analysis indicates that all three batches of crystals contain one molecule of crystallization of the corresponding solvent. These complexes change to the a form ifleft in water, but in doing so pass through an intermediate X form which has a unique X-ray pattern (i.e. is not the P form). The intermediate form is generated by heating the complexes to drive off the solvent molecule and allowing the resulting crystals to cool to room temperature. It would appear that the X-form is a dihydrate, as it loses 4% by weight at approximately 90°C and also at

X-RA Y POWDER DIFFRACTION 7

approximately 170°C However, these transitions reverse as soon as the crystals are cooled, and no X-ray pattern was obtained between 90°C and 170°C to determine whether the 13 form existed in this region.

A later study confirms the change from the a form to the 13 form at about 75°C9• The traces are similar to those reported previouslyI7.18.

1.2.2.2 Polymorphism in monoazo compounds. A great deal of work has been conducted on this group of compounds, much of it on disperse dyes. As in the previous review I , only examples confirmed with X-ray data are included.

Over the last few years there has been a tendency for higher temperatures to be used during disperse dyeing, and, as a result of this, there has been an impetus to obtain dyes suitable for high-temperature application. In turn, this has produced a search for polymorphs of existing disperse dyes that are stable at high temperatures. Indeed, in many cases the citation specifically states that the low-temperature polymorph is unsuitable for high-temperature dyeing. There are many methods of producing a high-temperature polymorph, but the favourite manner is simply by heating the low-temperature form in a liquid, often water. Sometimes the aqueous phase contains an additional compound.

In general these compounds can be placed in one of several sub-groups, the commonest of which would appear to be para-amino substituted azobenzene (1) with further substitution in the two phenyl groups.

3 2 2' 3 R 4 o-N=N-o-N~ , 5 6 6' 5' R

Substitution in the phenyl group originatingfrom the diazo compound. This is the left hand phenyl group in 1. Data exist for three polymorphs of CI. Disperse Red 73 (CI. 11116), (4-nitro-6-cyano, R = C2Hs' R' = C2H4CN in 1). Early work reported two forms, a and 13, the a to 13 transformation being produced by heating in water or hot milling with a dispersing agenfo. Subsequent work reports the existence of a r form, produced by heating in an excess of an aromatic hydrocarbon21 . The latter patent gives the most important peaks for all three forms. Those for the a and 13 forms agree with the earlier work, but the earlier data are preferred as they include complete traces which indicate the presence of weaker peaks.

CI. Disperse Orange 5 (CI. 11100) (1: 2, 6-dichloro-4-nitro, R = CH3,

R' = C2H40H) is dimorphic22 and the transformation is obtained by heating this dye with water containing an ionic surfactant.


Among other dimorphic azo dyes in which the a to fJ transformation is produced by heating with water are the following based on 1: 2-cyano-4-nitro, R = C2H4CN, R' = C2H4C6H/l; 2-bromo-4-nitro-6-chloro, R = R' = CH2CH2

COOCH/4; 4-nitro-6-cyano, R = C2H4CN, R' = C2H40CONHC2H4CN2S; 4-nitro, R = C2H4CN, R' = CH2C6H/6 ; and 4,6-dicyano, R = C2H4CN, R' = C2H40CONH/7• In the last example one may also transform the dye by heating it with water containing polyethylene glycol. For the last two compounds, the reported patterns are not very satisfactory, as only three lines of each of the a and fJ (transformed) modifications are reported and all without intensities.

The original form, a, of a dye (1: 2-chloro-4-nitro, R = C2Hs' R' = C2H4CN), changes to fJ by heating with water 28 • The same compound has been reinvestigated but the patterns of the two a forms do not agree, suggesting another polymorph29 ; however, this could be caused by different coupling conditions. The traces of the fJ forms appear to be the same, but the absolute values of f} are different. In fact, on converting these values to sin f}

(for Bragg's law) it is clear that one set of sin f} values is approximately 15% larger than the other. The immediate explanation is that radiation of different wavelengths was used (the wavelength of Co Ka is 16% longer than that of Cu Ka), but both patents specifically state that Cu Ka radiation was used. On the other hand, it does seem unlikely that there are two fJ phases that give similar patterns, with one set of inter-planar spacings approximately 15% larger than the other.

According to one source10 a dye (1: 2-chloro-4-nitro, R = C2H4CN, R' = C2H40COCH1) has two forms: one, a; on coupling and the second, fJ, on heating with water. A subsequent study3! states that it has three forms: a (on coupling), fJ (on boiling with water) and r (on recrystallization from ethyl alcohol); however, the patterns for both the a and fJ forms reported in these references do not agree with each other. The second studl! also gives a few lines (2f) values only) for three polymorphs of (1: 2,6-dichloro-4-nitro, R = C2H4CN, R' = C2HPCOCH1), where again thefJform is obtained by boiling the a form with water and the r form by recrystallization from ethyl alcohol.

Of course, heating with water is not the only way of producing a polymorphic change; the dye (1: 2-hydroxy-5-methyl, R = H, R' = COCH3) changes from the a to fJ form by recrystallization from dimethylformamide; while the dye (1: 4-nitro, R = C2Hs' R' = C2H40H) changes from the a to p form by recrystallization from methanol and from the a to r form by recrystallization from acetic acid32.33. Although both studies are from the same group of investigators, the earlier work32 gives the powder traces only. The latter12 also gives a table of2f}and dvalues of the strongest lines, but since the English translation omits the decimal points in the table one needs to give consideration to both versions.

In the case of the dye (1: 4-nitro-6-S02CH3, R = C2Hs' R' = C2H40COCH3) the a to p transformation is accomplished by heating the presscake with or


without milling34. For the disperse dye (1: 2-hydroxy-5-methyl, R = H, R' = COCH3) three patterns are available3s: one for a stable form, one for a metastable form and the third for the monohydrate, the last being produced when coupling is conducted at temperatures below 35°C.

Substitution with an acetoamido group in the phenyl group of the coupling component. This is the right-hand phenyl group in 1 giving 2. Again, the commonest method for transforming the initially produced a form to the fJ form is by heating the dye with water. All the cases reported are dimorphic. This is the method employed for two dyes: (2: 2-chloro-4-nitro, R = R' = C2H40COCH3)36 and (2: 2-cyano-4-nitro-6-iodo, R = R' = C2Hs)37, but the patterns from the latter are not very comprehensive.

3 2 2' 3' R

4o-N=N~N~ , 5 6 ~'R

NHCOCH3

2

Sometimes the transformation can be accomplished in more than one manner. For instance the dye (2: 2-cyano-4-nitro-6-chloro, R = R' = n-CSHll) can be transformed by heating the dye in water or an organic solvenes while for the dye (2: 2,5-dichloro-4-S02N(C2Hs)2' R = R' = C2HPH) the alternative to heating in water is to grind the dye at 80-90°C in the presence of a dispersing agene9 . The final example in this sub-group is the dye (2: 2-chloro-4-nitro, R = C2H4CN, R' = CH2CH=CHCl), where the a and fJ forms are stated to be obtained by different coupling conditions40.

Substitution of another group and acetoamide in the coupling component. The most common method for transforming the a to fJ form is again by heating in a liquid medium. For dyes (2: 2,4,-dinitro-6-bromo, 3' -methoxy, R = R' = C2HS)41, (2: 2-chloro-4,6-dinitro, 3'-methoxy, R = R' = C2H40CO CH3t 2 and (2: 2-bromo-4,6-dinitro, 3'-methoxy, R = R' = C2HP2COCH3)43 the transformation is brought about by heating with water, although in the last case an additive is recommended. While the patterns of both forms are given in all cases, the last report only includes four lines without intensities for each phase.

In the case of the dye (2: 2,4-dinitro-6-chloro, 3' -OC2HPCH3, R = CH3, R' = C2Hs)' the heating has to take place in an organic liquid44 while for the dye (2: 4-nitro-6-chloro, 3' -chloro, R = H, R' = CH2C(OH)HCHPC6Hs) the

10 ANALYTICAL CHEMISTRY OF SYNTHETIC COLORANTS

liquid is specified as chlorobenzene although vacuum drying is given as an alternative4s.

The disperse dye (2: 4-nitro-6-cyano, 3'-chloro, R' = CH2C(OH)HCH3) is claimed to have three forms46.47. However, the original coupling pr'ocess produces a so-called P form that is actually amorphous and can not be considered a polymorph. The a form is obtained from the P form by heating with water46, while the E form is produced by heating with water to obtain a liquid melt and holding until crystallinity is established47.

Substitution in both phenyl rings where the coupling component substituent is not acetoamide. In all the cases considered, the colorants are dimorphic and the a form is obtained on coupling while the second form is called p. In all except the last two cases, the a to p transformation may be obtained by heating with water, although an additive may be required for some, and it may not be the only method of producing the p form.

In the case of c.1. Disperse Red 65 (C.1. 11228) (1: 2-chloro-4-nitro, 6' -methyl, R = C2Hs' R' = C2H4CNt8 and c.1. Disperse Brown 1 (C.1. 11152) (1: 2,6-dichloro-4-nitro, 6'-chloro, R = R' = C2HPHt9 , the transformation may also be produced by milling at 80°C and for Disperse Brown 1 by heating in an organic liquid.

Other dimorphic colorants for which heating in water (with or without an additive) is sufficient include (1: 4-nitro, 6' -NHCOC6Hs, R = R' = C2H40 COCH3)SO; (1: 2,6-dicyano-4-chloro, 3'-chloro-6'-NHS02CH3, R = H, R' = C2H4CH3)SI; (1: 2-chloro-4-nitro-6-bromo, 2'-chloro, R = R' = C2H40H)52; (1: 2-chloro-4,6-dinitro, 3'-OC2HPCH3-6'-NHCOCH2CH1, R = H, R' = C2Hs)51; and (1: 2-cyano-4-nitro, 6' -chloro, R = R' = C2HPC(O)C2Hs)54. Transformation in the last compound may also be accomplished by recrystallization from I-hexanol.

In the penultimate example in this group, the a to ptransformation may be accomplished by heating the dye in organic solvents or by hot milling; the colorant involved is (1: 2-cyano-4-nitro, 3' -chloro, R = H, R' = C2H4CN)55.

The final example involves a dye (1: 2,6-dicyano-4-nitro, 6' -methyl, R = R' = C2Hs) which, it is claimeds6, exists in three polymorphs, a, p and y; however a is amorphous and so is not a polymorph.

Another group of azo compounds which have been investigated are those where a substituted diazo component has been coupled to a 3-cyano-4-

3 2 *CH3 CN

4 Q-N=N 0--N 0

5 6 OH 'R

3


methyl-6-hydroxy-pyrid-2-one (3). In the first two cases the compounds are dimorphic, the initially coupled form being a, and the second form, /3, being produced by heating. In the case of the dye (3: 4-methyl, R = H), the heating is in a liquidS7, while for (3: 2-nitro-4-methoxy, R = C2Hs) the paste is heateds8. In the dye (3: 4-COOC2H40CH2CH=CH2, R = CH2COOC2Hs) the two forms are named a and r, and the transformation is brought about by heating in solutions9. The disperse dye (3: 5-0S02C6Hs' R = CH3) has three polymorphs60, while (3: 6-nitro, R = C4H9) has four, a from the synthesis and the other three by recrystallization from dimethylformamide (j3 ), acetone (r), or hexanol (0)61. The dye (3: 4-COOCH2C6Hs, R = (CH2)3CH3) also has four polymorphs, a as prepared from the synthesis, /3 by recrystallization from ethanol and rand oby treatment with water (the former at 80-100°C and the latter at 130°C)62. As a final example, dye (3: 2-nitro-4-chloro, R = n-C3H7) is dimorphic63.

The disperse dyes 4 and 564.6S are both dimorphic, the transformation from a'to /3 being produced by heating with water.

H3 C -0- -0- *CH3 CN "CH r; ~ CO r; ~ N=N ~ 0

../ - - N H3 C \

OH H

4

5

Among colorants based on acetoacetanilide, c.1. Pigment Yellow 5 (C.1. 11660) is dimorphic66 both forms may be obtained directly from the synthetic step; however, the a form may also be obtained from /3 by recrystallization from toluene. c.1. Pigment Orange 36 (C.1. 11780) is also dimorphic67, the /3 phase is obtained by heating with water. There is another source of a powder pattern of this compound [PDF 36-1876]11, which is in very good agreement for the 13 phase. 6 also exists in two forms, a and 13, the conversion being obtained by heating in N-methyl pyrrolidine68 .

Among the colorants based on p-naphthol, probably the most studied is C.I. Pigment Red I (C.1. 12070) which exists in four polymorphs69: a [PDF 40-1548]69,13 [PDF 32-1807fo, r[PDF 32-1875fl, that can all be obtained from organic solvents, i.e., pyridine72, toluene and chlorobenzene, respectively. The


H2 H

O<~-hN=N_~:~:3NH nN'~O H9 \dH

CH3

6

oform [PDF 40-1549t9, is a discontinued commercial product. CI. Pigment Red 31 (C.1. 12360) exists in three forms73 : a 'non-equilibrium' form obtained by quenching a pyridine solution of the coupling product with water; an 'equilibrium' form obtained by heating the precipitate in water to 90°C; and a high-temperature form obtained by heating to 225°C. These forms are called phase I, phase II and phase III respectively.

7 exists in two forms, a and f3 74; the transformation from a to f3 may be accomplished by heating or by recrystallization from organic solvents; only X-ray diffractometer traces are given, with no listing of d or l. 8 is also

7

HO CONH -o-OCH3

Q-N=N-K -o

8

dimorphic75 , the change of polymorph being effected by heating with water, while 9 has three crystalline phases: coupling gives phase A which is amorphous, while phases B, C and D may be obtained by recrystallizing phase A from various organic solvents 76. The patterns of three phase of dye 10 are also available77 •

Among the azo-pyrazolone colorants, CI. Solvent Yellow 18 (CI. 12740) exists in three polymorphs78.79. The a and f3 forms can be obtained by recrystallization from dimethylformamide and chi oro benzene respectively,

X-RAY POWDER DIFFRACTION

o II HO CONH -o-N Cl

HN"-c-o- 1=3 - II*Cl C r; ~ N = N 'I ~ ,C 1-"';:: ~ _ _ HN,....-;

o . C Cl ~ /, \ \ Cl

o 9

13

while the r form is a commercial sample, the preparation of which is unpublished. Both.B and r may be obtained commercially. The disperse dye 11 has two forms80.8l • The earlier report80 contains poor quality patterns (four values of inter planar spacings, no intensities) of the two forms, called 'unstable' and 'stable' (to high-temperature dyeing). Coupling produces the unstable form, which changes to the stable form by heating in an organic solvent. The later reference8l gives patterns for two forms which, as far as one can judge from the four lines of the earlier reference, are the same as previously reported. However, in this case, both forms can be obtained on coupling at a suitable pH. The modifications are called A (equivalent to the earlier stable form) and B (equivalent to the earlier unstable form). The second reference provides better patterns.

Cl HO COOH

Q-N=N~ Cl 0

10

11

Poor quality X-ray data (four interplanar spacing values, no intensities) have been reported for two forms, unstable and stable, of two disperse dyes having the formula of 12. The stable form is obtained in both cases by heat-


12

ing in water. The specific dyes are (12: X = CH3, R = R' = C2H40H)82 and (12: X = H, R = C2H40H, R' = C4H9)83.

It is claimed that the dye (13: 3'-methoxy, 6'-NHCOCH3, R = R' = C2H40COC2H5) exists in three forms84.85. However, the form produced on coupling, called p, is amorphous rather than a polymorph. To produce the other two forms,pis heated in aqueous methyl alcohol: the a form is produced at 60°C84 and the r form at 30°C85. The patterns for the a and r forms are distinctive. It is also claimed that the benz homologue dye (14) occurs in three forms, but again the form called a, produced on coupling, is essential amorphous86.87. Heating the a form with water produces a crystalline p form86, heating p with water containing an organic solvent produces the r form87. 15 is dimorphic88, and in this case coupling gives the pform, while heating with water gives the a form; both are crystalline. 16 is trimorphic89, although the p

13

14

15

form is a disordered, poorly crystalline phase; a and p forms are produced by coupling at different pH values and r by mixing the presscake with organic liquids and milling. In this work the intensity values have not been corrected


16

for the background count. As a result, those reported are wrong and give the impression that P is a well-crystalline phase. The published traces indicate otherwise.

In addition to the azo colorants reported, work has been completed on various azo lakes. c.1. Pigment Red 53: 1 (C.1. 15585: 1) is a barium lake which has been shown to be dimorphic90 • C.I. Pigment Red 57:2 (C.1. 15850:2) is a barium lake for which it is claimed that there are four polymorphs, a,p 91, y92

and 8 93 . The polymorphs are produced at different temperatures of laking, room temperature, 27"C, 57°C and 78°C, respectively. As the temperature of laking is increased, the crystallinity of the sample gets better, a being very poorly crystalline. Although the references list the values of 2(} that look satisfactory, the accompanying traces are not entirely conclusive. In the case of the yand 8 forms, the patterns are not well characterized; many of the diffraction lines are common with similar relative intensities and this raises the possibility that neither is pure but contains a common phase. The analogous strontium lake (no c.1. name or number) is dimorphic9\ the forms being produced by different laking temperatures. Here the patterns are distinctive and the problem obtained with the barium lake does not arise.

17 is also dimorphic95, the high-temperature form,p, is obtained by heating the pigment after laking.

17

2+ Ca

2

1.2.2.3 Non-polymorphic azo compounds. X-ray data exist for many azo compounds for which only one crystalline form has been reported. Most of these are already characterized in the Colour Index1o; these include c.1. Solvents Yellow 2 (C.1. 11020)96 and Yellow 56 (C.1. 11021)97; c.1. Disperse Blue 165 (C.1. 11077)98 and c.1. Pigment Yellow 6 (C.1. 11670) [PDF 35-1772]99. c.1. Pigment Yellow 4 (C.1. 11665) has been reinvestigated


[PDF 33-1985foo, and the later pattern is preferred to the earlier one 101 as it has been successfully indexed. There are now three patterns for Pigment Yellow 1: (C.1. 11680)101; [PDF 32-1720]102; and [PDF 36-1854]11 and for c.1. Pigment Yellow 3: (C.1. 11710)101; [PDF 35-1771]103; and [PDF 36-1855]11. In the cases involving both pigments, the patterns are consistent, the preferred ones being those that have been successfully indexed from single crystal data: [PDF 32-1720]102 and [PDF 35-1771]103, respectively.

Other c.1. Pigments based on acetoacetanilide for which data have been reported include Yellow 98 (C.1. 11717) [PDF 36-1764]104; Yellow 73 (C.1. 11738) [PDF 36-1860]11; Yellow 65 (C.1. 11740) [PDF 38-1553]105; Yellow 74 (C.1. 11741) (twice), [PDF 36-1862]11 and [PDF 38-1554f06 (the patterns arising from the latter work are in agreement with the former, and are preferred, as the second pattern has been indexed); Orange 62 (C.1. 11775) [PDF 39-1562]; Yellow 154 (C.1. 11781) [PDF 36-1867]11; and Orange 60 (C.1. 11782)[PDF 39-1563).

X-ray patterns have been reported for a host of non-polymorphic azo colorants based onpnaphthol: namely c.1. Pigments Brown 2 (C.I. 12071)15; Orange 5 (C.1. 12075) [PDF 36-1875]11; and Red 4 (C.I. 12085) [PDF 39-1564). Although a second pattern has been reported for Red 3 (C.1. 12120) [PDF 36-1806]11 that confirms the earlier pattern [PDF 25-1879]107, the earlier one is preferred as it has been indexed. Other c.1. pigments in this group for which powder data are available include Red 114 (C.1. 12351) [PDF 36-1789]11; Red 112 (C.1. 12370) [PDF 36-1786t; Red 12 (C.1. 12385) [PDF 36-1802]11; Red 11 (C.1. 12430) [PDF 36-1803]11; Red 10 (C.1. 12440) [PDF 36-1804]11; Red 170 (C.1. 12475) [PDF 36-1793]11; Brown 1 (C.1. 12480)15; Red 146 (C.1. 12485) [PDF 36-1795t; Red 5 (C.I. 12490) [PDF 36-1805fl; Brown 25 (C.1. 12510) [PDF 36-1877fl; and Yellow 156 [PDF 36-1869t.

X-ray patterns have been reported for three non-polymorphic azo colorants based on pyrazolone, namely c.1. Disperse Yellow 16 (C.l. 12700)108 and c.1. Pigments Yellow 60 (C.I. 12705)109 and Yellow 10 (C.1. 12710) [PDF 40-1547]110.

In the Colour Index sub-group in which the colorants include an acid radical or salt of an acid radical, X-ray patterns are available for C.I. Pigment Green 10 (C.1. 12775) [PDF 36-1874t; Acid Red 2 (C.I. 13020) [PDF 27-1956]; Acid Orange 52 (C.1. 13025) [PDF 9-818]; Pigments Yellow 151 (C.1. 13980) [PDF 36-1868]11; Red 58 (C.1. 15825) [PDF 36-1787]11; Red 57 (C.1. 15850) [PDF 36-1791]11; Red 52 (C.1. 15860) [PDF 36-1788fl; Red 48.2 (C.1. 15865.2) [PDF 36-1801]11; Red 48.3 (C.1. 15865.3) [PDF 36-1800fl; Red 48.4 (C.1. 15865.4) [PDF 36-1785t; and Direct Yellow 9 (C.1. 19540) [PDF 9-813).

The Colour Index also contains colorants which have been named but for which there is no constitution number, and powder data are available for some of these, namely: c.1. Pigments Brown 32 [PDF 36-1878t; Red 166 [PD F 36-1797] II; Red 223 [PDF 36-1792t ; Yellow 129 [PDF 36-1865] II and


[PDF 39-1567]. In the case of Yellow 129 both patterns are in agreement, but the latter is to be preferred since it is more extensive.

There are other colorants in addition to those listed in the Colour Index, but in the following cases the corresponding patent claims that the colorant is dimorphic despite the fact that in each case coupling produces an amorphous material which is treated to become crystalline. Therefore, they are not dimorphic and the terms a and fJ are unnecessary. In four para-amino substituted azo-benzenes, (2: 2,4-dinitro-6-bromo, R = R' = C2HS)III, (2: 2,4-dinitro-6-bromo, R = R' = CH2CH=CH2)1l2, (2: 2,4-dinitro-6-chloro, 3'-OC2HPCH3), R = R' = CH2CH=CH2)1l3, and (2: 2,4-dinitro-6-bromo, 3'-OCH3, R = R' = C2H40CH2CH=CH2)114 the crystalline form (called fJ in each patent) is produced by heating the amorphous (a) phase in water or water containing a water-soluble organic solvent. The patterns of two other para-amino substituted azo-benzenes are also reported I IS: (1: 2,6-dicyano-4-nitro, 6'NHCOCH3, R = R' = C2Hs) and (1: 2,6-dicyano-4-nitro, 6' -NHCOC2H5, R = R' = C2H5).

In the case of substituted pyridone compounds (3), four colorants have been investigated: (3: 4-COOCH2C6Hs' R = CH3)116, (3: 2-nitro-4-bromo, R = C2H5)117, (3: 2-nitro-4-COOCsH II (n), R = CH3)118 and (3: 2-nitro-4-chloro, R = CH2C(C2Hs)HC4H9(n»119. The crystalline form (this time called a in the literature) is produced by heating the amorphous (fJ) phase in water. The pattern for the dye (3: 2-nitro-4-chloro, R' = n-C4H9) is available63 .

The four disperse dyes (18: X = CI, R = C3HPCH3)12O, (18: X = CI, R = C2HPC2HPH)12l, (18: X = CN, R = C3H60CH3)122 and (18: X = CN, R = C2HPC2HPH)122 are all amorphous on coupling (termed fJ form) but crystallized on heating in water (termed a form).

18

Dye (13: 6' -methyl, R = CH3, R' = C2H4COOC2HPC6Hs) is obtained from the amorphous form by heating in methyl alcohol123 and the same general technique applies to 14: (R = C2Hs' R' = C2H4CNY24.

There are two similar substituted acetoacetanilide pigments, (19: 2' ,5'dimethoxy)77 and (19: 3',6'-dimethoxy) [PDF 32-1983]125 for which patterns are available.

The pattern of a dichlorotriazine orange has been reported 126, and although not pigments themselves, the patterns for the chloro derivatives of c.l. Pigment Red 9 and c.1. Pigment Brown 1 are available127•

18 ANALYTICAL CHEMISTR Y OF SYNTHETIC COLORANTS

19

1.2.2.4 Azo pigment mixed crystals and solid solutions. As discussed above (section 1.1.2) X-ray diffraction powder technique is unequalled for investigating mixed crystals and solid solutions. Usually these are composed of two compounds with similar formulae.

In the case of dyes (1: 2-cyano-4-nitro, R = C4H9, R' = C2H4CN) and (1: 2-cyano-4-nitro, R = C2H5, R' = C2H4CN), X-ray patterns have been reported for two modifications of two different compositions128. The compositions are 50:50, for which the polymorphs are named a (unstable) and p (stable) and 20:80 for which the polymorphs are named y(unstable) and b'(stable)'28. The terms unstable and stable refer to dyeing conditions. The nomenclature is a little confusing as y and 15, although polymorphs of each other, are not polymorphs of a and p. There is no obvious similarity between the patterns at the two compositions, and so it would be wise to take the compositions as two mixed crystals rather than a solid solution across the range.

The pattern of a 50:50 mixed crystal of two dyes (1: 2,6-dicyano-4-nitro, 6'-NHCOCH3, R = R' = C2H5) and (1: 2,6-dicyano-4-nitro, 6'-NHCOC2H5, R = R' = C2H5) has been reported l15; the patterns of both pure dyes were mentioned in the previous section, but there is no similarity between any of the patterns.

A mixed crystal composed of 50:50 (3: 2-nitro-4-chloro, R = n-C3H7) and (3: 2-nitro-4-chloro, R = n-C4H9) has been obtained in four polymorphs63 . Normal coupling produces an a form while coupling in acid medium produces a p form. The yform is produced by heating a or p in water, although the a form has to be heated under pressure, while the 15 form is obtained by recrystallization from an organic solvent, but only patterns of the first three are included. Again, the patterns of the pure dyes were covered earlier in this revIew.

The pattern has been reported for a mixed crystal of 30% (20: R = C2H5) and 70% (20: R = n-C3H7)'29. The mixed crystal produced on coupling is amorphous (termed p in the original article), and heating it in water then produces a crystalline form (termed a).

There is a group of mixed crystal patterns based on 21. The patterns are for a 50:50 mixed crystal of X = CI, Y = H and X = H, Y = CI with Rand R' the same for both components are given for the dyes: (21: R = C2H5, R' = C2H40COC6H5)130; (21: R = R' = C2H40COCH3)131; (21: R = C2H5, R' = C2H40COCH3)1l2; and (21: R = C4H9, R' = C2H40C6H5)133.

X-RA Y POWDER DIFFRACTION

n HO F

O-OH 4C200C-U-N=N-)yN COOR

20

XVN R I" ')--N=N~N/ /;S ~- ......... ,

Cl R Y

21

19

A final example of a mixed crystal for which there is powder data is unusual in that the two molecules are not as similar. The data are for a 50:50 composition of(2: 4-nitro, R = CcH4CN, R' = CcH40COC6Hs) and (14: R = CcHs' R' = C~H40COC6Hs)[34.

1.2.3 Disa::.o compounds

Powder traces are available for two polymorphs, a and /3, of c.I. Pigment Yellow 12 (C.I. 21000)[3S; a second pattern exists for the a form [PDF 36-1856][ [. The two patterns for a are essentially the same. Patterns also exist for c.1. Pigments Yellow 13 (C.1. 21100) [PDF 36-1857][[, Yellow 17 (C.I. 21005) [PDF 36-1859][[ and Yellow 14 (C.I. 21095). For the last pigment, two patterns exist for the same polymorph [PDF 36-1858] that are in good agreemene 1.[36.

Among the disazo pigments listed in the Colour Index lO, without a published structure X-ray data have been obtained for C.I. Pigments Yellow 128 [PDF 36-1866][[, Red 144 [PDF 36-1796][[ and Brown 23 [PDF 36-1872][I.[s. The two different patterns for Brown 23 were taken using different recording techniques (Debye-Scherrer camera and diffractometer). Allowing for this, the patterns agree, but the latter is preferred as it is more extensive.

Data are also available for four other diarylide colorants. 22 is dimorphic, the product of the coupling reaction (a form) changing to p on heat treatment 137 • Heat treatment, this time in the presence of benzoic acid, causes the very poorly crystalline a form of 23 to change to the p form 138. The other two colorants in the group are based on 24. In both cases, (24: X = Y = CH])139 and (24: X = H, Y = OCH])[40 data exist for two polymorphs: the form obtained on coupling, a, is poorly crystalline and converts to a better crystalline form, p, on heating in an organic liquid.


COCH3 el Cl eaCH3

H3ea--O--NHeO~H-N=N--b---6-N=N-~HCONH--O--aeH3 22

23

Cl CaCH3 X CaCH3 Cl

Cl -6-N =N - ~H C ON H --P-N HC O~H - N= N -O-Cl

y

24

2 3

X~N=N~N=N~Y 6 5

25

It is claimed that colorant (3: 4-azobenzene, R = C3H60CH(CH3)2 exists in two forms l41 , but that obtained on coupling, A, is amorphous. Consequently only the B modification, obtained by heating A in water, is crystallographi-cally acceptable. .

There are several disazo dyes based on bis-azotriphenylene (25). Atypically, c.1. Disperse Yellow 23 (C.1. 26070) (25: X = H, Y = OH) changes from the metastable a form to the stable p form simply by storage in the mother liquorl42. The reported patterns for these phases are rather poor as the reference provides 20values offive or six lines with no intensities. In addition, the wavelength of the radiation used to obtain the pattern is not mentioned. c.1. Disperse Yellow 68 (C.1. 21005) is trimorphic; an early reference l43 reported the existence of two forms, termed a and c, but gave only three lines (interplanar spacings) without intensities. A later reference l44 claimed that there were three polymorphs, a,pand y, the r form being obtained by heating the a form in water. Also given were values of 20, and intensity for four to six spacings for each, but the wavelength of the X-radiation used was not stated. Assuming the radiation was Cu K a , it would appear that the earlier a and c forms are the later a and p forms. As the later reference l44 is slightly more comprehensive it is preferred.


Dye (25: X = H, Y = N(C2H5)C2H4CN) is dimorphic l45. X-ray traces for both forms have been published and show that on coupling the material is the almost amorphous f3 form, while heating in water produces the crystalline a form. Disperse Orange 29 (25: 3-methoxy, X = N02, Y = OH) is trimorphic l46; the a form is produced on coupling, while the f3 and r forms are produced by heating in water at different temperatures. The patterns for the f3 and r forms are distinctive and they are different polymorphs. The final dye of this group is c.1. Acid Orange 156 (C.l. 26501) (25: 3-methoxy-6-methyl, X = S03Na, Y = OCH3); this exists in two forms: a, on coupling, and f3 on heating in aqueous solution at pH 8.5-9.5 under pressure l47.

There are also X-ray data for disazo compounds based on 26. An X-ray trace is available on one form of (26: R = 3-PhNCO-2-0H-(l-naphthyl))148, while there are three polymorphs of (26: R = 27)149.150 in which coupling gives the a form; f3 and r are obtained on recrystallization from dimethylformamidel49 and phenyl nitrite, respective1yl50.

R-N=N-o-CH=CH-o-CH=CH-o-N=N-R

26

R =

HO CONHYCHl

-K CH~ o 27

28

Patterns are available for several disazo compounds based on cyanothiophene (28). Data are available for one crystalline form (called f3) of (28: X = N02, R = C2Hs' R' = C2H4CN)151; this is obtained by heating the coupled compound (called a) in water. However, a is amorphous and so this compound is not a polymorph. (28: X = N02, R = C2H40CH3, R' = C2H40COCH3) exists in three forms (two crystalline)152.153; coupling gives an amorphous form (a), heating in water at 60°C gives the f3 form, which is poorly crystalline152, while heating the a form in water at 80°C gives the r


29

form 153. The patterns recorded on the p and rforms are different. The final colorant in this group is (28: X = H, R = C2Hs' R' = C2H4CN); coupling gives the p form, which is very poorly crystalline, almost amorphous. Heating this form in water with a dispersant gives the a form 1s4, while heating in methyl alcohol gives the r form 1ss• Both a and rare crystalline.

29 is dimorphid s6 : both forms, a and p, are obtained on coupling, the p form at higher temperature.

Finally, during the course of some studies on Congo Red (C.I. Direct Red 28, c.I. 22120), Souaya, Moawad and Hanna reported the spacings of the strongest four lines, without intensities, of Congo Red and its iron, cobalt, copper and zinc complexes 157.

1.2.4 Azoic compounds

It would appear that powder patterns exist for many azoic compounds, both diazo and coupling components, and a list of these is given in Table 1.1. In all cases only one form has been reported at room temperature. All of the reported X-ray patterns of coupling components are presented in one excellent article, which includes not only lists of interplanar spacings and their intensities but also relatively large-scale diffractometer traces 1S8 •

1.2.5 Stilbene optical brighteners

These are usually symmetrically substituted triazinyl stilbene structures (30) in which R is often anilino or substituted anilino and M is usually H or Na. Often these compounds are obtained in a yellow amorphous or poorly crystalline form which causes discoloration of the detergent with which they are combined. However, they can also be obtained in white crystalline forms which are preferred.

In the earlier review I , it was observed that Tscharner had reported X-ray patterns for two products (30: R = anilino, R' = N(CH3)CH2CHPH, M = Na) in which he termed the two forms 'starting material' and 'a-modification' but that the a-modification was a monohydrate not anhydrous 1s9; the pattern of the latter has been verified subsequentlyl60. Subsequent to this earlier work,


Table 1.1 Azoic compounds

Azoic compound c.1. No. Reference

Diazo 44 37000 PDF 32-1565 Diazo 3 37010 PDF 22-1701 Diazo 6 37025 PDF 38-1960 Diazo 7 37030 PDF 38-1961 Diazo 37, Developer 7 37035 PDF 38-1962 Diazo 9 37040 PDF 1-553 Diazo 6 37107 PDF 11-933 Diazo 112 37225 PDF 9-794 Diazo 113 37230 PDF 9-807 Diazo 114 37265 PDF 7-549 Diazo 37270 PDF 24-1815 Diazo 36 37275 PDF 11-963 Coupling 1, Developer 5 37500 PDF 23-1785 Coupling 2 37505 PDF 11-888 Coupling 10 37510 PDF 11-885 Coupling 17 37515 PDF 11-891 Coupling 18, Developer 21 37520 PDF 11-877 Coupling 31 37521 PDF 11-875 Coupling 21 37526 PDF 11-876 Coupling 20, Developer 22 37530 PDF 11-874 Coupling 34, Coupling 41 37531 PDF 11-878 Coupling II 37535 PDF 11-883 Coupling 19 37545 PDF 11-890 Coupling 12 37550 PDF 11-889 Coupling 14 37558 PDF 11-884 Coupling 4 37560 PDF 11-887 Coupling 7 37565 PDF 11-886

R R'

N ')-NH I '\ CH =CH \ NH---{ N 'rN -Q- -y\\ N-{ rN - - N=\

R SO 3M S03M R

30

Tscharner and colleagues have published a patent161 in which the nomenclature is very confusing; it contains four patterns in all: (30: R = anilino, R' = NH(CH2)PCH3, M = Na, heptahydrate); (30: R = anilino, R' = NH(CH2)P CH3, M = Na, heptahydrate); (30: R = anilino, R' = NH(CH2)PCH3, M = Na, monohydrate); and (30: R = anilino, R' = NH(CH2)PCH3, M = Na, monohydrate). The confusion arises because both heptahydrates are termed a phases and both monohydrates are termed P phases, implying that there are two polymorphs of each compound. Of course this is not so as all four brighteners are chemically distinct rather than polymorphs of each other.

Two patents contain very similar patterns and use identical unusual nomenclature for (30: R = R' = aniIino, M = Na)162.163. They contain five X-ray


patterns without intensities; one is from a product made using a previously known technique and four products are claimed to be new. The pattern for the earlier product is essentially that quoted previously for the 132 phasel64 while the new forms are termed Rod Form A Hydrated, Rod Form A Anhydrous, Rod Form B, and Plate Form. The hydrated compound contains about 6% water, which is equivalent to three molecules of hydration. The pattern derived from Rod Form B is the same as the strongest lines obtained from the PI phase reported previouslyl65, and so may not be a new polymorph. The same two patentsl62.l63 include a pattern for (30: R = anilino, R' = morpholino, M = Na) and confirm a pattern reported previouslyl66.

Recent patterns for the two polymorphs, a and 13, of (30: R = o-toludino, R' = morpholino, M = Na)lfi7 also agree with those reported previouslyl68. However, in addition to these, there is a very comprehensive pattern available for an 'alpha-crystalline form' of the same compoundl69 which does not agree with either of the patterns in the previous references. None of the sources reports the chemical analyses, and so there must be some doubt as to whether the compounds are chemically identical (e.g. one could be an hydrate). A pattern of a 'white crystalline' form of (30: R = anilino, R' = NHCH3, M = Na) also has been reported 170.

Finally, there is an example of a triazolyl stilbene brightener (31) which exists in three forms; a, the normally prepared form which exists as light green crystals and the 13 and r forms which are yellowish white. X-ray data are given for all three, but only four interplanar spacings of each 171.

31

1.2.6 Quinacridones

1.2.6.1 Linear trans-quinacridone. At the time of the previous reviewl there was some evidence for the existence of seven polymorphs, a, d , 13, y, r' , 0 and 8, and so many X-ray patterns had been reported that it was decided to tabulate the best pattern for each. In the present chapter, more recent patterns are compared with those tabulated previously, and the patterns for several more polymorphs discussed; the additional ones are termed PI' ri, zeta and 'new'.

The more recent determinations for the a form 172 - 176 agree with those reported previously I and there are really no improvements in quality.


Several more patterns have been reported for the P form [PDF 36-1850fI.l73-177; [PDF 4l-l870f8 and are in agreement with the previous reviewl. However, the latter is more extensive and is to be preferred. The new PI form l79 is not very well characterized, the only difference between the original P and the new PI modification is that the latter has a close doublet at 5.50 A and 5.39 A whereas the former has a single peak at 5.52 A and the latter has two extra weak lines at 4.77 A and 4.24 A but not the one at 6.38 A. The line at 6.38 A does not appear in every pattern and may be caused by an impurity. The other differences could be attributed to the PI modification giving a better resolved pattern; this could be the result of larger particle size or better crystallinity. Since the PI form is heated in a methyl alcohol/water mixture at 95°C and subsequently vacuum dried at 80°C, the possibility of improved crystallinity cannot be ruled out.

Similarly the pattern of r-quinacridone has been reported on several occasions: [PDF 36-1851 ]".173-177; [PDF 41-1871 ]178; [PDF 25_1782]180-186. Any doubts about the pattern of r-quinacridone must now be considered settled. Chung and Scott l80 grew single crystals of the materials and were able to index the pattern that is now the definitive one. Subsequently, an additional pattern [PDF 41-1871]178 was indexed by the ICDD that is more extensive, probably because of better crystallinity of the specimen. It must be noted, however, that the listings by Thomas and Ghode l75 for rand r' are those of 2B, and the a, P and 8data in the same table are interplanar spacings (d). Additionally, in the earlier reviewl the pattern tabulated for r included a doublet near 6.5 A; four later references claim that this is actually a tripletI76.178.180.186. Doubts were cast on the existence of r' -quinacridone in the earlier reviewl since the main evidence for its existence was that the X-ray pattern contained a triplet oflines at approximately 6.5 A rather than the doublet found in the pattern of the r form. Soon after, it was shown that if the rform was irradiated with Cr Ka radiation, causing the pattern to be dispersed, the doublet became a triplet'78. Because of this evidence and the better patterns of r noted above, the existence of r' -quinacridone must be discounted.

Jaffe has claimed another form of quinacridone, r I' similar to the rform, and which he refers to as rll" However, there is very little difference between the X-ray patterns of these forms. The difference could be accounted for by the r I form having slightly poorer crystallinity or smaller particle size. As the process for obtaining rl from rn (i.e. r) includes milling, this may be the reason; certainly, the X-ray evidence for claiming r I as a new polymorph is not acceptable.

Further listings of X-ray data for the 8 form have become available. These are compatible with tabulated data I, but two of the listings give only the strongest lines, and it is noticeable that these lines are a fairly good fit to those of the r form. However, it appears that the 8 form has a weak line at 15.1 A, and this line cannot be indexed from the single crystal data of the rforml80. Consequently, this is really the only diagnostic line, assuming it is not caused


by an impurity. Interestingly, the fact that this is a weak line means that a list of strong lines alone does not characterize this form.

Two patterns have been reported for the & phase173.174 that agree with each other, but are from the same source. Although interplanar spacings from these patterns do not quite agree with those reported previousli, if one converts the interplanar spacings to values of20, the two sets are fairly close. Specifically, the latest values of20are higher than the earlier values by an average of 0.2°. This is greater than one would expect from the random experimental error (0.05°) given in the literature5• More interesting is the fact that the latest listings are almost a perfect fit to the better resolved patterns of r phase. Thus, there now seems to be some doubt as to the existence of & form, as the original data may be attributed to a slightly inaccurate set of values from the rform (possibly due to a misaligned diffractometer). Incidentally, one of the latest studies173 gives two lists of data for the & phase, one in the text and one in a table. The two lists of interplanar spacings in Angstrom units do not agree. The list in the table is correct, that in the text is actually a list of values of 20 (Cu Ka radiation).

The existence of another phase, zeta form, has been claimed173; however, the X-ray pattern is very similar to that of the a form. Comparing the two patterns, the strong line at 3.46 A in the pattern for a appears as a doublet (3.45 A and 3.35 A) in zeta. The strong line at 3.19 A in a appears as a doublet (3.18 A and 3.11 A) in zeta, and the zeta pattern contains an additional strong line at 2.11 A and two weak lines at 3.91 A and 2.83 A, not present in the pattern for the a form. These differences may be accounted for by the zeta form having a larger particle size or better crystallinity. Therefore, the powder diffraction evidence for the zeta form is not conclusive at this stage, since it could be a more crystalline specimen of the a form. Unfortunately, there are no available traces derived from the zeta form on which to test this hypothesis.

Finally, a 'new' form oflinear trans-quinacridone has been claimed 176. Here again there is a similarity between this pattern and that seen for the a form. There are two additional very weak lines in the 'new' pattern which could be caused by an impurity or better crystallinity. Therefore, it would be unwise to accept this 'new' form as a new polymorph without further study.

1.2.6.2 Substituted quinacridones. Since the last review 1 two substituted quinacridones have been given c.1. names and numbers, and both have been reclassified as indigoids1S7; however they are treated in this section. The two colorants of concern are 2,9-dichloroquinacridone (C.1. Pigment Red 209, c.1. 73905) and 2,9-dimethylquinacridone c.1. Pigment Red 122, C.I.73915).

Chlorinated quinacridones. A pattern for an unnamed form of 2,9-dichloroquinacridone (C.1. Pigment Red 209, c.1. 73905) is in the X-ray


powder data file [PDF 36-l783r I. Comparison of the published data with data published previously188 for a, fJ and r forms indicates that the new pattern corresponds to none of these, suggesting that this may be another polymorph. However the pattern is in very good agreement with a pattern of the a form of 3,1O-dichloroquinacridoneI89 and it could be that the sample received was mislabelled.

An X-ray pattern for the fJ form of 4,II-dichloroquinacridone (C.I. Pigment Red 207) [PDF 25-1600r 80 has been reported. This pattern has been indexed, but it is in poor agreement with that reported previously for the fJ form l90. On the other hand, it is in excellent agreement with the previously reported pattern for the 8forml91 . Since renaming polymorphs in this manner may lead to confusion, great care must be taken.

Methyl-substituted quinacridones. The pattern of two types of 2,9-dimethylquinacridone, types A and B, have been reported l91; these patterns fit those reported previously for forms IV and III, respectivelyl93. The powder data file contains a pattern for an unnamed form under the designation ofC.I. Pigment Red 122 (C.I. 73915) [PDF 36-1799]11. Comparison with previous data suggests that this is a poorly crystalline sample of form IV 193.

Additionally, X-ray powder traces have been reported for three polymorphs, a, fJ and r of N,N' -dimethylquinacridoneI94.

1.2.6.3 Miscellaneous data. Another pattern for 6,13-dihydroquinacridone has been reported l81 that agrees with those reported previouslyI95-197. Since the latest data do not contain measurements of intensities, the early data are preferred I95.196.

The only available data on a mixed crystal are that for a mixture of 60% quinacridone and 40% 2,9-dichloroquinacridoneI92 . The data do not agree with that reported previously for the same compositionl 98 although there are some similarities.

1.2.7 Dioxazines

The most comprehensive X-ray data published are on Dioxazine Violet (C.I. Pigment Violet 23, c.I. 51319) which is dimorphic. The stable fJ form is obtained from the unstable poorly crystalline a form either by mixing the latter with an organic compound having low solubility in water and heating, by forming the sulphate and hydrolysingl99, or by heating in the presence of an acetate ester2OO. Data are given for both forms. The pattern for the fJ form has been independently confirmed [PDF 36-1852]11. There are slight differences in the intensities of the diffraction lines of the latter compared with the other references but this could be attributed to different methods of collecting the data.

A pattern for C.I. Pigment Violet 27 is also available [PDF 37-1853]11.


1.2.8 Amino ketone compounds

Within this group, more X-ray data exist for Isoindoline (C.1. Pigment Yellow 110, c.1. 56280). An early reference201 claims that it is dimorphic: the normal phase, a, changing to P by heating; X-ray patterns for both are given. Subsequently, a third polymorph, y, was produced by treating a or p with liquid ammonia202 and patterns are given for all three phases. The more recent patterns for a and p agree with the previous ones, but contain sharper peaks. These are probably the best patterns for the a, p and y forms. All three patterns were confirmed subsequently in a patent reporting another way of producing y form203. References describing a fourth form, 0, along with further confirmation of the patterns of a, p and y204, and finally a fifth form, s 205, are also available. However the pattern reported for the s form is almost identical to that for the a form; the only difference being an extra line in the s form at 2Bequals 28.1°. In view of this small difference, which might be caused by a slight impurity, it would be unwise to accept the existence of the s form without further evidence. One pattern of this compound is found in the X-ray Powder Data File [PDF 36-1863]11 which is that of the a form.

In addition to patterns for c.1. Pigment Yellow 110, there are also patterns for c.1. Pigment Yellow 109 [PDF 36-1864f1 and c.1. Pigment Brown 38 15.

1.2.9 Anthraquinone compounds

Of the large number of commercially available anthraquinone dyes, relatively few have been characterized by X-ray powder diffraction.

There are two patterns available for alizarin (C.1. Mordant Red 11, c.1. 58000), [PDF 9-805, PDF 14-887]. These are in general agreement with each other, but the later data [PDF 14-887] have been indexed from single crystal data and are preferred. Patterns are also available for quinizarin (C.1. Pigment Violet 12, C.I. 58050:1) [PDF 33-1858] and quinalizarin (C.1. Mordant Violet 26, c.1. 58500) [PDF 9-820]. There are two patterns for dibromoanthanthrone (C.1. Pigment Red 168, c.1. Vat Orange 3, c.1. 59300) in the X-ray Powder Data File [PDF 36-1784, PDF 36-1794]11 which agree with each other with regard to the strongest lines, but show some differences among the weaker lines; they are in reasonable agreement with the data given by Warwicker 206. There is another pattern of this colorant in the form of a trace207 and although it is not possible to obtain accurate values of2Bfrom the trace, it is possible to match the PDF data to it. This suggests that PDF 36-1794 contains the best data. The study that gives the trace of dibromoanthanthrone also includes traces of dichloroanthanthrone and of a mixed crystal of 80% dibromoanthanthrone and 20% dichloroanthanthrone207 • The traces of the dibromo and dichloro analogues are completely different, while the trace from the mixed crystal is very similar to the dibromo compound. This suggests that a solid solution occurs between the mixed crystal and the dibromoanthanthrone.


Data are available for violanthrone (C.I. Pigment Blue 65, c.1. Vat Blue 20, c.1. 59800) dibromo-16, 17-dimethoxyviolanthrone (C.I. Vat Green 2, C.1. 59830) and 16,17-dimethoxyviolanthrone (C.1. Vat Green I, c.1. 59825)208 Vat Green I is dimorphic, the phase change being induced by milling208. There are two other patterns for violanthrone available206 [PDF 4-01 34fo9 which are in good agreement with each other where they overlap (both are really incomplete patterns); however, they do not agree with this latest pattern208 and so it is possible that violanthrone is dimorphic.

c.l. Disperse Blue 60 (C.l. 61104) has the formula (32: R = (CH2)PCH3).

A pattern for this dye and for a related one (32: R = (CH2)P(CH2)PCH3) is avaiiable210 together with patterns of a series of mixed crystals of the two, having ratios of I : 0.8, 1:1, 1: 1.2, 1: 1.5 and 1:5. These patterns suggest that there is probably a solid solution over the ratio range I :0.8 to I: 1.5, and since there are similarities between these patterns and that seen for the ratio 1:5, the solid solution may extend to include this composition. Interestingly, the patterns from the mixed crystals do not resemble those from the pure compounds. The benzanthrone dye, c.1. Solvent Orange 63 (C.1. 68550) is dimorphic; the f3 form is obtained from the a form by heating in sulphuric acid211 . Traces are given for both.

32

Another pattern of in dan throne (C.1. Pigment Blue 60, C.I. Vat Blue 4, C.I. 69800) has been reported [PO F 36-1885]11; comparison with previous reported patterns indicates that this is the a phase of Warwicker206 or ala phase of Honigmann212. The same original source ll gives the data for a form of dichloroindanthrone (C.l. Pigment Blue 64, c.l. Vat Blue 6, c.l. 69825) which agrees with the ala phase ofHonigmann212 . It is this pattern that is in the PDF [PDF 36-1886] although Honigmann's pattern212 is more extensive. Another pattern of flavanthrone (C.l. Pigment Yellow 24, c.l. Vat Yellow 1, c.l. 70600) has been reported [PDF 36-1881 ]11. It agrees with the one reported by Warwicker206. Also, a pattern has been reported for c.l. Pigment Orange 43 (C.l. 71107) [PDF 36-1879]11.

c.1. Acid Blue 324 is dimorphic213 , the forms being called 'old' and 'new'. The 'new' is obtained by heating a slurry of the 'old'.

Cis-naphthoylene bisbenzimidazole (C.I. Pigment Red 194, c.1. Vat Red 15, c.1. 71100) has been investigated four times21 4-217. Two studies214.215 agree


that it is dimorphic, and both give traces of the two polymorphs that are very similar to each other. 2(} values of the P form, which are consistent with the form used as the pigment Bordeaux Red are given215. This form can be obtained from the a form by ball-milling with mineral acid and simultaneous treatment of the amorphous product with an organic solvent, by boiling in high boiling point organic solvent 215 or by treating with a mixture of ethyl alcohol, alkali, surfactant and water 216 • Confirming patterns of the 2(}values (no intensities) are also available216.217 •

A pattern is also available for CI. Pigment Orange 52 [PDF 36-1880r I . I-Amino-2-methoxy-4-hydroxyanthraquinone has been shown to be di

morphic, having a andp forms 218 • The a form is obtained by methylation in acetone with subsequent reprecipitation from sulphuric acid, and the P form is obtained by methylation and recovery from acetone.

4A'-Dibenzylamino-I,I'-dianthraquinone (the dibenzyl derivative ofCI. Pigment Red 177, CI. 65300) exists in three forms, a as produced and P and r after milling the a form with salt followed by treatment with benzene or dimethylformamidell9 . Powder traces of all three forms are given together with a table which includes the values of 2(} and interplanar spacings; but, the listings of 2(} do not agree with the powder traces (though some individual values may agree) and recalculations show that the values of2(}do not always agree with the quoted values of inter planar spacings. Therefore, care must be taken with these data. Unfortunately, the traces are too small to remeasure the values of 2(}.

Some X-ray data have been collected on IA-diamino-2-benzoylanthraquinone and one of its derivatives220 • I A-Diamino-2-benzoylanthraquinone itself is dimorphic, and the two forms can be produced by precipitating the dye from sulphuric acid into water at different temperatures. The derivative studied, I A-bis (p-nitrophenylamino )-2-benzoylanthraquinone, is monomorphic, and small scale traces are given.

33, a disperse dye, is dimorphic having A and B forms 221 • The B form is obtained from A by treating in water, sometimes with the addition of a dispersant.

0»0 NH2 o-o~ :?" I I -...:::: -~ h

o N-S02-o-~ CH3 H -

33

Three X-ray patterns are given for sodium-l-amino-4-bromoanthraquinone-2-sulphonate222 : one for the A form as produced and two, Band C


after heating at different temperatures. Two patterns are available for 1-amino-2-[4-[N-(3-ethoxypropyl) sulphamoyl] phenoxy]-4-hydroxanthraquinone: a as prepared and p after heating223.

There is also an X-ray pattern for a mixed crystal of (32: R = CH2CH2 CHPR' where R' is CH3, C2H5 and CH(CH3)2 in the ratio of 1:1:1) and c.1. Disperse Blue 165 (C.1. 11077) (3: 2,6-dicyano, 4-nitro, R = R' = C2H5) in the ratio of 95:598 . The pattern is that of a poorly crystalline compound. This patent also contains the pattern of c.1. Disperse Blue 165 (C.1. 11077) as mentioned in section 1.2.2.3.

1.2.10 Perylene compounds

Most studies providing X-ray data involve c.1. Pigment Red 149 (C.1. 71137), which has been examined several times224-227. Initially it was reported to be dimorphic224 with a and P forms, the P form being obtained by heating the a form in an organic solvent. Subsequent work indicated that it was trimorphic225 with a third r form. This later study confirmed the patterns of the a and P forms; these patterns are preferred because they are more extensive and include better estimates of the intensities. In addition, there is a pattern of an unnamed form in the PDF [PDF 36-1798rl. This is the a form, and traces of the a form are also given in further references226.227 •

A further pattern (interplanar spacings and traces) is also available for c.1. Pigment Red 179 (C.I. 71130)228. Although there is general agreement between this and the one reported previously229 the line of strongest intensity in the earlier reference is not reported in the later one, and it may have been the result of an impurity.

An X-ray pattern is available for C.I. Pigment Red 178 (C.1. 71155) [PDF 39-1566].

Some work has been reported on mixed crystals involving c.1. Pigment Red 149 (C.1. 71137) (34: R = 3,5-dimethylphenyl). The X-ray pattern of 0.9% by weight (34: R = 4-ethoxyphenyl) in C.1. Pigment Red 149 has been reported226. This pattern is very similar to that of the a form of c.1. Pigment Red 149 but the relative intensities are sufficient to distinguish between them. In another study227, X-ray patterns are given for compositions of 90 mol% c.1. Pigment Red 149/10 mol% (34: R = H) and also of90mol% c.1. Pigment

34


Red 149/10 mol% (34: R = CH3)' Again, these mixed crystal patterns are similar but not identical to that of the a form c.1. Pigment Red 149 and suggests that there is a solid solution between these compositions and the a form ofC.I. Pigment Red 149.

An X-ray pattern has been reported without intensities for 1,6,7,12-tetrachloroperylene-3,4,9,1O-tetracarboxylic acid (containing 26.9% chlorine) in the p form230. Unfortunately, the pattern for the a form is not given for comparison. According to Dietz and U rban23 ', this P form is the stable phase in mixed crystals of tetrachloro-, trichloto- and pentachloroperylene-3,4,9,1O-tetracarboxylic acid. The example given is for a ratio of 81: 17:2 of these three compounds (chlorine content 25.1%), and the pattern reported without intensities agrees with that given previously230. It appears that the p form is stable at chlorine contents as low as 20.4%.

1.2.11 Indigoids

Very little data exist on this group of colorants. An X-ray pattern is available for c.1. Pigment Red 88 (C.1. 73 312) [PDF 36-1790]". It is also known that 2,2' -bis(naphtho-[2, I-c] thiophenylidene )-1, l' -dion (35 is dimorphic, the different forms being obtained by different methods ofpreparation232 •

35

1.2.12 Phthalocyanines

The interest in phthalocyanines has continued and they are continually being reinvestigated by X-ray powder techniques. In the more recent work, the primary area of investigation has tended to move from copper phthalocyanine to oxytitanium phthalocyanine because of the latter's property as a photoreceptor.

1.2.12.1 Metal-freephthalocyanine (CI. Pigment Blue 16, CI. 74100). In the earier review', it was noted that there was evidence for three polymorphs in phthalocyanine: a, p 233 and X forms234. A fourth form has been claimed233,

but Honigmann235 had already shown that this was a poorly crystalline a form. Since then, in addition to confirming patterns of these polymorphs becoming available, the existence of several more polymorphs have been claimed. In order of discovery, these have been named &, 71, r, modified 71


and r. One of the problems with the methods of confirming these various polymorphs is the lack of certainty regarding whether a single polymorph rather than a mixture is under examination.

The only patterns available in the powder diffraction file are those for the a form [PDF 36-1882]" andjHorm [PDF 37-1844]. The patterns for both of these have been confirmed several times236-240. In general, the discrimination of these patterns is not as good as those in the file, and for this reason the file patterns are preferred.

The various traces and listings of the X_form237-242 agree with the original234. Unfortunately, none of the patterns is very good, as none includes the values of the intensities (although these can be estimated from the various traces) and none of the specimens investigated is very crystalline.

An £ phase of the metal-free phthalocyanine has been reported twice236.243, only one source of which contains a trace236; this is important, as the pattern for £ phase can easily be confused with the X form unless the intensities are known. The two strongest lines of both forms occur at 2(} values of approximately 7.60 and 9.1°, having interplanar spacings 11.6 A and 9.7 A, respectively. In the X form, the second line is approximately 85% as strong as the first, while in the case of £ phase this ratio is about 55%. Without this distinction, the £ phase could easily be attributed to a better crystalline specimen of the X form.

The 17 form is very poorly characterized238.244. The spacings fit the X form and the traces238 could easily be a pattern from a better crystalline specimen of the X form. The earlier patent238 contains traces for both X and 17 forms, and although these look dissimilar, there is quite good agreement between this trace for the 17 form and the original trace for the X form234 as the latter specimen is slightly better crystallized. In these circumstances, it would be unwise to accept the 17 form as a polymorph on the X-ray evidence presently available.

Similar criticisms can be applied to X-ray data from the • form. This form has been reported four times237.239.245.246, three times from the same laboratory237.239.245. The interplanar spacings fit the X form within experimental error but, more interestingly, the traces from the same source are somewhat different. One239 contains more diffraction lines than the others237.245 and the extra lines could be caused by an impurity. The traces from the last specimens237.245 could easily be explained as originating from a slightly better crystalline specimen than the original trace of the X form234. Assuming that these three specimens are the same pure phase (as claimed), again it would be unwise to accept the. form as a new polymorph. However, ignoring the claims and simply examining the traces, it is reasonable to question whether the. phase was produced in two cases237.245 and to suggest that the X form was produced instead. The • form may exist in the other case239. The fourth study2~6 claims that the. form can be obtained in two types, called types I and II, and gives traces for both. These are similar to each other (type II being


more crystalline than type I) and to one of the earlier references239• However, the morphology of the two types is different. Type I is needle-shaped, while type II is equant (granular) and relative intensities are changed by morphology. These differences, and the better crystallinity, could account for the differences between the traces for the two types. Because of this, the pattern from type II is to be preferred, assuming that the rphase exists.

The pattern for modified rl4fJ is very similar to that for 77 phase and a slight impurity could cause the differences. Again, there are doubts as to whether this is a different polymorph.

The evidence for the r phase consists of a listing of28 values without either intensities or traces247. The list is a very good fit (within experimental error) to the original trace of Bryne and Kurz234 for the X form and, hence, conclusive evidence for the rform is not available.

1.2.12.2 Copper phthalocyanine (CI Pigment Blue 15, CI 74160) and its derivatives. When the previous review' was written, there were claims for the existence of nine polymorphs of copper phthalocyanine for two of which the evidence was unsatisfactory. Because of the large number of patterns and traces that were available, a tabulation of the 'best' data for the other seven polymorphs, which are named a,p, r, 8(called 88 in the table), &(or R), X and 7rwas provided'.

Since then, additional data have become available on all the earlier polymorphs, and the existence of five others has been claimed: p, a and three called 'new'. To differentiate between the last three, in this review they will be called new (W), new (SFHO) and new (SFHK) (the letters come from the initials of the inventors' surnames).

There are several other listings or traces given for the a form248-257 in addition to that given in the previous review' and two are given in the powder data file [PDF 22-1686; PDF 36-1883]" All are in good agreement. The preferred listings are probably [PDF 36-1883]'-"; the former study is the more extensive but the specimen in the latter has better crystallinity. The best trace (one from the best crystalline specimen) is neither of these255 but, unfortunately, it does not list all of the interplanar spacings nor provide the intensities.

There are several listings or traces for the p form248-25 1.257.258 in addition to the earlier review' and two occur in the powder data file [PDF 36-1881]" and [PDF 37-1846]. Of these, the last[PDF 37-1846] is by far the best. It is the most extensive and the pattern has been indexed from single crystal data. The others are in agreement.

Two further patterns for the rform252.257 both confirm the data in previous review'. Since neither provides an improvement, the earlier pattern remains the preferred one.

The two additional patterns for the 8 form are completely differene52.257 •

When examined, it appears that one is for the original 15K form252, which is structurally the same as the & (or R) form, rather than a new polymorph'.


In addition, the new trace given for this 15K (or E or R) form252 is also incorrect; it corresponds to that from the X form. The other pattern for the 8 form257 confirms that given in the review, with the exception of one line which was quoted in error as 7.85 A (Table 10.2)1 rather than as 8.75 A. The new trace257 lists the interplanar spacings, but not the intensities, for all the then existing polymorphs of copper phthalocyanine together with a new one.

The reported patterns for 8/52 and E and R 257 also confirm the existing pattern 1 but omit the intensities. The same applies to the confirming patterns for X form252 and "form252,259,260, Therefore, none is an actual improvement on those reported previouslyl,

Of the more recently discovered polymorphs, the p form has been reported several times252,261265 from the same source, As a result, it is difficult to know if the patterns are confirming patterns or simply repetitions, However, the X-ray pattern appears to be unique, and so the p form does seem to be a new polymorph,

A listing of interplanar spacings (no intensities) is available for a second new phase, a, and again the pattern appears to be unique257,

There are three 'new' phases. New (W)253 is structurally no different to the a form. It may have improved colouring properties but it is not a new polymorph. The new (SFHO) form260 gives a pattern which can be attributed to a better crystalline specimen of "form, The pattern from the third new (SFHK) form256 can be accounted for if the specimen is a poorly crystalline sample of the a form, Therefore, none of the three 'new' forms is acceptable as a structurally different polymorph.

A new pattern of CI. Pigment Green 7 (CI. 74260) has been reported [PDF 36-1870]11, This is a poly-chi oro copper phthalocyanine with 15~16 chlorine atoms in each molecule. A trace for this pigment is already available235 , and, although the patterns have some features in common, there are sufficient differences to give the impression of two similar but different structures. This could be a result of different numbers of chlorine atoms in the molecule. A pattern for CI. Pigment Green 36 (CI. 74265) has been reported [PDF 36-1871]11.

1.2,12,3 Nickel phthalocynanine (CI. 74160,1), Traces for three polymorphs of this compound have become available267 a, fJ and E. The first two agree with those reported previously268, and the third is a new polymorph. The later traces for the a and fJ forms are better than those reported previously.

1.2,12.4 Cobalt phthalocyanine (CI. 74160.2). Confirming traces for the a and fJ forms have been reported269 together with one for a new polymorph, E phase. No interplanar spacings or intensities are given for any of the phases; therefore, these patterns for the a andpforms are no improvement on those in the X-ray Powder Data File [PDF 22-1663 and PDF 14-948, respectively] .


1.2.12.5 Oxytitanium phthalocyanine. The X-ray data for this compound have been confused because different companies use different symbols for the various polymorphs, e.g. Toyo Ink, Dainippon and Sanyo use Greek letters a andp, while Mitsubishi, Konica and Canon use A, B, C and 0, and adoes not correspond to A and P does not correspond to B. In addition, later polymorphs have received a variety of names (m, y, M, I, X, Z-l and Z-2) and five new unnamed forms were published. This has caused the nomenclature to become chaotic. One reference270 has attempted to correlate the various polymorphs, but in doing so produced yet another set of symbols, (I, II, III and IV) for polymorphs published up to that point. As this particular patent starts to resolve the confusion, their nomenclature will be used in this section, where appropriate.

A further problem is that the patterns of various polymorphs have been reported several times, but not all are complete. As a result, it has been decided to re-examine and tabulate what is considered to be the best data for each polymorph (cf. Table 1.2), and to list the references containing those data. In general, the various patterns for the same polymorph are in agreement.

Type I represents the A form or pform; it has been frequently X_rayed27O--288 but some of the reported patterns are incomplete listings or small scale traces. The most extensive listing284 does not give intensities and the values reported here have been estimated from the trace.

Type II represents the B form or a form; again, it has been frequently Xrayed27O--278. 280--287. 289 and the quality of the pattern varies. The data given in Table 1.2 come from the most extensive listing278 • The intensities have been estimated from the trace.

Type III represents the C form. Although this form has been X-rayed several times27O--273.275.276.281.283, none of the listings generated is complete, most giving only the two strongest powder lines. However, one281 gives four out of the nine strongest powder lines. The position of the other lines and the intensities have been estimated from this trace to make the pattern in Table 1.2 as complete as possible.

Type IV represents the 0 form; it has been reported twice27o.283 but the pattern in Table 1.2 is from another source289. The positions of the seven strongest lines are taken from the reference; however, the intensities and positions of the other lines have been estimated for this review.

Although the m-form has been reported twice277.284, it is poorly crystalline. The reported traces consist of three broad peaks which coincide with the three strongest lines of type III. Under these circumstances, it would be unwise to accept the m form as a different polymorph.

The trace of the r form appears to be unique284 and its pattern is included in Table 1.2.

Both of the reports of the M form are from the same source285,286 and although they may not be independent, the pattern appears to be unique, The


Table 1.2 X-ray powder data for the polymorphs of oxytitanium phthalocyanine

Type I Type II Type III Type IV

d I d I d I d I

9.41 5 11.6 51 12.6 42 11.6 6 8.35 3 8.67 15 8.93 3 11.5 5 6.66 8 7.03 14 5.68 14 9.31 21 5.83 3 6.71 12 4.02 I 9.12 25 5.64 2 5.87 7 3.77 14 7.56 II 5.47 3 5.47 14 3.68 5 6.66 9 4.23 7 5.16 7 3.47 7 6.51 7 3.80 3 4.85 12 3.09 3 6.28 9 3.36 27 3.95 26 3.00 2 5.91 17 3.26 II 3.68 19 4.93 9 3.13 2 3.52 34 4.82 9

3.12 44 3.97 4 3.05 5 3.79 II 2.84 2 3.69 26

3.62 12 3.49 5 3.27 96 3.18 7 3.11 8 3.07 7 3.01 4

Y M New(T)

d I d I d I d I

12.1 6 12.3 43 9.83 52 11.6 34 5.0 3 6.24 13 6.51 17 8.43 3 3.71 10 5.01 9 6.24 29 7.08 5 3.28 23 3.71 20 4.93 14 5.68 7 3.12 6 3.28 75 3.72 33 5.41 4

3.11 10 3.29 \09 5.01 5 3.06 9 3.39 4

3.09 8 2.93 7 2.80 3

d, interplanar spacing; I, relative intensity.

same comment applies to the I form286.287 • The patterns for both are given in Table 1.2.

The X form is very poorly characterized270, as the pattern consists of one sharp peak and several small broad ones. The pattern could easily be that of a very poorly crystalline specimen of type IV, and so the existence of the X form must be doubted at this stage.

The X-ray evidence for both Z-l and Z-2 is of poor quality. In both cases, it consists of small-scale traces together with the values of 2(} given alongside each peak. Although both traces appear unique, their size prevents calculation of reasonable accurate values of intensities. In addition, some of the 2(}values given are incorrect.


There are five new unnamed forms274,279,282,288,289 which we will designate new (S), new (E), new (OKW), new (DTNNS) and new (FT), respectively. The letters in parentheses are the initials of the inventors' names274.

New (S) is very poorly crystalline, the pattern consisting essentially of one sharp peak and several small broad ones. It could easily be a pattern arising from a very poorly crystalline specimen of type IV, and the existence of this phase must be considered doubtful.

New (T)279 gives a unique pattern and may be considered to be a new polymorph. The pattern is given in Table 1.2.

Both new (OKW)282 and new (FT)289 are type IV, and the latter is slightly more crystalline than the former. The pattern of the latter is reported in Table 1.2.

The evidence for new (DTNNS)288 is a small-scale X-ray powder trace which could be attributed to a poorly crystalline specimen of Type IV.

X-ray evidence would indicate that, of the new unnamed phases, only new (T)279 can be considered to be an independent polymorph.

J.2. J 3 Miscellaneous compounds

There would appear to be three forms of the 1:2 nickel complex of I-nitroso-2-naphthol. Inman reports the X-ray patterns of two new polymorphs, A and B, the latter obtained by heating former29o • Also reported is a pattern from another polymorph which is referred to as MacQueen pigment. All three patterns are different. A British Patene91 quoted as equivalent to this reference gives an unsatisfactory pattern (six lines without intensities) for the B form only. Subsequently, Matlack292 reported all three patterns, but since the list of interplanar spacings and intensities is identical for both references290,292, the listings are probably not independent. Two further reports of the pattern for the B form have been published.293,294.

The polymorphism of some nickel acetoacetanilide dioximates (36) has been investigated by X-ray diffraction295 • The forms of preparation produce three polymorphs when R = 2' ,3' -dimethylphenyl, and two polymorphs when R = 2' -methyl-3' -chlorophenyl. In addition, this reference includes the pat-

H3C-C - C-CONH-R

II " O-N N-O / ,,/ ",

H Ni H " /" / O-N N-O

II " R-NHOC-C - C-CH3

36


terns for two polymorphs when R = phenyl, one arising from manufacturing, and the other by heating the initial form in boiling dimethylformamide.

A group of azine pigments (37) has been prepared where R = methyl and p-methylphenyl. According to one study296 both pigments are dimorphic, a and p, and the a form of both compounds is changed to p by heating in nitrobenzene. A later study297 includes the pattern of a r form (37: R = methyl) that is obtained by milling the a form. X-ray patterns for all three forms are included.

69c CONHC6Hs R

'/ I I N-Ni-O~O :::-.... I

~

N-N=CH eN

CH3

37

Data are available for two forms, a and p, of 5,7,5' ,7' -tetrabromo-8,8'dihydroxynaphthazine298. The pform is obtained by kneading the a form in a salt solution to which has been added a small percentage of an organic liquid. The patterns agree with those already reported299 •

X-ray data are available for three recording materials based on 3,3-bis[2,2-bis (p-substituted phenyl)ethanyl]-4,5,6, 7-tetrahalophthalide (38) all of which are dimorphic. All use a for the original form and p for the new form: (38: R = N(CH3)2' X = CI)3OO; (38: R = N(CH3)2' X = Br)301; and (38: R = pyrrolidino, X = CI)302. In addition, data are available for recording materials based on fluoran: 3-dibutylamino-6-methyl-7-anilinofluoran is dimorphic303,304 as is 3-dibutylamino-6-methyl-7-(4-fluoroanilino )fluoran305,

38


An X-ray diffraction trace has been reported for the dye (39: R = t-C4H9)306. Data are also available for the merocyanine-like dye 3-ethyl-5-[2' -(1' ,3'dithiolanylidene )]rhodanine307 and the polymethine dye l,3-bis (dimethylamino )trimethinium perchlorate308.

39

Finally, data have been reported on mixed crystals based on l,4-diketopyrrolo-[3,4-c]-pyrrole (40)309. The two compositions for which data are given are: 85% (40: R = 4-chlorophenyl)1l5% (40: R = 4-methylphenyl) and 85% (40: R = 4-methylphenyl)1l5% (40: R = 3-methylphenyl).

40

Acknowledgement

The author would like to thank Professor H.S. Freeman, without whose encouragement and assistance, this review would not have been completed.

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Chern. Abstr. 116,61573z. 288. K. Diamon, A. Tokida, K. Nukada, H. Nukada and Y. Sakaguchi (Fuji Xerox)(1 99 I) IP 3-

269061; (1992) Chern. Abstr. 116, 108316k.


289. Y. Fujimaki and S. Takenouchi (Konica Corp.) (1990) US 4,898,799. 290. CG. Inman (Hercules Inc.) (1967) US 3,338,937. 291. Hercules Inc. (1969) GB 1,144,770. 292. A.S. Matlack (Hercules Inc.) (1967) US 3,338,938. 293. R.K. Putney (Hercules Inc.) (1973) DE 2,306,988. 294. R.K. Putney (Hercules Inc.) (1979) US 4,148,666. 295. J. Kraska and W. Czajkowski (1984) Dyes and Pigments, 5,3. 296. R. Neeff, M. Rolf and W. Muller (Bayer AG) (1986) DE 3,430,800; (1986) US 4,614,547. 297. M. Tanner (Ciba-Geigy AG) (1989) DE 3,916,637. 298. H.-E. Baurecht and R. Homle (Bayer AG) (1976) DE 2,437,526. 299. P. Mertens and H.-S. Bien (Bayer AG) (1972) GB 1,270,173. 300. Y. Fujino, M. Shiraishi and K. Tsunemitsu (Yamada Chemical Co. Ltd) (1988) JP 63-

295672; (1989) Chern. Abstr., 110, 203038f. 301. Y. Fujino, Y. Uda, K. Mizuno and K. Tsunemitsu (Yamada Chemical Co. Ltd)(1989) JP 1-

168681; (1990) Chern. Abstr., 112, 384495j. 302. Y. Fujino, Y. Uda, K. Mizuno and K. Tsunemitsu (Yamada Chemical Co. Ltd) (1989) JP

64-85254; (1989) Chern. Abstr., 111, 123951d. 303. T. Numa (Nippon Kayaku Co. Ltd) (1985) JP 60-202155; (1986) Chern. Abstr., 104,

208835z. 304. Y. Tanabe and Y. Iwasaki (Yamamoto Chemicals Inc.) (1991) JP 3-195777; (1992) Chern.

Abstr., 116, 7926d. 305. T. Numa and H. Nimoto (Nippon Kayaku Co. Ltd) (1985) JP 60-217267; (1986) Chern.

Abstr., 104, 208836a. 306. M. Matsuoka, Y. Saitoh, H. Oka and T. Kitao (1990) Kobunshi Ronbunshu, 47 (11),875;

(1991) Chern. Abstr., 114, 83854c. 307. A. Whitaker (1975) ZeUs Kristallogr., 142, 442. 308. A. Whitaker (1977) ZeUs Kristallogr., 145,155. 309. F. Babler (Ciba-Geigy AG) (1988) EP 256,983.

2 Solid state NMR spectroscopy of synthetic dyes v

A. L YCKA, 1. JIRMAN and 1. STRAKA

2.1 Introduction

High-resolution NMR spectroscopy in liquids has been for many years one of the most frequently used methods in chemical structure determination. Therefore, it is not surprising that this technique has also been successfully applied to the analysis of synthetic dyes. IH NMR data have been summarized by Foris1 and Fedorov2 has collated some !3C NMR data of azo dyes. In addition, an application of two-dimensional NMR spectroscopy3 and multinuclear NMR studies4 of azo dyes has been published recently.

In contrast, solid state NMR spectroscopy has not been extensively used for structural analysis purposes. The main reason for this is the relatively wide lines in the NMR spectra of solids5 (e.g. typical line widths in the !3C NMR spectra of common organic compounds are on the order of 20 kHz). Such a broadening leads to loss of the chemical shifts and hence of information which is the main parameter in establishing a correlation of NMR spectra with the chemical and space structure of molecules. The reasons for the considerable widths of the signals in the NMR spectra of solid samples pertain to the presence of anisotropic interactions of the nuclear spins. These interactions are, in principle, present also in solutions, but the relatively fast and prevailingly isotropic molecular motion, typical of low viscous liquids, is able to average these interactions. Only isotropic average values of these interactions affect the NMR spectra of solutions. In the solid state, such a fast isotropic motion is essentially absent. Solid state NMR spectra shapes are affected also by interactions (dipolar and quadrupolar) that are not observed in the spectra in solutions and, in addition, by all interactions showing an anisotropic character.

Developments in the application of solid state NMR for structural elucidation became possible with the introduction of methods for high-resolution solid state NMR spectroscopy. High-resolution solid state NMR methods derive from the introduction of some motion into the system of nuclear spins. This motion is under experimental control and is coherent and anisotropic, contrary to the random and isotropic motion in solutions. The motion can deal with common coordinate space (rotation of a sample) or spin space (an irradiation of a sample by high-frequency magnetic fields). The consequence of this motion is the time modulation of molecular interactions that enables


averaging of selected interactions and suppressing, or at least simplifying, their effects on the spectrum. At present, the cross-polarization/magic angle spinning (CP/MAS) technique is the most frequently used method for the routine recording of solid state NMR spectra, giving results analogous to the high-resolution NMR spectra ofliquid samples. The objective of this chapter is to outline some applications of CP/MAS NMR spectroscopy for structure elucidation and some studies of the physical properties of organic dyestuffs.

2.2 Basic principles of cross polarization/magic angle spinning measurements

Anisotropic interactions of nuclear spins, which cause the broadening of NMR lines in the solid state, can differ considerably in both magnitude and their influence on spectra, according to the type of compounds and nuclei measured5• The most common technique in organic chemistry is the \3C CPI MAS NMR spectra measurement, and for this reason, the review will focus mainly on this method.

2.2.1 Dipolar interactions

Dipolar interactions are the main reason for the broadening of the NMR signals in the solid state of nuclei possessing spin 1= 112. In the case of 'H nuclei, broadening is caused by homonuclear interactions between protons, while in the case of \3C nuclei, broadening results from heteronuclear interactions between \3C nuclei and protons. Dipolar interactions are dependent upon the distance between interacting nuclei and on the orientation of the internuclear vector. In the case of common organic solids, this can be expressed by the dipolar Hamiltonian relationship'.

HD = Hcc + HCH + HHH (2.1)

The first term (Hcc) reflects homo nuclear dipolar interaction between \3C nuclei and, because of the low natural abundance level of \3C nuclei (1.1 %), it can be neglected in most cases. The second term (HCH ) corresponds to heteronuclear dipolar interactions between \3C and 'H nuclei and in strong external magnetic field is given by equation 2.26.

HCH =- (Po I 4Jr)YCYHli2LL rik-3(3 cos2 .9ik -l)Izilzk (2.2) i k

in which Yc and YH are carbon and proton magnetogyric ratios, Ii is the Planck constant divided by 2Jr, rik is the distance between interacting nuclei and t} ik is the angle between a \3C-'H vector and the direction of the external magnetic field. From equation 2.2, it follows that \3C-'H dipolar interaction leads to splitting of f3C signals, the extent of which depends on the magnitude and

SOLID STATE NMR SPECTROSCOPY 51

direction of the internuclear vector. The whole dipolar interaction for a given carbon is determined by the sum taken for all neighbouring protons with different distances and orientation ofthe internuclear vectors; this results in splitting of the signal into many components and significant broadening of the NMR signal occurs. In polycrystallinic or amorphous organic compounds, the typical half-widths of such broadened signals, assuming a rigid structure, are about 25 kHz7 and are independent of the strength of the external magnetic field. The last term in equation 2.1 corresponds to homonuclear dipolar interaction between 'H nuclei and is defined by equation 2.36•

HHH = II 2(.uo I 47r)r~1i2LLrik -3( 3 cos2 .9ik -Ii ~ 4-31 7.JZkJ (2.3) 1 < k l

This term must be taken into account even when recording l3C spectra, as it leads to important consequences (see below). In low viscous liquids, fast random rotational and translation movements cause averaging of the dipolar interaction via its orientation-dependent term. Its average isotropic value is equal to zero, which means that a dipolar interaction does not affect the shapes of the signals. The broadening in the l3C NMR spectra in the solid state caused by dipolar interactions between 'H nuclei can be removed using dipolar decoupling8. Dipolar decoupling results from the application of a strong highfrequency magnetic field having a frequency equal to the resonance frequency of the 'H nuclei during acquisition of the l3C free induction decay (FID). Dipolar decoupling in the solid state is analogous to that used in liquids to remove the influences of scalar interactions between l3C and' H nuclei. Dipolar interactions are much stronger than indirect spin-spin interactions, and this is the reason why the strength of the high-frequency field applied must be much greater in the case of dipolar decoupling. To ensure the efficiency of dipolar decoupling, the decoupling field strength must be high enough to minimize the effects of 'H-'H dipolar interactions. This is the reason for maintaining the term H HH of equation 2.1 for all dipolar interactions. Typical values of decoupling field strength vary in the range of 10--20 G (- 40-80 kHz). The dipolar decoupling is most effective when applied exactly to, or not far from, the resonance of the 'H nuclei. However, because of dispersion of the 'H chemical shifts (approximately 10 ppm), the dipolar decoupling cannot be exactly in resonance for all protons. A simple equationS was developed to address the residual broadening of signals. The second moment M'z of the decoupled line is given by equation 2.4:

(2.4)

in which M2 is the second moment due to l3C-'H dipolar interaction without decoupling and (J = arctan (lO,H/.1lOoH), where lO'H is the dipolar decoupling field strength and .1lO0H is the resonance offset (i.e. (J is the angle between the effective field and the z-axis).


2.2.2 Chemical shift anisotropy

The position of signals in an NMR spectrum is, in addition to other effects, determined by partial shielding of the external field by the electron cloud of an atom. This shielding is, in principle, anisotropic and can be described by the Hamiltonian relationship (equation 2.5)

----'B----' Hcs = I C1' B (2.5)

in which Bo is an external static magnetic field and ~ is the chemical shift tensor. An average value exists in the liquid state because of molecular rotations and, therefore, one isotropic chemical shift value (for which o;zo = 1/3 Tr (~), where Tr is the symbol for tensor trace) is observed. In the solid state, the position of signals in a spectrum is dependent upon the mutual orientation of a molecule and the external magnetic field. Poly-crystallinic or amorphous materials, containing a large number of molecules with different orientations, are usually used for experimental studies. In such materials the molecular orientation distribution leads to the distribution of resonance frequencies and a broadening of signals in the NMR spectra occurs. This broadening is proportional to external magnetic field and can be several kHz for commonly used field strengths (about 150 ppm for aromatic carbons7, i.e. 7.5 kHz for 4.7 T field strength corresponding to lH NMR resonance frequence 200 MHz).

The broadening caused by chemical shift anisotropy can be removed by using mechanical rotation of the sample. Rotation in the external magnetic field induces periodical changes in chemical shifts. If the rotation is fast enough with respect to the statil; line width, the spectrum is first order and is determined by the time average values of the chemical shifts. This average value for a)-nucleus with a given orientation is given by equation 2.67

(2.6)

in which p is the angle between the rotation axis and external field Bo; C1'Oi = 113 (C1'li + C1'2i + C1'3) is the isotropic chemical shift measured in the liquid state, where C1'ai' a = 1,2,3 are the principal values of the chemical shift tensor, and C1'pPi = A.i~ C1'li + A.;P2i + A.~i C1'3i' where A.ai' a = 1,2,3 are the direction cosines orientating the main tensor axes to the rotation axis. For rotation under the common anglep, it is possible to derive that the shape of the signals is the same as in the static case, but reduced by a factor 1/2 (3 cos2 P - 1). For a special angle, the so-called 'magic' angle (p = 54.7°), this factor equals zero, and the second term in equation 2.6 disappears. The average values of the chemical shifts are then equal to its isotropics values C1'Oi independent of the orientation of the nuclei.

In addition to the centre bands at positions given by isotropic chemical shift values, satellites (spinning sidebands, SSB) also appear in magic angle spinning (MAS) spectra. Their appearance is caused by modulation of chemical


shift interaction by sample rotation. These sidebands are located at integral multiples of rotational frequency from the centrebands. Their intensities at low spinning speeds reflect, approximately, the shielding anisotropy pattern; at high spinning speeds their intensities decrease with increasing speed. It is, therefore, advantageous to use sufficiently fast rotation, with respect to the width of a signal broadened by the chemical shift anisotropy effect, to avoid any complications in the interpretation of spectra caused by a great number of overlapping satellites originating from different centre bands.

It is important to note that the MAS technique is also able, at least in principle, to remove dipolar interactions. The limiting factor is the spinning speed. In the case of 13C nuclei broadened by dipolar interaction with protons, the spinning must be fast enough with respect to the proton line width (as determined by the term HHH in equation 2.1). From a comparison of typical proton linewidths (of the order of 20 kHz) with commonly available spinning speeds (1-8 kHz), it is clear that the application of the MAS technique for such a purpose is limited. Progress can be expected in the future by utilizing new commercially available probes which make rotational speeds of approximately 8-20 kHz possible.

2.2.3 Cross polarization

13C Nuclei belong to the so-called rare nuclei, together with, for example, 15N and 29Si. Typical features of these nuclei are a low natural isotopic abundance, a low magnetogyric ratio and long spin-lattice relaxation times. As a result of those properties, these nuclei give very weak signals in the solid state NMR spectra. Pines et al. 9 proposed the use of a cross-polarization technique (CP) to enhance such signals. The method utilizes the principle of double resonance reported by Hartmann and Hahn 10, where an indirect detection technique was used to enhance rare nuclei signals. The theoretical description of crosspolarization is usually based on thermodynamics and uses the concept of spin temperature. The spin temperature concept plays an important role in solid state NMR experiments and the basis for its development is described in standard texts6. 1I • NMR measurements utilizing cross-polarization can be divided into three steps9:

1. cooling of the I H nuclei system 2. temperature controlled contact of \3C and 'H nuclear sub-systems 3. detection of \3C nuclei signals.

The individual steps can be realized practically in several ways. The most frequently used combination is the one shown here; however, alternative techniques have been discussed by Mehring'2 and by Pines et al. 9

The pulse sequence of the standard experiment utilizing cross-polarization is shown in Figure 2.1.


(.7r12)x

Decoupling

CPx f\ .,........ Acquisition ,,, ~ -Figure 2.1 The pulse sequence for a standard experiment with cross-polarization, using 'H-"C

as an example.

The cooling of the lH nuclei system is realized using a ,./2 pulse, with subsequent application of the pulse with a 90° shifted phase (spin 10ck lO).

The spin temperature13 T' of the spin-locked lH nuclei is given by equation 2.7.

(2.7) in which Bo is the static magnetic field strength, T is the lattice temperature (equal to the initial temperature of the lH system) and B1H is the strength of the high-frequency magnetic field used for the spin-lock. Because the highfrequency magnetic field strength is usually much lower than that of the static magnetic field, the spin temperature T' is, therefore, much lower than the temperature of the lattice. Cross-polarization occurs when the larger and cooler lH system is brought into contact with the small and hotter 13C system. Contact is realized by keeping the lH spins locked, while simultaneously irradiating the carbon nuclei with a high-frequency field with a strength obeying the Hartmann-Hahn lO condition (equation 2.8)

YH B1H = Yc B 1C (2.8)

Fulfilment of the Hartmann-Hahn condition enables fast energy transfer between the two sub-systems, the whole energy of the spin system being conserved. Energy transfer is driven by dipolar interaction between 1 Hand 13C nuclei, which causes mutual spin flips between the energy levels in both subsystems. After a suitably long contact time (usually in the range of 1-5 ms), a common spin temperature is established. Its value is very close to the initial1H spin temperature because of the much higher heat capacity of the lH spin system. l3C magnetization is then achieved by employing the Curie law and Hartmann-Hahn condition (equation 2.9)13

Mc = Moc (YH/yd (2.9)

in which Moc is the 13C equilibrium magnetization corresponding to the lattice temperature. After cross-polarization the acquisition of the l3C signal is generated using a strong dipolar decoupling (eventually MAS). Besides giving direct signal enhancement, the use of cross-polarization is advantageous for another reason. The repetition time of the cross-polarization sequence is


limited by the proton spin-lattice relaxation time TI H, which is usually shorter than the analogous carbon relaxation time TIc. The total gain per unit time in the signal-to-noise ratio with respect to the conventional experiment, is about 103 for the J3C-IH systemJ3 under normal conditions.

2.2.4 Cross-polarization/magic angle spinning method

Cross-polarization/magic angle spinning (CP/MAS) is the most frequently used NMR technique to obtain high-resolution NMR spectra of rare nuclei (e.g. J3C, 15N, 29Si) in the solid state. It is based on the simultaneous use of high-power dipolar decoupling, cross-polarization and magic angle spinning. Schaefer et af. initially implemented this method and showed experimentally the advantages of the combination ofthe three previously known techniques 14.

The line widths in the CP/MAS NMR spectra in the solid state are usually 10-100 times higher than those in NMR spectra measured in the liquid state. Some subtle structural effects observable in solution spectra cannot be detected in the solid state NMR; other effects, e.g. the signals for small amounts of impurities, are also not detectable. In rigid crystalline compounds, the experimental line widths are 0.2-1 ppm in J3C CP/MAS NMR spectra. The main reason for this residual line width relates to insufficiently averaged dipolar interactions, chemical shift dispersion caused by packing variation in disordered systems (as it affects chemical shift), and susceptibility variationsI5-17•

The line widths and shapes of the peaks in J3C CP/MAS NMR spectra are influenced by effects typical of the solid state: conformational changes, inhibition of motions which are free in solutions, hydrogen bond formation, nonequivalent arrangement of the molecules in a crystal (molecular packing effect), etc. which lead to splitting, broadening and shifting of peaks. Some complications in structural interpretation arise from these factors. On the other hand, these effects allow various modifications of chemically equivalent compounds to be studied.

Quantitative analysis of CP/MAS NMR spectra is not as straightforward as in NMR of liquids. Peak intensities are affected by the cross-polarization kinetics, which are generally not the same for all nuclei (relative cross-polarization rates: CH3 (static) > CH2 > CH ~ CH3 (rotating) > C quaternaryl8) and thus they are generally not directly comparable. It is, therefore, recommended that the behaviour of the samples at different contact times be determined, followed by correction of the experimental intensities.

We should also briefly mention the question of high-resolution IH NMR in solids which differs from the situation in liquids where IH NMR data give complementary information to J3C NMR data and IH NMR spectra measurements are routinely used for structure determination. The proton spectra in the solid state are broadened by the strong homonuclear dipolar interactions between protons. This broadening cannot be removed as easily as the heteronuclear dipolar broadening of J3C bands. To average homonuclear dipolar


interactions, special multi pulse sequences13 must be used, and ultimately combined with the MAS (Combined Rotation and MultiPulse SpectroscopyCRAMPS5,13,19), The resolution in such spectra is strongly dependent upon the high-frequency field strength and exact adjustment of pulse lengths and phases for which special tuning techniques are required13 , Despite the high sensitivity of IH NMR spectroscopy and the considerable advancements in tuning techniques during the last few years, so far these techniques have not been routinely used in chemical analyses, Their use for chemical applications is also restricted (in addition to experimental difficulties) by the large residual line width compared to the small range of IH chemical shifts,

2.3 Examples of solid state NMR spectra of dyes

Figure 2,2a shows the conventional 13C CP/MAS NMR spectrum of compound 1. Because of the existence of the strong hydrogen bond, the two methyl groups are non-equivalent (8 (I3C) = 32.4 and 27,1), However, the differences in 8(13C) of the C=O groups probably coincide with the line widths and a single signal (8 (l3C) = 197,3) is observed, Note that all six carbons of the phenyl group give resolved signals (8 (l3C) = 141.6, 129,9, 127.0, 125,9, 118.4 and 113.7) in the solid state at 300 K.

In Figure 2.2b the spectrum of 1 recorded using the dipolar dephasing method5,19 is shown, This method helps in the assignment of signals arising from quaternary carbons (8 (13C) = 197,3 (C=O); 141.6 (C-NH) and 131.8

(b)

(a)

I 200.0

I 150.0

I 100.0

PPM

I 50.0

Figure 2.2 Conventional (a) and dipolar dephased (b) "C CP/MAS NMR spectra of compound I.


(C=N)) by suppressing the signals of carbons with directly bound protons. The sequence used for such measurements differs from the standard CP/MAS experiment (Figure 2.1) only in a delay between the cross-polarization contact and acquisition, during which the decoupler is turned off. Because of the strong dipolar interactions with directly bound protons during this delay, the magnetization of CHn carbons (n ~ 1) decays much faster than that of quaternary carbons. This delay must be adjusted experimentally to obtain a spectrum containing the selectively suppressed signals, typical values being in the range of 40-120 IJS. The suppression is not effective for mobile groups such as rotating CH3 groups, as can be seen in Figure 2.2b.

2

We have used20 selectively deuterated isopomers in the analysis of the l3C CP/MAS spectra of azo dyes. l3C CP/MAS NMR spectra of compound 2 (Figure 2.3) are shown as an example. In de ute rated Ci2H)s-N=N- compounds, C-2H carbons behave as non-protonated ones, giving appropriate signals (b\l3C)=128.8-130.3 (4C) and 115.6 (lC)) in the dipolar dephased spectra (compare spectra in Figure 2.3a and b); Full details are reported in the study20. Having used the selective deuteration and dipolar dephased spectra measurement, we were able to distinguish between CH group signals belonging to the active and passive components in this case.

Many dyes contain nitrogen atom(s) in their molecules, and the 99.6% naturally abundant isotope 14N (l = 1) can cause a broadening or splitting of the signal of carbon nuclei directly bonded to this nitrogen. This effect is caused by the non-zero quadrupolar momentum of the 14N nucleus, and can help in the assignment of signals in l3C CP/MAS spectra. This is illustrated in Figures 2.4 and 2.5. The signals for the C=O (8= 169.2) and =C-N (8= 136.5)


I 150.0

I 140.0

I 0

130.0 PPM

o I 120.0

I 110.0

Figure 2.3 Conventional (a) and dipolar dephased (b) "C CP/MAS NMR spectra of the aromatic region of compound 2 and dipolar dephased (e) "C CP/MAS NMR spectrum of its C6 ('H),-N=N

isotopomer.

3

groups in compound 3 give a typical' I :2' doublet. In contrast, no quadrupolar splitting was found for the ipso carbons21 in Ar-N=N-Ar'.

In other cases only a broadening of signals can be observed. The aliphatic parts of the 13C CP/MAS NMR spectra of compounds 2 and 4 are shown in Figure 2.5. CHzOH carbons (Figure 2.5a: b"(I3C) = 61.0 and 60.5; Figure 2.5b: b" (13C) = 62.6 and 57.3) give relatively narrow signals, while the signals of

I 180

(h)

(a)

SOLID STATE NMR SPECTROSCOPY

n

I 170

Figure 2.4

I 62.5

~n 1\

J I I I I I I

160 150 140 130 120 110 PPM

13C CP/MAS NMR spectrum of compound 3.

I 60.0

I 57.5 PPM

I 55.0

I 52.5

59

Figure 2.5 13C CP/MAS NMR spectra of the N(CH,CH,oH), groups of compounds 2 (a) and 4 (b).


4

CH2N (Figure 2.5a: 8(13C) = 53.2 (2C); Figure 2.5b: 8 (13C) = 59.4 and 53.6) groups are significantly broader.

The effect of 14N nuclei on the 13C CP/MAS is dependent on the magnetic field strength, and the resulting spectral pattern is affected by relative Zeeman and quadrupolar term contributionsl9 , the splitting being smaller at high fields.

Temperature is an important parameter when recording the NMR spectra of solids. Temperature changes can have a considerable effect on the spectral patterns connected with freezing (when cooling the sample) or the initiation of molecular motion upon heating the sample. Measurements of solid state NMR spectra at different temperatures can thus provide important information concerning molecular dynamics. Figure 2.6a shows the 13C CP/MAS NMR spectrum of compound 1 measured at 300 K. All six carbons of the phenyl group give clearly resolved signals at this temperature in the solid state, contrary to that observed in solution, where the two ortho- and the two metacarbons appear equivalent as a result of the fast rotation of the phenyl group about the C-N bond.

At elevated temperatures (Figures 2.6b and 2.6c), resolution of the signals at 8= 116.2 and 128.4 generated at 300 K (Figure 2.6a), and corresponding to the two ortho-carbons and two meta-carbons of the phenyl group appear, no doubt because of temperature induced rapid rotation of the phenyl group.

In addition to 13C NMR spectra, 15N CP/MAS NMR spectra can also provide valuable information concerning chemical structure and molecular dynamics. Because of the lower occurrence of nitrogen in organic molecules, 15N NMR spectra are not as crowded as 13C NMR spectra and are often easier to interpret. The chemical shift range is also higher (approximately 900 ppm in comparison with 250 ppm in the case of carbon). On the other hand, 15N NMR spectra show a much lower signal intensity than i3C NMR spectra and usually 15N-enriched samples must be used. We used 15N_ enriched samples for studies of tautomeric equilibria in the solid state. Figure 2.7 shows 15N CP/MAS NMR spectra of the 15N doubly labelled compounds 5-7 (20% 15Na and 10% 15Np). While 8C5N) in compounds 5 (model hydrazone compound) and 7 (model azo compound) are practically temperature independent, the analogous signals for compound 6 undergo considerable shifts as a result of temperature-induced changes in the azo-hydrazone equilibrium.


(c)

(h)

(a)

PPM

Figure 2.6 IlC CP/MAS NMR spectra of compound 1 recorded at 300 K (a), 323 K (b) and 348 K (c).

5


J (e)

(b)

(a)

i 100

1.

A

dt

i o

PPM i

-100

'* !

A

*" ~

'* "--

L o i

-200

Figure 2.7 ISN CP/MAS NMR spectra of compound 5 at 305 K (a); 6 at 194 K (b) and 358 K (e); and 7 at 306 K (d). The asterisks denote spinning sidebands.

6


Q-NH- O

\\ - N

7

In the 15N CP/MAS NMR spectra of compound 6 it can be seen that the signals arising from both nitrogen atoms are split, the splitting amounting to 3.4-5.8 ppm22 . This effect can be ascribed to the existence of two nonequivalent molecules in the unit cell. Such a splitting, as a result oftht: so-called molecular packing, is frequently encountered in the high-resolution NMR spectra of solids and can be caused either by the existence of two or more molecules per unit cell or by the fact that the molecule has a lower symmetry in the solid state. To distinguish between these possibilities, an X-ray structure should be determined.

As shielding anisotropy can be rather high in the case of 15N nuclei, relatively intense spinning sidebands (SSB) can be observed in 15N CP/MAS spectra. The shielding anisotropy powder pattern in trans-azobenzene was

I 600

I 400

*

I 200

PPM I o

I' -200

I -400

Figure 2.8 liN CP/MAS NMR spectrum of compound 8. The arrow denotes the centreband and the asterisks denote two folded spinning sidebands.


reported by Wasylishen et al.23 The shielding anisotropy for the two crystallographically non-equivalent molecules of trans-azobenzene is 925 and 880 ppm, respectively. Thus, the SSB can cover a wide spectral range. The practical consequence is the necessity to use wide spectral widths for measurements. Figure 2.8 shows the 15N CP/MAS NMR spectrum of the 15N_

8

monolabelled compound 8 (95% 15N). Narrower-than-required spectral width was used and folding of two spinning sidebands was observed. Their correct positions are depicted by dashed lines.

The centre band can be identified in the simplest way by using two different spinning speeds as the peak, the resonance of which is not affected by this parameter. Central peaks, in common, need not be the most intense signals when spinning speed is not high enough in comparison with the shielding anisotropy width.

The 15N CP/MAS NMR spectra of the 15Na-monolabelled (95% 15N) compounds 9-11 are shown in Figure 2.9. Compounds 10 and 11 exhibit two strong intramolecular hydrogen bonds24 with the same C=O group and give

Q N

N/ 'H

cxJ::JD o I ~

10

(e)

(b)


I -140

I -160

PPM

I -180

I -200

Figure 2.9 15N CP/MAS NMR spectra of compounds 9 (a), 10 (b) and 11 (el,

11

65


only one signal in the spectrum. In contrast to the 15N NMR spectra in solution24 the 15N CP/MAS NMR spectrum of compound 9 exhibits two signals (oC 5N) = -141.2 and -159.8), which can be interpreted as signals of two different azo-hydrazone forms, probably caused by the existence of two stable conformers differing in the orientation of the COOCH3 group. This is in agreement with the occurrence of two sets of signals in the 13C CP/MAS NMR spectrum of compound 924.

As in high-resolution NMR in solutions, two-dimensional measurements can be used in the NMR spectroscopy of solids, although their application is often limited by the lower sensitivity and resolution of solid state NMR spectra. Figure 2.10 shows the two-dimensional solid state CP/MAS NMR spectra of compound 1 measured using the pulse sequence given in Figure 2.11. This sequence enables chemical exchange and spin diffusion processes to be studied. We believe that the existence of off-diagonal peaks (cross-peaks) in Figure 2.10 provides evidence for rotation of the phenyl group in compound 1 at 300 K. This rotation is very slow and it is not evident in the one-dimensional spectrum (compare Figure 2.2a). Signals resonating at 118.4 and 113.7 ppm correspond to ortho-carbons and those at 129.9 and 126.9 ppm to the metacarbons of the phenyl group. The appropriate pairs of carbons mutually change their positions; the mixing time used was 500 ms. The other mechanism that could lead to the appearance of cross-peaks in the spectrum in Figure 2.10, namely spin diffusion, is improbable in this case.

All 13C and 15N CP/MAS NMR spectra shown in this chapter were measured at 50.3 and 20.28 MHz on a Bruker MSL 200 spectrometer. 13C and 15N chemical shifts were referenced against the carbonyl carbon signal of glycine (0 = 176.0) and to NH4Cl (0 = -352.5), respectively, by sample replacement.

2.4 Survey of published chemical shift data on the solid state

2.4.1 i3C NMR data

Chippendale et al. reported the 13C solid state NMR spectra of azobenzene and its derivatives21 . In trans-azobenzene, as well as in the symmetrically 4,4'disubstituted trans-azobenzenes 12, rapid rotation about the C-N bonds causes C-2/C-6 and C-3/C-5 to be equivalent and have the same chemical shifts in solution. In the solid state, rotation of the ring cannot occur (or is extremely slow on the NMR time scale) and molecules become locked in the planar trans configurations, in which C-2/C-6 and C-3/C-5 are no longer equivalent. Typical differences in the C-2/C-6 chemical shifts are 13-18 ppm, and in the C-3/C-5 shifts they are about 1-3 ppm. (The corresponding shifts in compound 2 which exists completely in the hydrazone form (Figure 2.2), are 4.6 and 3.0 ppm, respectively.) Using 2,2' -dimethylazobenzene as a model

I 150


I 140

I 130

PPM

o 0)

o 0

I 120

I 110

I 100

67

Figure 2.10 Part of the two·dimensional chemical exchange "C CP/MAS NMR spectrum of compound 1 recorded at 300 K.

y decouple decouple

L contact

preparation ~ ____ ~Il~ ________ ~~

evolution mixing detection (tll (t21

Figure 2.11 The pulse sequence used to investigate spin diffusion and chemical exchange processes in solids.


compound, it was shown that C-6 absorbs at higher field than C-2 in transazobenzene (8 (C-2) = 8 (C-6) = 122.9 in solution, 8 (C-2) = 130.7,8 (C-6) = 117.9 in the solid state). The l3C CP/MAS NMR spectrum of Disperse Orange 25 was recorded and proposed peak assignments made21 .

This non-equivalence greatly complicates the interpretation of the l3C CPI MAS NMR spectra of azo dyes differing in aromatic ring substitution, even for simple compounds where the chemical shifts measured in solution and their interpretation are available. The dipolar dephasing experiment permits the selective measurement of non-protonated carbon (i.e. with no directly bound hydrogen). We have used deuterated isopomers in the analysis of the l3C CP/MAS spectra of azo dyes20. l3C CP/MAS NMR spectra of 4-[N,N-bis (2-hydroxyethyl)-amino]azobenzene (2), 2-hydroxy-5-tert-butylazobenzene (7) 4-(N,N-dimethylamino)azobenzene (8), 4-methoxyazobenzene (13), and 4-hydroxyazobenzene (14) were recorded. The isotopomers of2, 7 and 14 with pentadeuterated unsubstituted aromatic rings were also studied. Dipolar dephased spectra of the de ute rated isotopomers were used for chemical shift interpretation in the solid state (see Figure 2.3). The change~ in crosspolarization dynamics caused by deuteration were investigated for 2. The spectra of 2,8, 13, and 14 were also recorded at elevated temperature. It was found that in 2, 8 and 14 rotation of the aromatic rings is induced at elevated tern pera ture.

~N C=C

\\J \'N-\ 8 )-x C-c

2 3

12

Q-\-o-0CH ~;; 3

13 14

Olivieri et al. have reported the l3C CP/MAS NMR spectrum of 1-phenylazo-2-naphthol (6) using standard and dipolar-dephased experiments25 . Hsieh et al. measured 4-(3' ,5' -dinitro-2' -hydroxyphenylazo )-3-hydroxy-2-naphthanilide (15)26 and Fedorov studied 4-(4-nitrophenylazo)-1-naphthol (16)27 in the solid state. The typical feature of all of these spectra is the strong overlapping of signals in the aromatic region of the spectra (8l3C) - 110-140 ppm). Therefore, it is difficult to assign positively l3C resonance in this region.


On the other hand such spectra do provide important information pertaining to azo-hydrazone tautomerism. The J3C CP/MAS NMR signals of C=OI C-OH are shifted downfield from the aromatic region and from the values of is (13C) ofthis carbon; the authors concluded that compounds 6,15 and 16exist predominantly in the hydrazone form.

OH

~ l0Y N~

¢ N02

16 15

Harris et al. investigated the 13C CP/MAS NMR spectra of l-(subst. phenylazo )-2-naphthols28 from the viewpoint of azo-hydrazone tautomerism and the effects of l4N_I3C residual dipolar couplings. CI. Pigment Red 1, CI. Pigment Red 3 and CI. Pigment Red 6 were studied in the form of pigmentary powder. Harris et al. 29 also used CI. Pigment Red 57: I as a model compound in testing a pulse sequence for generating 13C CP/MAS NMR spectra of proton-bearing carbons only. The sequence is based on difference spectroscopy and enables the observation of peaks which are otherwise severely overlapped by others.

Fedorov, Rebrov and Shen Liafang30 have reported an extensive set of 13C CP/MAS NMR data for arylazo derivatives of resorcinol, naphthols, naphthylamines, chromotropic acid and its cyclic modification, and of some formazans. Only negligible changes of is(l3C) were found on comparing solution and solid state spectra.

A correlation between the 13C CP/MAS NMR signal pattern3l and X-ray data was found for compounds 2 and 4. Sharp signals belong to -CHPH groups, while broadened ones (caused by residual dipolar interaction with l4N) correspond to NCH2- groups (Figure 2.5). Two values of is (13C) for both -CH20H and NCH2- were obtained, in agreement with X-ray data32.33 in which particular NCH2CH20H groups are different. The difference is small in the case of compound 2 but much greater in compound 4. The differences in is (J3C) are analogous, i.e. smaller for compound 2 and greater for 4.

The structure of a reagent known under the commercial name 'calcionca1cichrom' has been studied34 in solution and in the solid state using 13C CPI MAS NMR. The actual structure of this reagent differs from the earlier


supposed structure. The structure derived from the more recent study is compound 17.

17

Fedorov and Rebrov35 studied the l3C CP/MAS NMR spectrum of dithizone and compared the data obtained with those from X-ray studies.

The second major chromophore in dyestuff chemistry is the anthraquinone moiety. Chippendale et al. 36 studied anthraquinone itself (18) and some of its derivatives, i.e. I ,4-dimethoxy- (19), I ,4-dihydroxy-2,3-dimethyl- (20), 1,4-bis (n-butylamino)- (21), I ,4-bis(isopropylamino)- (22), 1,4,5,8-tetramino- (23) and I-dimethylaminoanthraquinones (24). In compound 18, the carbon atom chemical shifts formed in the l3C CP/MAS NMR spectra were practically identical, within experimental error, to the solution values and no additional splitting was observed; compounds 19 and 20 behaved in a similar manner to compound 18. The l3C CP/MAS NMR spectrum of 1,4-bis-(n-butylamino) anthraquinone contains many more peaks than that of the solution spectrum. In addition, some signals, which appear as singlets in solution, give pairs of signals, the distance between them being practically the same, i.e. 22.63 and 75.4 MHz. X-ray studies clearly showed that the compound crystallizes in the space group P2/c with four molecules in the unit cell, and that the asymmetric unit is a complete molecule (contrary to the possibility that two nonequivalent molecules exist in the unit cell). The spectral pattern of 1,4-bis (isopropylamino) anthraquinone is similar, whilst that of 1,4,5,8-tetraaminoanthraquinone is even more complicated; X-ray measurements are required to explain this. The residuaI 14N-l3C dipolar splittings for ipso aromatic carbons are 240-260 Hz and for aliphatic carbons are 181-204 Hz at 22.63 MHz.

Harris. et al. 37 have reported the l3C CP/MAS NMR spectra of 1,4-bis (n-butylamino) anthraquinone. The effect of two short-range and two longrange residual 14N-13C dipolar couplings was observed.

Law et al. 38 recorded the l3C CP/MAS NMR spectra of 1,5-diamino-4,8-dihydroxyanthraquinone (25) and of the corresponding tetrazonium-bis (tetrafluoroborate) (26) sulphate (27), and deprotonated form (28).


18 19

c¢?0 :H2CH2CH2CH3

I I "'" ~ ° NHCH 2CH 2CH2CH 3

20 21

~o :CH(CH3)'

I I "'" /'

NHCH(CH 3).

@NH2 o: I I

"'" ~

NH2 0 NH2

22 23

~H3)2

~ o

24

x 0 Y

#1 I'" ~ ~

Y X

Compound X Y

25 -OH -NH, 26 -OH -N;.BF4 27 -OH -N;.1I2(SO~ ) 28 =0 =N+=N


2.4.2 15 N N M R data

Wasylishen et al. 23 recorded the dipolar 15N NMR spectrum of a static powder sample using high-power decoupling and cross-polarization, and also the ISN CP/MAS NMR spectrum of (,sN)2-enriched trans-azobenzene (compound 12: X = H). The shielding anisotropy for the two crystallographically nonequivalent molecules of trans-azobenzene is 925 and 880 ppm. The isotropic shielding constants for the two non-equivalent molecules are the same within experimental error (W1/2 < 20 Hz). Principal components and orientation of the ISN chemical shift tensor are reported.

The ISN CP/MAS NMR spectra of ISN doubly labelled 3-methyl-l-phenylpyrazole-4,5-dione 4-phenylhydrazone (5) I-phenylazo-2-naphthol (6) 2-hydroxy-5-tert-butylazobenzene (7) and 4-hydroxyazobenzene (14) have been recorded and the temperature dependence of 8('sN) was followed 22. For compound 6, representing a mixture of the azo and hydrazone forms, the hydrazone content was calculated from the 15N chemical shifts of both nitrogen atoms at various temperatures. The two calculations gave identical results. In comparison with spectra recorded in solution, the hydrazone content in 6 is slightly higher in the solid state. Thermodynamic data were calculated using the temperature dependence of 1 n K (K = [hydrazone form]1 [azo form]). Some hydrazone content was found in 14 in the solid state, in contrast to the measurements in solution.

The IsN CP/MAS NMR spectra of compounds 9-11 have also been recorded24 (Figure 2.9). The results indicated that compounds 10 and 11 exist practically completely in the hydrazone form. For compound 9, evidence for an equilibrium mixture of azo and hydrazone forms was found. In this case, two IsN chemical shifts were detected in the IsN CP/MAS NMR spectrum. A possible explanation for this is the existence of two molecules in the unit cell that differ in their hydrazone content as a result of different orientations of the ester methyl group24.

Harris et al. 39 investigated the ISN CP/MAS NMR spectra of IsN selectiveenriched c.1. Pigment Red 57:1 (the monohydrated calcium salt of 1-(2-sulpho-4-methylphenylazo )-2-hydroxynaphthalene )-3-carboxylic acid). IsN chemical shifts provided direct evidence for the existence of the ketohydrazone structure.

Limbach et al. 40,41 studied the dynamic behaviour of porphin 29 and porphycen 30. Two separated signals (NH and N) were observed at 192 Kin compound 29, the difference in 8(15N) being 108 ppm, while at 356 K one sharp signal, as a result of fast exchange on the NMR time scale, was detected. Similarly, in compound 30 four signals of forms A-O were measured at 107 K and one sharp signal was found at 366 K.

Proton transfer kinetics in the free base meso-tetra-arylporphines42 have been studied by IsN CP/MAS NMR and the 15N CP/MAS NMR spectra of IsN-enriched phthalocyanine have been recorded at 153 and 300 K43.


29

~-

A C

1l 1l B 0

¢t9 ¢m ~- H H N N NI N

~ ~

30

2.5 Conclusions

The availability and use of modern instrumental techniques for recording solid state NMR spectra enables the NMR spectra of dyes in the solid state to be obtained with resolution nearly similar to that of solution spectra. Although the spectra in the solid state can be more complex than those in solution, the differences provide valuable information on the solid state structure of dyes. In addition it is advantageous to combine the results from CP/MAS NMR spectra and X-ray studies. We believe that the utility of CPI MAS NMR in generating key structural information on synthetic dyes isjust starting to be realized.

Acknowledgement

The authors thank Mrs V. Stant for her technical assistance.


References

I. A. Foris (1977) In The Analytical Chemistry of Synthetic Dyes, ed. K. Venkataraman, Wiley, New York.

2. L.A. Fedorov (1987) NMR Spectroscopy of Organic and Analytical Reagents and Their Complexes with Metal Ions. Nauka, Moscow (in Russian).

3. A. Lycka and J. Jirman (1991) In Color Chemistry, Ch. 10ed. A.T. Peters and H.S. Freeman. Elsevier, London.

4. A. Lycka (1993) Annu. Rep. NMR Spectr., 26247. 5. C Fyfe (1984) Solid State N M Rfor Chemists. CFC Press, Guelph. 6. A. Abragam (1961) The Principles of Nuclear Magnetism, Oxford University Press, London. 7. A.N. Garroway, W.B. Monitz and M.A. Resing (1979) Carbon-I3 N MR in Polymer Science,

ed. W.H. Pasika. A. c.s. Symposium Series, No. 103 p. 67. 8. L.R. Sarles and R.M. Cotts (1958) Phys. Rev., 111, 853. 9. A. Pines, M.G. Gibby and J.S. Waugh (1973) J. Chern. Phys., 59, 569.

10. S.R. Hartmann and E.L. Hahn (1962) Phys. Rev., 128, 2042. II. M. Goldman (1970) Spin Temperature and Nuclear Mqgnetic Resonance in Solids. Oxford

University Press, London. 12. M. Mehring (1976) High Resolution NMR Spectroscopy in Solids, Springer-Verlag, Berlin. 13. B.C Gerstein and CR. Dybowski (1985) Transient Techniques in NMR of Solids, Academic

Press, Orlando. 14. J. Schaefer, E.O. Stejskal and R. Buchdahl (1977) Macromolecules, 10,384. 15. D.L. VanderHart, W.L. Earl, and A.N. Garroway (1981) J. Magn. Reson., 10,361. 16. R. Voelkel (1988) Angew. Chern., Int. Ed. Engl. 27, 1468. 17. Y.J. Jiang, R.J. Pugmire and D.H. Grant (1987) J. Magn. Reson., 71, 485. 18. L.B. Alemany, D.M. Grant, R.J. Pugmire, T.D. Alger and K.W. Zilm (1983) J. Am. Chern.

Soc., 105,2133. 19. P. Granger and R.K. Harris (ed.) (1990) Multinuclear Magnetic Resonance in Liquids and

Solids-Chemical Applications. Kluwer Academic, Dordrecht. 20. J. Straka, B. Schneider, A. Lycka and J. Jirman (1991) Magn. Reson. Chern., 29, 500. 21. A.M. Chippendale, M.A. Mathias, R.K. Harris, K.J. Packer and B.J. Say (1981) J. Chern.

Soc., Perkin Tr. II, 1981, 1031. 22. A. Lycka, J. Jirman, B. Schneider, and J. Straka (1988) Magn. Reson. Chern., 26507. 23. R.E. Wasylishen, W.P. Power, G.H. Penner and R.D. Curtis (1989) Can. J. Chern., 67,1219. 24. A. Lycka , M. Necas, J. Jirman, J. Straka and B. Schneider (1990) Collect. Czech. Chern.

Commun., 55, 193. 25. A.C Olivieri, R.B. Wilson, I.C Paul, and D.Y. Curtin (1989) J. Am. Chern. Soc., 111, 5525. 26. B.R. Hsieh, D. Desilets and P.M. Kazmaier (1990) Dyes Pigm., 14, 165. 27. L.A. Fedorov (1991) Izv. Akad. Nauk SSSR, Ser. Khim. 1991,2302. 28. R.K. Harris, P. Jonsen, K.J. Packer and CD. Campbell (1986) Magn. Reson. Chern., 24, 977. 29. R.K. Harris, P. Jonsen and K.J. Packer (1984) Org. Magn. Reson., 22, 269. 30. L.A. Fedorov, A.I. Rebrov and Shen Lianfang (1992) Dyes Pigm., 18, 207. 31. A. Lycka, J. Jirman, H.S. Freeman, S.A. McIntosh and J. Straka (1994) unpublished data. 32. SA McIntosh, H.S. Freeman and P. Singh (1991) Dyes Pigm., 17, I. 33. S.A. McIntosh, H.S. Freeman and P. Singh (1989) Textile Res. J., 59,389. 34. L.A. Fedorov (1991) Izv. Akad. Nauk SSSR, Ser. Khim. 1991,2775. 35. L.A. Fedorov and A.I. Rebrov (1992) Izv. Akad. Nauk SSSR, Ser. Khim. 1992, 113. 36. A.M. Chippendale, A. Mathias, R.S. Aujla, R.K. Harris, K.J. Packer, and B.J. Say (1983) J.

Chern. Soc., Perkin Tr. II, 1983,1357. 37. R.K. Harris, P. Jonsen and K.J. Packer (1984) Org. Magn. Reson., 22,784. 38. K.Y. Law, S. Kaplan and I.W. Tarnawskyj (1991) Dyes Pigm., 17,41. 39. R.K. Harris, P. Jonsen and K.J. Packer (1987) J. Chern. Soc. Perkin Tr. II, 1987 1383. 40. H.H. Limbach and B. Wehrle (1987) Fresenius Z. Anal. Chern., 327 61. 41. B. Wehrle, H.-H. Limbach, M. KiicherO. Ermer and E. Vogel (1987) Angew. Chern., 99, 914. 42. H.-H. Limbach, J. Hennig, R.D. Kendrick and CS. Yannoni (1984)J. Am. Chern. Soc., 106,

4059. 43. R.D. Kendrick S. Friedrich, B. Wehrle, H.-H. Limbach and CS. Yannoni (1985) J. Magn.

Reson., 65,159.

3 Near IR spectroscopy M. MATSUOKA

3.1 Introduction

The identification of dye structure can usually be conducted using 'H and DC NMR, mass spectra and elemental analysis, but the chromophoric system of the dye can be determined with the aid of visible and/or near infra-red (NIR) absorption spectra. For instance, advances in quantum chemistry and the application of the newer techniques to dye chemistry have resulted in the development of new methodology for the identification of dye chromophores. In addition, the absorption spectra of dye chromophores can be predicted using straightforward molecular orbital (MO) calculations, such as those employed in the Pariser~Parr~Pople molecular orbital (PPP MO) method. The molecular design of dyes having a predetermined absorption spectra thus becomes possible, and, consequently, the identification of dye chromophores from their absorption spectra also becomes possible.

Infra-red (IR) spectroscopy, on the other hand, is not such a valuable method for the identification of the dye structure. Whilst empirical applications of 'functional group frequencies' are valuable especially for complex dye molecules, IR spectra can only be used to check and confirm structures which have been identified by other methods, such as mass spectrometry and NMR. Dye molecules are generally quite large, often having 50 or more atoms and multiple functional groups per molecule, and the resulting complex IR spectrum may not contain a 'fingerprint' that can be attributed to a specific compound. Consequently, the utility ofIR spectroscopy in the analysis of dye structures is now gradually diminishing.

The NIR spectra, similarly to ultraviolet and visible spectra, show a characteristic absorption curve depending on the chromophoric system involved, and can be evaluated in terms of the A.max value, & value, half band width, vibrational splitting, etc. There are many chromophoric systems which absorb in the NIR region, but their numbers are rather limited because they have only very recently been developed. NIR absorption spectra can be measured by using milligram quantities of samples and are assessed within the range from 400 nm to 900 nm. Evaluation of the NIR absorption curve is a valuable and convenient analytical method and a useful addition to mass and NMR spectra determinations.

76 ANALYTICAL CHEMISTRY OF SYTNTHETIC COLORANTS

In this chapter, the utility of NIR spectroscopy in the analysis of dye chromophores is reviewed.

3.2 Chromophoric systems of NIR dyes

The relationship between color and structure is one of the most important factors in the design of NIR dyes, a class of colorants which have recently become important in functional materials for diode laser technology. The NIR dyes are a new category of dyes, the synthetic design of which advantageously utilizes new methodology.

The color-structure relationship of dyes was initially rationalized by the chromogen theory in 1876, which helped to establish our understanding of dye chemistry. The resonance theory established by Buryl in 1935 developed the chemistry of ,,-electron and aromatic systems and contributed greatly to the development of new synthetic dyes; almost all of the dye chromophores commercialized today were developed using the concepts of resonance theory. Although resonance theory can be applied qualitatively to evaluate the chromophoric system, a quantitative interpretation of absorption spectra is preferably required for the design ofNIR dyes. The Amax of a dye for projected use as a photo receiver must be predicted correctly for its effective application in systems involving a diode laser which emits single wavelength laser light in the 780-830 nm range.

Advances in the quantitative prediction of the absorption spectra of dye chromophores are attributable to the development of the PPP MO theory in 1953.2 The absorption spectra of any dye chromophore can be quantitatively calculated by the PPP MO method. The method is similar to the simplest Hucke! molecular orbital (HMO) method; both methods deal with only the ,,-electrons, independent ofthe a-electrons. The PPP MO method differs from the HMO method in considering electron repulsion effects. The self-consistent field method is employed in the PPP MO method; the approximate set of the LCAO coefficients is first obtained by the HMO calculation method, and then these values are improved by repeated calculations evaluating the electron repulsion energy until no further improvements in the set of LCAO data results. The resultant MOs are then said to be self-consistent. Transition energies can then be calculated from the orbital energies and the electron repulsion terms. At this stage, the calculated transition energies are still not precise enough to predict the observed values; configuration interaction (CI) treatment is then applied to give better correlation of calculated and observed values.

The energy difference between the ground state and the first excited singlet state gives the transition energy for the first absorption band, which usually corresponds to a single electron transition from the HOMO to LUMO. Similarly, the second absorption band corresponds to the transition from the next HOMO to LUMO, or that from the HOMO to next LUMO. Each of the

NEAR IR SPECTROSCOPY 77

transition energies can be similarly calculated and they correlate well with the observed values. The use of the PPP MO method is limited to chromophores having a coplanar structure. Steric effects can usually be considered when evaluating the non-planar geometry of molecules.

The potential utility of the PPP MO method for studying dye chromophores was summarized by Griffiths3, and practical applications of this method for various dye chromophores have been described by Fabian and Hartmann4 • We have recently published a text5 (in Japanese) which describes the design offunctional dyes with the aid ofPPP MO calculations. Practical applications of the PPP MO method for the design of some NIR dyes are described in the sections below.

3.2.1 Intramolecular charge-transfer chromophores

Intramolecular charge-transfer (CT) chromophores can be defined as chromophores in which Jr-electron densities move from a donor moiety to an acceptor moiety accompanying the first excitation. Examples are generally found in quinoid, azo and indigo chromophores. For instance, in quinoid chromophores the first absorption band corresponds to single electron transfer from the HOMO to the LUMO of the molecule, and the substituent effects can be evaluated by the energy differences between these two states; this is important because the HOMO and LUMO energy levels are affected by the introduction of substituents.

A good linear correlation between the observed first excitation energy (~EmaX> and the PPP MO calculated values (~EI) has been determined5 for a number of intramolecular CT chromophores. A good linear correlation also generally exists between the first excitation energy (~EI) and the singly excited configuration energy (~ELUMO-HOMO) accompanying the excitation from HO M 0 to L UMO. This shows that ~EI depends markedly on the character of the HOMO and LUMO, and that substituent effects can then be defined in terms of the ~EI and the energy levels of the HOMO and LUMO. Once the substituent effects are evaluated quantitatively by the PPP MO method, the intramolecular CT character of the first absorption band can be confirmed.

Another approach which can be used to design molecules showing a large bathochromic shift of the absorption band involves evaluation of the Jrelectron density changes accompanying the transition. Substitution of acceptor groups at positions showing increased Jr-electron density, or substitution of donor groups at positions showing decreased Jr-electron density results in a bathochromic shift, depending on the electron-withdrawing or electrondonating strength of the substituent. An example is the indo naphthol dye6

shown in Figure 3.1, which shows the intramolecular CT character of the first absorption, where positive values denote an increase of Jr-electron density and negative values a decrease. Introduction of an electron-withdrawing group at the positive positions and/or of an electron-donating group at the negative positions thus produces a bathochromic shift in the first absorption band,


0.05 0.11 0.02·0.07

0 0.11 - '0.ON~NR2 0.11 _ 0.1~0.19

0.03 0.02 . 0.01 . 0.08

·002 0.03

(a)

AXN-O-NA' o (b)

Figure 3.1 (a) 7r·Electron density changes accompanying the first excitation of an indonaphthol dye. (b) The structure design considerations for an indonaphthol NIR dye: A, acceptor; D, donor.

depending on the strength of the groups involved. The Amax value can be quantitatively calculated by the PPP MO method, and the molecular design of dyes on the basis of absorption spectra thus becomes possible. The general structural design considerations for indonaphthol NIR absorbing dyes are shown in Figure 3.1b. In practice, some NIR-absorbing indonaphthol dyes have been synthesized (cf compounds 18,22 and 23 below). These results were effectively applied in the development of new NIR absorbing dyes based on other chromophores.

The basic chromophoric systems of cyanine dyes have been rationalized by the Dewar-Knott rule7 and summarized by Griffiths3• Klessinger8 re-evaluated the cyanine chromophores by the PPP MO method, and the results have been summarized by Fabian and Hartmann4•

The chemistry of NIR dyes is summarized in Infra-red Absorbing Dyes 9

which deals with the synthesis, characteristics and applications ofNIR dyes. The absorption spectra of 192 NIR dyes in solution and on vapor-deposited thin films have also been reported lO and a review of NIR absorbing dyes, including radical chromophores, has been recently published II.

3.3 NIR spectra of dyes

Various NIR polymethine dyes became accessible in the 1930s and proved to be useful in photographic sensitization. Some of these NIR dyes were found to contain natural porphyrin moieties or various related cyclic chromophores which could be synthesized by conventional methods. A wide variety ofNIR dyes have since been synthesized as photo receivers for the diode laser. Development of the gallium-arsenic semiconductor laser (diode laser), which emits laser light at 780-830 nm, has made possible the development of new optoelectronic systems, including laser optical recording systems, thermal writing display systems, laser printing systems, etc. The development of new types of NIR dye has, therefore, been anticipated as a source of functional materials for high-technology applications. New NIR dyes include cyanine, quinone, metal complex, cationic, azo, polycyclic and miscellaneous chromophores.

NEARIRSPECTROSCOPY 79

3.3. J Cyanine dyes

Polymethine cyanine dyes have the general structure shown in compound 1 and absorb over a wide range of wavelengths, from 340-1400 nm. In compound 1, R denotes a heteroaromatic residue and the A.max of the dye is predominantly affected by the electronic characteristics ofR. The length of the ethylene unit in the conjugating bridge also significantly affects A.max; NIR absorption can generally be attained where n is greater than three.

f\. . f\ R • C=CH-(CH=CH).-C~ ....... R

'N...... N+ I I R R

The color-structure relationships of cyanines and related dyes have been quantitatively evaluated by Fabian and Hartmann using the PPP MO method4 • The structural changes affecting the color of cyanine dyes have also been summarized by Griffiths3•

Over the past century, cyanine dyes have been used mainly as photosensitizers for silver halide photography, and a very large number of such dyes have been developed in line with technological advances in the photographic industry. On the other hand, there is now much demand for the use of cyanine dyes as functional dyes for new technology.

The basic chromophoric system of cyanine dyes can be evaluated quantitatively by the MO method to determine the energy levels of the frontier orbitals and the 7r-electron density changes accompanying the first transition. Cyanine dyes are an odd-altern ant system and each element can be divided into two groups (starred and un starred) at each position (as shown in dye 1). The 7r-electron densities are decreased at the starred position and increased at the un starred position in the first transition, generally caused by the oneelectron transition from the HOMO to the LUMO. Introduction of donor groups at the starred positions or of acceptor groups at the un starred positions thus produces a bathochromic shift in A.max depending on the relative polar nature of the substituents. Increase in the number of conjugating units (n) gives an enlargement of the 7r-conjugating system, and produces a bathochromic shift in A.max • Some examples of systems producing NIR absorption in cyanine-type dyes are shown in Figure 3.2 and Table 3.1.

Heptamethinecyanine dyes 2 also absorb infra-red light. They absorb at 730-820 nm in solution, and their solubility in organic solvents is largely affected by the nature of the N-alkyl substituent, the heteroaromatic ring and the counter anion. It is generally thought that perchlorate salts dissolve


R=CH- (CH=CH)3-R

R= ,,-8 > CJ(\ > 0:>= ::--.. N I

I Et Et

920 om 818 om X = Se 790 om X=S 763 om X = eMe, 741 om

X=O 695 om

Figure 3.2 Effect of heterocyclic substituents (R) on the absorption spectra of cyanine dyes.

Table 3.1 Absorption maxima of some cyanine type dyes"

Structure Y

as bS 'C(Me), dC(Me)2 'CH=CH

cry YJ) I .}-(CH=CHh-CH =< I ~ N N .0

I I R R

2 'CH=CH 'CH=CH

~ ~ V:,~CH=CH-DCH-CH::::l--. .. ~ N ~ N I CI I Et 3 Et CIO;

~ =<{-O ~ V:.~CH # CH::::l--.,.~ N N I 0- I Et Et 4

~ 0r-f0 ~ ~N~CH~CH::::l--.N~

I 0- I Et Et

5

Meo~ _, CH-CH=CH -Q-NMe1

" '. Me , . ,

"'_0" CIO' j-Pr 6

a""J measured in methanol. b,d,g measured in methylene chloride,

x R

I Et CIO. Et I Me CIO. Me I Et Br Et ClO. Et

A. max

(nm)

757 758 738 740 817 818 820

872

708

807

728


more readily in methylene chloride (as a relatively non-polar solvent) than in methanol. Indolenine derivatives have better solubility than benzothiazole or quinoline derivatives. The indolenine NIR dyes 2c and 2d have good properties with respect to solubility, heat resistance and lightfastness. The N-alkyl substituent affects not only the solubility but also the aggregation of the dye molecules. Introduction of a larger alkyl chain in cyanine dyes prevents the crystallization caused by aggregation.

Although cyanine dyes have high molar absorptivity, high fluorescence quantum yield and a large Stokes' shift, the lightfastness and chemical stability of cyanine dyes is generally poor. Structural modifications are required to improve these properties. Better lightfastness results when the cyclic conjugating bridges shown in dye 3, and/or a carbocyclic ring, as in the squarylium 4 and croconium dyes 5 are introduced to replace the ethylene unit of dyes 2. The styryl type azulenium dye 6 affords another variation that absorbs in the NIR region.

Singlet oxygen quenchers such as dithiol nickel complexes can be used to improve the lightfastness of NIR cyanine dyes. Cyanine dyes possessing a nickel complex as a counter anion have also been reported l2 to have superior characteristics for practical use in recording media for optical recording systems.

Polymethine NIR dyes are used as dye laser materials. The dye laser was developed by Sorokin in 1966 and over 600 kinds of dyes are known to be suitable for this application. As a result, lasering action ranging from the

Table 3.2 Heptamethinecyanine infrared absorbing dyes for dye laser materials"

Structure

~~>-(CH=CHh-CH=<X~ ~N N~

I I R R

y-

~(CH=CH)l-CH ~ N d....N~ I I Et Et

{Xs V s~ I )-CH=CH ~ CH-CH=< V Cl::::"" N N Cl

I NPh, I Et Et

• Measured in methanol. b Measured in dimethyl sulfoxide.

x

o C(Me), S

Y R

Me Me

Br Et

A.maxa

Lmax (nm) (nm)

681 720-864 741 775-940 759 793-900

817 865-969

928 970-1145

825b 858-1030

82 ANAL YTICAL CHEMISTRY OF SYTNTHETIC COLORANTS

Table 3.3 Effect of me thine substituents on the Am" of dyes 7, 8 and 917

-(Xx xx) y I }-P-CH =<* I y ~ N * N .0

* I z- I R R

o 0 croconium dyes -~H~ 7 P=

0-

squarylium dyes 0 8

-~H~ P~

0-

* cyanine dyes 9 P= ~CH=CH)r

Dye Substituent Calculated Observeda ~b Observed~

A max(nm) A m,,(nm) X Y R 7-8 7-9 8-9

7a CH=CH H Et 731 832 101 8a CH=CH H Et 680 724 44 108 9a CH=CH H Et 671 706 35 126 18 7b S H Et 709 771 62 8b S H Et 650 663 13 108 9b S H Et 617 651 34 120 12 7c C(Me), H Me 764 8c C(Me), H Me 629 135 9c C(Me), H Me 638 126 -9

a Measured in acetonitrile b Observed Am" - calculated Am,,'

ultraviolet to the NIR regions is available and is widely used in spectrophotometrical and medical applications.

Some examples ofheptamethinecyanine NIR laser dyes are given in Table 3.2, together with their spectral properties13 .

The synthesis of squaryliuml4 and croconium dyes 15 has been reported. The squarylium dyes have an almost planar structure through all1l'-conjugated systems, as shown by X-ray analysis l6. Correlations between the structure and properties of croconium and related cyanine dyes have been demonstrated by the PPP MO method 17 •

The effects of substituents and solvents on the Amax of dyes 7-9 are summarized in Table 3.3. The introduction of a croconic moiety into the conjugated methine chains of cyanine dyes produces a 120-126 nm bathochromic shift; introduction of a squaric moiety produces only a 12-18 nm bathochromic shift but gives a 9 nm hypsochromic shift in dye 8e. The


croconium dyes 7a-c absorb at a much longer wavelength than the corresponding squarylium dyes 8a-c and cyanine dyes 9a-c. These results were predicted by using the PPP MO method. The half band width value (A1/2) at Amax of these dyes increases in the order croconium > cyanine > squarylium. A narrow absorption band is important for use as a dye laser material and in recording media for laser optical storage. The prediction of AI/2 values by the PPP MO method has been reported l8 .

The IT-electron density changes accompanying the first excitation of dyes 7b and 9b have been calcula ted 17 (Figure 3.3). The electron density of7b decreases at the starred positions and increases at the unstarred positions, as defined by Dewar's rule. The croconium and squarylium dyes can be evaluated as having the same chromophoric system as that of the corresponding cyanine 9. Compounds 7a-c can be considered to have two carbonyl groups as an acceptor group at the un starred positions and the oxide group as a donor group at the starred positions. These substituent effects are anticipated to produce a large bathochromic shift in Amax in comparison with the cyanine dyes 9a-c. The squarylium dyes 8a-c have one carbonyl group at the unstarred position and an oxide group at the starred position. Therefore, the croconium dyes 7a-c can be expected to absorb at much longer wavelength than the squarylium 8a-c, which, in turn, are expected to absorb at longer wavelengths than the cyanine dyes 9a-c. Some of these dyes are useful as dye media for optical recording systems and as photo sensitizers for electrophotography. Dye 7b has been reported to have good photosensitivity from the visible to the NIR region. Compounds 7a-c and 8a absorb at wavelengths above 700 nm and can be employed as optical recording media.

Some pyrylium dyes absorb in the NIR region. Carcogenopyrylomethine dyes of the general formula given in compound 10 absorb at much longer wavelengths than the corresponding cyanine dyes. The effects of the heteroatom X on the Amax of10 follow the orderTe (1010 nm) > Se (910) > 0 (798)

Q S>oa:- 01l2 I ·C H :::::,... +#003

.N I -0.000'

EI

7b

Figure 3.3 1f-Electron density changes accompanying the first excitation of dyes 7b and 9b.

84 ANAL YTICAL CHEMISTRY OF SYTNTHETIC COLORANTS

10

Ph

MO,N-Q-( CH ~CH )~TO' V

Ph

11

o R-OCH-CH=CH- CH ~,!S

° \ Et

12

R3

I Me Me C=CH-(CH=CH) 0 oY n - R,

Rl I _ CH,CH,O

13

> NR (748). While the monocarcogenopyrylomethine dyes (11) absorb at much shorter wavelengths than the corresponding carcogenopyrylomethine dyes (for n = I, log & is 4.97 at 790 nm; for n = 2, log & is 5.19 at 870 nm), they still absorb in the NIR region.

The structurally similar chromophoric system of the azulenium dye 619,

which absorbs at 728 nm, has been reported as the charge-generating material (CGM) in organic photoconductors for laser printers and/or optical recording media.

Some merocyanine dyes of the general formula of 12 absorb in the NIR region (716 nm)13. The first transition consists of a strong intramolecular CT character.


The indo line-type cyanine dye (13) has been reported as an electrochromic dye20 . The color--colorless system of 13 is generated by ring cleavage of an intermediate spiro ring.

3.3.2 Quinone dyes

Dyes based on the quinone chromophore are extensively used in the modern dyestuff industry , and a wide range of colorants has been commercialized. The chemistry of quinone dyes was initially reviewed in depth by Venkataraman21 and later updated by Gordon and Gregory22, although both reviews pre-date many developments in NIR quinone dyes.

Quinone molecules are well known as electron acceptors. Introduction of electron donor and/or acceptor substituents into the quinone nucleus produces visible to NIR absorption with intramolecular CT character. 1,4-Naphthoquinone and 9,10-anthraquinone are the main chromophores for these colorants and their chromophoric system has been described quantitatively using the PPP method23 •

The first synthesis of an NIR 1,4-naphthoquinone dye was reported by Griffiths et al. in 197824• 5-Amino-8-anilino-l,4-naphthoquinone (14) was obtained by the reaction of 5-amino-2,3-dicyano-l ,4-naphthoquinone with aniline in ethanol and was found to absorb at 759 nm in acetone.

(Q;N 0 CN

0... 1 1 CN

PhHN 0

14

n o HN~ w: o HN'6 :/1

0...

15

Matsuoka et al. 25 reported a new type of deep-colored bis-ring-closed dye of type 15 which absorbs at 725 nm. The type of ring-closure reaction required has produced many NIR quinone dyes, e.g. 16 and 17.

Indonaphthol dyes are well known for the cyan color they produce in photography and they were extensively studied by Weissberger et al. 26. Application of the PPP MO method to the molecular design of such dyes indicates that the introduction of acceptor groups into the quinone moiety and/or of donor groups into the aniline moiety should produce a bathochromic shift of Amax. A practical development of this is the introduction of a 2-carboxamido substituent to produce a bathochromic shift of about 120 to 150 nm; the

86 ANALYTICAL CHEMISTR Y OF SYTNTHETIC COLORANTS

~ ~ s¢.~ ~~ ;/ ~

.;:' II/- S

&"" a a ""~ 1'-': ~I /-

16 17

X Y ;/ NR:!

ZSN-O-"~ 0

~

~ /; 0

18 19 '-': NR2

0 0 /-

0 NR:!

20 21

resulting dyes 18a absorb at 690-730 nm. The 2,3-dicyano derivative 18b (X = Y = CN, Z = 0) absorbs at 795 nm6• A 212 nm bathochromic shift of Amax results from the introduction of two cyano groups into the quinone moiety. 18b is very unstable and gradually reduces to the corresponding leuco compound.

Yoshida et al. have reported the synthesis of cyan orne thine derivatives of indonaphthol (18c; X = Y = H, Z = CH(CN)2) that absorb between 720 and 760 nm in chloroform27 • A strong intramolecular CT character of the first excitation was confirmed by the results ofPPP calculations. They also reported the synthesis of a series of quinoid ligands to give NIR metal complex dyes; the quinones used were 5,8-quinolinediones (19), 1,2-naphthoquinones (20), l-aza-9,IO-anthraquinones (21), azaindonaphthols (22; R' = Me, R = Et or R' = H, R = Me) and 3-phenyliminopyridino[2,3-a]phenothiazines (23). These compounds absorb in the visible region as free ligands, but absorb in the NIR region on formation of a I: I or 1:2 (metal:ligand) metal complex with Ni2+ or Cu2+. The bathochromic shifts range from 30 to 250 nm. The & values attained on metal complex formation are I to 10 times those of the free ligands. These are new types of NIR dye and some data on them are summarized in Table 3.4.

NEARIRSPECTROSCOPY 87

Table 3.4 Spectral data for the complex formation of metal salts with 22 in 99% EtOH 26

Free ligand Complex

Amjnm (lim,,) Metal salt Am,/nm (lim,,) M:L AAmax Rli

22a 635 (21300) Cu(ClO4), 776 (144000) 1:2 141 6.8 22a 635 (21300) Ni(CI04), 775 (118000) 1:2 140 5.5 22a 635 (21300) Ni(BF4)2 778 (113000) 1:2 143 5.3 22b 600 (16600) Cu(ClO4), 772 (144000) 1:2 172 8.7 22b 600 (16600) Ni(ClO4), 745 (85000) 1:2 145 5.1

R' Q-S_ oSNO-N~ "BN-O-NR' N\ ;; N\ ;;

22 23

X 0

~ 4Q) I I N-R ::::-.... ~ ::::-.... ~ ©NR O

S o NH2 ;/ I I ~ 24 ::::-.... ~

0 X

25

Some anthraquinone derivatives absorb light above 700 nm. These include the 1,4-diaminoanthraquinone (N-alkyl)-3' -thioxo-2,3-dicarboximides 24, employed as deep-colored dichroic dyes for guest-host liquid crystal displays and optical recording media; the well-known indanthrene pigments 25, employed as optical recording media; and 2-arylamino-3,4-phthaloylacridones 26 used for deep-colored disperse dyes or pigments. Tetrakis(arylamino)anthraquinones (27) absorb at 740 nm and are useful as organic color filters for diode lasers. Naphtho[I,2-b]phenazin-5-ones (28) absorb near 700 nm. The branched conjugated quinoid dyes 29 and 30 absorb strongly at 770 and 782 nm, respectively.

3.3.3 Metal complex dyes l2

The benzene dithiol metal complexes (31) are well known as metal ion indicators and they absorb in the NIR region. Metal dithiolene complexes (32) which absorb at 700-1200 nm have also been examined as NIR dyes. The electronic structure of dithiolene metal complexes is shown by the two


o NHAr

26

27

o

28

o

29 o

o 30

31

equivalent resonance forms. The chelate component is a stable aromatic 101l'electron system and it can be reduced to mono- and di-anions. The neutral and mono-anion dyes give an NIR absorption having log & > 4. The NIR absorption is very sensitive to substituent effects, which cause 1l'-ftI' transitions. Electron-donating substituents produce a bathochromic shift of Am.x to 1000-1300 nm depending on their donating strength. The central nickel chelate ring has an approximately square-planar structure, but the phenyl substituent is largely twisted out of the 1l'-plane. A hypsochromic shift is caused by steric


hindrance between substituents and the chelate ring. The A.m•x of 32 can be varied from 780 to 1300 nm by appropriate substitution into the phenyl rings. These dithiolene metal complexes act as an effective single oxygen quencher, and they can be used, together with cyanine or other NIR dyes, for the dye media of optical recording disks. The dithiol metal complexes (31) also absorb at the NIR region; substituent effects in these compounds show a similar tendency to those of 32, but their A.1/2 values are larger.

The phenylenediamine nickel complex dyes (33) absorb at 750-800 nm and also act as effective singlet oxygen quenchers for cyanine dyes. Quinone-type metal complex dyes were described in section 3.3.2.

33

32

3.3.4. Phthalocyanine and naphthalocyanine dyes

Phthalocyanines (Pc) are important colorants as dyes and pigments. Their structural analogy to the natural pigments such as the porphyrins is of great interest in academic research and also in regard to their applications as colorants. The chemistry of phthalocyanine compounds was reviewed by Moser and Thomas28 in 1963, and further advances were reviewed by Booth29 in 1971 and Gordon and Gregory22 in 1983. Recently, Luk'yanets30 surveyed and summarized the absorption spectra of phthalocyanines, naphthalocyanines (Nc) and related compounds.

Metal-free phthalocyanines absorb at 698 nm in I-chloronaphthalene and at 772 nm in the solid state. The metal complexes generally absorb at much shorter wavelengths, but some, such as lead phthalocyanine, absorb afmuch longer wavelengths than metal-free phthalocyanine. Shift into the NIR region may occur with mUltiple substitution but is more generally observed with annelation.

Pc (34) R = t-heptyl, M = VO R = 1,4-(OMek2,3-CI2, M = H,H R = 1,4-(OBu)2' M = Sn (OSiEt)2

Nc (35) R = H, M = Ge(OSiEt)2 R = 2-heptylundecanoyl, M = Si (ORJ)z R = SiR) or (COZR)n, M = Si (ORJ)2 R=H,M=AICI

809nrn 750nrn 695 nrn (log e 4.64) 779 nrn (log e 5.27) 780nrn 798 nrn 800nrn 774nrn


A large bathochromic shift is produced on changing from phthalocyanines to 2,3-naphthalocyanines (35). 2,3-Naphthalocyanine derivatives are potentially very important as organic materials for electro-optical applications such as optical recording media, organic photoconductors, color filter dyes and photosensors. They have been modified to give improved solubility by the introduction of branched long-chain alkyl groups into the naphthalene rings and/or of trialkylsiloxysilane into the central core.

2,3-Naphthalocyanine itself was first synthesized by Luk'yanets et al. 31

from 2,3-dicyanonaphthalene. Substituted naphthalocyanines are generally synthesized from substituted dicyanonaphthalenes. Kenney et al. 32 prepared new types of bis(alkylsiloxyl)silyl-naphthalocyanines (R = SiR3, COR, etc.) 2,3-Dicyanonaphthalene is converted to 1 ,3-diiminobenz[fJisoindoline, which tetramerizes to give dichlorosilylnaphthalocyanine (SiNcCI2) in the presence of tetrachlorosilane. The acid hydrolysis of SiNcCl2 results in SiNc(OH)2' which reacts with trialkylsilylchloride to give SiNc(OSiR3)2 (36).

34

35 36

The color-structure relationship of phthalocyanine chromophores has been studied by MO calculations;3 A.max values, molar absorptivities and changes in 1r-electron density accompanying the first transition have been calculated. The calculations indicated that the 1r-electrons migrate from the center of the molecules towards the aromatic rings on the periphery. Metallation, which reduces the electron density at the inner nitrogen atoms, is predicted to produce a hypsochromic shift, and this is observed experimentally. The extent of the A.max shift to shorter wavelength depends on the electronegativity of the metal. In contrast, electron-withdrawing groups such as chlorine at the periphery of the molecule are predicted to give rise to a bathochromic shift. Nucleophilic displacement of the chlorine atoms in a perchlorinated Pc gives polyalkylthio- or polyarylthio-Pc which absorb in the NIR region33. Annelation also produces a bathochromic shift. In principle, however, the 18 1r-electron inner ring system of the phthalocyanines deter-


mines the bathochromicity, but polysubstitution by strong electron-donating groups in the phenyl rings can also produce a large bathochromic shift in ph thalocyanine chromophores.

Tetraphenylphthalocyanine absorbs at 715 nm. A bathochromic shift of about 20 nm is produced by the introduction of a benzo group into the benzene ring of 34. Benzannelation of phthalocyanine brings about a large bathochromic shift of 67 nm in 35 (R = H) and 22 nm in 37 (R = H) and increases the molar absorptivity as a result of the enlargement of the 1(

conjugated systems. Annelated phthalocyanines are fluorescent at room temperature.

CI R'

-t)LN=N~NRIR2 X S -y_

R4

38

X R' R2 R] R'

C(CN)=C(CN), Et Et H H

b N~C I CN : =c = ° Et Et H H

CH I y "" c y = SO, H ROMe NHAc

37 Measured in dichloromethane. R2 = CH(Me)Bu(n).

3.3.5. ko dyes

Only a few azo dyes display absorption maxima at more than 700 nm. Griffiths and BeII034 synthesized NIR monoazo dyes such as 38a-c (a: 725 nm, log & 4.88; b: 700 nm, 4.83; c: 778 nm, 4.92 in CH2CI2). Absorptions at longer wavelength are produced if the azo dyes contain strong electrondonor groups in conjugation with strong electron-withdrawing groups on the opposite end of the molecule. These dyes show positive solvatochromism, as Amax values are displaced to longer wavelengths (by around 25 nm) on changing the solvent from CH2Cl2 to the more polar DMSO.

Another series ofNIR azo dyes are the polykisazo dyes. Examples ofNIR absorbing disazo dyes are 39 (710 nm, log &4.33 in CHCll) and 40 (706 nm in EtOAc)ll. .

The NIR absorption of the above dyes is due to 1(-7r'* excitation and many of the azo dyes in this category have intramolecular CT character. The absorption intensity, however, indicates a marked electron delocalization in the ground and the lowest energy 1(-7r'* excited state, which increases on switching from the neutral arylazobenzenes to cationic diazacyanine dyes.


39

40

3.3.6. Miscellaneous chromophoric systems

Insertion of a 7r-conjugation unit such as -HC=CH- and -C == C- into cationic dye chromophores such as diphenylmethane and triphenylmethane dyes produces a large bathochromic shift and these compounds absorb in the NIR region. The shift in A.max depends on the chromophore, but is within the 70-100 nm region. These chromophoric systems are similar to those of cyanine dyes, and substituent effects in them can be predicted by the PPP MO method.

Dyes 41 are of particular interest as dye media for optical recording since they exhibit very intense absorption about 800 nm (log &_5)11. In agreement with the PPP calculations, a large bathochromic shift is observed following the preparation of rigid analogues of 41, e.g. the fluorenyl dyes 42 which absorb at 900-1000 nm but with a lower & value.

Rl

42 41

Dyes oflow symmetry composed of two (or more) loosely joint fragments may display intramolecular CT bands. In such a case, one fragment is a donor and the other one is an acceptor and partial electron transfer occurs upon 7r-1i* excitation. As would be anticipated, the donor and acceptor strength of the


fragments determine the absorption wavelength. The Amax values of the azamonomethines, for example, increase on converting 43a (R, = R2 = H) to 43b (R, = R2 = HC=CH-CH=CH), viz. from 755 nm (log Ii 4.41) to 850 nm in CH2CI2, caused by the increased donor strength of the aryl substituent. The bathochromicity is increased if N is replaced by CCN.

Junek and co-workers35 synthesized various carbodinitriles which absorb above 700 nm (e.g. 44: 755 nm, log Ii 3.90 in CH2CI2).

CI 9:1 Et:zN~--": ~~ CN

CI CN

43 44

Another series of NIR chromophores are zwitterionic compounds", e.g. 45.

c:)rf·u Me CN

~ ~Y O=U---~y

X Z 0

45 46

Few intermolecular CT chromophores are known that are of practical use as coloring materials. The relationship between color and constitution of intermolecular CT dyes has been quantitatively evaluated by means of the PPP MO method36 • Matsuoka et al. synthesized NIR dyes composed of the carbazole-naphthoquinone CT complex dyes 46 (X = HorCH = N - N (Ph)2; Y = CN and Z = N02 or H; or Y = Cl when Z = N02). These dyes generally show a broad absorption band and exhibit two Amax in the visible and NIR regions. The Amax values were mainly affected by combinations of an appropriate donor and acceptor and showed bathochromic shifts depending on the strength of the donor and acceptor. X -ray studies confirmed the 1: 1 composition ofthese CT complexes37 • There are, evidently, many possibilities for the combination of various types of donor and acceptor to give NIR intermolecular CT complex dyes.

Staab and co-workers38 have described donor-acceptor cyclophanes giving broad bands in the NIR region. The compounds show positive solvatochromism, e.g. 47 (827 nm in n-C6H,4; 1094 nm in DMSO). Cyclophanes such

94 ANAL YTICAL CHEMISTR Y OF SYTNTHETIC COLORANTS

N~CN NC - CN

M8:!N \ /. NM8:!

47

as 47 call to mind electron donor~acceptor complexes which are formed between donor and acceptor compounds in less polar solvents. They may also be deep-colored because of a broad and weak intermolecular CT band.

3.4 Conclusion

NIR dyes are a new category of dye chromophores which have resulted from the development of the diode laser as a light source. The full shape of the NIR absorption spectra is very important in the design of dye materials for optoelectronic systems such as laser optical recording, thermal writing display, laser printer and laser reading systems. The molecular design of NIR absorbing dyes by means of the PPP MO method is valuable in predicting absorption spectra data such as A.max and 8 max ' The MO method is also useful in predicting substituent effects necessary to produce the bathochromic shifts required to give absorption in the NIR region.

New applications of NIR dyes now being studied include charge-generation materials for organic photoconductors in the diode laser printer, photoinitiators for polymerization, decolorizable toners, leuco NIR dyes for laser reading systems, and photodynamic therapy.

The future ofNIR dyes is a very important and continually developing area for dye chemists and material scientists.

References

I. C.R. Bury (1935) J. Arn. Chern. Soc., 57, 2116. 2. R. Pariser and R.G. Parr (1953) J. Chern. Phys., 21, 466, 767; J.A. Pople (1953) Trans.

Faraday Soc., 49, 1375. 3. J. Griffiths (1976) Colour and Constitution of Organic Molecules. Academic Press, London,

p.240. 4. J. Fabian and H. Hartmann (1980) Light Absorption of Organic Colorants. Springer-Verlag,

Berlin, p. 162. 5. S. Tokita, M. Matsuoka, Y. Kogo and H. Kihara (1989) Design of Functional Dyes by the

PPP MO Method. (Japanese). Maruzen, Tokyo. 6. S.H. Kim, M. Matsuoka, T. Yodoshi, K. Suga and T. Kitao (1989) J. Soc. Dyers Colourists,

105,212. 7. M.J.S. Dewar (1950)J. Chern. Soc., 1950,2329; 1952, 3532, 3544; E.B. Knott (1951) J. Chern.

Soc., 1951, 1024. 8. M. Klessinger (1966) Theoret. Chirn. Acta (Berlin), 5, 251.

NEAR IR SPECTROSCOPY

9. M. Matsuoka (ed.) (1990) Infrared Absorbing Dyes, Plenum, New York. 10. M. Matsuoka (1990) Absorption Spectra of Dyes for Diode Lasers. Bunshin, Tokyo. II. J. Fabian, H. Nakazumi and M. Matsuoka (1992) Chem. Rev., 92,1197.

95

12. K. Namba (1990) In Infrared Absorbing Dyes, ed. M. Matsuoka. Plenum, New York, p. 57. 13. S. Yasui (1987) Shiki:cai Kyokaishi, 60, 212 (in Japanese). 14. V.H.E. Sprenger and W. Ziegenbein (1967) Angew. Chem., 78,581. 15. G. Alfred (Agfa-Gevaert AG) (1970) Ger. Offen. 1930224. 16. J. Bernstein, M.T. Kendra and c.J. Kekhardt (1986) J. Phys. Chem., 90, 1069. 17. S. Yasui, M. Matsuoka and T. Kitao (1988) Dyes Pigments, 10, 13. 18. C. Lubai, C. Xing, H. Yufen and J. Griffiths (1989) Dyes Pigments, 10, 123. 19. K. Katagiri, Y. Oguchi and Y. Takasu (1986) Nippon Kagaku Kaishi, 1986,387 (in Japanese). 20. M. Hayami and S. Torigoshi, JP 1,158,538, 61-25036. 21. K. Venkataraman (ed.) (1971) The Chemistry of Synthetic Dyes. Academic Press, London. 22. P.F. Gordon and P. Gregory (1983) Organic Chemistry in Colour. Springer-Verlag, Berlin. 23. Y. Kogo, H. Kikuchi, M. Matsuoka and T. Kitao (1980) J. Soc. Dyers Colourists, 96, 475,

526. 24. K.Y. Chu and J. Griffiths (1978) J. Chem. Res., 1978, (S) 180, 1978 (M), 2319. 25. K. Takagi, M. Kawabe, M. Matsuoka and T. Kitao (1985) Dyes Pigments, 6,177. 26. C.R.Barr, G.H. Brown, J.R. Thirtle and A. Weissberger (1961) Photog. Sci. Eng., 5,19. 27. K. Yoshida and Y. Kubo (1989) Senryo to Yakuhin,34, I (in Japanese). 28, F.H. Moser and A.L. Thomas (1963) Phthalocyanine Compounds. Reinhold, New York. 29. G. Booth (1971) The Chemistry of Synthetic Dyes, Vol. 5, ed. K. Venkataraman. Academic

Press, New York, p. 241. 30. E.A. Luk'yanets (1989) Electronic Spectra of Phthalocyanines and Related Compounds.

Catalogue (in Russian), Cherkassi. 31. E.!. Kovshev, V.A. Puchnova and E.A. Luk'yanets (1971) Zh, Org. Khim., 7, 369. 32. B.L. Wheeler, G. Nagasubramanian, A.J. Bard, L.A. Schechtman, D.R. Dininny and M.E.

Kenney (1984) J. Am. Chem. Soc., 106, 7404. 33. P. Gregory (1991) In Color Chemistry, ed. A.T. Peters and H.S. Freeman. Elsevier Applied

Science, London, p. 193. 34. K.A. Bello and J. Griffiths (1986) J. Chem. Soc. Chem. Commun., 1986,1639. 35. H. Junek, G. Vray and G. Zuschnigg (1988) Dyes Pigments, 9,137. 36. M. Matsuoka, T. Yodoshi, L. Han and T. Kitao (1988) Dyes Pigments, 9, 343. 37. M. Matsuoka, L. Han, T. Kitao, S. Mochizuki and K. Nakatsu (1988) Chem. Lett., 1988,905. 38. H.A. Staab, R. Hinz, G.H. Knaus and C. Krieger (1983) Chem. Ber., 116, 2785, 2835.

4 Mass spectrometry R.B. VAN BREEMEN

4.1 Introduction

Since the last review of mass spectrometry of textile dyes', extraordinary developments have taken place in the field of mass spectrometry. New desorption ionization and ion evaporation methods have been introduced, including fast atom bombardment (or liquid secondary mass spectrometry), plasma desorption, laser desorption, thermospray, electro spray, and ion spray. These new ionization techniques complement the older ionization methods of electron impact, chemical ionization, desorption chemical ionization and field desorption by fa<.:ilitating the direct analysis of polar, ionic, nonvolatile and thermally labile compounds, such as sulfonated azo dyes, without the need for prior derivatization. Although these new ionization techniques have expanded the range of compounds that can be analyzed using mass spectrometry, there is still no single technique that is suitable for the analysis of all classes of dyes. Therefore, gas-phase ionization techniques such as electron impact and chemical ionization remain necessary and useful for the analysis of volatile compounds.

Because desorption and ion evaporation ionization methods eliminate the need for derivatization of non-volatile dyes, less time is required for sample preparation, and artifacts resulting from incomplete derivatization or the formation of derivatization by-products are eliminated. Also, non-volatile impurities or degradation products present in dye preparations can be easily identified. With the ability to generate gas-phase ions of polar and nonvolatile compounds, analytes weighing up to several hundred thousand daltons can now be measured using mass spectrometry, which facilitates the analysis of high-molecular-weight dyes.

As a result of the broad range of ionization techniques and other mass spectrometric methods available on commercial mass spectrometers, essentially all synthetic dyes can be analyzed by using mass spectrometry. Molecular ions provide molecular weight confirmation and fragment ions show structural features of the analyte. High-resolution mass spectrometry with exact mass measurements can be used to provide the elemental composition of the molecular ion or fragment ions. Since only submicrogram quantities of analytes are typically required, mass spectrometry can be used even when sample quantities are quite limited.

MASS SPECTROMETRY 97

Often, the ionization method used in the analysis of a particular dye is guided by the mass spectrometry facilities available to the chemist. Since no single ionization technique has emerged as the method of choice for the mass spectrometric analysis of all classes of synthetic dyes, and since several ionization methods might be suitable for a particular dye, mass spectrometric analysis of synthetic dyes will be discussed in this chapter as a function of the ionization method used. To assist in the selection of suitable ionization methods for a particular class of dyes, Table 4.1 provides a summary of dye classes that have been analyzed using mass spectrometry, the type ofionization method used and the literature references. Except for a few historically important references, only those mass spectrometric studies of synthetic dyes published since the review published in 19771 will be considered.

Table 4.1 Applications of mass spectrometric ionization techniques to the analysis of synthetic dyes and related compounds

Dye class

Unsulfonated azo dyes

Sulfonated azo dyes

Other sulfonated dyes Cationic dyes

Coumarins

Cyanine dyes Anthraquinone dyes

Disperse dyes

Indigoid dyes Benzyl[cd]indole dyes Xanthane dyes Dispersants and other dye additives

Ionization method

EI LD DCI NCI PD TS FAB DCI EI EStIS FAB FD TS FAB FAB PD TS LD TS EI DCI FAB LD TS EI FAB TS EI EI FAB FAB

Reference

2,3,6,13,14 47 20 17 51 15,54,55,59,60 18,35,51 21 II 63-66 19,32-34,38~2 23,24 15,55-58 27,30,34,37 21,31,43,51 51,52 9,55 47 15,55 5,12 19 18,27,41 47 15,55,59 10 19,21,44 15,44,55,60 8 7 31 45

DCI, desorption chemical ionization; EI, electron impact; ESIIS, electrospray or ion spray; FAB, fast atom bombardment; FD, field desorption; LD, laser desorption; NCI, negative ion chemical ionization; PD, plasma desorption; TS, thermospray.


4.2 Ionization methods

4.2.1 Electron impact

The most frequently used ionization technique in mass spectrometry is electron impact (EI), a procedure carried out on analyte molecules in the gas phase. The sample may be introduced into the ionization source of the mass spectrometer by means of a gas chromatograph (GC-MS), evaporation from a direct insertion probe, as a gas from a reservoir inlet, or through a particle beam interface. The energy for ionization and fragmentation is derived from gas-phase collisions between analyte molecules and energetic electrons (usually at 70 eV). Instead of electron capture, which only occurs with very low energy electrons, collisions between a high energy electron and an analyte molecule result in expulsion of both the impacting electron and an electron from the analyte to form a radical cation or molecular ion M+·. The energy transferred to most analyte molecules during electron impact ionization is typically sufficient to produce fragmentation with subsequent elimination of neutral molecules and free radicals that provide structural information about the analyte.

The primary limitation of EI mass spectrometry is the requirement of sample volatility. Although some analytes can be vaporized upon heating in vacuo, most polar, non-volatile and thermally labile compounds will decompose, and only the corresponding pyrolysis products form gas-phase molecules that can be ionized and detected. In other cases, molecules in the gas-phase fragment so extensively during EI ionization that no molecular ions are detected. Because molecular ions are frequently not detected using EI mass spectrometry, alternative, 'softer' ionization techniques have been developed to reduce fragmentation. For instance, desorption ionization techniques have been introduced to facilitate the mass spectrometric analysis of non-volatile and thermally labile compounds.

For the analysis of volatile dyes, dye intermediates, and by-products from dye synthesis, EI mass spectrometry and EI with GC-MS remain both widely available and effective techniques. For example, Youngless et ae and Haessner et at. 3 used EI mass spectrometry to analyze volatile azo dyes. Havlickova et al. 4 used EI mass spectrometry with direct insertion probe sample introduction to identify the by-products formed during the synthesis of a red vat dye. Romanov et al. 5 studied the fragmentation of cyanine dye intermediates and Amer et al.6 used GC-MS to analyze aminobenzenesulfonic acid dye intermediates as their sulfonyl chloride derivatives. In a recent study by Naef1, molecular ions and structurally significant fragment ions were observed for benz[cd]indole dye systems.

High-resolution EI mass spectrometry was used for the structural analysis of trace amounts ofindigoid dyes of molluscan origin extracted from contemporary and ancient textiles8• These data provided insight into the methods


used in the generation of ancient dyed textiles. Voyksner et aU used EI GCMS to obtain molecular ions and structurally significant fragment ions of seven out of thirteen photodegradation products of Basic Yellow 2. Six polar photodegradation products were identified using thermospray LC-MS that could not be detected using EI. In a related study, Freeman and Hsu lO used EI GC-MS to separate and identify volatile photodegradation products of several disperse dyes following their extraction from polyester and nylon substrates.

Sugiura and Whiting I I prepared methyl esters of a series of sulfonated arylazonaphthalenes and successfully analyzed them using EI mass spectrometry. Pyrolysis mass spectrometry has been carried out using EI to form ions of the pyrolysis products. In these cases, no intact molecular ions of the original dye were detected. For example, Haessner et al. 12 used pyrolysis mass spectrometry with EI ionization to analyze a series of six N-monosulfoalkylsubstituted cyanine dyes. Molecular ions of the dyes were not detected, and the most abundant ions observed resulted from the elimination of the polar N-(sulfoalkyl) substituent. Abdel-Megeedl3 used EI mass spectrometry to analyze a series of 4-arylazopyridylpyrazolones and identified structurally significant fragment ions. However, molecular weight confirmation was not possible because the sample decomposed during analysis.

EI can be used during liquid chromatography-mass spectrometry (LC-MS) in conjunction with a particle beam LC-MS interface. In this interface, a thermal nebulizer is used to convert the mobile phase to a spray which is gradually desolvated, leaving behind an aerosol containing particulates of the analyte. The resulting particle beam is enriched with respect to sample particles as the solvent vapor is removed in a momentum separator. Finally, the sample particles enter the heated ionization source where they are rapidly vaporized and ionized using electron impact or chemical ionization. The advantage of particle beam LC-MS is that analytes separated during HPLC can be analyzed as they elute from the HPLC column without the intermediate steps of fraction collection and solvent evaporation. However, the disadvantages of EI remain, especially the requirement of sample volatility. Yin on et al. 14 used particle beam LC-MS with electron impact ionization to analyze a series of commercial dyes consisting primarily of azo dyes. In this case, molecular ions were usually detected as well as structurally significant fragment ions l5 •

4.2.2 Chemical ionization

During chemical ionization (CI), analyte molecules are ionized by gas-phase collisions with reagent gas ions. The reagent gas ions are generated using EI ionization of gases such as isobutane, methane or ammonia at relatively high pressure (approximately I Torr)16. Protonated analyte molecules, [M + Hr, are usually produced instead of molecular ions. Because the transfer of a proton

1 00 ANALYTICAL CHEMISTRY OF SYNTHETIC COLORANTS

in the gas-phase adds less energy to the analyte molecule than interaction with high energy electrons, less fragmentation occurs in CI than in EI, and CI is known as a softer ionization technique. Therefore, CI is sometimes used for molecular weight confirmation, which complements the structural information obtained through fragmentation during E1. Less commonly used, negative ion chemical ionization usually involves the formation of radical anions M-' by electron capture of low energy electrons in the ion source by analyte molecules. Like EI, the primary limitation of chemical ionization is the requirement that the sample molecules enter the gas-phase prior to ionization.

Negative ion chemical ionization mass spectrometry was used by Brumley et al. 17 to study the negative ion chemistry of seven non-sulfonated azo dyes containing 2-naphthol. Next, tandem mass spectrometry (MS-MS; see description below) was used following collisional activation (CAD) of abundant M-' ions to generate fragment ions corresponding to cleavage at the azo bond. For example, abundant ions were observed at rnIz 157 and rnIz 91, corresponding to the 2-naphthol moiety and the phenyl ring plus ~ nitrogen from the -N=N- bond, respectively.

MS-MS is of great value in producing fragment ions for structure determination when only molecular ion species are formed in the ionization source. During MS-MS, ions at one particular rnIz value in a mixture of ions can be isolated in one stage of the mass spectrometer. Next, fragmentation of the selected ions may be carried out in the gas-phase by CAD with other gas molecules, and then structurally significant fragment ions derived from the selected ion precursors are separated and detected in the next stage of the mass spectrometer. These MS-MS spectra are typically free from contaminating ions from the original mixture.

4.2.3 Desorption EI and CI

Desorption electron impact (DEI) and desorption chemical ionization (DCI) are compatible with mass spectrometers equipped for either EI or CI. The analyte is introduced directly into the ionization source as a liquid or solid deposited on a tungsten or other metal filament. The filament containing the sample is placed inside the ionization source but not adjacent to the source, unlike the solid direct insertion probe technique used in EI and CI. As the filament is resistively heated in the EI beam or CI plasma, neutral molecules are immediately ionized just above the sample surface as they desorb from the condensed phase into the gas phase.

DCI and DEI can be used for marginally volatile samples which would otherwise require derivatization to increase their stability and volatility in standard EI or CI analysis. Alternatively, DEI and DCI can be used on derivatized samples that will not provide molecular weight information during standard EI or CI. Although DCI and DEI require that the sample form


gas-phase molecules prior to ionization, desorption and ionization take place almost simultaneously so that fragmentation is reduced compared to standard EI or CI.

DCI was used by Freeman and Sokolowska-Gajda18 in the analysis of photodegradation products of c.l. Acid Orange 60 and c.1. Acid Green 25, following irradiation of these dyes in dimethylformamide or on nylon 66 fiber. Because of the low quantities of the isolated degradation products, the high sensitivity of mass spectrometry was required. Iso butane DCI formed protonated molecules of polar photodegradation products that were low in molecular weight (i.e. m/z less than 400) such as l-hydroxy-2 aminobenzenesulfonamide, which was a degradation product ofC.!. Acid Orange 60. Higher molecular weight and more polar compounds, such as Cr(III)-containing products of c.1. Acid Orange 60, did not form molecular ion species using DCI.

Freeman et al. 19 used isobutane DCI to form abundant protonated molecules, molecular ion radicals and structurally significant fragment ions of six anthraquinone dyes. For example, see the positive ion DCI mass spectrum of an anthraquinone disperse dye in Figure 4.1. In this study, no sample ions were observed using either EI or CI mass spectrometry. In other studies,

10 0 R e 1 a t 8 0 v e

A b 6 0 u n d a n 40 c e

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250 300 350 400 450 500

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Figure 4.1 Positive ion desorption chemical ionization (DCI) mass spectrum of I Jig of a highenergy disperse dye. Isobutane was used as the reagent gas. (Reproduced from reference 19 with

permission.)


Roach et al. 20 analyzed semi-volatile azo dyes using DCI, and SokolowskaGajda and Freeman21 used positive ion DCI mass spectrometry to analyze mono- and disulfonated naphthol azo dyes. Abundant protonated molecules were detected which confirmed the expected molecular weight.

4.2.4 Field desorption

Unlike EI, CI, DCI or DEI, samples for field desorption (FD) do not require volatilization prior to ionization. Instead, ionization takes place simultaneously with desorption. For FD analysis, the analyte is dissolved in a solvent and applied to an emitter covered with thousands of graphite or silicon microneedles. The solvent is evaporated, leaving behind the solid (or liquid) sample coated on the emitter. Next, the emitter is subjected to a powerful electromagnetic field (107-108 V/cm) which abstracts electrons from the sample to form molecular ions, M+·. Simultaneously, the emitter can be resistively heated to assist in the desorption of sample ions. When the emitter is heated, thermal desorption sometimes leads to the evaporation of cations such as Na + or K+ that can combine with sample molecules in the gas phase to form cationized molecules including [M + Nar, [M + Kr, etc. Protonated molecules are also observed for some compounds using FD mass spectrometry. The theory and use ofFD mass spectrometry have been reviewed by Beckey22. FD is not compatible with LC-MS.

Fragmentation during FD is significantly reduced compared to EI or CL so that molecular ions are often the only ions detected. FD is therefore a soft ionization technique, because little excess energy is added to the molecules during ionization/desorption. For example, see the positive ion FD mass spectrum of an anthraquinone disperse dye in Figure 4.2, in which molecular ions but no fragment ions were detected.

The development ofFD facilitated the formation of molecular ions having rnJz well beyond 1000. This technique is limited to mass spectrometers equipped with high accelerating potentials such as magnetic sector instruments. FD was the method of choice for the desorption and analysis of polar, non-volatile and thermally labile compounds, until the development of fast atom bombardment ionization during the 1980s. The disadvantage of FD is the expense and handling of the fragile emitters. However, FD is still used for applications involving complex mixtures, low-molecular-weight polymers and compounds that lack a good proton acceptor or donor. Also, FD is useful for the analysis of compounds that are too high in molecular weight to be desorbed easily using DCI or DEI or do not form molecular ions during EI or CI ionization. Mathias and coworkers23 and Schulten and Kummler24 used FD to analyze underivatized sulfonated azo dyes. Monosulfonated dyes produced abundant molecular ions M+' and protonated molecules, [M+Hr. If the sample contained sodium ions, then abundant [M+Nar ions were detected. A naphthalene disulfonated acid produced a base peak corre-

100

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60 R b

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MASS SPECTROMETRY

02N OH

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150 200 250

M+'

300 350

376

103

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Figure 4.2 Positive ion field desorption (FD) mass spectrum of 0.5 pg of an anthraquinone dye, which is one of the two isomers contained in commercial Disperse Blue 77. Note the absence of

fragment ions and background noise.

sponding to [M+Hr. Few fragment ions were detected in these analyses, as would be anticipated.

4.2.5 Fast atom bombardment and liquid secondary ion mass spectrometry

Since its introduction by Barber et al. 2s- 27 , F AB mass spectrometry has become a standard method for the analysis of polar, non-volatile and thermally labile compounds28 • During F AB, sample ions are des orbed from a liquid matrix, usually glycerol, as a result of bombardment by energetic atoms (usually xenon at 4-10 kV). The matrix functions as a solvent that replenishes the supply of analyte at the site of bombardment and can facilitate the formation of protonated or deprotonated analyte molecules. If energetic ions are substituted for the fast atoms, then this technique is called liquid secondary ion mass spectrometry (liquid SIMS). The matrices, analytes, mass analyzer requirements and the mass spectra obtained using F AB and liquid SIMS are essentially identical.

During bombardment by energetic neutrals or ions, energy is distributed into the matrix and then transferred to the sample through intermolecular collisions. Although the energy of the bombarding particle is large, the analyte ions that are desorbed from the matrix are low in energy and little fragmentation occurs. Therefore, F AB and liquid SIMS are soft ionization techniques.

Preformed ions, protonated or deprotonated species, molecular ion radicals and cationized molecules have been observed in F AB and liquid SIMS


mass spectra. Disadvantages include the frequent detection of matrix ions, matrix ion clusters and occasional observation of adducts of sample molecule and matrix. Other disadvantages resulting from the use of a matrix include oxidation/reduction reactions between the matrix and analyte and strong matrix effects (different classes of analyte require different matrices). Advantages ofF AB and liquid SIMS include the ability to desorb and analyze intact molecular ion species of thermally labile and non-volatile compounds, more fragmentation than FD for structural determination and the long-lasting production ofanalyte ions that facilitates MS-MS measurements.

F AB has been coupled to a direct liquid introduction LC-MS system called continuous-flow F AB mass spectrometry29. The use of continuous-flow F AB mass spectrometry has not yet been demonstrated for the analysis of synthetic dyes. However, because of the wide use of static F AB in the analysis of synthetic dyes, this technique shows considerable promise for the analysis of mixtures of dyes and dye derivatives from a variety of sources.

In early applications ofF AB mass spectrometry to the analysis of synthetic dyes, glycerol was used as the matrix and mass spectra were obtained for naphthalene sulfonates3o , an anthraquinone monosulfonate27 , a series offluorescent, water-soluble xanthane dyes3J and a series of sulfonated azo dyes containing up to five sulfonate groupS32. Monaghan et al. 32 .3J and Haessner et al. 34 found that negative ion F AB mass spectra were superior to positive ion spectra for the analysis of anionic sulfonated azo dyes as their sodium salts or free acids. In addi tion to [M -N a] ,fragment ions were detected corresponding to [S03]-' [HSOJ]- and cleavage at the azo linkage. In subsequent studies, Monaghan et al. 35 extended the use of negative ion F AB mass spectrometry to analyze monophosphonated azo dyes. In another early FAB/SIMS study, Scheifers et al. J6 demonstrated the utility ofliquid SIMS for the identification of several types of organic dye.

Disulfonated oxonole dyes were analyzed by Borsdorf et al. 37 using positive and negative ion F AB mass spectrometry and a glycerol matrix. Because these compounds were ionic and non-volatile, they could not be analyzed using EI or CI mass spectrometry. Negative ion FAB mass spectrometry produced sulfonate ions, [M-H]-, and when lithium salts were analyzed, [M-Li]was identified. Abundant fragment ions were detected, including isocyanate and sulfanilate ions and ions resulting from cleavage along the methine chains of the oxonole molecule. In contrast, positive ion F AB mass spectra contained molecular ions species and fragment ions in very low abundance.

--------------------------------------------------~ Figure 4.3 (a) Negative ion fast bombardment (FAB) mass spectrum of I ~g of a high-energy disperse dye analyzed in I ~I of the matrix 3-nitrobenzyl alcohol. (b) Negative ion B/E linked scan (MS-MS analysis) of M-' shown in (a) following collisional activation (CAD) to promote fragmentation. Compared to the standard F AB mass spectrum of this dye, matrix ions have been eliminated and only sample ions are detected. (Reproduced from reference 19 with permission.)

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In another study, negative ion FAB mass spectrometry and MS-MS with CAD were used by Freeman et al. '9 to analyze 15 disazo direct dyes for cotton. The F AB matrices diethanolamine and 3-nitrobenzyl alcohol were found to provide more abundant sample ions than either glycerol or thioglycerol. Both radical anions M-' and [M-Na]- were detected. For example, the negative ion F AB mass spectrum of a high energy disperse dye in Figure 4.3a shows a molecular ion M-' and several fragment ions. The most abundant ions in the mass spectrum shown in Figure 4.3a were formed from the 3-nitrobenzyl alcohol matrix, which is a common characteristic of F AB mass spectra.

Following collisional activation to promote fragmentation, Freeman et al. '9 used CAD and B/E linked scanning (a type ofMS-MS) to detect fragment ions of selected molecular ion precursors of high-energy disperse dyes. An example of one of these MS-MS analyses is shown in Figure 4.3b and contains abundant structurally significant fragment ions that are free from matrix ions. In another study, Rivera et al. 38 used FAB mass spectrometry and FAB combined with CAD and MS-MS to identify a variety of organic compounds including several sulfonated azo dyes that had been added to drinking water. The detection limits for these compounds ranged from 500 ng to l/-lg.

Freeman and Sokolowska-Gajda '8 used positive and negative ion FAB mass spectrometry with 3-nitrobenzyl alcohol as the matrix to obtain protonated and deprotonated molecules of c.1. Acid Green 25 and the intact metalized azo dye c.1. Acid Orange 60. Photodegradation products of these dyes were also detected using F AB mass spectrometry. For the lower molecular weight degradation products (i.e. less than mJz 400), matrix ions interfered with the detection of the analytes, so that positive ion isobutane DCI mass spectrometry was more useful for molecules in this mass range. In another study, Sokolowska-Gajda and Freeman21 used positive ion FAB mass spectrometry and 3-nitrobenzyl alcohol as the matrix to confirm the molecular weight of six disperse dyes and four cationic dyes formed from mono- and di-sulfonated naphthol azo dyes. The base peak in each mass spectrum of the disperse dyes was usually the protonated molecule. The cationic dyes typically lost the iodide counter ion to form abundant cations that confirmed molecular weights.

Shimanskaset al.39 found that the use of an ion exchange column to remove impurities and generate the free acid form of sulfonated azo dyes improved the intensity ofFAB mass spectra of these compounds. Similarly, Kawaguchi and Sait040 found that the use of reversed phase and ion exchange chromatography to obtain pure sulfonic acid azo dyes in their free acid form was useful for the formation of protonated molecules during positive ion liquid SIMS. A nearly saturated solution of glycerol matrix provided the best SIMS spectra.

Recently, Freeman et al. 41 compared the negative ion FAB mass spectra of 15 sulfonated azo anthraquinone acid and direct dyes using thioglycerol, glycerol, diethanolamine and 3-nitrobenzyl alcohol as F AB matrices. Thioglycerol was found to be the best general matrix for mono-, di-, and trisulfon-


ated dyes having molecular weights in the range 300-700 daltons. Molecular ion species M', [M-H] and [M-Na]- were detected, in addition to abundant fragment ions formed by cleavage at or adjacent to the azo linkages. For sulfonated dyes weighing 700-900 daltons, 3-nitrobenzyl alcohol and thioglycerol were sometimes good matrices. However, no matrix investigated consistently facilitated the formation of abundant sample ions for dyes weighing more than 900 daltons. In a similar study, Richardson et al. 42

analyzed two mono sulfonated and eight disulfonated azo dyes using liquid SIMS and six different matrices (glycerol, thioglycerol, 3-nitrobenzyl alcohol, diethanolamine, 2-hydroxyethyl disulfide and a mixture of dithioerythritol and dithiothreitol). The matrix 3-nitrobenzyl alcohol provided the best results. In order to obtain useful mass spectra, a minimum dye concentration of 0.4 or 4 ~g/~ 1 (dye in matrix) was required for mono- or di-sulfonated azo dyes, respectively.

Isotope-exchange experiments can be carried out in the liquid matrix during FAB or liquid SIMS to identify active (exchangeable) hydrogen in organic molecules and as an aid to understanding some of the chemical reactions that take place during desorption using these methods. Following this approach, Bentz and Gale43 analyzed nine cationic dyes by using liquid SIMS mass spectrometry and either glycerol or deuterated glycerol as the matrix. The number of active hydrogens in each dye could be determined by the shift in the measured mlz value for each preformed cation. For example, the cation of diethylphenosafranin was detected at mlz 343 using a glycerol matrix but shifted to mlz 345 when [d8]-glycerol was used, which indicated that diethylphenosafranin contained two active hydrogens (on the -NH2 group). In addition, these deuterium labeling experiments could be used to follow fragmentaion pathways and identify if reduction was taking place in solution or during the desorption process.

Gurka et al.44 used positive ion F AB with a matrix consisting of either thiogycerol or glycerol and N, N-dimethylformamide to identify Disperse Blue 79 and one of its derivatives in HPLC fractions from an aqueous dye discharge from an industrial site. Another dye derivative could be analyzed using thermospray, but no useful information was obtained using FAB mass spectrometry. F AB mass spectrometry has also been used to identify dispersants and other additives in commercial dye samples45 .

4.2.6 Laser desorption

Laser desorption (LD) resembles F AB and SIMS, except that photons are substituted for energetic atoms or ions. Typically, photon energy is absorbed by a sample matrix and then transferred to the analyte molecules, which are simultaneously desorbed from the solid state and ionized. Alternatively, neutral dye molecules can be desorbed by a pulsed infra-red laser, followed by photoionization using an ultraviolet laser. In either case, LD is a soft ioniza-


tion technique. Although few applications of LO mass spectrometry to the analysis of synthetic dyes have been published, this technique is potentially useful for the analysis of synthetic dyes in mixtures or on textile surfaces. At this time, there are no commercially available instruments for LC-MS using LO.

Bennett et al. 46 used laser microprobe mass spectrometry to detect dyes present in a multicomponent mixture. Several laser shots were used and the resulting mass spectra were averaged to produce a mass spectrum for qualitative analysis. Each dye in mixtures containing up to four dyes could be detected. Quantitative analysis was not carried out because of differences in the ionization efficiencies of the different dyes. Oale et al. 47 used two-step laser desorption photoionization to examine azo, anthraquinone, phthalocyanine and coumarin dyes. Both molecular weight and structural information were obtained. Fragmentation could be enhanced by increasing the power density of the second laser or by decreasing its wavelength.

4.2.7 Plasma desorption

Plasma (or fission fragment) desorption (PO) mass spectrometry, pioneered by MacFarlane and Torgerson48 , utilizes high-energy heavy ion bombardment to desorb and form secondary ions of non-volatile analytes. Samples are typically prepared for analysis by electro spraying onto a thin substrate such as aluminum foil, Nafion49 , polypropylene or Mylar50 • No combination of PO with LC-MS has been reported for the analysis of textile dyes.

mCf undergoes nuclear fission spontaneously to form two new elements which are emitted at 50-80 MeV approximately 1800 apart. While one fission fragment strikes a detector to begin a timing circuit in a time-of-flight mass spectrometer, the other fragment penetrates the foil or other thin membrane supporting the sample and then the sample. Sample ions are formed and accelerated, and then detected in a time-of-flight analyzer. This is a highly efficient ionization technique, because a single fission event along the axis of the mass spectrometer results in a mass spectrum. Typically, thousands of such events occur. The spectra are acquired and then signal-averaged to produce a mass spectrum. Ions weighing up to 20 000 daltons or beyond may be formed using this method. Advantages of plasma desorption include high sensitivity, no liquid matrix requirement and the ability to analyze very high mass samples (at least 20 000 daltons). The disadvantages are that plasma desorption must be carried out at low resolution on a dedicated time-of-flight mass spectrometer, little fragmentation occurs and no MS-MS measurements are possible. Because little fragmentation is observed in PO mass spectra, only limited structural information can be obtained.

One of the advantages of PO over FAB ionization is the absence ofa liquid matrix which forms abundant ions that might interfere with the detection of analyte ions. For example, positive ion PO and liquid SIMS mass spectra of

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o '.D

11 0 ANALYTICAL CHEMISTRY OF SYNTHETIC COLORANTS

the aromatic dye chrysoidin are compared in Figure 4.4. Abundant glycerol matrix ions were detected in the liquid SIMS mass spectra at rnJz 93 (base peak and rnJz 185. In the PD mass spectrum, protonated Chrysoidin is the base peak, and no abundant matrix ions were detected. Gale and coworkers5l

found that F AB ionization of certain aromatic cationic dyes using a glycerol matrix resulted in enhanced abundances of artifacts corresponding to [M + 1 r and [M + 2r that were probably products of reduction reactions that occurred in the matrix during F AB. Because PD was not carried out using a liquid matrix, no reduction products were observed.

A mixture of three cationic dyes, Rhodamine B, Methyl Violet and Methylene Blue, were analyzed by Beug-Deeb et al. 52 using positive ion PD mass spectrometry. Samples (80 nmol) of each dye were electro sprayed onto aluminized Mylar for analysis. Abundant preformed cations of each dye in the mixture were detected during the analysis, as well as some less abundant fragment ions. However, the intensities of peaks for each dye were different, despite the presence of equimolar quantities. These variations were believed to be dependent upon the homogeneity of the electro sprayed mixture, the mass of the analytes (lower molecular weights tended to produce more abundant ions) and the chemistry of the desorption process. Therefore, quantitative analysis of dyes using PDMS is predicted to be difficult52 .

4.2.8 Thermospray

During thermospray, liquid solvent (often from an HPLC system) is vaporized as it passes through a heated capillary tube53 • The superheated vapor emerges as a supersonic jet that is progressively desolvated as it moves toward a skimmer at the opening of the mass spectrometer. Analyte ions released from droplets of the supersonic mist (often by ion evaporation) enter the mass spectrometer through the skimmer and are analyzed according to their massto-charge ratio. In order to enhance ionization, external ionization may be applied prior to the skimmer by using a heated filament or discharge electrode.

Direct injection thermospray was used by Gurkaet al.44 to identify Disperse Blue 79 and a debrominated analog isolated by HPLC from an aqueous azo dye discharge into the environment from an industrial site. Another derivative that could not be identified completely using thermospray formed protonated molecules during FAB mass spectrometry.

Yinon et al. 15 analyzed a series of mono- and disulfonated azo dyes and single examples of anthraquinone, coumarin, xanthene, methine and arylmethane dyes using positive ion thermospray mass spectrometry. Detection limits for peaks in reconstructed total ion chromatograms using successive full scan analyses ranged from 0.05 ng for the anthraquinone dye Disperse Blue 3 to 20 ng for the azo dye Disperse Orange 13. Mass spectra of four mono- and disulfonated azo dyes were presented which were obtained using 1-10 J.lg of each dye. The mass spectra of sulfonated azo dyes showed protonated

100.0

50.0

mlz

MASS SPECTROMETRY

S03 Na

b-N=N-O-N=NS_~ NH-Q o ~ Ii S03Na

Acid Blue 113 C.1. 26360

638

660

682

500 520 540 560 580 600 620 640 660 680 700

III

1438

Figure 4.5 Positive ion thermospray mass spectrum of Acid Blue 113. Protonated molecules were detected at mlz 682, and protonated molecules in which one or two sodium cations were replaced by protons were detected at mlz 660 and 638, respectively. (Reproduced from reference 15 with

permission.)

molecules or sodium adducts and abundant fragment ions such as [MHNaS03r. Exchange ofNa for H was also frequently observed. For example, the positive ion thermos pray mass spectrum of Acid Blue 113, a disulfonated azo dye, is shown in Figure 4.5. In another study, Yinon et al. 54 used repellerinduced collisional activation thermos pray mass spectrometry to increase the number of fragment ions for structural determination. Some fragment ions were similar to those obtained using thermospray with CAD, while other fragment ions resembled those obtained with EI with a particle beam interface l4 •

Thermospray ionization with MS-MS was used by Ballard and Betowski55

to analyze 16 commercial dyes and samples of liquid wastes from dye manufacturing. No chromatography was used. By using collisional activation and MS-MS following thermospray ionization, fragment ions of selected precursor ions could be detected that were free from contaminating ions. Fragmentation pathways for several classes of dyes were discussed, including azo dyes, methine dyes, arylmethane dyes, anthraquinone dyes, coumarin dyes and xanthene dyes. Detection limits for several of these dyes were determined using full scans in both positive ion and negative ion modes. For


example, the limit of detection of Disperse Blue 3, an anthraquinone dye, was 20 ng for the protonated molecule and 1300 ng for the radical anion in negative ion mode. The negative ion mode was typically at least one order of magnitude less sensitive than the positive ion mode.

Since sulfonated azo dyes cannot be separated using gas chromatography or analyzed using EI or CI because of their high polarity and non-volatility, the most widely used LC-MS interface for the analysis of synthetic dyes has been thermospray. However, because of the low sensitivity reported for the thermospra y analysis of sulfonated azo dyes 15,56, Groeppelin e tal. 57 modified the normal thermospray interface by lining the stainless steel tubing at the beginning of the thermospray interface with fused silica capillary. Using this more inert surface at the beginning of the interface, they improved the limit of detection for a monosulfonated azo dye from 10 J.Lg to approximately 600 ng.

In another study, McLean and Freas58 modified the thermos pray system by restricting the vapor exit orifice and adding a needle-tip repeller electrode to the interface. As a result, on-column detection limits of 5-20 ng were achieved with a signal-to-noise ratio of 10 for disulfonated azo dyes. It should be noted that, unlike Groeppelin et af.57 McLean and Freas58 used selected ion monitoring instead of scanning a specified range of mJz values.

Voyksner59 demonstrated the application of thermospray LC-MS to the analysis of azo, disazo and anthraquinone dyes in waste water, soil and gasoline. Subsequently, Voyksner et al. 9 used thermospray LC-MS to separate and determine the molecular weights (as protonated molecules) of photodegradation products of Basic Yellow 2. Quantities ranging from 50-500 J.Lg of dye mixture were injected per LC-MS analysis. Because only protonated molecules were detected, GC-MS with electron impact ionization was carried out to obtain molecular ions and structurally significant fragment ions of the seven most volatile derivatives.

A quantitative thermospray LC-MS assay was developed by Betowski et al.60 to measure Disperse Red 1 and its degradation products in the effluent from a municipal wastewater treatment plant. MS-MS methods, induding product ion scanning and selected reaction monitoring, were carried out using a triple quadrupole mass spectrometer to increase the sensitivity and specificity of measurements. The limits of detection were 600 pg, 2 ng and 180 pg for single quadrupole scanning, product ion scanning and selected reaction monitoring, respectively.

4.2.9 Electrospray and ion spray

The electro spray ionization interface for mass spectrometry was developed by Whitehouse et al. in the laboratory of Fenn61 • Ion spray was developed by Bruins et al.62 and is essentially a pneumatically assisted electrospray. In both techniques, a charged aerosol beam is formed by applying a potential of several kilovolts to the end of a small capillary through which the LC eluent

MASS SPECTROMETR Y 113

flows. As the liquid flows through the capillary at rates of several microliters per minute (up to 100 Ill/min in ion spray), a fine mist of charged droplets is generated by the applied electrical field. Ions are evaporated into the gas phase as field strengths produced by coulombic repulsion between ions within a droplet exceed the solvation energy of the ions in solution. After desolvation or ion evaporation, sample ions, which may be multiply charged, enter the mass spectrometer analyzer region and are separated according to their massto-charge ratio. Ion spray and electro spray are soft ionization processes that produce abundant sample ions with high sensitivity and little fragmentation. As discussed below, these techniques show high sensitivity in the negative ion mode for the analysis of sulfonated azo dyes. Application to other classes of dye has yet to be demonstrated but should be feasible for the more polar compounds such as cationic dyes.

Ion spray LC-MS of a series of sulfonated azo dyes was carried out by Bruins et al. 6J using reversed phase HPLC separation on-line with ion spray mass spectrometry and MS-MS in a triple-quadrupole mass spectrometer. Abundant [M-H] - were detected and the limits of detection for mono- and disulfonated dyes ranged from 10-50 ng for full scans. Using MS-MS following collisional activation, S03· were detected, at mJz 80, that were characteristic of sulfonated azo dyes. Selected ion monitoring and selected reaction monitoring were used to detect S03· ions in order to identify sulfonated azo dyes in environmental samples.

In a similar investigation, Edlund et al.64 developed a quantitative LC-MS procedure to screen municipal wastewater for sulfonated azo dyes. Reversed phase HPLC coupled on-line with ion spray mass spectrometry was used to form [M-Na]- and [M-2Naf- according to the number of sulfonate groups on the analyte. Collisional activation of precursor anions produced S03· fragments plus additional fragment ions of each dye. The detection limits of several sulfonated azo dyes were on the order of 50 ppb in municipal wastewater.

Lee et al.65 interfaced capillary zone electrophoresis (CZE) to an ion spray mass spectrometer using a liquid junction interface and demonstrated the utility of this instrument for the analysis of several classes of polar compounds including sulfonated azo dyes. Lee et af. 66 then applied CZE-MS and CZE-MS-MS to the analysis of a series of sulfonated azo dyes at the low picomole level. Ion spray produced exclusively [M-Hr and [M-2Ht for mono- and disulfonated azo dyes, respectively. Because no fragmentation was observed, collisional activation was used to form structurally significant fragment ions that were detected using MS-MS on a triple quadrupole mass spectrometer. Fox example, the CZE-MS-MS spectrum of[M-2Hf- of Acid Blue 113 is shown in Figure 4.6. CAD of the doubly charged precursor ion produced fragment ions formed by cleavage at the azo linkages and at the SOJH group. Using selected ion monitoring instead of full scans, sulfonated azo dyes could be detected at the femtomole level.

Q) 0 c: as "0 c: :l


100

80

60

297 1- --

S03H 180 :

O:--~=N ON=NONH-o : 0 O S03H

. __ I

156 Acid Blue 113 FW -637

(M-2H) 2-317

..0 156 <C Q)

40 > ~ CD a: 20 80 297

o 50 80 100 120 140 160 180 200 220 240 260 280 300 320 350

MIZ 317 - 1060 COUNTS

Figure 4.6 CZE-MS-MS analysis of [M-2Hf- from Acid Blue 113 formed using ion spray. Fragmentation was induced using collisional activation (CAD). (Reproduced from reference 65

with permission.)

4.3 Conclusion

There is no one ionization method that is ideal for the analysis of all classes of dye, therefore, the dye chemist and mass spectrometrist must work together to select the best ionization method that will provide the required mass spectrometric data. It is clear from this survey that some ionization methods are best suited for molecular weight determination, while others provide primarily fragment ions and structural characterization. If MS-MS is available, then abundant molecular ions might be formed first for molecular weight confirmation, then the molecular ions could be fragmented using collisional activation to generate fragment ions. In other cases, dyes in mixtures containing impurities or degradation products might be analyzed using LC-MS or GC-MS so that chromatographic separation could be carried out with the power of mass specific detection or mass selective quantitation.

LC-MS is a powerful complement to GC-MS for the on-line separation and mass analysis of mixtures of non-volatile, polar and thermally labile compounds. Unlike the comparatively mature GC-MS systems, LC-MS is undergoing rapid changes and development. The most widely used LC-MS methods for the analysis of synthetic dyes have been thermospray, electro spray and particle beam. In addition to functioning as interfaces for


LC-MS, thermospray and electro spray are also ion evaporation ionization techniques.

Recently, mass spectrometric techniques have improved dramatically so that molecular weights and some structural information may be obtained for virtually all types of synthetic dye. New ionization and LC-MS techniques have made possible measurements that might have been unimaginable just a few years ago. Besides incremental improvements in sensitivity and performance of LC-MS and MS-MS systems, mass spectrometric methods will probably be extended over the next few years to facilitate the direct analysis of synthetic dyes adsorbed on surfaces, including covalently bound colorants.

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357-65. 4. L. Havlickova, A. Kolonicny, A. Lycka, l. lirrnan and l. Kolb (1988) Dyes and Pigments, 10

1-11. 5. N.N. Romanov, l.S. Shpileva, E.K. Mikitenko, Ukr. Khim. Zh. (Russ. Ed.), 54 (1988)738-41. 6. A. Amer, E.G. Alley and e. U. Pittman (1986) 1. Chromatogr., 362 413-18. 7. R. Naef (1991) Dyes and Pigments, 16, 183-96. 8. P.E. McGovern, l. Lazar and R.H. Michel (1990) J. Soc. Dyers Colourists, 106,22-5. 9. R.D. Voyksner, TW. Pack, e.A. Haney, H.S. Freeman and W.N. Hsu (1989) Biomed.

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18, 253-7.

5 Electron spin resonance spectroscopy H.S. FREEMAN and R.D. BEREMAN

5.1 Introduction

Although review papers exist pertaining to the use of electron spin resonance (ESR) spectroscopy in polymer science, to detect transient radicals formed during the initiation and propagation steps of polymerization and in polymer degradation I and in biochemistry2-3, a similar compilation of published papers on applications of ESR in dye chemistry has not been published. This chapter presents such a survey, with emphasis on applications rather than theory or instrumentation. A brief summary of the basic terminology of ESR spectroscopy will be given, however.

ESR spectroscopy can be used, in principle, to detect any system having a net spin angular momentum. Examples are free radicals, molecules in the triplet state and many transition metals, all of which can be detected by ESR in a solid or liquid medium to give information about the electronic structure and environment of the system in question. This means that ESR is a convenient method for characterizing the molecular motions of small molecules in a polymer matrix, as well as for assessing changes in their molecular environment.

5.2 Basic principles

ESR spectra are produced because an unpaired electron has magnetic properties resulting from its electronic angular momentum, which is the sum of the spin angular momentum and orbital angular momentum. In most cases, spin angular momentum alone is considered, since the orbital angular momentum is negligibly sma1l4 • When the unpaired electron is placed in a magnetic field H, the energy of that electron can be expressed by equation 5.1

E= ± gfiHl2 (5.1)

where fi is the Bohr magnet on (eh/4lrmc), and g is a dimensionless proportionality constant between the angular momentum and the magnetic moment of the electron and equals 2.002319 for a free electron. Equation 5.1 represents the two energy levels for an electron in a magnetic field. It is


1 >(!) ffj----K z LJ.J

o

hv

Hr

MAGNETIC FIELD •

1 Ea=-gPH 2

Ej3 1 -- gPH 2

Figure 5.1 Electronic energy level diagram showing Zeeman splitting for a free electron under the conditions of variable external magnetic field and constant frequency4

also referred to as the Zeeman energy levels for an electron, and the energy difference (tlE) between these two levels is given by equation (5.2)

tlE = gfiH (5.2)

tlE increases linearly with the intensity of the magnetic field as illustrated in Figure 5. 1.1t is possible to induce transitions between the two energy levels by applying electromagnetic radiation having an energy equal to gfiH:

(5.3)

where vis the microwave frequency and Hr is the magnetic field at which the difference or resonance condition is satisfied. The resonant absorption can be detected either by fixing the frequency and varying the magnetic field or by fixing the magnetic field and varying the frequency. In a typical ESR experiment, the frequency is fixed and the energy levels are 'tuned' by changing the magnetic field. Most commercial ESR spectrometers operate in the microwave region, using X-bands (about 9500 MHz) as the irradiation source.

5.2.1 Spin relaxation and line broadening

The ESR signal can be detected only when there is a population difference between the Zeeman energy levels. At thermal equilibrium, there are initially more spins in the lower level. The relative population of the two levels is

ESR SPECTROSCOPY

determined by the Boltzman distribution:

N(+1I2)=ex (_6.E)=ex (_gPHr) N(-1/2) p kT P kT

119

(5.4)

where N (+ 112) and N (-112) are the number of spins in the upper and lower spin energy levels, respectively, k is the Boltzmann constant, and Tis absolute temperature. At room temperature, the energy splitting (gPHr) is so small that the population ratio is almost unity, indicating that the difference in the population is very small. This population difference is directly related to the intensity of an ESR signal, and it increases with decreasing temperature. Therefore, there is always a gain in intensity by cooling a sample to lower temperature'.

As the energy absorption proceeds, the populations in the upper and lower levels become equal and the ESR signal disappears (saturation effect), unless the excess spins in the upper level are able to lose energy and the system returns to Boltzmann distribution at thermal equilibrium. Processes which restore this equilibrium are known as relaxations and they are characterized by two relaxation processes. One process by which the spins in the upper level lose energy externally is called spin-lattice relaxation. This can be defined in terms of the spin-lattice relaxation time T,:

(5.5)

where M= is the z component of the macroscopic magnetization of the sample, and T, is the parameter characterizing the rate of decay of M= into an equilibrium value Mequ' when the magnetic field H= is off. The Heisenberg uncertainty principle is used to explain the origin of line broadening caused by spin-lattice relaxation:

(5.6)

where M is the lifetime of the energy state in question and 6.E is the energy width of that state. A small T, value results in a large 6.E, or a large fluctuation in energy levels. This fluctuation corresponds to the line broadening in the ESR spectrum. The line width (6.H) of the absorption peak is inversely related to the spin-spin relaxation time T,:

6.H = (h/gP) 6.v- (h/gP) (l/2n") (liT,) (5.7)

Under the conditions oflow microwave power, line widths are usually related to a spin-spin relaxation mechanism. When two neighboring electrons in different energy states are processing at the same frequency, the magnetic field caused by each electron induces a transition in the other. This phenomenon is called mutual exchange of spin states and it reduces the lifetime of each spin state without changing the total number of spins in the levels. In other words, the ensemble of spins precessing in the magnetic field cannot precess in phase permanently because of magnetic interaction between spins. Each electron,


experiencing a slightly different local magnetic field, begins to destroy the phase coherence. As a result of this process, the perpendicular component of the magnetization (MJ decays to zero. The time required for complete dephasing is called the spin-spin relaxation time T2• This relaxation process can be defined using the following equation:

(5.8)

where T2 is the parameter describing the rate of decay of the x component of the bulk magnetization (M.). The spin-spin relaxation phenomenon is mainly responsible for the observed line broadening, particularly in the case of stable free radicals, and it produces an absorption peak that is described by a Lorentzian function5•

5.2.2 The g-value

If each unpaired electron in a paramagnetic molecule behaved as a free electron, then all resonances would be expected to occur at the same value of the field. However, this is not observed because there are small orbital contributions to the magnetic moment, caused by the magnetic interaction between the spin momentum and the magnetic field generated by the orbital motion6• This phenomenon is called spin-orbital coupling. It is convenient, however, to assume that the resulting magnetic moment is produced by a pure spin angular momentum. The effective g-value (ge) thus can be expressed as a function of the microwave frequency and magnetic field intensity at resonance:

ge = hvlfiH, (5.9)

The effective g-value is often characteristic of a particular radical and can be used in determining the nature of a paramagnetic compound. In free radicals, the unpaired electron is delocalized over the whole molecule and behaves like a free electron. Therefore, the g-values of free radicals are very similar to the theoretical value for a free electron of ge = 2.00232. The g-values of nitroxide radicals are somewhat higher (2.0050-2.0060) since the unpaired electron is essentially localized in the p-orbital of the nitrogen atom. On the other hand, transition metal ions have g-values which differ significantly from that of a free electron?

5.2.3 Hyperfine coupling

A multiplet, called hyperfine structure, is often observed in ESR spectra. The hyperfine structure is often useful in identifying particular paramagnetic molecules or in improving understanding of the electronic structure of the molecule. It results from an interaction between the electron spin magnetic moment and the magnetic moments of nuclei having permanent nuclear spin

ESR SPECTROSCOPY 121

angular momentum, I. Common examples are hydrogen (IH = 112) and nitrogen (IN = 1). The interaction between an unpaired electron and a magnetic nucleus is called nuclear hyperfine interaction.

When a magnetic field is applied to the system, the nuclear moments can take 21 + 1 orientations with respect to the direction of the magnetic field. The hyperfine structure of the ESR results from the fact that the electron spin magnetic moment interacting with the nucleus experiences 21 + 1 different total fields according to the orientations of the nuclear spin in the static magnetic field.

Taking into account the hyperfine interaction, the resonant magnetic field Hr can be expressed as follows:

(5.10)

where H' is the resonant magnetic field at a = 0, a is a hyperfine splitting constant in units of gauss, and M[ is the value ranging from -J to + J in unit increments4 •

5.2.4 Anisotropic effects

The ESR spectra of solid state systems show anisotropy and are greatly dependent upon the orientation of paramagnetic species in a magnetic field. The typical anisotropic systems are free radicals or transition metals in a solid matrix. In these oriented systems, the corresponding anisotropy is determined by the g-value and hyperfine splitting. Hence, the most general expression representing the Zeeman splitting and nuclear hyperfine interaction for nitroxides such as 1 is given by equation (5.11)

H= PHgS + SAl (5.11)

where H, S, and I are vectors and 9 and A are second-order tensors. When paramagnetic compounds are dissolved in a low viscosity solvent,

the anisotropy is averaged out by rapid molecular reorientations. Thus, the position of the spectrum in the field and the magnitude of the hyperfine splitting are determined by the average values of the diagonal elements of the 9 and A tensors5•


5.3 Applications

ESR has been used to help characterize:

1. the mechanism of fading of azo dyes 2. the nature of the triplet state of irradiated dyes 3. the mechanism of dye sensitization 4. the environment of metal ions in metallized dyes 5. the mechanism of energy transfer between donor and acceptor molecules.

The following is a summary of the results of those investigations.

5.3.1 Sensitizing and desensitizing dyes

Lu Valle et al. 8 used ESR to study the ability of 34 cyanine and merocyanine dyes (e.g. 2 and 3: R = Me, Et; X = 0, S) to initiate the polymerization of acrylonitrile in the presence of visible light and to determine the source of the ESR signal previously observed in crystalline samples of these dyes. Of the dyes evaluated, only dyes similar to 3 were effective initiators in the presence of visible light. All of the xanthene dyes (desensitizers) required light with wavelengths longer than 5 x 103 A. It was also reported that the absence of a radical signal at g = 2 or g = 4 meant that it was not possible to say whether the dye-induced polymerization of acrylonitrile occurred by an electron-transfer or energy-transfer mechanism when CHCl3 was used as the medium, and that the ESR signal in the crystalline dyes resulted from impurities readily removed by recrystallization.

Me

~S>=CH-C=CH-CH=C~=~ ~N O~A -00 '~I~

I T "I I Et R-N N-R Et

Y

EtHN

X

2 (Rodamine 6G ) 3 R = Me, Et; X = 0, S

Lu Valle et al. 9 extended their work in this area to include an extensive range of donor-acceptor complexes of sensitizing/desensitizing dyes (79 sensitizing and 20 desensitizing) and related compounds, using chloranil as the principal acceptor. The donor-acceptor complexes exhibiting paramagnetic properties gave a sharp peak at g = 2.0033. The spectra of three of the complexes exhibited hyperfine structure, and many others contained a broad peak attributed to an oxidation-reduction reaction.


4 (Me 540) 5

Sarna 10 used ESR to assist in the elucidation of the mechanism by which 4 (MC 540) facilitates the photodynamic termination ofleukemia cells. Specifically, the formation of the anion radical of MC 540 and its properties were studied, including the nature of the excited state of MC 540 that causes electron transfer reactions leading to reduced nicotinamide adenine dinucleotide. In this study, MC 540 was demonstrated to be an efficient singlet oxygen sensitizer capable of abstracting an electron from a variety of electron donors in its excited state. In addition, it was proposed that the photodynamic efficiency of MC 540 was attributed to the formation and decay of oxidizing species photosensitized in cell membranes by this dye, and that MC 540 may be an example of a regiospecific (site specific) Fenton reaction.

Tani and Sano" showed that the ESR signal resulting from placing the cyanine sensitizing dye 5 on the surface of AgBr microcrystals increased as the size of the l-aggregates of the dye increased. It was also found that lightinduced electron-transfer from these aggregates to octahedral AgBr microcrystals produced positive holes in the aggregates, giving rise to the observed ESR signal. Further, it was suggested that the behavior of positive holes in these aggregates accounts for the decrease in the photographic sensitivity with increasing size of l-aggregates. The authors also reported the use of ESR to determine that the decay of dye-positive holes was a second-order reaction that followed the reaction:

(5.12)

where Xo and X were the ESR signal intensities at time 0 and time t, respectively.

ESR has been used to assess the importance of the triplet state of vat dyes such as 6--7 in the photosensitized degradation of cellulosel 2• Triplet-triplet energy transfer to anthracene was observed for 6 and 7 in boric acid glass; no such transfer was observed with Cibanone Yellow R, Caledon Yellow G, or Caledon Gold Orange 6, even though their luminescence spectra were consistent with the formation of triplet states. This led the authors to suggest that the orientation between the latter dyes and acceptor molecules do not assist in the necessary triplet energy transfer process. It was concluded that the sensitizing dyes do form singlet oxygen as an intermediate in the dye-sensitized photodegradation of cellulose, and that the degradation process is the result of


f\ a S\\ "=/

~N N~ ifs a

~I ::,...

~ @oH'N~OH ~ I I '-': 'H

::,... ~

a

6 (C. I. Vat. Yellow 2 ) 7 (C. I. Vat. Blue 12 )

both singlet-oxygen and hydrogen-abstraction mechanisms originating from the triplet state of the dye.

5.3.2 Azo dyes

Heijkoop and Beekl3 employed ESR to provide direct evidence for the formation ofhydrazyl and aminonaphthoxy radicals upon the light-induced degradation of monoazo dyes (8: R = (S03Na)I,2)' The g-values, 2.0038 in each case, reported were consistent with the formation of hydrazyl radicals. The authors also reported hyperfine splitting constants for both the hydrazyl radicals and aminonaphthoxyl radicals. These data indicate that a low oxidation/reduction equilibrium constant accounts for the inability of one of the dyes (R = 4,8-(S03Na)2) to form a detectable concentration of the corresponding aminonaphthoxy radical.

8

In related work l4 the fading mechanism of paint films containing a mixture of Pigment Red 170 and titanium dioxide was investigated using ESR to explain the accelerated fading of films containing a mixture of inorganic and organic pigments compared with films containing only an azo pigment or titanium dioxide. From the results of the ESR analyses it was concluded that (i) the combination of Pigment Red 170 and Ti02 produced paramagnetic species in paint films even at room temperature; (ii) the concentration of the observed paramagnetic species correlated with the level of fading in paint


Pigment Red 170

films; (iii) the g-values of the mixture varied (g :::: 2.175 to g :::: 2.651) with temperature and method used to prepare the colored films; and (iv) the paramagnetic species appeared to be O2- adsorbed on the surface ofTi02 and 0- originating from TiO;. Fading occurs when these reactive species attack the azo pigment.

5.3.3 Triarylmethanedyes

In a short communication, Antonucci and Talley 1 5 reported the use ofESR to determine the factors affecting intersystem crossing to a triplet state (detrimental to laser action) and to determine which type of dye gives a llM :::: ±2 signal. This study employed xanthene dyes (Rhodamines), oxazines (Cresyl Violet, Oxazine I), and the triphenylmethanes Crystal Violet and Malachite Green. It was concluded that the triplet ESR signal in the spectrum of Rhodamine 6G at a concentration of 5 x 10-5 M indicated that this signal can be attributed to the monomer rather than an aggregate of the dye. It was also found that Crystal Violet gives a strong signal between 1612 and 1637 G, but that the oxazine dyes were ESR inactive and afforded very little to no phosphorescence.

Schmidt extended studies on the triplet spectra of Rhodamine dyes and their aggregates 16. For Rhodamine 6G (2), he observed two different ESR triplet spectra depending on its concentration. The spectra recorded at low and high concentrations were attributed to monomeric and associated dye molecules, respectively. The concentration dependence of the ESR spectra and the use of triplet exciton theory based on those spectra allowed the author to draw conclusions about the structure of the associated molecules (aggregates). He found that the molecules within the aggregates existed as twisted sandwich structures and calculated the twisting angles using the equation:

((z* y*Y 1 cos (2¢) = 2 2 -I (Z-Y) (5.13)

where Yand Z are the zero-field splitting energies of the dye monomers, and Y* and Z* are the corresponding parameters of the associated molecules


e o

Br

9 (Phloxine semiquinone anion)

eN

H2N--o-tO-¢1 I' '\ NH2 7, ~

NH2 10

having an even number. The twisting angles for Rhodamine 6G increased from 17° to 30° as the dye concentration was increased from 10-4 M to 10 I M.

Kimura and Imamura l7 recorded the ESR spectra of the xanthene dye semiquinone anion 9, and found that the spectrum consisted of three wellresolved hyperfine lines having 1 :2: 1 intensity ratio in the temperature range of -80 to +60°C. The lines possessed a Lorentzian shape and equal line widths in the -50 to +5°C temperature range. Below -60°C, the lines exhibited anisotropic broadening. A dependence of the line width on solvent viscosity was also found. The authors proposed that the observed variations in line widths were the result of dissociation and/or distortion of hydrogen bonding between semiquinone anions and hydroxylic solvent molecules.

Uribe et al. 18 used ESR to analyze poly(vinylbutyral) and nylon films containing leucocyanides of triphenylmethane dyes (cf to) following irradiation with 6OCO r-rays. These workers found that by separating the ESR signals derived from the polymer matrix from the spectrum derived from the dye precursor, the number of spins associated with a free radical produced in the substituted triphenylmethyl radical could be determined as a means of dosimetry. Line widths of 10-15 gauss and ag-value of 1.99 were reported for the leucocyanides studied.

5.3.4 Acridine dyes

The triplet states of acridine dyes (11: x = H, CH 3: y = H, CH3, OEt: RI = H, Ph, NH2: R2 = H, CH3) were investigated at 90 K using ESR 19. The dyes were excited to the triplet state in the ESR cavity by irradiation using a high pressure xenon lamp equipped with a filter giving a 475 nm cut off. The results

11


of this study indicated that the observed triplets resulted from ,. ~ ,.* transitions, and that placement of a methyl group in the two and seven positions resulted in a detectable contribution to the triplet spin density through hyperconjugative or inductive effects. Zero-field splitting parameters were calculated from the recorded spectra, and their relationship to the distribution of the triplet spin density in a group of five acridine dyes was discussed.

The synthesis and ESR analysis of nitroxide spin-labeled acridine orange dyes (e.g 12) have also been reported20 • The temperature dependence of the ESR spectra of the spin-labeled dye led the authors to conclude that the molecules underwent anisotropic and rapid rotation. It was also shown that the attachment of the spin-labeled dye to DNA caused the label to undergo slow and anisotropic rotation, and that the ESR spectrum of the spin-labeled dye in the presence of native DNA could be calculated with good agreement between the observed and simulated spectra.

12

5.3.5 Miscellaneous dyes

The ESR spectra of the copper(II) complex oftetrasulfonated phthalocyanine were recorded following the incorporation of the dye into wooF l to determine the environment of the metal ion. After studying the curves generated in the g = 2 and g = 4 regions using of frozen HplDMF (4:1) solution, the authors concluded that the dye molecules resided in the amorphous or plastic phase of the Merino wool structure. Based on ESR experiments conducted at room temperature, it was concluded that the amorphous phase must be rigid enough to keep the dye molecules stationary on an ESR time scale. Further, it was suggested that spectra in the g = 2 region arose from the presence of associated (polymeric, dime ric) and monomeric structures, and that spectra in the g = 4 region arose from ~M = 2 transitions within the triplet state arising from the dipolar coupled eu(II) ions of dimeric species.

Larach and Turkevich22 used ESR to investigate Rose Bengal-sensitized ZnO layers and found that spectrum recorded at 77 K correlated well with the dye absorption spectrum and the photoconductivity spectrum recorded at room temperature. The authors also conducted experiments designed to


Rose Bengal (CI Acid Red 94 )

distinguish between the two proposed mechanisms for the dye sensitization of ZnO: electron transfer and resonance transfer. The results of those experiments showed that coating of ZnO with Rose Bengal and irradiation caused a large increase in the ESR signal atg = 1.96, a signal attributed to Zn+, and that oxygen also caused a rise in the g = 1.96 signal presumably by facilitating a transfer of electrons from Zn clusters to oxygen on the surface. Interestingly, the authors concluded from experiments involving the kinetics of the rise of the g = 1.96 signal upon illumination as a function of O2 pressure that both of the mechanisms under examination can account for the formation of the paramagnetic species.

Coles and Nicholls23 analyzed temperature-dependent ESR spectra of 310 nm light-irradiated dyed wool and found that it was possible to distinguish between the wool and dye signals when Basic Red 2 (13; and azine dye), Basic Blue 4 (4, an oxazine dye) and anthraquinones (15; Mordant Red 3 and l-aminoanthraquinone, 16) were used. Interestingly, it was not possible to distinguish between the ESR signals of wool and the azo dyes Basic Orange I (17), Acid Red 88 (18) and Mordant Blue 44 (19) or Basic Yellow 2 (20) and Basic Violet 14 (21) diphenylmethane and triphenylmethane dyes, respectively. It was found that these results correlated with the photoconductivity behavior of the dyes. Specifically, when the n-type carriers (13-16; conduction is the result of mobile electrons acting as charge carriers) were applied to wool, an ESR spectrum of wool only was observed, while the p-type carriers (17-21; where the photoejected electrons are kept immobile in an electron trap within associated dye molecules and the resulting positive holes act as the charge

13 (Basic Red 2 ) 14 (Basic Blue 3/4 )


15 (Mordant Red 3 ) 16 (1-Aminoanthraquinone)

17 (Basic Orange 1 ) 18 ( Acid Red 88 )

19 (Mordant Blue 44 ) 20 (Basic Yellow 2 )

21 ( Basic Violet 14 )

carriers) do not promote an interaction between wool and its charge carrier, and, as a result, signals for both the p-type carriers and wool were observed.

Coles and Nicholls24 conducted a similar study of the 9 dyes (13-21) on Nylon 6 film. The dyed films were irradiated with 310 nm light for 4 h at room temperature in the presence and absence of air, with the same g-values and line widths being observed. However, the intensity of the signals in the spectra generated in the presence of air was less than that obtained in vacuo. Unlike the study conducted with dyed wool, the ESR spectra recorded on dyed nylon film were found reflect the adsorbed dye only. Changes in the current passing through dyed films before and after irradiation were measured and it was


concluded that charge transfer conductivity was responsible for the observed light-induced paramagnetic properties. Interestingly, it was also suggested that the extent of dye fading might be related to the mobility of the charge carrier through a textile substrate.

HO~. ~ N .... O

~ I I N I H

23 22

24

ESR has been used to characterize the diffusion of non-ionic molecules of type 1 (R = NH2) and 22. This work led to a useful expression for determining the time required for a stable free radical to reorient through an angle of 1 radian (viz., the rotational correlation time, TR) in a solid matrix and for describing Brownian translational diffusion25 :

kT TR =---

61[1]a~ (5.14)

where aR is the molecular hydrodynamic radius for rotation and 1] is the viscosity of the medium; and

(5.15)

where aD is the molecular hydrodynamic radius for diffusion. McGregor et al. 25,26 then incorporated the two nitroxides (1 and 22) into the

backbone of several colored spin probes (23 and 24: R = S03Na, CO2, Na, N02). Studies involving compound 23 in PET and nylon matrices led to the conclusion that the Stokes-Einstein equation (equation 5.15) should provide a better description of translational motions than the description of the rotational motions available through the use of equation 5.16

R=~ 81[1]a~ (5.16)


An increase in activation energy of rotation was observed at temperatures close to the Tg determined from dynamic mechanical measurements, and it was found that free rotation of the probe was strongly hindered until temperatures well above Tg were reached. In nylon fibers containing a significant level of water, probe rotation was more consistent with the 'free volume' theory than with models involving the rotation and diffusion of molecules in 'solventfilled pores or channels'.

When this work was extended to the more rigid colored probes 24 (R = S03Na, C02Na, N02) it was found that these larger molecules undergo Brownian diffusion in the slow tumbling region, experienced decreased rotational motion and required higher temperatures to reach the rapid tumbling region. Reduced rotational motion was attributed to either strong ionic interactions with polymer chain ends (for the two ionic dyes) or intermolecular interactions (for the non-ionic dye). It was also found that in the hightemperature region, the probes responded to a merged a + f3 relaxation, and it was shown that the volume of polymer segment that must move to permit the colored probe to move into space created in the polymer matrix by this movement is 1.5-2.0 times the volume of the diffusing probe.

5.4 Conclusion

Even from this very brief review, it is clear that the types of problem in dye chemistry for which ESR can help provide solutions are quite varied. The only limitations seem to be the need for a paramagnetic system and the ingenuity of the researcher. The use of ESR in characterizing dye-polymer interactions is no doubt one of the most underutilized applications of this interesting technique. This is probably because interpreting the resulting data is by no means a simple matter.

References

1. B. Ranby and J.F. Rabek (1977) ESR Spectroscopy in Polymer Research. Springer-Verlag, New York.

2. D.J. Kosman and R.D. Bereman (1981) In Spectroscopy in Biochemistry. ed. J. Ellis Bell. CRC Press, Boca Raton, FL.

3. H.M. Swartz, J.R. Bolton and D.C. Borg (1972) Biological Applications in Electron Spin Resonance. Wiley-Interscience, New York.

4. J.E. Wertz and J.R. Bolton (1972) Electron Spin Resonance: Elementary Theory and Practical Applications. McGraw Hill, New York.

5. P.L. Nordio (1976) In Spin Labeling: Theory and Applications. ed. L.J. Berliner. Academic Press, New York.

6. A. Carrington and A.D. McLachlan (1979) Introduction to Magnetic Resonance. Chapman and Hall, London.

7. P.L. Kumler (1980) In Methods of Experimental Physics. Vol. 16A, ed. R.A. Fava. Academic Press, New York.

8. J.E. Lu Valle, A. Leifer, P.H. Dougherty and M. Koral (1962) J. Phys. Chern .. 66(12),2403.


9. J.E. Lu Valle, A. Leifer, M. Koral and M. Collins (1963) J. Phys. Chern., 66(12), 2635. 10. T. Sarna, B. Pilas, C. Lambert, E.J. Land and T.G. Truscott (1991) J. Photochern. Photobiol.

A: Chern., 58, 339. I I. T. Tani and Y. Sano (1991) J. Appl. Phys., 69(8), 4391. 12. B. Garston (1980) J. Soc. Dyers Colourists, 96,535. 13. G. Heijkoop and H.C.A. van Beek (1977) Recueil, J. Royal Netherlands Chern. Soc., 96(3),85. 14. S. Okamoto and H. Ohya-Nishiguchi (1990) Bull. Chern. Soc. Jpn.. 63(8), 2346. 15. F.R. Antonucci and L.G. Talley (1973) J. Phys. Chern., 77(22), 2712. 16. H. Schmidt (1976) J. Phys. Chern .. 80(27), 2957. 17. K. Kimura and M. Imamura (1974) Bull. Chern. Soc. Jpn., 47(6),1358. 18. R.M. Uribe, W.L. McLaughlin, A. Miller, T.S. Dunn and E.E. Williams (1981) Radiat. Phys.

Chern., 18(5-6), 101 I. 19. H. Schmidt (1970) Photochern. Photobiol. 11, 17. 20. S. Noji and K. Yamaoka (1980) J. Sci Hiroshima Univ., Ser. A, 44(1),101. 21. J.A. DeBolfo, T.D. Smith, J.F. Boas, and J.R. Pilbrow (1974) Magn. Reson. Relat. Phenorn.,

Proc. 18th Ampere Congress, Nottingham. 22. S. Larach and J. Turkevich (1969) Appl. Opt. Suppl., No.3, 45. 23. R.B. Coles and C.H. Nicholls (1976) J. Soc. Dyers Colourists. 92,166. 24. R.B. Coles and C.H. Nicholls (1975) J. Soc. Dyers Colourists, 91,19. 25. R. McGregor, T. Iijima, T. Sakai, R. Gilbert and K. Hamada (1984) J. Mernb. Sci .. 18,129. 26. S.-D. Kim (1989) Synthesis of Bulky and Rigid Spin Probes and a Study of Their Mobility in

Nylon 6 Film by ESR. Ph.D. thesis, Fiber and Polymer Science Program, NGrth Carolina State University at Raleigh, University Microfilms, Ann Arbor, MI.

6 Microspectrophotometry H.-D. WEIGMANN, Y.K. KAMATH, and S.B. RUETSCH

6.1 Introduction

Most information about the transport and distribution of dyes in textile substrates comes from studies on bulk materials in either yarn or fabric form. Kinetic data are obtained through the exhaustion of the surrounding dye bath, by dyestuff extraction from the dyed substrate or by dissolution of the dyed material in appropriate solvents. Spectrophotometric measurements involving appropriate techniques provide the required parameters. These techniques are inadequate, however, when more detailed information on dye distribution within a fiber cross section is required or when studying mechanisms of dye diffusion. In such cases, it is necessary to investigate the substrate on a microscopic level, using a microspectrophotometer to acquire the necessary experimental results. As Peters et al. 1 stated in one of their important contributions to the study of the diffusion of dyes into polymeric materials, 'One of the most informative techniques for studying diffusion requires the measurement of the concentration distributions set up during a normal sorption process'. This concentration distribution, or concentration profile, in the dyed substrate can be obtained only with microscopic techniques, and in some of their initial work, McGregor, Peters, and Petropoulos2 used microdensitometry of dyed polymer films. Recent improvements in instrumentation have extended these studies to the investigation of concentration profiles within individual textile fibers.

In this chapter, we will review microspectrophotometric methods that are being used to quantify the distribution of dyes and other compounds absorbing in visible or ultraviolet light. Also included is a review of microfluorometry, another method recently introduced as a means of quantifying the distribution of compounds on fiber surfaces or within fiber cross sections. Microfluorometry uses mainly incident rather than transmitted light and measures the intensity of fluorescence emission.

6.2 Microdensitometry and microspectrophotometry

6.2.1 Instrumentation

One of the first microspectrophotometers used to measure dye concentration profiles in monofilaments was assembled by Luck' using commercially


available components. Microdensitometry involves photography of cross sections taken in a bright-field microscope with a magnification of approximately 300 x. Dye concentration profiles were obtained by scanning the crosssectional photographs with a Joyce Loebl model Mark 3B microdensitometer4 • In earlier work, McGregor et al. 2 used a modified version of the automatic Joyce Loebl double-beam microdensitometer designed by Walkers. The modifications described by McGregor et al. enabled the instrument to measure much smaller specimens than the original design.

Modern microspectrophotometric systems usually consist of a high-power microscope incorporating all the methods of illumination and image formation with both transmitted and incident light. Modular design makes it possible to interchange important functional elements on both the illumination side and the measuring side. Various instruments are commercially available, such as the Leitz MPV microscope photometer and the Zeiss Universal Microspectrophotometer System (UMSP).

A typical schematic of the light paths in microspectrophotometers is shown in Figure 6.1 for the UMSP 80-D. The system includes a xenon lamp followed by a UV -monochromator capable of producing beams from 240-850 nm. The light beam goes through filters and variable luminous field stops to a mirror, where it is reflected and focused onto the sample. The sample is located on a microscope slide attached to a scanning stage that can move the specimen through the light beam at a range of constant speeds. After going through the specimen, but before entering the detector, the beam passes through a variable measuring diaphragm that controls the beam size that is actually measured. In cross-sectional scanning, beam size determines the resolution of the measure-

PMT -DETECTOR

PHOTOMETER HEAD

SH2 LF2 LF1

VARIABLE MEAS. DIAPHRAGM

EYEPIECE

UV-MONOCHROMATOR ( ........... m)

Figure 6.1 Light paths in the Zeiss UMSP 80-D micro spectrophotometer. LF, Luminous-field stop; SH, shutter; F. filter.

MICROSPECTROPHOTOMETR Y 135

ment and the extent of optical distortion, as we will discuss below. The smaller the beam size that can be tolerated, the better the concentration profile will be. The extent to which the beam size can be reduced depends on the dye concentration in the sample, the extinction coefficient of the dye and the thickness of the section. As in all microspectrophotometers, the light can be dispersed before or after passing through the specimen with the aid of interference filter systems or monochromators on the illuminating side or on the image side.

6.2.2 Analysis of in situ dye spectra

A comparison of dyestuff spectra in a fiber of film cross section with those of the same dyestuff dissolved in solvents of different polarities can provide interesting data about dyestuff-polymer interactions and can actually be used to generate information about the polymer structure. Luck6 studied Bromophenol Blue in nylon 6, which forms a strongly pH-dependent acid/base equilibrium. Loss of a proton produces a dianion and results in a shift of the absorbance maximum from 438 to 590 nm. Luck found that, in nylon 6 monofilaments, the dyestuff exists as the free phenol in the peripheral regions of the fiber cross section where dyestuff concentration is high, whereas towards the center of the fiber, at low dye concentration, the anionic form prevails. Microspectrophotometry provides spectral data that can be interpreted in terms of the position of this acid/base equilibrium, thus permitting statements about the internal pH of the fiber.

An interpretation of such data in terms of fiber structure is provided by Feichtmayr7 who postulates that the nylon monofilament structure includes micelles with positive surface charges and suggests that these charges, which are due to end groups, are localized at the interface between those micelles. Using another triphenylmethane dyestuff (bis-dimethylamino-fuchson), Feichtmayr observed considerable changes in the in situ spectra of the dyestuff at various concentrations in polyacrylonitrile films (Figure 6.2), and interpreted these spectral shifts in terms of interactions of the dyestuff with its environment. At low dyestuff concentrations (curve 1), the spectrum of the protonated dyestuff was observed, suggesting that the sulfonic acid groups of the polyacrylonitrile easily transfer their protons to the dyestuff molecules. At high dyestuff concentrations, on the other hand, the dyes are incorporated mainly as neutral molecules (curves 2 and 3). As seen in curve 4, the dyestuff can be totally transferred into its ionic form when the dyed film is immersed in boiling water. According to Feichtmayr, in polyacrylonitrile cationic dyestuffs exist in three different states: (i) free solvated dyestuff cations; (ii) associated dyestuff cations (dimers and polymers); and (iii) dyestuff cations in the form of ion pairs. Using the spectral composition of Victoria Pure Blue (Basic Blue 7) in polyacrylonitrile films (Figure 6.3), he showed that apart from free dyestuff cations, dyestuff association occurs throughout the dyeing process.


Absorbance

0.20

4

0.16

0.12

0.08

0.04

0 700 600 550 500 450 400

A. (nm)

Figure 6.2 Influence of the concentration of bis-dimethylamino-fuchson on its spectra in polyacrylonitrile film'. Curves I to 3: concentration increasing; curve 4: after immersing film in boiling

water.

log E

5.0

4.5

4.0

3.5 14000 16000

700 650 600

18000 20000 22000

550 500 A. (nm)

Figure 6.3 Concentration dependence of the spectrum of Victoria Pure Blue (Basic Blue 7) in polyacrylonitrile film7 Concentration (molll x 10 5): -, 200; - - -. 20; .... , 4; _. -. 2.

MICROSPECTROPHOTOMETR Y

Table 6.1 Monomer and aggregate content of Acridine Orange (Solvent Orange 15) in various synthetic polymer fibers·

Fiber Amount of dye x ]02 Monomer(%) Dimer ('Yu) (mole/kg fiber)

Acetate 1.70 65 35 Nylon 6 0.84 58 42 Polyester 0.70 53 47 Acrylic 2.0 33 67 Polyvinyl alcohol

Skin 3.5 90 10 Core 3.5 54 46

137

Ohtsu et al.8-1O published extensively on the use of micro spectrophotometry to establish the state of dyestuffs in the cross sections of various natural and synthetic fibers. They studied the dyeing of various polymeric fibers with Acridine Orange (Solvent Orange 15) in free base form and found three peaks in the visible absorption spectra of fiber cross sections, located at 443,470 and 499 nm. They named these peaks the crystal band, j3 band, and a band, respectively, and attributed absorbance at these wavelengths to the monomeric state, the aggregate state and dye crystals within dyed polyvinyl alcohol (PYA) fibers8• The authors also showed differences in the ratio of monomeric to aggregate states of Acridine Orange between the skin and the core of the PYA fiber and suggested that crystal aggregates are formed in microvoids that exist in the interior of the fiber and that these aggregates can be observed by electron microscopy. As Table 6.1 shows, monomeric and aggregate states of Acridine Orange exist in a number of synthetic fibers. The spectra of metal complexed dyes obtained from cross sections of wool fibers using microspectrophotometry also show the existence of dye in the monomeric, aggregate or crystalline state lO •

As might be expected, micro spectrophotometry has developed into an extremely powerful tool of forensic science. In comparisons of fiber samples introduced as evidence in legal proceedings, careful and detailed characterization is crucial; the distribution of dyestuffs within fiber cross sections, as well as the spectra of dyes within fibers, can be critically important"-15•

6.2.3 Dye concentration profiles infibers andfilms

The ability to determine dye concentration profiles in individual filaments or films is of considerable interest in fundamental approaches to the study of dyeing kinetics, which requires the determination of actual diffusion coefficients and their variation with dye concentration. Techniques are now available that overcome difficulties associated with producing cross sections of uniform thickness and geometrical shape, which are necessary for quantitative microspectrophotometry. A number of authors have determined dye concentration profiles in films or fibers and have evaluated these profiles in terms of


dye diffusion coefficients as a function of time, location in the fiber and dye concentration. Among the first attempts to quantify dye penetration into fibers or films are the studies of Luck3, Jelley and Pontius l6 , and Olofsson 17 .

According to Peters et al. I, however, these results have not been suitable for detailed analysis in terms of the theory of diffusion, and he and his group of coworkers set out to do a definitive study of dye diffusion in polymer films using a microdensitometric technique. In several papers, McGregor et al.2.18 describe details of the microdensitometric technique developed.

6.2.3.1 Preparation of cross sections. In order to obtain a representative dye concentration profile in fiber or film cross sections, it is critical to arrest the diffusion process as soon as the substrate is taken out of the dye bath. This is usually achieved by temperature reduction, rapid drying or freeze-drying2.19.

The substrate is then embedded in an appropriate support stystem, such as an epoxy resin or by step-wise formation of an ice block around the sample4. Obviously, any kind of embedding procedure would have to retain the concentration profile as it exists at the end of the dyeing period. In other words, support resins have to be cured at low levels of exothermic reaction. In situations where moisture-sensitive polyamide fibers or films are involved, the material has to be dried to immobilize the dye in situ.

The success of microspectrophotometric or microdensitometric techniques in achieving quantitative dye concentration profiles depends on the quality of the cross sections. It is obvious that one of the major considerations for cross section quality is the uniformity and reproducibility of its thickness. The necessary precautions that have to be observed in order to produce acceptable cross sections have been discussed in some detail by a number of authors I.2,4.2o.21. Cross sections are cut with a microtome using diamond, steel or glass knives. To section fibers contained in an ice block, the microtome must have a freezing attachment and the microtome blade must be cooled before cutting. The most suitable sections are obtained when blade and block temperature are about equal4. For substrates that swell significantly in water, like cellophane films, a swelling and freeze-drying technique has been developed that avoids section distortion during drying2.

6.2.3.2 Optical distortion of concentration profiles. During the microspectrophotometric scanning of dyestuffs in cross sections of fibers or films, the concentration profiles are distorted by the light path through the microscope. This distortion effect can lead to considerable deviations between true and measured concentration profiles22-23 • More recently, a mathematical model has been developed that permits extrapolation of a concentration profile to the geometric edge of the cross section, thus eliminating distortion of concentration profiles in films24. Kuhnle and Schollmeyer25-27 extended this analysis to include distortions on cross sections of cylindrical geometry (fibers).

Distortion of measured concentration profiles is caused by light beam convergence and by too large a slit width at the measuring diaphragm22-24.


50

40 •

30

L [Jl]

20 •

10 • •

• O+-----~----~----r---~r----,r----,

o 1 0 20 30 40 50 60

S [Jl]

Figure 6.4 Dependence of distortion parameter L on slit width S24,

Provided the sensitivity of the instrument and the various experimental parameters (extinction coefficient of the dye, cross-sectional thickness, etc.) permit the use of a very small slit width, its contribution can be largely eliminated (Figure 6.4). Under such conditions, only the convergence of the light beam has to be considered as the source of distortion.

To visualize the distortion phenomenon, Navratil et al."4 suggest the following 'thought' experiment, illustrated in Figure 6.5. While the cross section remains stationary, the measuring slit and the condenser move simultaneously in the horizontal direction. The schematic in Figure 6.5 shows that the light cone encounters the cross section non-uniformly, which causes distortion of the concentration profile, Two conclusions can be drawn: first, the extinction at point B is different from zero, even though the geometric edge of the cross section does not lie underneath the measuring slit; and second, the extinction value depends on the angle fJ as well as on the thickness h of the cross section.

A number of simplifying assumptions are made for the various models24 16

of the distortion phenomenon:

• the light cone going through the cross section IS ideal as a first approximation

• the height of the light cone can be adapted to the thickness of the cross section (Kohler illumination) by adjusting the condenser

• the refractive index of the immersion liquid, no, is approximately equal to that of the polymer cross section

• the inclined rays in the light cone have the same intensity as the central ray.


r E

A B I C

I I

,i, t . . , I I \' •

,. I -", '.

. I." '. .' I h '

,.' .' I \ . I I •

.' '.

" , • I I •

, \,. ;;. . '.

J:L I

D

x

jL I ____ 1

E

.'

/1\ .' I

.'

.' '. .' "-, , ,

2 3

Figure 6.5 Schematic representation of distortion of extinction curve (concentration profile) as the result of light beam convergence'4. I. slit; 2. cross section of thickness h; 3. cone-shaped light

beam; L. distortion parameter.

Having made these assumptions, the angle p of the light cone with the optical axis can be calculated from no and the numerical aperture A:

A = nosinpl2 (6.1)

A is the smaller value of the numerical aperture of the condensing lens or the objective.

Based on these assumptions, the extinction E(r) of the distorted profile can be calculated from the true concentration distribution C(r):

£(r) = V [C(r)]. (6.2)

Details for the calculation of the integral operator Vand its actual values can be found in references 25-27. According to Schollmeyer and Kiihnle27 , the integral operator V depends on a number of experimental variables:

h thickness of microtome section R fiber radius A numerical aperture no refractive index of the immersion oil e extinction coefficient of the dyestuff in the fiber a, b length of the sides of the rectangular measuring diaphragm r measuring position in the fiber


7r----------------------------------------,--------, 6

5

4

2

E (R)

a b £ C o

(~) Fiber edge Ro

e

o~----__ --__ ~~~~---------L~~~ o 5 10 15 20 25

R (Ilm)

141

Figure 6.6 Calculated effect of increasing the area of a square measuring diaphragm on extinction of a Fickian concentration profile26 • Curve a is the actual profile; curve b shows the distortion produced in accordance with the model, a cone oflight at an infinitely small diaphragm

setting; curves c-h for diaphragm with sides I, 2, 3,4, 5, and 6 11m, respectively.

Kuhnle and Schollmeyer26 show a theoretical concentration profile, reproduced in Figure 6.6, curve a, which corresponds approximately to dyeing PET fibers with disperse dyes assuming a constant diffusion coefficient and constant surface concentration Co- Curve b shows the distortion produced in accordance with his model, a cone of measuring light at an infinitely small diaphragm setting. Curves c-h show the effect of increasing the diaphragm opening, assumed to be square (a = b). With increasing diaphragm area, greater extinction per unit area is measured in the interior of the fiber than with the measuring light cone. In the fiber edge zone, on the other hand, where the measuring diaphragm covers the fiber cross-sectional area only partially, the extinction is lower. The effect of the size of the measuring diaphragm becomes negligible as it moves outside the fiber.

Figure 6.7 shows an example of the measured average extinction values E(r) of a PET fiber cross section after dyeing with c.l. Disperse Red 6028. The experimental points clearly deviate from the non-distorted extinction computed according to the distortion model of Kuhnle and Schollmeyer, with the major distortion occurring at the fiber edge.

As Kuhnle and Schollmeyer pointed out26, it is difficult to define exactly the geometrical edge of the fiber cross section and thus to determine the shifts in


2 ,,-----------------------------r-------,

a 0.0

E

0.5 r/R

1.0

Fiber edge Ro

•

Figure 6.7 Comparison of measured extinction (points) with theoretical non-distorted extinction (line) for a PET fiber dyed with c.r. Disperse Red 60".

a=b i!JmJ

6

5

2

o -1 -2 -3 -4 b [101m]

Figure 6.8 Correlation between diaphragm size (a = h) and the shift Sin extinction maximum'6.

the extinction maximum of a distorted Fickian concentration profile, as represented in Figure 6.6. However, averaging the measured shifts on both edges of the fiber cross section shows the theoretically postulated linear correlation (Figure 6.8) between the size of the quadratic diaphragm and the shift in the extinction maximum.

The problems of identifying the surface concentration are not as serious for polyamide fibers dyed with acid dyes, where a definite saturation concentration is reached rather rapidly. While distortion caused by light beam convergence is still a problem at the edge of the fibre and as the concentration profile moves into the fiber, the real surface concentration of the cross section

c .2 co ~

c CD (.) C o ()

CD >o

o


5 10 15 20 25

Distance (11m)

Figure 6.9 Time dependence of the penetration of c.1. Acid Red I into a nylon 66 fiber4

is reflected in the extinction value when the light cone has completely moved into the fiber.

6.2.3.3 Presentation and analysis oj diffusion profile data. The microspectrophotometer provides transmission data at a wavelength at or near maximum absorbance as a function of distance into the film or fiber cross section. These data are transformed to absorbance values, which are directly proportional to dye concentration. This is illustrated (Figure 6.9) by some dye penetration profiles4 • The time dependence of penetration of an acid dye (CI. Acid Red No.1) into the interior of a nylon 66 fiber is clearly seen. It is important to establish that the Lambert-Beer law is observed and to make corrections for any deviations if necessary. Ifthe data are expressed as relative concentration c = ctc", (where C is the dye concentration at penetration distance x at time t, and C'" is the surface dye concentration assummg saturation at the surface) as a function of a new variable TJ, where

TJ = x t2/i (6.3)

then all penetration data at various diffusion times should fall on a single curve as in Figure 6.10, and this sort of plot can act as an additional check on the accuracy of the data. As Peters et a/. I discussed, this is a sensitive test to establish that the system adheres to the mathematical model of diffusion from a constant surface concentration into a semi-infinite plane.

The value of the diffusion coefficient D(c) for any value of relative concentration c, can be obtained from the plot in Figure 6.10 by graphical integration. The diffusion coefficient-concentration relation can be obtained using the method of Matan029 as modified by Crank30 :

D(c) =-2(d TJ)r' TJ dC c=c, de 0 (6.4)


1.01--o-..x,.,.~

0.8 x

t. D

8 0.6 l) x -l) 0.4

D

x 0.2 )(

a 0 t.

0 0 0 .2 .3 .4 .5 .6 .7 .8 .9 1.0

x/2 {t

Figure 6.10 Relative dye concentration as a function of penetration distance x at various dyeing times t (min)'. x, 64; A, 49; D, 36; 0, O.

This method of determining the diffusion coefficient from dye concentration profiles has been employed extensively by workers using micro densitometry or micro spectrophotometry. The Matano equation is definitely applicable to a system where diffusion occurs from a constant surface concentration into an infinite plane sheet. Applying this equation to calculate diffusion coefficients from concentration profiles in fiber cross sections, however, will produce some errors because of the difference in geometry, and a different equation should be used. Assuming a constant surface concentration Coo, radial dye diffusion coefficients can be determined using an equation describing diffusion into a cylinder30:

(6.5)

In this equation, C is the dye concentration at r, the radial distance from the center of the filament, a is the filament radius, D is the dye diffusion coefficient assumed to be constant, t is the diffusion time, andPn are the roots of the Bessel function of the first kind of order zero ['o(fin) = 0]. Values of 'oCr Pn/a) and 'l(fin) are listed in tables or are calculated from polynomial approximations. Newton's method of reiteration is used to find values of Dtla2 from C/Coo values obtained from the dye concentration profiles at fixed intervals along the filament radius. The value of D is readily calculated from the Dtla2 values, since both t and a are known.

6.2.4 Applications

6.2.4.1 Dye diffusion studies. In applying the microdensitometric technique to dye diffusion in polymer films, McGregor et al. 2 explored a number of systems in some detail. The authors clearly established the considerable disadvantages of apparent or integral diffusion coefficients (15) based on rate


6

0-<D

.!!? CA (gl100 g) '" E

~ 4 '" 0

x :§: 0

2

o o

Salt Concentration, S (gil)

Figure 6.11 Variation of D(c) with salt concentration2b•

of dyeing curves alone without considering dye penetration, if they are to be used as guides to diffusion behavior and to understanding diffusion mechanisms. When they applied microdensitometry to the diffusion of Chlorazol Sky Blue FF (Direct Blue 1) into cellulose films, they were able to demonstrate that the effect of increasing salt concentration on the diffusion of the dye into the film is completely different from that reflected by the apparent diffusion coefficient 152h • Using 15 in such investigations would give a totally misleadin.g impression of the effect of salt on the diffusion process, as seen in a comparison of Figures 6.11 and 6.12.

In studies of the diffusion of an acid dye (C.1. Acid Red 18) in nylon 66 films, McGregor et al. explored the concentration dependence of the diffusion coefficienec • This information is accessible only through an analysis of dye concentration profiles during diffusion. McGregor et al. postulated that amine and amide dyeing occur by different mechanisms and can be treated as simultaneous but independent transport processes. They showed that the diffusion of dye bound to the amide group conforms approximately to Fick's law, the diffusion coefficient being independent of dye concentration. In the amine dyeing mechanism, on the other hand, the diffusion coefficient increases rapidly as saturation of the amine dye sites is approached.

Another study of the diffusion of direct dyes into cellulose films by McGregor and Petersl8 provides a further example of how the microdensitometric technique helps in understanding diffusion mechanisms. Although


3.0

0-CD .!!! '" E E-o )(

10

1.0

o

Salt Concentration, S (gil)

Figure 6.12 Variation of fj with salt concentration2b. e, Present results; ., calculated from Neale's results.

conventional dyeing rate measurements suggest that diffusion conforms to simple dyeing theory, large deviations from the expected ideal behavior become apparent when microdensitometry is used. In this case, the authors interpreted the discrepancies in terms of diffusion processes involving a simultaneous reversible internal absorption process with partial immobilization of a fraction of the dye molecules. In the case discussed, the anomalies arise from the fact that the rates of these sorption processes are similar to the rate of transport itself.

Harwood, McGregor, and Peters3l studied the absorption of cationic dyes by acrylic films, using a microdensitometric technique, to determine the kinetics of dyeing. They established that the diffusion coefficient calculated from concentration profiles depends on concentration and explained this dependence through an ionic transfer or exchange mechanism of hydrogen or sodium ions for the dye ions.

McGregor, Peters, and Ramachandran32 studied the diffusion of disperse dyes in polymer films using microdensitometry. The films were dyed in aqueous dye dispersions in the presence or absence of carriers and also in solutions of dyes in organic solvents. In the presence of dyeing accelerants, dyeing showed marked deviations from the Fickian sorption model, which may be associated with the simultaneous diffusion of carrier and dye and the resulting extensive swelling of the cellulose triacetate film at the diffusion front of the carrier. In other words, the diffusion of dye into the polymer film is


determined by the rate of advance of the carrier-swollen region. This behavior resembles that of a particular case of diffusion into a crosslinked polystyrene resin33 .

6.2.4.2 Dye diffusion anisotropy index. Microspectrophotometry makes it possible to determine dye diffusion coefficients in both the radial and axial directions of a fiber, thereby providing interesting insights into the structure through which the dye molecules must diffuse. Radial diffusion coefficients can be determined from dye concentration profiles in the fiber cross section; the corresponding axial diffusion coefficients are obtained from longitudinal dye concentration profiles with dye diffusion restricted to end-on penetration by embedding the fiber in an appropriate matrix. The so-called dye diffusion anisotropy index, defined as the ratio of the two directional diffusion coefficients, is shown as a function of filament draw ratio in Figure 6.13, and it immediately becomes apparent that the undrawn nylon 6 filaments are already quite anisotropic with respect to diffusion34 • As other workers have noticed35.36 , a maximum in anisotropy is reached at a draw ratio of approximately 2, with a rapid decrease in anisotropy as the filaments are drawn to a higher level; note that in this context the anisotropy appears to decrease with increasing dyeing temperature and that at 900e the index become essentially independent of draw ratio.

Another way of expressing the difference in the response of directional dye diffusion to structure development during drawing is shown in Figure 6.14. Here the relative changes with drawing in the dye diffusion coefficients at 800 e for the two directions are plotted as a function of draw ratio. It is quite apparent that the radial dye diffusion coefficient shows only a modest change during initial drawing, but drops off quite dramatically between draw ratios 2 and 3. In contrast, the axial diffusion coefficient shows a much more immediate precipitous decrease upon drawing to a draw ratio of 2, with much smaller further changes in the higher deformation regimens. This difference in behavior is the reason for the maximum in the anisotropy index at a draw ratio of2. It appears that the change from a spherulitic to a microfibrillar structure places only minor restraints on the lateral transport of dye molecules through the non-crystalline domains in the fiber, while the imposed orientation of the basic structural elements interferes quite strongly with transport in the longitudinal direction. The drop-off of the radial diffusion coefficient at higher draw ratios possibly reflects orientation of tie molecules in the amorphous domains within the microfibrils, and the eventual formation of a highly oriented intermicrofibrillar phase. Diffusion in the axial direction is probably already so impeded by the formation and orientation of microfibrils that further orientation and formation of intermicrofibrillar material has only a minor additional effect.

The observation that the anisotropy index maximum at draw ratio 2 disappears for dye diffusion at 900 e suggests that the diffusional barriers that become operational in the axial direction upon drawing to a draw ratio of 2


6

4

2

O~ __ ~ __ ~ __ -J ____ L-__ ~

2 3 4 5

DRAW RATIO Figure 6.13 Dependence of dye diffusion anisotropy index on draw ratio for dyeing of nylon 6

monofil at different temperatures34 , +, 50 (0C); 0, 70; 11,80; 0, 90,

can largely be overcome at elevated dyeing temperatures. This indicates the importance of using microspectrophotometry to probe diffusion behavior in both directions, not only over a wider temperature range but also with dye molecules of different sizes.

6.2.4.3 Measurement of molecular orientation using dichroic dyes. One of the key microstructural features affecting dye diffusion in fibers is the level of molecular orientation in the noncrystalline (amorphous) regions. It is well known, however, that quantitative characterization of the non-crystalline regions is problematic and that there is a lack of consistency in the data from

MICROSPECTROPHOTOMETRY 149

1·0 ,,-_

2 3 4

DRAW RATIO Figure 6.14 Relative change in radial (x) and axial (0) dye diffusion coefficients with drawing of

nylon 6 monofil at 80°C34. D" diffusion coefficients of undrawn filament.

the various methods used for this purpose. One of these methods is based on the optical dichroic properties possessed by many dyes, resulting in a strong orientational dependence of light absorption. When a polymer structure is doped with such a dichroic dye, anisotropic intermolecular forces or geometrical requirements impart the orientation of the dye molecules onto the polymer molecules. This is particularly true for dye molecules with strong geometric anisotropy, such as c.1. Disperse Yellow 23 (1). Such probe dye molecules are incorporated into the polymer substrate either by introducing them into the melt prior to extrusion or by absorption into the substrate from an aqueous dye bath solution. Dichroic measurements of dyed fibers or films are made in simple polarized light, recording transmission values for the parallel and perpendicular components.

Studies at TRI/Princeton used microspectrophotometry to determine amorphous orientation functions of high-speed spun and spun-and-drawn polyester yarns based on dichroic ratio measurements37 • Procedures were established for measuring transmission data in the two major directions of


Width of measuring slit

• =========::;:+:;;~======== Cover glass

Fiber Immersion fluid

I" 1'1 I : \ Glass slide I I, I M-- In focus

I~out of focus

Light sou ree

Figure 6.15 Light paths during measurement of transmission of polarized light with a narrow measuring slit37•

vibration of the polarized light which avoided the concern of Kobayashi et aU8 Based on their analysis of the previously reported 'overdyeing phenomenon' which results in decreases of the dichroic orientation function with increasing dye uptake38-41, Kobayashi and his colleagues felt that this phenomenon was an experimental artifact arising from stray light. In microspectrophotometric studies at TRI, we carefully control the beam size entering the photomultiplier to a very narrow and elongated slit, which can be as small as 1 ~m in width. This narrow slit is placed parallel to the fiber axis, exactly in the center of the fiber, to ensure that the measurement detects the light beam traveling through the fiber diameter, maintaining the same path lengths, as is shown in Figure 6.15. The beam is focused on the edge of the fiber so that the light does not reflect from the fiber surface.

Transmission values T for the dyed fiber and the control, for both vibrational directions of the light beam, are converted into absorbance:

Absorbance = log (TcontrolTdyed)

The dichroic ratio D is given by

D = absorbance 11/ absorbance.!.

The dichroic orientation function!a(D) is then calculated as

!a,D) = (P2 (cos ~»aJdichroic = (D-l)/(D+2)

(6.6)

(6.7)

(6.8)

Salem et al. 37 suggested that the apparent decrease in dichroic orientation function with increasing dye uptake occurs because dye molecules align themselves differently with the polymer chains at different levels of uptake. While initial or low uptake results in excellent alignment, as dye uptake increases, 'surplus' dye molecules are absorbed in an increasingly more random manner. Provided that dichroic ratios are determined at low concentration, amorphous orientation functions determined by dichroic ratio and birefringence measurements correlate quite well (Figure 6.16).


f.(D) 1.0~----------------------------~~

0.8

0.6

0.4

0.2

0.0 -+-''--.---.---......----,r--..---"T"""---.---.---......-~ 0.0 0.2 0.4 0.6 0.8 1.0

f.(B)

151

Figure 6.16 Correlation between amorphous orientation functions determined by the dichroic dye and birefringencelX-ray methods for high speed spun (e) and spun-drawn (0) polyester

yarns". m = 1.15, R = 0.96.

6.2.4.4 Environmental and light fading. Microspectrophotometry has been used successfully to establish mechanisms of environmental fading42.43.

Anthraquinone dyes such as c.1. Disperse Blue 3 react with environmental pollutants such as ozone or NOx : ozone resulting in a ring-opening reaction and total loss of color, and NO, in a replacement of substituents through a series of steps leading to a color shift from blue to pink (Figure 6.17). To explain the various phenomena observed during the exposure of dyed polymer fibers to ozone, Haylock and Rush44 proposed that the initial destruction of dyestuff molecules in the peripheral regions sets up a concentration gradient towards the outside of the fiber. This permits diffusion of dyestuff towards the surface where it acts as a scavenger of incoming pollutant molecules and is destroyed in the process. Obviously this mechanism, which discounts the diffusion of the pollutant into the interior of the fiber, can be applicable only at very low gas concentrations and high dyestuff mobility.

Kamath et al. 42 used microspectrophotometry to explore the applicability of this so-called surface reaction mechanism to the ozone fading of disperse dyes in nylon. As seen in Figure 6.18, diffusion ofCI. Disperse Blue 3 in a ringdyed nylon 6 fiber at 90% relative humidity and 40°C does indeed occur within the framework of fading times that have been investigated; the change in the


o If CXC

-OH

C-OH

" o ABSO:BANC~

2

~ o NHR

NOx Faded (Red)

~ C¢¢0 OH

I I ~ ~ ~

1

360

o NHR

Unfaded (Blue)

720

Figure 6.17 Color shift on exposing c.1. Disperse Blue 3 to NOx42. Ring opening (by ozone) and substituent replacement (by NOx ) reactions are shown.

TRANSMISSION %

."------ .... ,, ",'" "-

'" " """ 48 h diffused "

DISTANCE CJJm) Figure6.18 Diffusion ofC.1. Disperse Blue 3 within a ring-dyed (-) nylon 6 fiber after 48 h (--)

at 90% relative humidity and 40°C42.

INTENSITY !%l

80

20


{undyed

°0~------~10~------2~0--------3~0--------4~0------~50

DISTANCE (j.Lm)

Figure 6.19 Experimental dye concentration profiles produced by ozone fading (0.2 ppm OJ) for varying times42 .

concentration profile after diffusion for 48 h is clearly noticeable. The diffusion coefficient of the dye within the fiber under fading conditions calculated from such concentration profiles and an assumed diffusion coefficient of ozone have been used in a mathematical model designed to match the experimental dye concentration profiles caused by ozone fading (Figure 6.19). The experimental results did not correlate with the mathematical model based on the surface destruction model. A more detailed mathematical model involving the simul-taneous diffusion of ozone and the dyestuff was used by Bevans et al. who showed that ozone penetration into the fiber and destruction of dye molecules in the interior both have to be postulated to explain the experimental dye concentration profiles43 .

At higher pollutant concentrations, there is no question that pollutant diffusion and in situ dye destruction is the prevailing mechanism of fading. This is clearly seen in Figure 6.20a, which shows a series of cross sections after progressively longer fading times under the influence of a concentration of 10 ppm NO< at 90% relative humidity42. Corresponding dye concentration profiles are shown in Figure 6.20b, and comparison with the diffusion coefficient of this dye (C.I. Acid Blue 127) shows the predominant influence of pollutant diffusion.

Typical cross-sectional scans for the undyed, dyed, and dyed and faded fibers are shown in Figure 6.2142. All scans display optical edge effects indicated by the intensity maximum A and the minimum B, which occur when the scanning beam traverses the focusing rings at the edge of the fiber. These

(a)


INTENSITY (percent)

80

40

20

(b) 0

(' Undyed

16 h

20 40 60

DISTANCE ().Lm)

Figure 6.20 (a) Photomicrographs and (b) micro spectrophotometric scans of dyed nylon 6 fiber cross sections exposed to 10 ppm NO, at 90'Yo relative humidity for successively longer times'2•

edge effects result from differences in the refractive indices of the fiber and the embedding medium. The real specimen edge is located at the inflection point of AB25. The intensity profile between C and the real edge of the fiber is generated by fitting the curve CD to a quadratic expression and extrapolating it to the fiber edge. Intensity profiles corrected in this way, though not corrected for distortion effects as discussed above, can be used as a first approximation to calculate dye concentration profiles in the fiber cross section.

In order to calculate the dye content of a fiber from an intensity scan, the cross section is assumed to be divided into a series of concentric rings of width dr, as shown in Figure 6.22a. The dye content of the ith ring is proportional to

2trr j drb (6.9)

where b is the optical thickness of the cross section, which is proportional to the absorbance In /o//. The optical thickness of a uniformly dyed unfaded fiber is the same at any position in the cross section (Figure 6.22b). However, for the

INTENSITY (arb. units)

A


Undyed

Dyed, faded

Dyed, unfaded

DISTANCE

155

Figure 6.21 Typical microspectrophotometric scans for undyed, dyed, and dyed and faded nylon 6 fiber cross sections showing edge effects and method for locating the real fiber edge4'.

(a)

(b)

(c)

1IlllllllllllllllllllllllllllllltLn~

FlllllWJJ~ b· = Ln.!2...

I Ii

Figure 6.22 Parameters for calculating dye content in a fiber cross section42 (a) Optical volume of the ring is 21lT;drb;; (b) cross section of unfaded fiber; (c) cross section of faded fiber.

faded fiber cross section illustrated in Figure 6.22c, the optical thickness varies across the radius of the section. Therefore the dye content of the unfaded cross section is proportional to

(6.10)


and for the faded fiber, the dye content is proportional to n

"f.21['idrbi i~l

The fractional dye loss is given by

l n j "f.r.b.

:~ =1- '~±,: I~l

(6.11 )

(6.12)

where M, is the amount of dye lost at time t and M", is the amount of dye at time O. A comparison of dye fading losses determined from cross-sectional scans with a skein-dissolution method shows good agreement, demonstrating the validity of these calculations42 •

6.3 Microftuorometry

6.3.1 Background

Fluorescent dyestuffs have found wide application for solving specific problems in biology, medicine and laser technology. Fluorescent compounds are also used in textile applications as optical brighteners as well as in dyeing technology. The development of modern microfluorometric instrumentation has opened a way to employ fluorochromes for quantifying the deposition of compounds on fiber surfaces. The penetration of fluorescent dye into the cross sections of individual fibers can also be quantified using these instruments, and determination of the autofluorescence of specific compounds within natural or synthetic fibers is also feasible.

Absorption oflight at an appropriate wavelength produces an excited state of a fluorescent compound, which can lose the absorbed energy either in radiationless transitions, which usually cause heat formation, or by the emission of radiation in the form of fluorescence or phosphorescence. The processes occurring during energy absorption, internal conversion or intersystem crossing, as well as fluorescence and phosphorescence, are represented schematically in the well-known Jablonski diagram shown in Figure 6.2345 .

Fluorescence is emitted as a consequence of electrons dropping from the lowest vibrational level of the first excited state to one of the vibration levels of the ground state. The fluorescence spectrum is a mirror image of the absorption spectrum with shifts to longer wavelengths.

Fluorescent dyes can be used as specific indicators for the presence of charged or reactive groups, or they can be used to study dye diffusion into fibers where microspectrophotometric measurements using transmitted light


Singlet states

Absorption hVA :

Exciting light

. Triplet states

-hvp Phosphorescence

-hy~ Radiating transitions

~ Non.radiating transitions Vibrational states = Electron states

Figure 6.23 Jablonski diagram of energy changes caused by light absorption4S •

157

cannot be made or are not sufficiently sensitive. Fluorescent tracers have been used extensively to characterize qualitatively the surface deposits on natural and synthetic fibers46-53. Incorporating a fluorochrome into certain surface deposits, particularly micellar structures, protects it from deactivation processes and leads to an increase in quantum efficiency t/J. Quantum efficiency is defined in terms of the rate constants for fluorescence activation k f , and the combined rate constants for deactivation processes kd:

t/J=~ kf - kd (6.13)


Deactivation processes lead to the return of the excited state of the molecule to its ground state without fluorescence emission. This could involve collisions of excited molecules with each other or with solvent molecules, or various internal conversion processes within the excited molecules. Any interaction with the environment that protects the excited molecules from deactivation, or at least decreases the rate at which deactivation occurs, increases quantum efficiency.

Figure 6.24a clearly shows the effect of microenvironment on the quantum efficiency of a fluorescent compound C.I. Fluorescent Brightener 113 in solution. Introducting 1 % ethylene oxide/pro propylene oxide (EO/PO, 10/90) block copolymer into an aqueous solution of CIFB-113 dramatically increases emission intensity. The block copolymer forms micelles in which the insoluble PO block is stabilized by the soluble EO block. The tracer molecules incorporated into these micelles are excited and are much more protected in this environment from energy transfer through collisions. The increased protection not only increases quantum efficiency, but also causes the blue shift shown in Figure 6.24b (where the sensitivity of the photomultiplier is increased so as to compare the wavelengths of maximum fluorescence intensity in the two environments).

The structure of aggregates formed by EO/PO block copolymers has been studied using fluorescent probes54 • Concentration-dependent structural or conformational changes in the polymer solutions were detected by shifts in the wavelength of maximum emission intensity caused by depolarization of specific fluorochromes with aromatic ring structures. Depolarization is caused by changes in microviscosity or micropolarity of the environment of the

CIFB-113 in water

500 600

WAVELENGTH (nm)

100

450 480

500

(b)

CIFB-113 in water (1.4 kV)

WAVELENGTH (nm)

Figure 6_24 Fluorescence spectra of CIFB-113 in water and in a I % aqueous block copolymer (EO/PO. 10/90) solutionS'. (a) Increase in intensity; (b) blue shift.


PMT -DETECTOR

VlS-MONOCHR.2 (J&O-&!Onm)

FILTER 2

UMSP 80-FL

UV-MONOCHROMATOR 1 (2--1

'----y---='+='----1 VARIABLE MEAS. DIAPH.

PHOTOMETER HEAD

LAMP SH2 Lr""2

LAMP

Figure 6.25 Modification of the Zeiss UMSP 80 microspectrophotometer for incident illumination and fluorescence intensity measurements. LF, Luminous-field stop; SH. shutter; F, filter.

fluorescent molecules, which reflect transitions in polymer conformation as the result of aggregation or micelle formation.

6.3.2 Methods of measurement

Quantitative determination of fluorescent compounds on fiber surfaces or within fiber cross sections is made possible by the development of illumination attachments to existing microspectrophotometric equipment. Figure 6.25 shows the introduction into the UMSP 80 system of a monochromator for incident illumination of a specimen and the resulting excitation of fluorochromes. The emission spectrum resulting from excitation at various wavelengths can be resolved by a monochromator on the image side. Similar and somewhat less elaborate is the operation of the PLOEMOPAK fluorescent illuminator attachment to the Leitz MPV system. This attachment with its various filters, diaphragms and beam splitting mirrors is shown in Figure 6.26. The excitation filters that are available can be used to narrow the excitation wavelength range over a considerable part of the UV/visible spectrum. Again, the excitation beam is projected onto the specimen which is mounted on the movable scanning stage. The size of the emission beam that enters the photometer is controlled by a variable measuring diaphragm as described above for the microspectrophotometer.

Figure 6.27 illustrates the effect of curvature for a cylindrical fiber55 • At two different positions, XI and X 2' across the fiber diameter, the same slit opening (ox) gives two different lengths, oLI and OL2 along the curved surface of the


EXCITATION FILTER (3110-380 nm)

TO PHOTOMETER

SUPPRESSION FILTER

/" 0 UV LAMP O\~-tt-tt--;~-~=:j:j::~~r'-DICHROIC BEAM SPLITTING

MIRROR

SPECIMEN STAGE

Figure 6.26 Diagram of the PLOEMOPAK fluorescence illuminator attachment to the Leitz MPV microspectrophotometer55•

Scanning slit

":-.~ iii i iii i iii i , ,~" L 1 , i

I V !I x' ----~--~~;. !

R

I i i ,

I

Figure 6.27 Effect of fiber curvature on measured intensity at different positions55•


EMISSION INTENSllY

DISTANCE

Figure 6.28 Microfluorometric scan, using a narrow slit parallel to the fiber axis, across a longitudinally mounted round nylon 66 fiber treated uniformly with CIFB-113 55 .

fiber corresponding to angles 8(}] and 8(}2' The light intensity received by the slit is proportional to the fiber surface area covered, which, in turn, is proportional to the length 8L of the curved surface covered by the slit. The recorded intensity Ix is given by

I, = I cos () 8L = I cos () l~~ ) 8x (6.14)

and since

8L= R8fJ

then

I, =IR cos () l::) 8x (6.15)

Here I is the emission intensity, 8L18x is the ratio of the surface area of a curved surface to the area of the corresponding flat surface, R is the fiber radius, and () is the angle corresponding to position x along the radius. This analysis suggests that as the fiber edge is approached, fluorescence intensity will increase considerably. Figure 6.28 shows a scan across a longitudinally mounted round nylon 66 fiber treated uniformly with the tracer CIFB-113, the scan made with a narrow slit (2 x 200 Ilm) parallel to the fiber axis55 • The scan clearly reveals a considerable increase in fluorescence intensity near the edge of the fiber, showing the validity of the analysis above.


o • \ \

Small beam focused on dome of fiber to view detail

Fiber Scanning -----+- direction

- Slit to obtain average intensity

Figure 6.29 Two modes for continuous fluorescence intensity scanning along a fiber55 •

Two scanning modes for continuous measurements of fluorescence intensity along a fiber are illustrated in Figure 6.29. For detailed information about the distribution of a substance deposited on the fiber surface, a small beam is focused on the dome of the filament. The fiber moves through this beam, providing a representative distribution pattern for that location on the filament.

As an alternative to using a focused beam of small size to generate detailed information, the second scanning mode shown in Figure 6.29 - with the measuring diaphragm opened to a slit long enough to cover the whole width of the fiber - yields an average fluorescence intensity as a function of position along the length of the fiber. With this procedure, focusing problems exist at high magnification, and the information is less accurate because of the edge distortion discussed above (Figure 6.28). Another problem in both scanning modes is the potential contribution of fluorescence from the opposite side of the fiber51 .56•

6.3.3 Applications

6.3.3.1 Longitudinal scans. An important application of micro fluorometry is the use of fluorochromes to detect deposits - of spin finishes, for example -on fiber surfaces. The fluorescent compound is either incorporated into the finish formulation prior to deposition, or in cases where aftertreatment with the fluorochrome does not disturb the distribution, it is incorporated after application. An example of fluorescence intensity scans along individual filaments is shown in Figure 6.30, where the effect of winding speed on the thinning of a finish film was investigated. A stain-repellent finish containing the optical brightener CIFB-113 as a tracer was applied to a nylon 66 yam on an


100~~ (u)

50

1 0

100 (h)

~ 50 'c ::I

..c:i ... ~ -- 0 > ~ en z 100 (e) W

~ ~

~ W (,,) 50 z w (,,) en w a: 0 0 :::l ....I LL

10 (<II

50

o DISTANCE ~

Figure 6.30 Microfluorometric scans along nylon 66 filaments treated with stain-repellent finish containing CIFB-113 at constant finish throughput but different winding speeds55 • Winding speed

(m1min): (a) 500; (b) 1000; (c) 2000; and (d) 4000.

industrial threadline55.57 • The finish was metered to maintain the add-on level constant at 1.25%. As the winding speed was increased at constant polymer throughput, the nominal draw ratio of the filaments increased and with it their specific surface area. Since the radial dimensions of a cylinder change as 1IL1/2, an increase in length L at constant volume should decrease the filament


10 110

1.0

0.8

0.6

0.4

• \ \

\ '. , , , • .... ....

.... ....

--- ---• 0.2 +--~-T"""-...----,r---___,.-....... -....,

o 1000 2000 3000 4000

WINDING SPEED (m/min)

Figure 6.31 Comparison of measured (e) and calculated (-) film thicknesses {proportional to I draw/ lorigina,)55.

diameter and the finish film thickness according to the reciprocal of the square root of the draw ratio. Since fluorescence intensity is linearly related to finish film thickness, the ratio of the intensities Idraw/loriginal should correspond to the ratios of corresponding calculated film thicknesses. As Figure 6.31 shows, the correlation is quite good considering the non-uniformity of the scans.

In order to make quantitative statements about the thickness of surface deposits, several assumptions have to be made. We assume that fluorochrome distribution is uniform throughout the surface deposit and that the fluorescence intensity is directly proportional to the thickness ofthe deposit film. The latter assumption has been tested in a number of cases, and, as shown in Figure 6.3255, there is a linear correlation between fluorescence intensity and the thickness of films of Pluronic F-65. Film thickness was measured using interference microscopy and was limited to film thicknesses above 0.1 fJ.m. In other systems, the linear correlation between fluorescence intensity and film thickness is lost at higher film thicknesses; the finding that fluorescence intensity eventually reaches a constant level independent offilm thickness has been attributed to autoquenching. In most cases, deposit levels are well within the linear range, suggesting that measured fluorescence intensity levels are indeed representative of deposit thickness. These correlations vary, of course, depending on the system being studied, and direct comparisons between different fluorochromes and different surface deposits can be made only if calibration curves are determined. In most of the studies we have reported55.57,

it is adequate to show relative distribution patterns without going through the elaborate calibration procedure.


FLUORESCENCE INTENSITY (%)

14

12

10

8

• 6

4

2 3

FILM THICKNESS (~m)

FLUORESCENCE INTENSITY (%)

3

2

o~~~~~~~~~~~~~ 4 0.1 0.2 0.3 0.4 0.5 0.6 0.7

FILM THICKNESS (~m)

Figure 6.32 Correlation between fluorescence intensity and thickness of Pluronic F ·65 films containing CIFB·113 55 .

6.3.3.2 Dye concentration profiles. In situations where other absorbing species in the fiber cross section interfere with the determination of dye concentration profiles using microspectrophotometry, fluorescent tracers and incident light can sometimes be used to study diffusion behavior within a substrate. We have employed this method in efforts to characterize oxidative damage of hair fibers caused by exposure to various oxidizing media58 . Since the oxidative scission of disulfide bonds in the matrix of hair cortical cells produces increased hydrophilicity and accompanying fiber swelling, diffusion rates increase significantly as a result of such oxidative damage. Figure 6.33 shows cross sections of untreated and oxidatively treated hair fibers that were subsequently dyed in a 0.1% aqueous uranin solution at 500 e for 5.5 h. Uranin (the sodium salt ofCI. Acid Yellow 73) can be excited in the blue region with a filter of 450--490 nm, producing a maximum emission intensity at 540 nm. The fluorescence intensity scans for these fiber cross sections (also shown in Figure 6.33) can be used to calculate diffusion coefficients, provided ring dyeing is achieved. In the case of the most heavily oxidized sample, shorter dyeing times than those shown are obviously needed. As we have pointed out, the changes in diffusion behavior can be used as a means of quantifying oxidative damage within the overall hair fiber structure.

6.3.3.3 Autofluorescence andfluorescence of polymer degradation products. Another application of micro fluorometry is quantification of the autofluorescence of a fiber substrate. In some fibers, such as wool and hair, the autofluorescence of a particular amino acid or its degradation products can be used as a means to determine the extent to which UV irradiation has caused damage within the fiber cross section or in the peripheral cuticular region59 •


FI (%)

100

80

40

100] SO

40

(a) Unbleached

20~ O+-----~----~--________ ~ ____ __

o 20 40 60 80 100

100

80 (c) (c) ch creme, 30 minutes

40

20

Ot-----~----__ ----__ ----~----~ o 20 40 60 80 1 00

DISTANCE ( ~m )

Figure 6.33 Micrographs and scans of cross sections of unbleached and bleached hair fibers dyed in 0.1% aqueous uranin". (a) Unbleached; (b) bleached in 6% H,o" 4 h; (e) bleached with bleach

creme, 30 min.

A particularly interesting application of microfluorometry involves the formation of fluorescent oxidative degradation products after UV irradiation of polyester fibers. As Figure 6.34 shows, one-sided exposure to UV irradiation produces a focusing effect in which the fiber acts as a cylindricallensl9 •

Similar focusing effects occur during the light fading of dyes, as shown in Figure 6.35 19• Quantification of such degradative damage involves crosssectional scanning from the exposed side to the opposite side of the fiber using


Figure 6.34 Focusing effect during one-sided exposure of polyester fibers to UV light".

Figure 6.35 Focusing effect on exposing a dyed polyester fiber to light fading conditions l '.

appropriate excitation wavelengths. UV stabilizers that absorb in various ranges of the UV light spectrum can be used to eliminate or at least delay the polymer degradation process. We are currently exploring the importance of the location of UV stabilizers with regard to their effectiveness in preventing polymer and/or dye degradation 19• In these studies, we are using UV microspectrophotometry to detect the distribution of the UV stabilizers within fiber cross sections.


6.4 Conclusion

Modern instruments have made microspectrophotometry an extremely important and easily accessible method for measuring dye transport phenomena within fiber cross sections. The introduction ofUV microspectrophotometers with quartz optics extends this analysis to compounds that do not absorb in the visible light, i.e. which cannot be considered as dyes. In analyzing dye concentration profiles within fiber cross sections, optical distortion caused by light beam convergence should be taken into consideration.

Comparisons of dye spectra in fiber or film cross sections with spectra of the same dye in specific solvents can provide interesting information about dye/ polymer interactions and can be used to obtain information about the polymer structure. Dye diffusion anisotropy in fibers, established by measuring dye diffusion coefficients in both radial and axial directions, can also provide an interesting insight into the structure through which dye molecules must diffuse. The use of dichroic dyes yields information about molecular orientation in non-crystalline domains, and results obtained by this method are comparable to those obtained by other optical techniques, such as methods involving birefringence. In addition, microspectrophotometry can provide quantitative information regarding the sites within fiber cross sections of dye destruction as the result of environmental and light fading reactions.

Microfluorometry is an extension of microspectrophotometry in which fluorescent compounds and incident light are employed. The penetration of these compounds into the fiber cross section can be quantified, which is particularly useful in situations where other absorbing species in the fiber cross section interfere with the determination of dye concentration profiles. Introducing fluorescent compounds into surface deposits such as spin finishes permits an analysis of the relative or absolute distribution of finish film thickness along a fiber or film specimen. Autofluorescence within the fiber can be used as a means of determining the formation or disappearance of fluorescent compounds under the influence of chemical or physical degradation processes.

In general, it can be said that microspectrophotometry provides the means for quantitative microscopic analysis of absorbing or fluorescing species within fiber or film cross sections, thus yielding insight into transport processes and polymer/dye interactions that cannot be obtained using conventional macroscopic techniques.

References

1. R.H. Peters, J.H. Petropoulos and R. McGregor (1961) J. Soc. Dyers Colourisls. 77,704. 2. R. McGregor, R.H. Peters and J.H. Petropoulos (1962) Trans. Faraday Soc. 2, 58, (a) 771,

(b) 1045. (c) 1054. 3. W. Luck (1955) Melliand Texlilber., 36, 927, 1028, 1267.


4. J.P. Bell, W.C Carter and D.C Felty (1967) Textile Res. 1.. 37, 512. 5. A. Walker (1955) Exp. Cell. Res., 8,568. 6. W. Luck (1960) Angell'. Chern., 72, 57. 7. F. Feichtmayr (1974) Len~inger Ber., 36, 229. 8. T. Ohtsu, K. Nishida, K. Nagumo and K. Tsuda (1971) Kolloid Z. Z. PO/I'm., 249, 1077:

(1972) Ibid., 250, 860. 9. T. Ohtsu, K. Nishida, K. Nagumo and K. Tsuda (1974) Colloid Polym. Sci., 252, 377.

10. T. Ohtsu, K. Nishida, and K. Tsuda (1976) Colloid Polym. Sci., 254, 967. II. H. Amsler (1959) Archil' Kriminologie, 124, 85. 12. R. Halonbrenner and J. Meier (1973) Kriminalistik, Aug., p.l. 13. K.W. Smalldon and A.C Moffat (1973) J. Forensic Sci. Soc., 13,291. 14. R. Macrae. R.J. Dudley and K.W. Smalldon (1979) J. Forensic Sci .. 24, 117. 15. H. Suchenwirth and H.J. Bruck (1968) Archil' Kriminologie, 142, 16. 16. E.E. Jelley and R.B. Pontius (1954) J. Phot. Sci .. 2, 15. 17. B. Olofsson (1953) Medd. Svensk Textilforskninginst .. 29. 18. R. McGregor and R.H. Peters (1962) Trans. Faraday Soc., 60, 2062. 19. S.B. Ruetsch (l99\) TRIIPrinceton, unpublished results. 20. C Oster and A.W. Pollister (eds) (1956) Physical Techniques in Biological Research. Vol. 3,

Academic Press, New York. 21. J. Hiller and M.E. Gettner (1950) J. Appl. Phys., 21, 889. 22. P.W. Lange (1950) Svensk Papperstidning. 53. 749. 23. S. Ausunmaa and P.W. Lange (1952) Sl'ensk Papperstidning, 55, 217. 24. J. Navratil, F. Giirtner, and B. Milicevic (1974) Melhand Textilber .. 55, 463. 25. G. Kuhnle. E. Schollmeyer and H. Herlinger (1977) Optik, 47,477. 26. G. Kuhnle and E. Schollmeyer (1977) Optik. 49.163. 27. E. Schollmeyer and G. Kuhnle (1978) Optik, 52. 133. 28. E. Schollmeyer and G. Kuhnle (1979) Ber. Bunsenges. Phys. Chern., 83. 322. 29. C Matano (1932) Jap. 1. Phys., 8, 109. 30. J. Crank (1956) The Mathematics of Diffusion, Clarendon Press, Oxford. 31. R.J. Harwood, R. McGregor and R.H. Peters (1972) J. Soc. Dyers Colourists. 88. 288. 32. R. McGregor. R. Peters and CR. Ramachandran (1968) 1. Soc. Dyers Colourists, 84.9. 33. A. Peterlin (1965) Polym. Lell., 3, 1083. 34. R.A.F. Moore, S.B. Ruetsch and H.-D. Weigmann (1985) TRIIPrinceton. unpublished

results. 35. Y. Talzagi and H. Hattori (1965) 1. Appl. Polym. Sci .. 9, 2167. 36. D.C Prevorsek. P.J. Harget, R.K. Sharma and A.C Reimschuessel (1973) Macromol. Sci.

Phys., 88, 127. 37. D. Salem, S.B. Ruetsch and H.-D. Weigmann (1992) TRIIPrinceton, unpublished

results. 38. Y. Kobayashi. S. Okajima and K. Nakayama (1967) J. Appl. Poly. Sci., 11,2507.2533. 39. D.R. Morey and E.V. Maskin (1951) Textile Res. 1.. 21, 607. 40. D. Heikens (1952) Dichroism of Dyed Cellulose Fihers. Drukkerj F.A. Schotanus & Jens,

Utrecht. 41. S. Okajima and Y. Kobayashi (1952) Bull. Chern. Soc. Jap .. 25, 268. 42. Y.K. Kamath, S.B. Ruetsch and H.-D. Weigmann (1983) Textile Res. 1.,53,391. 43. R.S. Bevans, CJ. Durning, R.A. Moore and Y.K. Kamath (1986) Columbia University and

TRIIPrinceton. unpublished results. 44. J.C Haylock and J.L. Rush (1976) Textile Res. 1., 46.1. 45. H. Zollinger (1987) Color Chemistry, VCH Verlag, Weinheim. 46. H. Ruile (1957) Melhand Textilber .. 38, 442. 47. S. Kokot, M. Matsuoka, U. Meyer and J. Zurcher (1975) Texti/l'eredlung, 10, 127. 48. S. Kokot, U. Meyer and J. Zurcher (1976) Textile Res. J., 46, 233. 49. J. Garcia-Dominguez, M.R. Julia and A. de la Maza (1976) 1. Soc. Dyers Colourists.

92,433. 50. F.R. Rothery and M.A. While (1983) J. Soc. Dyers Colourists, 99, II. 51. U. Meyer (1982) Textilveredlung, 17,440. 52. D.J. Evans (1989) Textile Res. 1., 59, 569. 53. L.A. Holt and I.W. Stapleton (1988) 1. Soc. Dyers Colourists, 104.387. 54. N.J. Turro and C 1. Chung (1984) Macromolecules, 17,2123.


55. Y.K. Kamath, S.B. Ruetsch and H.-D. Weigmann (1993) Textile Res. 1.. 63, 19. 56. J. Zurcher-Vogt (1991) CherniefasernITextilind., 41, 93. 57. H.-D. Weigmann (1991) Textile Chern. Colorist, 23, 4,15. 58. H.-D. Weigmann and S.B. Ruetsch (1994) TRIIPrinceton, unpublished results. 59. K.F. Ley and K. Schafer (1990) Proc. 8th Int. Wool Tex. Res. Can!, Christchurch, NZ, Vol

I, p. 225.

7 Emission spectroscopy K.P. GHIGGINO

7.1 Introduction

Emission spectroscopy is amongst the most sensitive of all analytical techniques and is widely used to characterize organic and inorganic colorant materials as well as to probe chemical structure, molecular environment and monitor degradative changes l -4. In concert with other spectroscopic techniques, luminescent measurements in particular offer advantages over other analytical methods in providing a rapid and non-destructive means of providing information at the molecular level. In recent years there have been significant advances in increasing the information that can be obtained from emission spectroscopy studies. The introduction of new light sources (e.g. picosecond and femtosecond lasers) has enabled time-resolved fluorescence measurements to become commonplace, and improved capabilities in data processing and display have allowed the resolution and analysis of complex luminescent mixtures.

The objective of this chapter is to provide a review of some of the recent advances in luminescence studies, particularly in the area of time-resolved fluorescence measurements and to illustrate their application to a significant class of fluorescent synthetic colourless 'dyes', viz. fluorescent whitening agents.

7.2 Principles

The photophysical processes, including absorption and emission of radiation, occurring in organic molecules can be discussed with reference to the simplified J ablonskii diagramS given in Figure 7.1. This diagram depicts the electronic energy levels and the vibrational energy sub-levels available to a typical aromatic luminescent molecule. Under ambient conditions, molecules exist in their lowest available electronic energy level (ground state). This singlet electronic state, so designated because the available valence electrons are paired with opposite quantum mechanical spins, is labelled the So level. Upon irradiation with photons of energy E, absorption may occur from the ground state of initial energy E j to a higher singlet excited state level of final energy Ef • The energy of this transition equals that of the incident photon

172 ANALYTICAL CHEMISTRY OF SYNTHETIC COLORANTS .. I!!

Internal • &II Conversion

~ I Vibrational Relaxation

J __ = • • I!! &II

• E c:: .t----.g &II ~ L!:ifj~-~

E-< E-.!.

~l 1 Crossing

u 5- -u

~ I-0

rE- I-Intersyste

Crossing - ......

m • • E &II

c ~§ 0 .:::

~ 0':::

.::: '" • E! ~ .r-- ..0 :9'0 ~

• 1 >~ 2

m , ~

~~ Phosphorescence

~ ~

Figure 7.1 lablonskii diagram showing the excitation and relaxation pathways available to a complex aromatic molecule. Molecules are normally resident in the So ground-state level before

absorption of a photon occurs.

according to equation 7.1. In this equation, the frequency v and wavelength A of the photon are related to the energy of the transition by the fundamental Planck's constant h and the speed of light c.

E = IEf - Ed = hv = ~ (7.1)

Following the initial absorption of a photon, the processes of internal conversion and vibrational relaxation may occur in which the molecule undergoes non-radiative transitions between nearly iso-energetic states of the same multiplicity and then rapidly loses energy via many vibrational collisions with the surrounding medium. This generally leaves the molecule in the lowest vibrational level of the first excited electronic state (SI) from which both nonradiative and radiative processes may occur.

The molecule may dissipate energy non-radiatively to the ground state by internal conversion and vibrational relaxation as described above, in which

EMISSION SPECTROSCOPY 173

case no emission is detected. However it may undergo the same transition (SI ~ So) radiatively, producing fluorescence. The frequency vofthis observed luminescence is given by equation 7.1 and the emitted photon has an energy also equivalent to the difference in energy between the molecule's initial (E) and final state (Er)' Fluorescence is a rapid process, occurring on a time-scale of picoseconds to hundreds of nanoseconds (10-12-10-7 s).

A further possibility is that of intersystem crossing, promoted by spin-orbit coupling, which requires that there exists a close-lying energy level in the triplet manifold. All triplet levels have the valence electrons with parallel spins. Once within the triplet manifold, rapid vibrational deactivation to the lowest vibrational level of the TI state will occur. Intersystem crossing back to the singlet manifold followed by non-radiative vibrational relaxation may follow. The radiative transition TI ~ So can also occur, which produces phosphorescence. Because this transition involves a quantum mechanically forbidden reversal of spin, it is a comparatively slow step occurring on a time scale typically of seconds.

As can be seen from Figure 7.1, phosphorescence and fluorescence occur to a distribution of vibrational levels of the ground electronic state and hence emission over a range of frequencies is possible, resulting in an emission spectrum. The spectral intensity distribution and overall intensity will be determined by inherent transition probabilities together with perturbations caused by the surrounding environment.

The excitation/de-excitation processes can be considered as competing steps, each characterized with its own rate constant. This is shown in Table 7.1 for fluorescence, where M and M* represent the molecule in the ground and excited state and where superscripts I and 3 refer to the singlet and triplet state of the molecule, respectively. The experimentally determined quantum yield (rPf = photons emitted/photons absorbed) and lifetime (Tr) of fluorescence may then be defined in terms of these rate constants as given below.

,/,. _ kr I"f -

(kr +k[SC +k,c +kd) (7.2)

(7.3)

Table 7.1 The excitation/de-excitation process for fluorescence

Step Rate constant

'M+hv~ 'M* labs Absorption

'M* ~ 'M+hv k, Fluorescence 'M*~'M kJC Internal conversion 'M* ~'M* klSC Intersystem crossing

'M* ~ products kd Dissociation, isomerization, etc.


The fluorescence or phosphorescence emission spectrum is a plot of the respective luminescence intensity against the wavelength of the luminescence. Both are probes of the ground-state vibrational levels of the molecule. There may exist a mirror symmetry between the absorption spectrum profile, corresponding to SI f-- So vibronic transitions, and that of the fluorescence profile, as the result of the SI ~ So vibronic transitions, if there is minimal change in the nuclear configuration of the molecule upon excitation.

A hyper surface illustrating the spectral and temporal characteristics of fluorescence from a molecule is illustrated in Figure 7.2. If the detector used

(f)

c Ql -c

>. -(f)

c Ql -c

z

~,

Wavelength

Figure 7.2 Three-dimensional hypersurface illustrating the distribution of emission intensity as a function of wavelength and time for a luminescent molecule. Time zero corresponds to initial

excitation by a 8-function excitation pulse.


collects all photons emitted following excitation, as is the common case, then a steady-state emission spectrum will result. This spectrum is in effect a sum of the spectral distribution of photons emitted over all times following absorption oflight. If a cross section is taken of the x-z plane at a certain time (single y value), then a time-resolved emission (TRE) spectrum would result. For a single compound, successive spectra, recorded at different times after excitation, would show an unchanging profile but a diminishing intensity as excited state molecules lose their energy as fluorescence. If there were more than one species with different spectral and decay characteristics, the TRE spectra would in fact change with time. In this case, the observed steady-state emission spectrum will have contributions from all species.

In the time domain, the fluorescence emitted at a specific wavelength may be monitored as it diminishes as a function of time after excitation. This is equivalent to taking a slice in the y-z plane (single x value) and is called a fluorescence decay curve, since its profile reflects an exponentially decaying intensity with time. The fluorescence parameter associated with decay curves is the lifetime (,,), defined in equation 7.3, whereas the quantity connected with the steady-state emission spectrum is the quantum yield (~,) of equation 7.2.

The fluorescence (and phosphorescence) spectrum. intensity and lifetime are characteristic of the particular chromophore and are also sensitive to the surrounding environment enabling them to act as molecular probes. It is apparent from equations 7.2 and 7.3 that a knowledge of both the quantum yield of the process and the lifetime is required to calculate the rate constants for the various emission and non-radiative pathways.

7.3 Techniques

7.3.1 Steady state measurements

Spectrophotometers for recording steady state fluorescence and phosphorescence spectra have been well described in the literatureJ·6 • The essential features of such an instrument are depicted in Figure 7.3. Light from a suitable lamp (e.g. high pressure xenon or mercury lamp) is passed through a monochromator, in order to select the excitation wavelength, and directed into the sample chamber. For emission polarization experiments, polarizing films can be placed into the excitation and emission paths as appropriate. Monochromatic light is focused into the sample and luminescence is monitored, usually at right angles to the direction of excitation.

The photodetector output signal may be displayed on a chart recorder as a plot of signal intensity versus wavelength although in most modern instruments direct interfacing and display on a computer is common. A fluorescence/phophorescence spectrum results from the scanning of the emission monochromator at a fixed excitation wavelength, while an excitation


chan L ___ --l~!9lrecorder

Figure 7.3 Schematic representation ofa steady-state emission spectrophotometer. SI, S2: slits; MI, M2: excitation and emission monochromators; BS, beam splitter; Ll, L2: lenses; PI, P2: polarizers; F I: filter; QC: quantum counter; PMT I, PMT2: reference and sample photomultiplier

tubes; amp: amplifier.

spectrum can be recorded by scanning the excitation monochromator at a fixed emission wavelength. Appropriate correction for the wavelength sensitivity of the monochromators and detector can be incorporated to produce true undistorted spectra7

Advances in computational data manipulation and graphical representation of luminescence data have allowed the total information present in steady state excitation and emission spectra to be analyzed8• This requires a three-dimensional surface containing excitation and emission wavelength information together with intensity data. Such diagrams are illustrated in Figure 7.4 for a two-component fluorescent mixture. These plots contain considerably increased amounts of spectral/intensity information and are generated on conventional instruments by recording emission spectra using a range of excitation wavelengths and representing the data as surfaces or contour diagrams. The optimum excitation and emission wavelengths for isolating a particular species are readily obtained, while for more complex mixtures identification of the number of species and their spectral characteristics are facilitated. Resolution and analysis of multiple emitting components can also be achieved using factor analysis methods such as principal component analysis9• This has recently been demonstrated to be a powerful technique for spectrally resolving fluorescent mixtures lO• With the use oflaser excitation sources and luminescence analysis techniques, concentrations less than 10-10 molar of fluorescent species can be detected.

(a)

Fluorescence Intensity

(arb. units)

300

200

(b)

400

Excitation Wavelength (nm)

480

400

320

400


Rhodamine 110

480 560 640 Emission Wavelength (nm)

v

480 560 640

Emission Wavelength (nm)

Figure 7.4 (a) Total emission and (b) contour plot for a mixture of fi-carboline and rhodamine 110 in aqueous solution.


7.3.2 Time-resolved measurements

Phosphorescence, which occurs on the millisecond to second time-scale, can be isolated and studied by simply incorporating a mechanical/electronic shutter in the steady state spectrophotometer of Figure 7.3 to alternatively excite and monitor the emission after a delay during which fluorescence (which occurs on a nanosecond time-scale) has relaxed. However, in order to study the kinetics of fluorescence emission, more sophisticated techniques are required. One ofthe earlier methods, now regaining popularity, is phase modulation 11.12.

Here, fluorescence is excited with light of intensity that is modulated at high frequency, typically 10 MHz. Because of the finite lifetime of the excited state, there is a short delay before emission of fluorescence occurs. This results in a difference between the phase of the modulated fluorescence emission and that of the modulated exciting light. The measured difference in phase angle, 8, for a single exponentially decaying species is then related to the modulation frequency,/, and the fluorescence lifetime, 'r' by equation 7.4. The technique can be extended to extract-out multiple decaying components 11 .

tan(8) = 2,. f'r (7.4)

A technique employed by earlier workers in the field was that of pulse sampling, where a gaseous discharge lamp provided an excitation pulse of a few nanoseconds duration. The resultant luminescence could then be followed by a photomultiplier connected to a fast-sampling oscilloscope. More elegant extensions of this technique include the use of picosecond mode-locked lasers as excitation sources and streak-cameras as detectors, allowing picosecond time resolution 13.1 4 •

A technique that has been used widely is that of time-correlated single photon countingl5 - 17 which can provide data of high signal-to-noise quality and resolve lifetimes to a few tens of picoseconds. Instrumentation is readily available commercially but a time-resolved spectrometer using this technique and constructed in our own laboratories is illustrated schematically in Figure 7.5 18- 2°. The commercially available laser source is a mode-locked argon-ion laser synchronously pumping a cavity-dumped dye laser. Mode-locked Nd: Yag and Ti:sapphire lasers can also be used to provide tunable light pulses of a few picoseconds duration at repetition rates up to 80 million laser pulses/so The laser pulses are used to excite the sample under study and the resulting sample fluorescence is spectrally dispersed through a monochromator and detected by a fast photomultiplier tube.

In the time-correlated single photon counting technique, electronic pulses synchronized with the laser pulses are used to initiate a voltage ramp in a timeto-amplitude converter (T AC), while electronic pulses arising from fluorescence photons incident on the photomultiplier tube act to terminate the voltage ramp. The amplitude of the resulting voltage pulse from the T AC, which is stored in a memory of a multichannel analyzer (MCA) operating in the pulse-height analysis mode, will be directly related to the time between

sample

data acquisition computer

data

computer

EMISSION SPEc:.TROSCOPY

mode locked+Ar

cavity dumper dye laser

MeA

FLUORESCENCE INTENSITY

100

80

60

40

20

o 320 360 400 440 480

WA VELENGTI-I (nm)

stop

20.0

Figure 7.5 Schematic representation of a time-resolved fluorescence spectrometer.

179


excitation of the sample by the laser pulse and the detection of a fluorescence photon. Collection of many such events at a fixed emission wavelength will result in a histogram in the MCA, reflecting the probability of emission as a function of time after excitation by the laser pulse, i.e. a fluorescence decay curve. The fluorescence decay curve must then be analyzed to extract the decay parameters.

By collecting only those photons arriving within a set 'time-window' after excitation (selected by placing voltage discriminators on the output of the T AC) and scanning the emission monochromator, a time-resolved emission spectrum may be recorded. Alternatively, collection of fluorescence photons at a number of emission wavelengths and at various times after excitation can result in the three-dimensional hypersurface illustrated in Figure 7.5, displaying the spectral, temporal and intensity information simultaneously. In the inset diagram in Figure 7.5, the grow-in with time of a new fluorescent species (an excited state dimer or 'excimer' with emission maximum near 400 nm) formed from the interaction of initially excited (fluorescence maximum 340 nm) and ground-state acenaphthyl chromophores in a synthetic aromatic polymer (poly (acenaphthylene)) is illustrated20• The combination of such excitation and detection systems with a confocal microscope can allow full spectral and temporal resolution to be achieved on a sub-micron scale2l

allowing the spatial distribution of fluorescent species to be characterized in, for example, a single textile fibre or biological cell.

The main limitation in time resolution for time-correlated single-photon counting is the jitter in transit times of photoelectrons in the detecting photomultiplier tube. With modern multichannel plate photomultipliers, instrument response functions of 50 ps can be obtained. Extraction of the effects of the finite instrument-response function on the detected fluorescence decay profile is then only required for quite short decay times. The distortion resulting from the finite time response of the instrumentation p(t) on the observed fluorescence decay profile let) is given by equation 7.5 where th~ recorded fluorescence intensity l(t) is a convolution of the true decay G(t) law and the instrument response function P(t).

l(t) = f~p(n x G(t-n dt' = p(t) ® G(t) (7.5)

Therefore sample molecules excited by photons in the initial period are still decaying, while others are just being excited by later photons. The observed fluorescence intensity at time t is, therefore, a summation of the true decay curve G(t) originating at all times t' < t on the response function. The aim of the lifetime analysis is to take likely mathematical forms for G(t), containing adjustable parameters, and reconvolute them with the collected response function P( t). This trial calculated observed decay curve y(t) is then compared with the measured l(t), and the parameters iteratively adjusted so as to minimize some fitting criterion. The parameter optimization in the iterative


reconvolution is carried out in these studies via a non-linear least squares technique usually based on the Marquardt algorithm, and goodness-of-fit criteria involve the reduced chi-square, inspection of residuals and autocorrelation plots or other statistical parameters such as the Durbin-Watson factor l5 • The mathematical form for G(t) may be a single exponential for a single emitting species, or a proposed kinetic scheme might lead to a sums of exponentials or non-exponential fluorescence decay function.

7.4 Applications to fluorescent whitening 'dyes'

The fluorescence quantum yield and lifetime are sensItIve to competing photo physical and photochemical pathways as reflected in equations 7.2 and 7.3. Measurement of these two parameters thus provides a way of obtaining information on the rates of photoinduced relaxation processes. In addition, these parameters, together with the fluorescence spectral characteristics, can be influenced by the surrounding environment. Thus, fluorophores can act as probes of biological and polymer rigidity and polarity.

One example of the application of such measurements is the elucidation of the energy relaxation pathways in fluorescent whitening agents (FWAs) which are a widely used class of colourless dyes. FW As are compounds which absorb solar radiation in the near ultraviolet region and fluoresce in the blue. For many materials manufactured from paper and natural textile fibres, any undesirable yellow colouring can be counteracted effectively by these fluorescent dyes to enhance their white appearance. Considerable research effort has been undertaken to enhance the performance of these dyes and to understand their photochemistry22.2J. A particular problem with the application ofFWAs to wool fabrics is that while the natural off-white appearance of wool can be counteracted very effectively by the application of these compounds, their use also results in FW A-sensitized sunlight photodegradation of the fibre2J-25 • Although many studies have been undertaken in an attempt to reveal the mechanisms which would explain the poor light-fastness of FW Awhitened wool, a basic knowledge of the photo physics and photochemistry of FW As themselves is necessary to fully understand the processes involved.

Studies on the stilbene-based fluorescent whitening agents 1 (Uvitex NFW, Ciba-Geigy) and 2 (Leucophor PAF, Sandoz) are discussed here26.27 • The principal initial mechanism for photo fading of these 'dyes' is photoisomerization about the ethylenic double bond of the fluorescent trans-isomer to form the non-fluorescent cis_isomer22.23.26. In Table 7.2 the effect of solvent on the fluorescence quantum yield, fluorescence lifetime, and trans ~ cis photoisomerization quantum yield for 2 are reported. The fluorescence lifetimes are all less than I ns and have been measured in this case with a picosecond Nd:glass laser excitation source and a streak camera. Solvent polarity decreases from water to dioxane while ethylene glycol has a similar polarity to


PhO f-=N H +

N O>-N S03- Na

MeO _ OMe )-=N ~H H 0 N=:{

Na+ -03S N-<ON H N=(

OPh

2

methanol but is much more viscous. It is apparent from Table 7.2 that decreasing solvent polarity (or increasing viscosity) leads to an increase in fluorescence yield and lifetime but a decrease in the photoisomerization yield. By using equations 7.2 and 7.3, and with a knowledge of the fluorescence lifetime and the quantum yields of fluorescence and photoisomerization, the rate constants for fluorescence, non-radiative decay and photoisomerization can be calculated.

It is apparent from the data in Table .7.2 that the rate constants mainly affected by solvent polarity and viscosity are those for non-radiative decay and photoisomerization. In stilbene-type molecules, excitation of the transisomer can lead to either fluorescence or mutual rotation of the aromatic rings about the double bond which can lead to a twisted configuration that

Table 7.2 The effect of solvent on the fluorescence quantum yield, fluorescence lifetime and trans -+ cis photoisomerization quantum yield for 2 at 298 K

Solvent 'r " Tr k, kn' k, (x 1O-9 s) (x 108 s-')

Water 0.20 0.184 0.37 5.4 22 5.0 Methanol 0.49 0.135 0.59 8.3 8.6 2.3 Ethanol 0.59 0.091 0.69 8.5 6.0 1.3 DMF 0.71 0.049 0.68 10.4 4.3 0.72 Dioxane 0.94 0.98 9.6 0.6 Ethylene glycol 0.70 0.058 0.68 10.3 4.4 0.85

'r, Fluorescence quantum yield; ", trans -+ cis photoisomerization yield; T" fluore-scence lifetime; rate constants for radiative (k,), non-radiative (kn,) and photoiso-merization (k,) processes.


may then either decay non-radiatively to reform the ground state transisomer or undergo further rotation to form the cis-isomer28 • If the intermediate configuration has charge-transfer character, it will be stabilized in more polar solvents, and the energy barrier to intramolecular twisting and thus photoisomerization and non-radiative decay will be reduced. An increase in viscosity will also impose a frictional barrier to rotation about the ethylenic double bond, allowing the fluorescence process to compete more effectively with the isomerization processes.

The fluorescence emission and excitation spectrum of 1 in aqueous solution (dashed line) and a solid poly(vinyl alcohol) (PV A, solid line) film are illustrated in Figure 7.6. There is a marked red shift of28 nm of the excitation spectrum and an increase in fluorescence spectral structure for 1 in the polymer environment compared to solution. A similar red shift in excitation of the 'dye' is observed on incorporation in a wool fibre27 • The lack of such a large red shift in homogeneous viscous solvents indicates that binding of the dye to the polymers is responsible for the change in spectral properties rather than any rigidity effect. The excitation red shift and increase in vibrational structure are consistent with an increase in molecular planarity upon binding, which would induce a more extensive conjugated electron distribution in the dye molecule. Similar results are observed for 2.

In solid PV A films, photoisomerization of 1 and 2 is inhibited and fluorescence decay times of 0.78 ns and 0.72 ns are observed, respectively. When applied to wool fabric at 0.02% on-weight-of-fabric, the fluorescence decays

Excitaticn Emission

.£ , , , (fl , c::: Q) , c , . Q) . , > ,

~ . Q)

. , , , 0: , , , . ,

, , , , -- ...... - ... ,

240 300 360 420 480

Wavelength (nm)

Figure 7.6 Fluorescence emission and excitation spectra of 1 in aqueous solution (- - -) and poly(vinyl alcohol) film. (-).

184 ANAL YTiCAL CHEMISTR Y OF SYNTHETIC COLORANTS

become non-single exponential with a short Trcomponent of 0.33 ns (64%) and 0.39 ns (80%) contributing most to the initial fluorescence intensity for 1 and 2 respectively. Since these decay times are shorter than those observed in PV A films or any of the solvents studied, it suggests that a majority of photo excited dye molecules are involved in quenching interactions with the fibre environment. It has been suggested that one possibility is that non-radiative energy transfer occurs from the photoexcited 'dyes' to natural pigments in the wool, which could provide a pathway for the sensitized photodegradation ofFWAwhitened wool 26•

The fluorescence and photochemical properties ofpyrazoline- and collmarin-type whiteners have also been studied29• For pyrazoline whiteners, using fluorescence polarization measurements, it was determined that excitation energy transfer can occur between whitener molecules at the concentrations normally used on wool. The photodegradation of the whiteners could also be determined by monitoring the loss of fluorescence from the whitener with irradiation. Through these studies, the role of oxygen and moisture on photofading rates can be determined29 •

The discussion above serves to illustrate the type of information which can be obtained for synthetic dyes from both steady state and time-resolved luminescence measurements. Spectral, temporal and intensity information can be used as a probe of the dye environment and molecular configuration. Quantitative measurements of fluorescence yields and lifetimes can provide rate constant data for excited state processes to help elucidate photochemical mechanisms. With the availability of picosecond and femtosecond laser sources, sophisticated detection methods and data analysis procedures, there now appears no limit on the breadth and detail of information which can be obtained from luminescence measurements. Some of the most exciting current developments are in the extension of the spectral and time-resolved techniques outlined above to the micron and sub-micron spatial level using confocal imaging techniques21 • This will allow detailed luminescence analyses of synthetic dyes on a fabric surface and within, as well as on, single textile fibres and other polymer and biological substrates.

Acknowledgements

The author acknowledges financial support for this work from the Australian Research Council and the Australian Wool Research and Development Corporation. The contributions by previous research students P. Skilton, K. Smit and T. Smith whose work is reviewed herein is gratefully acknowledged.


References

1. 1.R. Lakowicz (ed.) (1991) Topics in Fluorescence Spectroscopy. Vol. I, Plenum Press, New York.

2. B.M. Krasovitskii and B.M. Bolotin (1988) Organic Luminescent Materials, VCH, Weinheim.

3. G.G. Guilbault (1973) Practical Fluorescence and Theory. Dekker, New York. 4. M.A. Winnik (ed.) (1985) Photophysii'al and Photochemical Tools in Polymer Science, NATO

ASI Series C. Vol. 182. Reidel, Dordrecht. 5. 1.G. Calvert and 1.N. Pitts (1966) Photochemistry. Wiley, New York. 6. CA. Parker (1968) Photoluminescence of Solutions. Elsevier, London. 7. K.P. Ghiggino, P.F. Skilton and P.l. Thistlethwaite (1985) J. Photochem., 31 113. 8. G.W. Suter, A.1. Kallir and U.P. Wild (1983) Chimia, 37, 413. Hitachi Technical Data Sheet

FL No. 25. Hitachi Ltd, Tokyo. 9. W.H. Lawton and E.A. Sylvestre (1971) Technometrics. 13,617.

10. 1. Drew, A.G. Szabo, P. Morand, T.A. Smith and K.P. Ghiggino (1990) J. Chem. Soc. Faraday Trans., 86, 3853.

II. 1.R. Lakowicz and 1. Gryczynski (1991) In Topics in Fluorescence Spectroscopy, Vol. I, ed. 1.R. Lakowicz, Plenum Press, New York, pp. 293-335.

12. K.P. Ghiggino, A.l. Roberts and D. Phillips (1981) Adv. Polym. Sci., 40,69. 13. G.R. Fleming, 1.M. Morris and G.W. Robinson (1977) Aust. 1. Chem .. 30, 2337. 14. T.M. Nordlund (1991) In Fluorescence Spectroscopy. Vol. I, ed.l.R. Lakowicz. Plenum Press,

New York, pp. 183-260. 15. D.V. O'Connor and D. Phillips (1984) Time Correlated Single Photon Counting. Academic

Press, London. 16. 1.N. Demas (1983) Excited State Lifetime Measurements, Academic Press, New York. 17. D.l.S. Birch and R.E. Imhof(1991) In Topics in Fluorescence Spectroscopy, Vol. I, ed.l.R.

Lakowicz, Plenum Press, New York, pp. 1-95. 18. K.P. Ghiggino, T.A. Smith and G.l. Wilson (1990) 1. Modern Optics, 37, 1789. 19. K.P. Ghiggino, P.F. Skilton and E. Fischer (1986) J. Am. Chem. Soc .. 108, 1146. 20. K.P. Ghiggino, S.W. Bigger, T.A. Smith, P.F. Skilton and K.L. Tan (1987) In Photophysics

of Polymers, ed. CE. Hoyle and 1.M. Torkelson, A. c.s. Symposium Series No. 358, Ch. 28. Am. Chern. Soc., Washington.

21. K.P. Ghiggino, M.R. Harris and P.G. Spizzirri (1992) Rev. Sci. Instrum., 63, 2999. 22. R. Anliker and G. Muller (guest ed.) (1975) Fluorescent Whitening Agents. Vol. 4 of Environ-

mental Quality and Safety. ed. F. Coulston and F. Korte. George Thieme, Stuttgart. 23. I.H. Leaver and B. Milligan (1984) Dyes Pigments. 5, 109. 24. 1.H. Leaver (1978) Photochem. Photobiol .. 27, 451. 25. B. Milligan (1980) Proc. Int. Wool Res. Con!. Pretoria, V, p. 167. 26. K.l. Smit and K.P. Ghiggino (1987) Dyes Pigments. 8, 83. 27. K.1. Smit and K.P. Ghiggino (1991) J. Polym. Sci., Part B: Polym. Phys., 29, 1397. 28. D.J.S. Birch and 1.B. Birks Chem. Phys. Lett., 38,432. 29. 1.H. Leaver (1977) Aust. J. Chem .. 30, 87.

8 Identification and analysis of diarylide pigments by spectroscopic and chemical methods C. NICOLAOU and M. DA ROCHA

8.1 Introduction

8.1.1 Historical background

Diarylide pigments are a class of colored organic disazo compounds prepared from substituted 4,4' -diaminobiphenyls (1: X = CI, CH3, OCH3; Y = H, CI) and acetoacetanilide or its derivatives (2: R I = H, CH3, OCH3, CI; R 2 = H,

1 2

H3C OH X Y HO CH3 1 Rl H'-I( 'J=\ k l--H R

qN~N .. N~N.N~N'¢-~ I A 0 Y X 0 A R2

R2 R3 R3

3

CH3, OCH3, OC2H5, CI; R3 = H, OCH3, CI). Diarylide pigments can be represented by the general structure, 3. The term 'diarylide' signifies a disazo compound whose azo groups are linked to an acetoacetarylide molecule, which is usually referred to as the coupler. Table 8.1 lists various diarylide pigments found in the Colour Index] and demonstrates the numerous permutations possible by changing the substituents. In addition, co-couplings are also prepared by the simultaneous reaction of two different couplers with the same aromatic diamine to form asymmetric pigments such as c.1. Pigment Yellow 174 and Yellow 188.

c.1. Pigments Yellow 12, 13 and 14 were first patented in 1911. They were commercialized2 in the late 1930s and early 1940s. c.1. Pigment Yellow 12 is the dominant pigment among this class of colorants, and finds significant use in the printing ink industry.

IDENTIFICATION OF DIARYLIDE PIGMENTS 187

Table 8.1 List of diarylide pigments found in the Colour Index (based on 3)

c.r. pigment name c.r. number X Y R' R2 R3

Yellow 12 21090 CI H H H H Yellow 13 21100 CI H CHJ CHJ H Yellow 14 21095 CI H CH3 H H Yellow 17 21105 CI H OCH3 H H Yellow 55 21096 CI H H CH3 H Yellow 63 21091 CI H CI H H Yellow 81 21127 CI CI CHJ CHJ H Yellow 83 21108 CI H OCH3 CI OCHJ

Yellow 87 * CI H OCH3 H OCH3

Yellow 90 * * * H H H Yellow 106 * CI H * * * Yellow 113 21126 CI CI CHJ CI H Yellow 114 21092 CI H H H H

H CHJ H Yellow 121 * CI H CI H H Yellow 124 21107 CI H OCHJ OCH3 H Yellow 126 21101 CI H H H H

H OCHJ H Yellow 127 21102 CI H CHJ CHJ H

OCHJ H H Yellow 136 * CI H * * * Yellow 152 * CI H H OC2HS H Yellow 170 21104 CI H H OCHJ H Yellow 171 21106 CI H CH3 CI H Yellow 172 21109 CI H OCHJ H CI Yellow 174 21098 CI H CH3 H H

CH3 CH3 H Yellow 176 21103 CI H CH3 CHJ H Yellow 188 21094 CI H H H H

CH3 CH3 H Orange 14 21165 OCH3 H CH3 CH3 H Orange 15 21130 CH3 H H H H Orange 16 21160 OCH3 H H H H Orange 44 * OCH3 H H CI H Orange 63 21164 OCH3 H H CH3 H

• Unknown group (proprietary information).

Diarylide pigments account for about 80% of all the yellow pigments used in the USA; the most widely used are c.l. Pigments Yellow 12, 13, 14, 17 and 83. c.l. Pigment Orange 16 is the most often used diarylide orange.

8.1.2 Method of manufacture

Since c.1. Pigment Yellow 12 is the most important diarylide pigment, its preparation will be used to illustrate the reactions involved in the synthesis of this class of product. The standard method of preparation involves tetrazotization of the diaminobiphenyl derivative (in this case 3,3' -dichlorobenzidine, 4) with nitrous acid at O°C to form the corresponding tetrazonium salt


(5). The nitrous acid is formed in situ from the reaction of hydrochloric acid with sodium nitrite (see equation 8.1). The tetrazonium salt solution is then reacted under controlled conditions with the coupler, in this case acetoacetanilide (6). A finely dispersed slurry of the coupler is prepared in sodium acetate buffer by precipitating an alkaline solution of the coupler with acetic acid, just prior to the addition of the tetrazonium chloride. The 'coupling' reaction temperature is usually controlled to 5-lOoC, but it can be as high as 35°C depending on the coupler and amine used (see equation 8.2). The pH of the coupling reaction is typically between 3.5-6.5. Higher coupling temperatures and pHs are used to increase the rate of reaction, but at the risk of decomposing the tetrazonium chloride. Surfactants or water soluble additives are sometimes used during the coupling to enhance the rate of reaction. An excess of coupler is often used (3-5%) to ensure complete utilization of the tetrazonium chloride and thus decrease the formation of decomposition products. Some of the unreacted coupler remains in the final product. The pigment slurry obtained is often digested by heating to near 100°C to obtain the most desirable particle size. The mixture is then washed free of saIts with water.

(8.1)

Cl

0- N=:N-b-Q--NEN Cl- + 4 H20

Cl 5

o ~ 9 -0 " 2 CH3 -CCHz-CNH ~ A + 2 NaOCCH3

5 6 (8.2)

H3C OH Cl HO CH3

O~~N~N-b-Q-N"NX;~~ ... 2 CH3COOH + 2 NaCl

I.&' 0 Cl 0 V c.1. Pigment Yellow 12


The major impurities from these reactions are unreacted coupler and 3,3'dichlorobenzidine. In some cases the surfactants used are not completely washed out and small quantities remain in the final product. Incidental byproducts may be formed from the decomposition of the tetrazonium chloride and from the hydrolysis of the acetoacetarylide used. By-products of concern are 3,3' -dichlorobiphenyl and aromatic amines. The 3,3' -dichlorobiphenyl is formed by the reduction of 3,3' -dichlorobenzidine tetrazonium chloride, for example, in the presence of formate ions. The use offormate buffers has thus been phased out of commercial practice. Aromatic amines (other than 3,3'dichlorobenzidine) may be formed from acid and base hydrolysis of the coupler or by the thermal decomposition of the coupler.

3,3' -Dichlorobenzidine and its salts are classified by the American Conference of Governmental Industrial Hygienists (ACGIH) as industrial substances suspected of carcinogenic potential to man. It is also classified as a Cancer Suspect Agent by the Occupational Safety and Health Administration (OSHA). Regulations by the United States Environmental Protection Agency (EPA) limit the maximum allowed amount of 3,3' -dichlorobiphenyl in diarylide pigments to 125 ppm. In actual practice, most diarylide pigments sold today contain less than 50 ppm of this impurity. OSHA regulations effectively limit the maximum permissible level of 3,3' -dichlorobenzidine in diarylide pigments to 0.1 %. Pigments containing higher amounts would have to be classified and labeled as suspected carcinogens in the USA.

8.2 Analytical methodology of diarylide .pigments

Like many other commercial products, today's diarylide pigments often contain one or more additives which impart the physical properties needed for a particular application.2.3 Occasionally the shade of a particular pigment is modified by the presence of a small amount of a related pigment. This is done by using more than one coupler or amine during the manufacturing process. Therefore, their analysis must include methods for the identification and quantitation both of the colorants and of their non-colored additives. To the best of our knowledge. the analytical chemistry of modern diarylide pigments has not been previously reported.

The most commonly used non-colored organic additives are long-chain aliphatic diamines (7: R = C 12-C18 aliphatic hydrocarbon chain), and triamines (8: R = Cll-C18 aliphatic hydrocarbon chain), rosins, metallic rosinates and sulfonated anionic surfactants. Recently. a new type of polymeric ether amine-based additive (9) has been introduced. These types of additive are mixtures of homo logs and isomeric compounds making the identification and quantitation of each individual compound rather difficult. Determination of such additives by gravimetric analysis using solvent extration is often sufficient.


7

,..,(CH2h-NH2 R-N,

(CH2h-NH2

8 9

The most commonly encountered inorganic additives are clay, barium sulfate and titanium dioxide all of which are quite stable to heat and can be easily determined by ashing the samples in a furnace at about 650°C. Most clay materials lose about 13% of their weight when heated at this temperature. This weight is accounted for in the final calculation of the clay content.

Since the matrix of commercial diarylide pigments can be complex, a combination of several analytical techniques is required to determine their composition. Although a general analytical scheme can be used for most samples, it is often necessary for the analyst to develop and employ new techniques that can be applied to specific samples. Techniques used for these analyses may include some or all of the following: infra-red spectroscopy (IR), visible spectrophotometry (UV-VIS), thin layer chromatography (TLC), high performance liquid chromatography (HPLC), gas chromatography (GC), mass spectrometry (MS), elemental analysis, solvent extraction, ash determination and other wet chemical methods such as reduction or distillation with soda lime. The specific application of these techniques is noted below.

• Visible spectrophotometry is used mainly for assay determination, provided a single colorant is present. For this purpose dilute solutions of the pigments in a-dichlorobenzene are prepared. Mixtures of diarylide pigments cannot be accurately analyzed quantitatively by this technique since they have very similar spectra but different absorptivities.

• Infra-red spectroscopy is used mostly for identification purposes and also for semi-quantitative analysis of mixtures of two diarylide pigments.

• Thin layer chromatography is used to separate and identify mixtures of pigments based on their R f values and their fluorescence properties under ultraviolet light. This technique is also applied for the quantitative analysis ofunreacted coupler, aromatic amines and aliphatic fatty amines.

• Elemental analysis is used to determine the elemental composition of purified pigments. Ash determination is used to estimate the amount of inorganic materials.

• High performance liquid chromatography is utilized for the analysis of trace impurities, such as aromatic amines.

• Gas chromatography is the technique of choice for determining trace amounts of polychlorinated biphenyls (PCBs).

• Mass spectrometry is used for the identification of additives, such as rosins and aromatic amines, fragmentation products from the reduction of the pigment and also for identification and quantitation of PCBs.


• Solvent extraction is used for extracting organic additives, such as surfactants, rosins and aliphatic amines. Methanol, ethanol and acetone are often used for this purpose, since most diarylide pigments have very low solubility in these solvents.

Special analytical techniques are needed in the case of so-called easily dispersible diarylide pigments (mainly c.1. Pigment Yellow 12). Such pigments have been treated with fatty amines to form a Schiff's base derivative by reacting two moles of amine with one mole of pigment, as shown in equation 8.3. The concentration of the Schiff's base form of the pigment is usually 5-30% of the product. As a result, the final product consists of a mixture of unreacted aliphatic amine, C.I. Pigment Yellow 12 and the corresponding Schiff's base derivative (10). Complete analysis of this type of pigment requires the determination of each of these components. To accomplish this, the sample is extracted with aqueous methanol to remove the unreacted amine, followed by hydrolysis of the Schiff's base derivative with acidified alcohol. This releases the fatty amine as the acid salt, which is then removed by washing the pigment with alcohol (see equation 8.4). After complete hydrolysis and clean-up, the pigment content is determined by visible spectrophotometry. The difference in pigment content before and after hydrolysis gives the amount of fatty amine present as Schiff's base. Hence the amount of Schiff's base pigment in the original sample can be determined.

H}C OH Cl HO CH}

~~lN"N-b--Q-N'>N~~~ +2RNH2

VO ClOV

H,C NR Cl I RN CH;

~~~N-'N-b-Q-N'>Nr:~~ Vo ClOV

10

+2H20

(8.3)

(8.4)

+ 2 RNH2. HCl


Other chemical methods employed to analyse diarylide pigments involve reduction of the azo groups to release the aromatic diamine and an amino derivative of the coupler used. Under the strong acid conditions used, substantial hydrolysis of the coupler to the corresponding aromatic amine also occurs. This technique is used when identification of the pigment by the above methods cannot be achieved. The products of the reduction can be identified by TLC, HPLC and/or GCMS. Zinc in hydrochloric acid, tin in hydrochloric acid (SnCI/HCl) and sodium dithionite have been used to reduce the azo linkage of pigments, including diarylide pigments.4.5.11 The azo linkages of diarylide pigments can also be reduced to amino groups by distillation with soda lime. In this case the amido group of the coupler used is easily hydrolyzed to form the corresponding aromatic amine. For example, the major products from soda lime distillation of c.1. Pigment Yellow 12 are 3,3'-dichlorobenzidine and aniline.

8.3 Analysis of diarylide pigments by infra-red spectroscopy

The theory and practice of infra-red spectroscopy has been well documented6--9 and will not be covered here. IR spectroscopy is a very convenient technique for the analysis of pigments in general. It is quick, easy to use, fairly inexpensive and requires only about a milligram of sample. Since IR spectroscopy is rather insensitive to as much as 10-20% of impurities, it is possibie to identify a single pigment without purification. However, if a mixture of two pigments is suspected, sample purification prior to IR analysis is recommended.

8.3.1 Sample preparation

Isolation of the pigment from a commercial sample is usually a necessary step. For samples in which the pigment is the major constituent, organic additives are first extracted with solvents such as methanol, aqueous methanol, acidified methanol or acetone. The pigment is then isolated by high speed centrifugation. Tetrahydrofuran, chlorinated aliphatic hydrocarbons and glacial acetic acid have been used to extract polymeric additives.

If the sample contains inorganic additives such as clay, titanium dioxide, barium sulfate, etc., a small amount of the pigment can be extracted using hot a-dichlorobenzene. After filtration and evaporation of the solvent, the pigment obtained is free of inorganic additives and can be analyzed by IR spectroscopy.

For this, small amounts of the isolated pigment are mixed with potassium bromide and pressed into a pellet (disc). Normally, spectra of good intensity are obtained by mixing and grinding 1-1.5 mg of pure pigment with 250 mg of potassium bromide. Typically, 120-150 mg of the ground mixture is placed into a 13 mm die and pressed for about I min at a pressure of 8 tons.

IDENTIFICATION OF DIAR YLIDE PIGMENTS 193

If a mixture of two pigments is present, the sample size can be increased to 3 mg per 250 mg of potassium bromide. The IR spectrum is then recorded in the range of wavelengths where the minor pigment component has a characteristic absorption band that is absent in the spectrum of the major component. For quantitative analyses it is necessary to prepare a series of standards ranging from 0 to 30% of the minor component. The total sample weight used to prepare the pellets must remain constant for the standards and sample.

8.3.2 Characteristic bands of the I R spectra of diarylide pigments.

IR spectra of several diarylide pigments have been published. 10 The IR spectra of diarylide pigments are quite complex, and it is not practical to assign specific bond or ring vibrations to all the absorption bands. The IR spectra of the most commonly used diarylide pigments, namely c.l. Pigments Yellow 12, 13.14,17,83 and c.l. Pigment Orange 16 are illustrated in Figures 8.1-8.6.

1C10 0 ,--------- - -- ------.---. -- - ----- _ .. _--_._---_._--------,

80

60

40

20

00+---_-- ~-~---,__- ,-.--_---~~-~~-~-~__I

40000 3500 3000 2500 2000.0 1 BOO 1600 1400 1200 1000 800 600 400 300.0

Frequency cm- 1

Figure 8.1 IR spectrum of c.I. Pigment Yellow 12: 0.4% in KBr (I 10 mg KBr disc).

60

40~

o:+j--,---~~-_,--,--,--,--,-~-~-~-~ 4000.0 3500 3000 2500 20000 1800 1600 1400 1200 1000 800 600 400 300.0

Frequency cm- 1

Figure 8.2 IR spectrum of c.I. Pigment Yellow 13: 0.4';;', in KBr (110 mg KBr disc),


100.0-,---------------------------------,

O.O+----::-::".,---~--__r--_,_---r-___.-__r--.__-~-_,_-~-__r___1 4000.0 3500 2000.0 1800 1600 1400 1200 1000 800 600 400300.0

Frequency em-1 3000 2500

Figure 8.3 IR spectrum ofC.1. Pigment Yellow 14: 0.4% in KBr (110 mg KBr disc).

100.0,--------------------------__ -,

.,SO

~ ~ 60

~ 40

iI. 20

0.0 +---.----,---,----,--,--.----=-----,----,,--r--r--.--,.......l 4000.0 3000 2500 2000.0 lS00 1600 1400 1200 1000

Frequency em-1 SOO 600 400 300.0

Figure 8.4 IR spectrum of C.1. Pigment Yellow 17: 0.4% in KBr (110 mg KBr disc).

100.0 .... ----------

m u

80

g 60 .~

~ 40

at 20

o.o+--~--~--~-_.-~-~--".~-~-~-~-~.,__~_i 4000.0 3500 3000 2500 2000.0 1800 1600 1400 1200 1000

Frequency em-1

Figure 8.5 IR spectrum of c.1. Pigment Yellow 83: 0.4% in KBr (I \0 mg KBr disc).

100.0,----------------------------,

O.O+--~.__-~--~-_r-~-~-"-~-~---r-~---r---r_l 4000.0 3500 3DOO 2500 2000.0 1800 1600 1400 1200 1000 800 600 400 300.0

Frequency em-1

Figure 8.6 IR spectrum of C.1. Pigment Orange 16: 0.4% in KBr (I \0 mg KBr disc).


The spectra are characterized by a very strong band, usually a doublet, between 1495 cm 1 to 1510 cm I. This band can be assigned to C=C skeletal vibrations of the aromatic rings. The small sharp band near 1600 cm I, sometimes a doublet, is also attributed to C=C vibrations of the aromatic rings. The strong sharp band near 1665-1670 cm· 1 is caused by c=o stretching vibrations of the amido group (usually referred to as the amide I band). The medium band near 1545~1555 cm 1 (sometimes obscured by the strong absorption band of the aromatic ring vibrations) can be attributed to a combination of N-H and C-N stretching vibrations of the amido group (usually referred to as the amide II band). The N-H overtone vibrations occur between 3100-3000 cm I, but they are rather weak. They can be observed if the sample concentration is increased. The absorption band of the azo group -N=N- occurs between 1575 cm 1 and 1630 cm 1 but is very weak and has no diagnostic value.

The position of the carbonyl band of the acetyl group of acetoacetarylides is found between 1705 cm -I and 1730 cm I. This band is absent in the spectra of the diarylide pigments indicating that in the solid state these pigments exist in the enol tautomer. In this form, the absorption band of the C=C-OH group is usually observed between 1550-1585 cm I. Diarylide pigments having methoxy groups attached to the aromatic rings exhibit characteristic bands between 1020-1035 cm 1 attributed to the O-CH] stretching vibration. A band near 2840 cm- I may also be observed from the C-H stretching vibration ofthe methoxy groups.

The spectra also exhibit a characteristic pattern of bands between 1180 cm- I

and 1310 cm I. c.1. Pigment Yellow 13 exhibits a series of six bands, whereas c.1. Pigments Yellow 12, 14, 17 and 83 have only five bands in this region.

Some characteristic absorption bands of the most widely used diarylide pigments are listed in Table 8.2. These bands are specific to a particular pigment and may be used to determine whether a mixture of diarylide pigments may be present in a sample. The minimum detectable amount in a mixture of two diarylide pigments is approximately 10%, depending on the specific pigments present.

Figure 8.7 shows the spectrum ofa mixture of85% c.1. Pigment Yellow 13 and 15% C.I Pigment Yellow 83. The characteristic band of the latter at 1400 cm 1 is easily observed.

8.4 Analysis of diarylide pigments by visible spectroscopy

Since today's diarylide pigments are seldom found in the pure toner form, it is often necessary to carry out their assay. Visible spectroscopy is the preferred technique for this analysis and requires the use of pure standard materials which, unfortunately, are not available from any laboratory chemical supplier and must be generated by the analyst. Purification may be accomplished by


Table 8.2 Characteristic absorption bands of some commonly used diarylide pigments

Wavenumber CI. pigment name (em ')

P.Y.12 P.Y.13 P.Y.14 P.Y.17 P.Y.83 P.O.16

2840 wsh v w br vw br 1590 msh 1462 msh 1460 msh 1447 msh vwbr v w sh m sh 1438 wsh 1400 msh 1329 m brd 1215 wsh w sh wsh msh s sh 1130 m sh 1122 v w br 1114 vwbr 1035 m br 1030 m br 815 m br 635 msh 557 wbr 543 w br

br, broad; d, doublet; m, medium; s, strong; sh, sharp; v, very; w, weak.

100.0--,~~~~~~~~~~~-~~--

90

80

70

60

30

20

10

4000.0 3500 3000 2500 2000.0 1800 1600 1400 1200 1000 800 600 4%300.0

Frequencyem-1

Figure8.7 IR spectrumofa mixtureof85%C.I. Pigment Yellow 13 and 15%C!. Pigment Yellow 83: 0.4% in KBr (110 mg KBr disc). The characteristic band ofC!. Pigment Yellow 83 is marked.

recrystallization from a solvent such as a-dichlorobenzene but, often, washing a laboratory prepared pigment with a mixture of methanol and water will yield a product of high purity.

Since diarylide pigments have limited solubility in a-dichlorobenzene, a micro-analytical balance is needed to weigh small quantities (5 mg or less).

IDENTIFICATION OF DIAR YLiDE PIGMENTS 197

Use of conventional analytical balances would require weighing larger quantities of sample and more solvent.

For assay determination, the sample is dissolved by boiling it for several minutes with o-dichlorobenzene (a fume hood is essential for this operation). The solution is allowed to cool, diluted to volume in a volumetric flask, and its absorbance is measured at the wavelength of maximum absorption. The purity is calculated as follows:

. Ax 100 Percent punty = (a) x (b) x (c)

where A = absorbance of the sample; a = absorptivity of the pigment being tested (in 1 mg-'); b = path length (in cm) of the sample cell; c = concentration of the sample solution in mg 1-'.

Visible spectroscopy, however, is not an acceptable technique for identification purposes, as most diarylide pigments have the same absorption spectrum with the wavelength of the maximum absorption between 430 and 450 nm (see Figure 8.8). The absorptivities of the most commonly used diarylide pigments are listed in Table 8.3. Mixtures of diarylide pigments cannot be accurately assayed spectrophotometrically because of the differences in their absorptivity.

0.70 -,--------------------------,

" u o ~

€

0.6

0.5

0.4

£ 0.3 .0

"" 0.2

0.1

0.00 -+---.---,-----.---,--,---.---,---,-----,---'T----.---,---,----.----l 300.0 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600.0

Wtlvelength nanometers

Figure 8.8 Visible spectrum orc!. Pigment Yellow 12: 5.840 mg I-I in dichlorobenzene.

8.5 Thin layer chromatography of diarylide pigments

Thin layer chromatography (TLC) is a simple, inexpensive and effective analytical technique for identifying unknown pigments, and for determining additives, reaction by-products and unreacted intermediates. Typically, a


Table 8.3 Absorption coefficients in a-dichlorobenzene

C.I. pigment name

Yellow 12 Yellow 13 Yellow 14 Yellow 17 Yellow 83 Orange 16

Wavelength of maximum absorption (nm)

430 430 432 434 439 451

Absorptivity (1 mg-I em-I)

0.1 \07 0.1015 0.1076 0.1048 0.0862 0.1 \08

small amount of sample solution is applied to the TLC plate, along a line marked lightly with a pencil about lcm from the edge of the plate. Solutions of reference standards are also spotted alongside the sample. The plate is eluted with preselected solvent(s) in a closed tank.

Unknowns are identified by comparing the R f value with that of known standards. Quantitation is achieved by the use of appropriate standards of known concentration by compairing the intensity of the sample spot with that of the standard. The identification of diarylide pigments, and the determination of unreacted coupler, aromatic and aliphatic amines by TLC is described below.

8.5.1 Identification of diarylide pigments

F or identification purposes, the pigment is first purified by solvent extraction. Methanol, acidified alcohol or acetone may be used. About 1 mg of the purified pigment is dissolved by boiling in 10 ml of o-dichlorobenzene in a closed tube. A 5 ~I sample of the solution is applied to a silica gel TLC plate along with solutions of known reference standards. The spotting solvent is evaporated off in an oven at 105°C for 5 min. The plate is developed in a mixture of toluene:acetonitrile:methanol (94.5:5:0.5) or toluene:methanol (99: 1). After the solvent has traveled about 8 cm from the origin line, the plate is removed from the tank, the solvent front marked and residual solvent on the plate evaporated off. The plate is examined under shortwave ultraviolet light for the presence of fluorescent compounds. Certain diarylide pigments exhibit fluorescence properties which can be used, in conjunction with the Rf values, to identify specific pigments.

It is important to emphasize that TLC is not an absolute identification technique since, in a given solvent system, two or more pigments may have the same R f value. For a more definite identification it is necessary to use two or more solvent systems. Alternative solvents consist of toluene, a mixture of toluene:acetonitrile:acetone (96:4:0.1), or toluene:isopropanol (99: 1).

Physical mixtures of two pigments will exhibit only two spots on the thin layer chromatogram. Un symmetric pigments (co-couplings) usually show


three spots, one corresponding to the unsymmetrical molecule and the others corresponding to the two possible symmetrical molecules.

Easily dispersed pigments also show more than one spot because of the presence of the Schiff's base derivatives. In this case the Schiff's base must be hydrolyzed prior to analysis. Physical mixtures of two pigments will only have two spots.

8.5.2 Determination of un reacted coupler in diarylide pigments

For the determination ofunreacted coupler, approximately 0.1 g of pigment is placed in a 20 ml culture tube containing 10 ml of o-dichlorobenzene and is shaken with a Vortex mixer for 1 min. The tube is placed in a beaker of hot water and sonicated for 10 min, cooled and centrifuged for a few minutes at 2500 rpm. A 51-l1 sample of the solution is then applied a silica gel plate along with standard solutions of the coupler being determined. After drying, the plate is developed in a mixture of toluene:methanol (85: 15) until the solvent front travels about 8 cm from the origin line. The solvent is allowed to evaporate and the plate is exposed to ammonium hydroxide fumes for 1 min in a closed container. The plate is then sprayed with 0.1 % azoene (Fast Violet B Salt) solution in methanol. The coupler will appear as a yellow spot. Using the appropriate standards, about 0.5% of coupler can be detected by this procedure.

8.5.3 Determination of aromatic amines in diarylide pigments

Commercially available diarylide pigments usually contain two aromatic amines; the substituted diaminobiphenyl used to prepare the tetrazonium chloride and the amine generated by the hydrolysis of excess coupler.

Sample preparation is the same as that used for extraction of the coupler (see above) but further treatment of the extract is necessary. After centrifugation, 5 ml of the supernatant o-dichlorobenzene solution is pipeted into' a second culture tube and extracted with 3 x 3 ml portions of 5% sulfuric acid. The sulfuric acid extracts are combined in a 20 ml culture tube and rendered alkaline by adding a few drops of concentrated ammonium hydroxide. After cooling, the solution is extracted with 4 x 3 ml of diethyl ether. The ether extract is transferred into a scintillation vial and the solvent is evaporated at 50°C using a stream of air. The residue is dissolved in 2-5 ml of methylene chloride. Sonication may be required to ensure complete dissolution. A 5 I-ll sample of the methylene chloride solution is applied to a silica gel plate along with appropriate standards of the aromatic amines being determined. The standards must be dissolved in the same solvent as the sample. Solutions containing 1-10 ng mIl of these standards are usually prepared.

The plate is developed in a mixture of toluene and acetonitrile (92:8). The solvent front is allowed to travel 8 cm from the origin line and the plate is


removed from the tank. The solvents are evaporated and the plate is exposed to nitrous acid fumes (formed by adding a few drops of sulfuric acid to 1 g of sodium nitrite contained in a closed container) for about 30 s in a closed container. The plate is exposed to a stream of air to remove the excess of nitrous acid and then sprayed with a 0.1 'X) solution of Marshall's Reagent (NI-naphthylethylenediamine dihydrochloride). This procedure is carried out in a fume hood.

The amines will appear as bluish to pink spots. Using the appropriate standards, as little as 10 ng of the aniline derivative can be detected.

8.5.4 Analysis offatty diamines and triamines in diarylide pigments

Detection offatty diamines and triamines or other long-chain aliphatic amines in diarylide pigments is an important step in the identification of these pigments and often signifies the presence of Schiff's base derivative of the pigment with the amine. TLC is a quick, sensitive and convenient technique for the detection of fatty amines. The unreacted fatty amine is extracted from the pigment with 50% methanol/water at about 60"C. The extract is centrifuged, and the solution is filtered and evaporated to dryness. The residue is dissolved in methanol and the resulting solution is spotted on a silica gel TLC plate, together with appropriate standard solutions of known amines. Such standard solutions are prepared in methanol at concentrations varying from 0.2 to 2 mg mtl. The plate is spotted with 5 or 10 ) .. tl portions of sample and standard solutions.

The TLC plate is placed in a covered tank containing the developing solvent. A good developing solvent for these fatty amines consists of toluene: acetic acid:methanol:water (25: 15:5:2). The solvents are added and mixed in the order noted. Variations of this solvent mixture may be used to achieve the desired separation. The developing solvent is allowed to travel about 8 cm from the origin, the plate removed from the tank and the solvent evaporated in a fume hood.

The plate is then sprayed with 0.1 % ninhydrin (1,2,3-triketohydrindene) solution in methanol and placed in an oven at 100'Yu for about 3 min. The ninhydrin reacts with primary amino groups to form intense dark red colors. The R f values of the spots are used to identify the sample by comparison with the standards. Since the fatty amines are mixtures of homologs and isomers, quantitation is rather difficult. A semi-quantitative estimation of these amines may be obtained based on the intensity of the spots ..

8.6 General scheme for the analysis of diarylide pigments

The following analytical scheme has been used successfully to analyze several samples of commercially available diarylide pigments. It must be emphasized


that this scheme may require modification in certain cases, depending on the type and number of additives present.

Step 1. The sample is dried in an oven at about 100°C and the amount of volatile matter is determined. A small amount of moisture is often found.

Step 2. The IR spectrum of the dried sample is obtained and the major pigment component is identified by comparison with reference spectra. The IR spectrum often provides information on the presence of additives. For example, large bands near 2800-3000 cm l

attributed to aliphatic C-H groups signify the presence of aliphatic amines or rosins. A sharp band near 3710 cm 1 indicates the presence of clay.

Step 3. The sample is extracted with methanol or SOD;') methanol/water to remove additives; aqueous methanol is used. since the Schiff's base derivatives offatty amines with diarylide yellow pigments are soluble in methanol. Usually, between 0.5-1.0 g of sample is extracted three times with about 40 ml of solvent using an ultrasonic bath for dispersion. The mixture is then centrifuged and the supernatant liquid is filtered. The solvent is evaporated and the residue is weighed. Analysis of the residue by IR spectroscopy reveals the type of functional group present. Strong absorption bands between 1510-1590 cm 1 and 1400 cm 1 suggest the presence of carboxylate groups (usually from metallic rosinates). Sulfonated anionic surfactants show strong absorption bands near 1200 cm l ; other characteristic bands caused by sulfonate groups occur between 1020-1050 cm I.

Sulfonated anionic surfactants are readily soluble in water and can be isolated by extracting with water. Aliphatic fatty diamines and triamines are not easily detected by IR spectroscopy as they do not have strong characteristic absorption bands. The presence of aliphatic diamines and triamines can be determined by TLC using ninhydrin as the visualizing agent (see section 8.5).

Step 4. If aliphatic fatty amines are present, they may be separated from the other additives by extracting the residue obtained in step 3 with methylene chloride. Metallic rosinates are somewhat insoluble in methylene chloride, but rosin acids are readily soluble and will be extracted together with the fatty amines. The methylene chloride soluble matter is treated with dilute hydrochloric acid in order to convert the amines to the hydrochloride salt. The acid solution is then extracted with ether to remove the rosin acid. Complete identification of the fatty amines can be achieved by acetylation and analysis of the acetyl deviratives by GClMS.

Step 5. The methylene chloride insoluble matter from step 4 may contain metallic rosinates which can be dissolved in methanol and filtered.

(b)

(a)

(b)

(a)


~ 1x105 c til "0

5x104 c :J .0 oCt:

0 50

Q) 3x105 u iij 2x105

"0 c

1x105 :J .0 oCt:

0

41 105 135 -.... -....

100 150 Mass/charge

TIC

4 6 8 10 12 time(min)

14

/ 259 302

/

300

16 18

Figure 8.9 (a) Total ion chromatogram and (b) mass spectrum of a selected mass peak of gum rosin.

Q) 6x1 O~ u 5x10 iij 4X10~ 1? 3x10 :J 2x105 ~ 1x105

55

----91 ,........

100

TIC

187

------

200 MasS/charge

/269

304 /"

300

0~==4~~6~~8~~1~0~~1=2~1=4~~1~6==1~8==~2~0-=~22 time(min)

Figure 8.10 (a) Total ion chromatogram and (b) mass spectrum of a selected mass peak of partially hydrogenated rosin.

After evaporation of the solvent, the methanol soluble material is dried and weighed. The metallic rosinates are analyzed by IR spectroscopy. Complete identification requires analyis by GC/MS in order to distinguish the type of rosin present. For GC/MS analysis, the rosinate salt is first converted to the free acid form or the methyl ester prior to analysis. Figure 8.9 and 8.10 show the total ion chromatogram (TIC) of gum rosin and partially hydrogenerated rosin,


respectively, and the mass spectrum of a major peak from each TIC. Abietic acid, a major constituent of rosins, has a molecular ion of302 as indicated by the mass spectrum in Figure 8.9. The partially hydrogenated rosin has a molecular ion of 304, as shown in the mass spectrum in Figure 8.10.

Step 6. The pigment sample after extraction with methanol or methanoV water is dried at 100°C and analyzed by visible spectrophotometry to determine the pigment content. The difference in pigment content before and after extraction gives an accurate estimation of the amount of additives removed by extraction. If the pigment content after extraction is 99-100%, most of the additives have been removed and no further extraction is needed. If the pigment content is not close to 100% then it is likely that inorganic additives or Schiff's base derivatives are present.

Step 7. To test for the presence of a Schiff's base derivative, the extracted pigment from step 6 is heated with alcoholic concentrated hydrochloric acid at about 80°C. Usually about 200 mg of pigment, 10 ml of ethanol and 10 ml of concentrated hydrochloric acid are used in a 250 ml beaker covered with a watch glass. The time required to hydrolyze the Schiff's base varies with the pigment present. Hydrolysis requires between 30 min and 2 h. A change in the shade of the pigment from orange-yellow to yellow indicates that hydrolysis is nearly complete. After hydrolysis, the mixture is diluted with about 100 ml of methanol and filtered through a membrane. The filtered pigment is washed free of acid with methanol and dried. The pigment content of the dried sample after hydrolysis should be close to 100%, if inorganic additives are not present. The difference in pigment content before and after hydrolysis gives the amount of aliphatic amine present as Schiff's base.

Step 8. If the pigment content after hydrolysis is less than 99% the sample is ashed at 650°C to determine if inorganic additives are present. The ash obtained is analyzed by IR spectroscopy to check for the presence of barium sulfate, titanium dioxide, clay or other materials that absorb in the IR region. The metals present in the ash can be determined by atomic absorption, X-ray fluorescence or other methods. Aluminum and silicon are found if clay is present. Barium, calcium, zinc, sodium and iron are often present. Calcium, zinc, barium and aluminum salts of rosins are commonly used as additives in commercial pigments.

Step 9. The purified pigment obtained in step 7 is analyzed by TLC and IR spectrocopy to determine if small amounts of other diarylide pigments are present. Successful analysis requires that the total amount of volatiles, additives, pigment, and unreacted starting materials add up to 100%.


8.7 Identification of diarylide pigments by reduction

Identification of diarylide pigments by IR spectroscopy requires that reference spectra be available to match the spectrum of the unknown. If known reference standards are not available, identification can be achieved by reduction of the azo groups. This converts the pigment back to the starting aromatic diamine and the amino derivative of the coupler. Zinc or tin with hydrochloric acid and sodium dithionite have been used as the reducing agents. Under the strong acid conditions used, the initially formed amino derivative of the coupler is easily hydrolyzed to the corresponding aromatic amine. The reduction ofC.!. Pigment Yellow 12 is given as an example to illustrate the reaction.

About 25 mg of pigment is refluxed in a mixture of o-dichlorobenzene (10 ml) and concentrated hydrochloric acid (5 ml) in the presence of zinc metal, under a nitrogen atmosphere, until the yellow color disappears. The acid solution is rendered alkaline with sodium hydroxide and extracted with methylene chloride. After clean-up the methylene chloride solution is analyzed by GC/MS or TLC to identify the aromatic amines. 3,3'-Dichlorobenzidine and aniline are the amines obtained from the reduction ofC!. Pigment Yellow 12 (equation 8.5).

I Reflux in o-dichlorobenzene (8.5)

Cl

6' H3C

H2N-b-Q-NH2 CH3 HCl

+

:)-NH2

.. H2Ni:: +

Cl H2O

NH

6

8.8 High performance liquid chromatography of aromatic amines in diarylide pigments

0

The most widely used aromatic amine for making diarylide pigments is 3,3'dichlorobenzidine. Aromatic amines are found in trace amounts in diarylide pigments as a result of incomplete tetrazotization of the starting amine,


usually 3,3-dichlorobenzidine, and also from decomposition of the acetoacetarylide used. These amines can be analyzed both by HPLC and TLC

8.8.1 Determination of 3.3'-dichlorobenzidine in diarylide pigments

The amine is extracted from the pigment by dissolving (or dispersing) about 100 mg of sample in 10 ml of hot o-dichlorobenzene in a culture tube, using an ultrasonic bath. The mixture is extracted three times with 5% sulfuric acid solution by shaking and centrifuging. The sulfuric acid solution is removed, transferred to another culture tube and rendered alkaline with ammonium hydroxide. The alkaline solution is extracted twice with diethyl ether. '[he diethyl ether extract is evaporated and the residue is dissolved in 5 ml acetonitrile. This solution is then analyzed by HPLC using appropriate standards. The HPLC conditions listed here may be modified, if necessary, to obtain the desired separation. This method can be applied to the analysis of other aromatic amines.

Column: LC-18 (octadecylsi1icon) 5 microns particle size, 15 cm x 4.6 mm l.D. or equivalent

Mobile phase: 48% 0.1 M sodium acetate (pH 5.5, containing 0.05% triethylamine) in acetonitrile

Elution mode: isocratic Flow rate: 1.5 m1 min I

Detector: ultraviolet-visible variable wavelength, set at 286 nm.

8.8.2 Determination of2.5- dimethoxy-4-chloroaniline (DMCA) in C. I. Pigment Yellow 83

Trace amounts of DMCA may be present in Cl. Pigment Yellow 83 as the result of hydrolysis of the excess coupler (2,5-dimethoxy-4-chloro acetoacetanilide) used in the coupling process. Aromatic amines derived from other couplers may also be analyzed in a similar way.

About 100 mg of pigment is dispersed in 10 ml of hot o-dichlorobenzene in a culture tube using an ultrasonic bath. After centrifugation, a 5 ml sample of the supernatant solution is removed and extracted with 5% sulfuric acid solution in a culture tube. The sulfuric acid solution is washed once with methylene chloride to remove residual o-dichlorobenzene. It is then rendered alkaline and extracted with diethyl ether. The ether solution is transferred into a vial and evaporated to dryness. The residue is dissolved in 5 ml of 10% 1-propanol in isooctane and analyzed by HPLC

For samples containing surfactants or polymeric additives the following extraction procedure is more efficient. About 1 g of sample is extracted by stirring with 30 ml of 1 N hydrochloric acid in a beaker for 30 min. The extraction is carried out twice and the combined extract is treated as described above prior to analysis by HPLC using the conditions below.


Column: Supelco LC-CN(cyanopropyl), 5 microns particle size, 25 cm x 4.6 mm i.d. or equivalent

Mobile phase 90% isooctane (containing 0.0 I % triethylamine), 10% I-propanol

Elution mode: isocratic Flow rate: 2 ml min-I Detector: ultraviolet-visible variable wavelength, set at 254 nm.

Standards ofDMCA are prepared in 10% I-propanol in isooctane. About 10-20 III sample and standard solutions are injected into the HPLC.

8.9 Gas chromatography of 3,3'-dichlorobiphenyl in diarylide pigments

Diarylide pigments derived from 3,3' -dichlorobenzidine may contain a few parts per million (50 or less) of 3,3' -dichlorobiphenyl, as a result of the inadvertent reduction of the corresponding tetrazonium chloride in the coupling process. Since polychlorinated biphenyls are regulated by the US Environmental Protection Agency (EPA), quantitation of such impurities is essential for control purposes. The 3,3' -dichlorobiphenyl is conveniently extracted from pigments by dispersing about 0.5 g of pigment in 6 ml of hexane in a culture tube and then adding about g ml of concentrated sulfuric acid. It is important to disperse the sample first in hexane as lower chlorinated biphenyls are not very stable in concentrated sulfuric acid. The tube is shaken until all the pigment is dissolved. After dissolution the mixture is shaken for 10 min and centrifuged. The hexane layer is removed and transferred into a 10 or 25 ml volumetric flask. The sulfuric acid solution is extracted two more times with hexane. The combined hexane extract is diluted to volume and dried with anhydrous sodium sulfate. The extraction procedure takes about 40 min. The hexane solution is analyzed by GC with electron capture detector using the following conditions.

Column: fused silica, DB-5, 60 m x 0.25 mm i.d., 0.25 microns film thickness

Detector: Ni63 electron capture Inlet temperature: 265°C Inlet pressure: 60 PSI Temperature program: initial temperature 150°C; hold for 3 min;

programmed to 260°C at gOC/min; final hold 10 min.

Carrier gas: Make-up Gas: Injection mode: Injection volume:

helium, flow rate 1.5 ml min-I argon/methane 95/5, flow rate 45 ml min-1

splitless. 2.5 Ill.


Other columns have also been used such as SP21 00, SP2250, OV -17 and PTE-5. Standard solutions of 3,3' -dichlorobiphenyl in hexane are prepared at concentrations ranging from 0.5-2 ng 1-1. The retention time on SP2100 (0.25 microns film thickness, 15 m long, 0.2 mm i.d.) column is 7.5 min.

Whenever a new manufacturing process is introduced or an unknown sample is analyzed it is recommended that the presence of 3,3 -dichlorobiphenyl be confirmed by GC/MS analysis.

Acknowledgements

The authors wish to thank Hugh Smith (Sun Chemical, Cincinnati, OH) for his useful suggestions, editorial assistance and encouragement; Yun Fang (Sun Chemical, Staten Island, N.Y.) for helping to draw the structures and Isabelle Barrett (Staten Island, N.Y.) for helping to type the manuscript.

References

I. Colour Index (1988) Pigments and Solvent Dyes. 3rd edn, Society of Dyers and Colourists and American Association of Textile Chemists and Colorists. Charlesworth, Huddersfield, U K.

2. P.A. Lewis (1988) Pigment Handbook, Vol. 1: Properties and Economics. 2nd ed. John Wiley, New York.

3. R.B. McKay (l991)J. Oi/Col. Chern. Assoc. 5, 176. 4. K.G. Hargreaves (1952) 1. Oil Col. Chern. Assoc. 35, 139. 5. F. Muzik (1965) Chemicky Prumysl. 15, 151. 6. L.J. Bellamy (1968) Advances in Infrared Group Frequencies. Methuen, England. 7. L.J. Bellamy (1958) The Infrared Spectra of Complex Molecules, 2nd ed. John Wiley, New

York. 8. N.B. Colthup. L.H. Daly and S.E. Wiberly (1964) Introduction to Infrared and Raman

Spectroscopy, Academic Press, New York. 9. A.D. Cross (1960) Introduction to Practical Infrared Spectroscopy, Butterworths, London.

10. J.T. Vanderbeg, D.G. Anderson, J.K. Duffer, J.M. Julian, R.W. Scott and T.M. Vaickus (Infrared Spectroscopy Committee of the Chicago Society for Coatings) (1980) An Infrared Spectroscopy Atlas for the Coatings Technology, Federation of Societies for the Coatings Technology, Philadelphia.

II. R.F. Straub, R.D. Voyksner and J.T. Keever (1993) Anal. Chern .. 65,2131.

Index

acetoacetanilide pigments 17 acid dyes see electron spin resonance; mass spectrometry; microspectrophotometry; X-ray data

anthraquinone compounds see mass spectrometry; near infra-red spectroscopy, solid state NMR; X-ray data

azo compounds see also diarylide pigments; solid state NMR; X-ray data non-polymorphic 15

azo-hydrazone tautomerism see solid state NMR

basic dyes see electron spin resonance; mass spectrometry; microspectrophotometry

bromophenol blue 135

charge-transfer (CT) chromophores cyclophanes 93 Dewar-Knott rule 78 donor/acceptor combinations 93

c.1. Pigment Red 1 69 c.1. Pigment Red 3 69 C.I. Pigment Red 6 69 c.1. Pigment Red 57: 1 69, 72 CP/MAS see solid state NMR CRAMPS see solid state NMR crystal see also X-ray powder diffraction

definition 2 modification 3

diarylide pigments additives

inorganic 190, 192 organic 189 see also rosins

characterization infra-red fingerprints 193-196 visible spectroscopy 197 thin layer chromatography 196, 198

c.1. names and structural information see also X-ray data: disazo compounds

Pigment Orange 14 187 Pigment Orange 15 187 Pigment Orange 16 187, 194, 196 Pigment Orange 44 187 Pigment Orange 63 187

Pigment Yellow 12 187,188,191,193, 196

Pigment Yellow 13 187, 193, 196 Pigment Yellow 14 187,194,196 Pigment Yellow 17 187,194,196 Pigment Yellow 55 187 Pigment Yellow 63 187,194,196 Pigment Yellow 81 187 Pigment Yellow 90 187 Pigment Yellow 106 187 Pigment Yellow 113 187 Pigment Yellow 114 187 Pigment Yellow 121 187 Pigment Yellow 124 187 Pigment Yellow 126 187 Pigment Yellow 127 187 Pigment Yellow 136 187 Pigment Yellow 152 187 Pigment Yellow 170 187 Pigment Yellow 171 187 Pigment Yellow 172 187 Pigment Yellow 174 187 Pigment Yellow 176 187 Pigment Yellow 188 187

determination of aromatic amines 199-200 determination of aliphatic amines 200 determination of unreacted coupler 199 general analytical methods

assay determination 197 chemical reduction 192 elemental analysis 190 gas chromatography 190 HPLC 190 infra-red 190,192-195 mass spectrometry 190 purification 195-196 solvent extraction 192 step by step analysis 200-203 thin layer chromatography 190 visible spectroscopy 190

general structure 186 identification by chemical reduction 204 importance 186-187 infra-red spectra 194, 195, 196 isolation from commercial products

removal of organic additives 192 separation from inorganic additives

192 manufacturing by-products 189

INDEX 209

Schiff's base derivative 191 synthesis 187-188 visible absorption data

absorption coefficients 198 Pigment Yellow 12 197

3,3'-dichlorobenzidine see also HPLC analysis concentration limits in diarylide pigments

189 diazotization and coupling 187-188 reduction product of Pigment Yellow 12 204

structure 188 3,3' -dichlorobiphenyl

by-product of diarylide pigment synthesis 189

extraction and GC analysis 206 diffraction data

international centre for (ICDD) 3 powder data file (PDF) 3 presentation of 5 reliability of patterns 4

dipolar interactions 50 see also solid state NMR

disperse dyes see mass spectrometry; microspectrophotometry; X-ray data

electron spin resonance spectroscopy basis principles

anisotropic effects 121, 126, 127 Bohr magneton 117 Boltzman distribution 119 Brownian translational diffusion 130 effective g-value 120 Heisenberg uncertainty principle 119 hyperfine coupling 120 J -aggregates 123 mutual exchange of spin states 119 proportionality constant 117 rotational correlation time 130 spin angular momentum 117 spin orbital coupling 120 spin relaxation 118, 119, 120 stokes-Einstein equation 130 twist angle 125, 126 Zeeman energy levels 118 see also line broadening

dyes analysed acridines 126-127 I-aminoanthraquinone 129 azonitroxides 130 Cl. Acid Red 88 129 Cl. Basic Blue 3/4 129 Cl. Basic Orange I 129 Cl. Basic Red 2 128 Cl. Basic Violet 14 129 Cl. Basic Yellow 2 129 Cl. Mordant Red 3 129

Cl. Pigment Red 170 124 Cl. Vat Blue 12 124 C.l. Vat Yellow 2 124 merocyanines 122 phthalocyanines 127 Rhodamine 6G 122 Rose Bengal 128 sensitizing and desensitizing 122-124 triarylmethanes 125-135

g-values donor-acceptor complexes 122 free radicals 120 hydrazyl radicals 124 leucocyanides 126 nitroxide radicals 120 pigment combinations 125 transition metals 120

requirements 117 solvent effects 121, 126

emission spectroscopy applications

photodegradation of whitening agents 184

relaxation pathways in whitening agents 181

basic principles see also lablonskii diagram emission polarization 175 fluorescence decay curve 175 fluorescence emission spectrum 174 fluorescent lifetime 178 internal conversion 172 phase angle 178 photon counting 178 quantum yield 173 vibrational relaxation 172

compounds analysed beta-carboline 177 coumarin-type whiteners 184 Leucophor PAF 181-182 pyrazolone-type whiteners 184 rhodamine 110, 177 Uvitex NFW 181-182,183

data fluorescence decay times 183-184 fluorescent lifetime 182 quantum yield 18l

diagrams of spectrophotometers steady state emission 176 time resolved 179

techniques confocal imaging 184 steady state measurements 175-177 time resolved measurements 178-181_

fluorescent whitening agents see emiSSIOn spectroscopy; microspectrophotometry

210

HPLC analysis determination of 3,3' -dichlorobenzidine

205 determination of 2,5-dimethyl-4-chloroaniline 205

Huckel molecular orbital method use in NIR spectroscopy 76

infra-red spectroscopy see infra-red spectroscopy; diary Ii de pigments

lablonskii diagram in emission spectroscopy 171-172 in microfluorometry 157

line broadening ESR 118-119 solid state NMR 50

mass spectrometry BIE linked scanning 105, 106 C. r. d yes anal ysed

Acid Blue 113 111, 114 Acid Green 25 101, 106 Acid Orange 60 101, 106 Basic Yellow 2 99, 112 Disperse Blue 77 103 Disperse Blue 79 107 Disperse Red 1 112

collision activation 100, 106 dye classes vs ionization method 97 fab matrix selection 104, 106 GC/MS 98, 114 ionization methods

chemical ionization (CI) 99 desorption CI and EI 100-102 electron impact (EI) 98 electrospray 112-113 fast atom bombardment (F AB)

103-104,107 field desorption (FD) 102-103 ion spray 113 laser desorption 108 liquid secondary ion 103-104,107 plasma desorption 108 pyrolysis 99 tandem (MS-MS) 100,114-115 thermospray 110-112

isotope exchange experiments 107 LC/MS 99,104,114-115 spectra

BIE linked scan 105 desorption chemical ionization 101 field desorption (FD) 103 negative ion FAB 105 plasma desorption 109 thermospray 110

sulfonated dyes 101, 104

INDEX

microspectrophotometry see also lablonskii diagram anisotropic index 147-148 applications

diffusion coefficients 147 dye diffusion in polymers 144 mechanisms of environmental fading molecular orientation measurements 148-150

C.I. dyes studied Acid Blue 127 153 Acid Red 1 143 Acid Red 18 145 Acid Yellow 73 165 Basic Blue 7 135, 136 Direct Blue 1 145 Disperse Blue 3 151-152 Disperse Red 60 141, 142 Disperse Yellow 23 149 fluorescent brightening agent 113 158 Solvent Orange 15 137

diffusion coefficients 143-146 dye concentration profiles 137 historical utility 133 instrumentation 133-135 in situ analysis 135-137 microdensitometry 134 microfluorometry

calculation of diffusion coefficients 165 detection of surface deposits 162 distribution of UV stabilizers 167 fiber cross-sections 167 fluorescent tracers 157 historical utility 156-159 instrumentation 159, 160 methods 159-162 quantification of fiber degradation 165 quantum efficiency 157

numerical aperture 140 optical distortion 138-143 preparation of cross-sections 138 spectra

c.r. Basic Blue 7 136 bis-dimethylamino fuchson 136 c.r. Disperse Blue 3 152

near infra-red dye systems see also sol vatochromism azomonomethines 93 azulenium dyes 84 azo dyes 91-92 carbodinitriles 93 cyanine dyes 79-85

croconium dyes 81 heptamethinecyanine dyes 81 indoline-type dyes 85 merocyanine dyes 84 monocarcogenopyrlomethine dyes 84

INDEX 211

pyryJium dyes 83 squrylium dyes 81,82

for laser materials 81, 83 f1uorenyl dyes 92 metal-complex dyes 86-89 for liquid crystal displays 87 for optical recording media 81,83,84,87,

92 for organic photoconductors 84 phthalocyanines and naphthalocyanines

89-91 pi-electron density changes 78, 83 potential new applications 94 quinone dyes 85-87 technical properties 81, 90 zwitterionic compounds 93

near infra-red spectroscopy electromagnetic range 75 pi-pi· transitions 88,91,92

Pariser-Pople-Parr molecular orbital method use in NIR spectroscopy 76, 77

polymorphism see X-ray powder diffraction

rosIns mass spectra 202

solid state NMR spectroscopy see also line broadening basic principles

chemical shift anisotropy 52 cross-polarization 53 dipolar decoupling 51 free induction decay 51 Hamiltonian relationship 50, 52 Hartmann-Hahn condition 54 heteronuclear dipolar interactions 50 homonuclear dipolar interactions 50 magic angle 52 quadrupolar splitting 58, 60 shielding anisotropy 63-64, 72 spin temperature 54

dye systems analysed anthraquinones 70, 71 azo-hydrazone tautomerism 60,

68-69, 72 calcion-calcichrom 69 meso-tetra-arylporphines 72 monoazo compounds 68-70 trans-azobenzene 66

methods combined rotation and multi phase

spectroscopy, (CRAMPS) 56 cross polarization/magic angle spinning (CP/MAS) 50-56

dipolar dephasing 56-58,68 two-dimensional CP/MAS 66, 67

pulse sequences

cross-polarization 54 spin diffusion and chemical exchange

67 spectra

13C CP/MAS 56,57,59,61 I'N CP/MAS 60,62-66

spinning sidebands 52, 63 solvatochromism 91,93 solvent dyes see micro spectrophotometry;

X-ray data

trimorphic 14,20,21 see also X-ray powder diffraction

vat dyes see electron spin resonance; X-ray data

X-ray data aminoketones

c.l. Pigment Brown 38 28 c.l. Pigment Yellow \09 28 c.l. Pigment Yellow 1 \0 28

anthraquinones alizarin 28 benzanthrone 29 c.l. Acid Blue 324 29 C.I. Disperse Blue 60 29 c.l. Pigment Orange 43 29 c.l. Pigment Orange 52 30 c.l. Pigment Red 177 c.l. Vat Green I 29 c.l. Vat Green 2 29 c.l. Vat Red 15 29 dibromoanthrone 28 dichloroindanthrone 29 f1avanthrone 29 indanthrone 29 quinalizarin 28 quinizarin 28 vii anthrone 29

azine pigments 39 azoic compounds 22, 23 diketopyrrolopyrrole 40 dioxazines

c.l. Pigment Violet 23 27 C.I. Pigment Violet 27 27

disazo compounds bon acid-based 22 c.l. Acid Orange 156 21 c.l. Direct Red 28 22 c.l. Disperse Orange 29 21 c.l. Disperse Yellow 23 20 c.l. Disperse Yellow 68 20 c.l. Pigment Brown 23 19 c.l. Pigment Red 144 19 c.l. Pigment Yellow 12 19 c.l. Pigment Yellow 13 19 C.I. Pigment Yellow 19 19

212

X-ray data contd c.1. Pigment Yellow 128 19 cyanothiophene-based 21

f1uorans 39 indigo ids

c.r. Pigment Red 88 32 MacQueen pigment 38 mixed crystals 18,27,31 monoazo compounds

c.r. Acid Orange 52 16 c.r. Acid Red 2 16 c.r. Disperse Blue 165 15,31 c.r. Disperse Brown I 10, 17 c.r. Disperse Orange 5 7 c.r. Disperse Red 65 10 c.r. Disperse Red 73 7 c.r. Disperse Yellow 16 16 c.r. Pigment Brown I 16, 17 c.r. Pigment Brown 2 16 c.r. Pigment Brown 25 16 c.r. Pigment Brown 32 16 c.r. Pigment Green 10 16 c.r. Pigment Orange 5 16 c.r. Pigment Orange 36 II c.r. Pigment Orange 60 16 c.r. Pigment Orange 62 16 c.r. Pigment Red I II c.r. Pigment Red 3 16 c.r. Pigment Red 4 16 c.r. Pigment Red 5 16 c.r. Pigment Red 9 17 c.1. Pigment Red 11 16 c.1. Pigment Red 12 16 c.1. Pigment Red 31 12 c.r. Pigment Red 48:2 16 c.1. Pigment Red 48:3 16 c.r. Pigment Red 52 16 c.r. Pigment Red 53: I 15 c.r. Pigment Red 57 16 c.r. Pigment Red 57:2 15 c.1. Pigment Red 57:2 15 c.r. Pigment Red 58 16 c.r. Pigment Red 112 16 c.r. Pigment Red 114 16 c.r. Pigment Red 146 16 c.r. Pigment Red 166 16 c.r. Pigment Red 170 16 c.r. Pigment Red 223 16 c.r. Pigment Yellow I 16 c.r. Pigment Yellow 3 16 c.1. Pigment Yellow 4 15 c.r. Pigment Yellow 5 11 c.1. Pigment Yellow 6 15

INDEX

16 16 16 16 16 16

c.1. Pigment Yellow 10 c.r. Pigment Yellow 60 c.r. Pigment Yellow 65 c.r. Pigment Yellow 73 c.r. Pigment Yellow 74 c.1. Pigment Yellow 98 c.r. Pigment Yellow 129 c.r. Pigment Yellow 151 c.1. Pigment Yellow 154 c.1. Solvent Yellow 2 15 c.r. Solvent Yellow 18 12 c.r. Solvent Yellow 56 15

16-17 16 16

naphthazide-type pigment 39 nickel dioximates 38 nitroso and nitro compounds

c.1. Disperse Yellow 42 3, 6 c.1. Pigment Brown 22 6 c.r. Pigment Green 8 6

optical brighteners 22-24 perylenes

c.1. Pigment Red 149 31-32 c.r. Pigment Red 178 31 c.r. Pigment Red 179 31

phthalide pigments 39 phthalocyanines

c.r. Pigment blue 15 34 c.r. Pigment Blue 16 32 c.1. Pigment Green 7 35 cobalt complex 35 nickel complex 35 oxytitanium complex 36-38 tau form 33 X-forms 32

polymethines 40 q uinacridones

c.1. Pigment Red 122 26,27 C.I. Pigment Red 207 27 c.r. Pigment Red 209 26 6,13-dihydro 27 N,N'-dimethyl 27 gamma forms 25 mixed crystal 27

rhodanines 40 see also diffraction data

X-ray powder diffraction advantages and disadvantages 2-4 basic concepts 1-2 Bragg's law I interplanar spacings 1,2 polymorphism

definition 2 in monoazo compounds 7-15

zeta pattern 26

Analytical Chemistry of Synthetic Colorants

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