Solution infrared spectroelectrochemistry: A review

Talanra, Vol. 38, No. 11, pp. 12094218, 1991 0039-9140/91 s3.00 + 0.00 Printed in Great Britain. All rights reserved copyright Q 1991 Pcrgamon Press pk

SOLUTION INFRARED SPECTROELECTROCHEMISTRY: A REVIEW

KBvINAsHLEY

Department of Chemistry, San Jose State University, San Jose, California 95192, U.S.A.

(Received 30 November 1990. Revised 20 February 1991. Accepted 13 March 1991)

Summary-Infrared (IR) spectroelectrochemical techniques have seen extensive use in studies of electrode surface. processes. They have also been employed, albeit Css frequently, to investigations of redox species dissolved in solution. The application of IR spectroscopy to electrochemical solution processes represents a special challenge, for absorption of IR radiation by the solvent is a sign&ant interference to detection of vibrational modes of dissolved analytes. It is also di5cult to maintain potentiostatic control of the system in specially designed thin-layer spectroelectrochemical cells. Solution IR spectroelectrochemical experiments are important for investigations of redox systems in which it is desired to spectroscopically monitor the structures of dissolved products, intermediates and reactants involved in electrode reactions. Such experiments have been conducted on biochemical, inorganic, organic, and other systems. In this paper some examples of applications of IR spectroelectrochemical studies of solution species in the above areas are presented, and experimental aspects are discussed.

In situ spectroscopic techniques for studying electrode processes were first introduced in the 1960’s in an effort to provide a better means to elucidate electrode reaction mechanisms and kinetics. These first experiments employed W- visible spectroscopy coupled with an electrochemical cell,’ and were used to monitor solution concentrations of redox species dissolved in solution. By using a W-visible probe, it became possible to follow the time- and potential-dependent spectra of reactants, intermediates, and products of electrode reactions. Species in the diffusion layer could be monitored spectroscopically, and therefore it was possible to elucidate electrode reaction mechanisms and kinetics; investigations of this kind were previously much more difficult without the advantage of an optical probe. A tremendous amount of activity in the area of W-visible spectroelectrochemistry followed for the next several years, *s3 but the experimental emphasis later shifted to phenomena occurring at the electrode surface. Since W-visible spectroelectrochemical methods are largely insensitive to surface species, other techniques were required for surface studies. This change in emphasis led to the development of surface infrared spectroelectrochemistry, as well as other surface-sensitive methods. Much less emphasis has been placed on infrared studies of solution species, but in the past five years or so a few examples of such investigations have appeared.

The past decade has seen numerous advances in the development of infrared (IR) spectroscopy as applied to electrochemical systems. Much of the progress in this field has been due to the development of the Fourier transform instrument, which was unavailable during the first W-visible experiments in the mid-1960’s. Most of the effort has focused on the characterization of electrode surfaces by IR techniques,4as since surface phenomena are supremely important in electrochemical systems. These surface infrared studies have the advantage of being conducted in situ, and have served to comp- lement other in situ surface spectroscopic probes such as surface-enhanced Raman scattering (SERS),6 optical second-harmonic generation (SHG),’ and x-ray methods.’ These in situ techniques are seen as having an advantage over ex situ methods which require that the spectroscopy be performed in air, gas, or in ultra- high vacuum (i.e., outside the electrochemical cell). The reason for this is that in an ex situ experiment the electrode environment might be different from its environment in situ. In ad- dition to being an in situ technique, IR spectroelectrochemistry also has an advantage over other in situ spectroscopic techniques in that experiments can be performed on either smooth or rough electrodes, and a great deal of information concerning identities and orientation of species at electrode surfaces is obtainable. Thus IR spectroscopy, as applied to electrode sur-

1209

1210 KEVIN ASHLEY

faces, has emerged as a very powerful probe of electrochemical surface processes.

Applications of IR spectroscopy to electrochemical processes occurring in solution (i.e., in the diffusion layer near the electrode surface) have been relatively few when compared to surface studies. The principal reason for this is that the strong absorption of IR radiation by the solvent can interfere with the absorption by the dissolved redox species. Studies of solution processes in electrochemical systems are extremely important in such fields as organic, inorganic, and biological chemistry. Other spectroscopic techniques to monitor solution electrochemical processes in situ have been limited to methods such as W-visible spectroelectrochemistry (mentioned above),9 electron spin res- onance (ESR),‘O and Mossbauer spectroscopy.” IR spectroscopy offers an advantage over these and other spectroscopic methodologies in that much more detail regarding molecular structure of products, intermediates and reactants of electrode processes is attainable when IR radiation is employed as a spectroscopic probe. While ESR and Mossbauer techniques can give detailed structural information in many cases, it is generally difficult to conduct these types of experiments on electrochemical systems. W-visible spectroelectrochemical methods are much more straightforward, but other spectro- scopies (such as IR) are better suited for provid- ing structural information regarding species involved in electrode-mediated reactions.

In this paper, some examples of applications of IR spectroelectrochemistry to studies of redox species dissolved in solution are presented. Also, considerations on the design of IR spectroelectrochemical cells for solution studies, as well as in&mental considerations, are presented. The systems chosen for discussion represent examples of solution IR spectroelectrochemical investigations in the areas of inorganic, organic and biological chemistry.

CELL DESIGN AND INSTRUMENTATION

Two principle IR spectroelectrochemical cell designs which have been employed for investigation of solution redox species in electrochemical systems are external reflectance and transmission cells. It is also possible to use internal reflectance, but this sampling technique is generally not feasible owing to the low electri- cal conductivities of available internal reflection elements. A special internal reflection IR spec-

troelectrochemical cell design has been demonstrated to be quite useful for surface studies,12 but applications of this design to solution studies are not recommended due to the possibility of interference from surface adsorbed species. Prime considerations in the design of a useful cell for solution applications include high electrode conductivity and optimized throughput of the IR radiation through the solution chamber of the electrochemical cell.

External reflection cells

The external reflection IR spectroelectrochemical cell is amenable to investigations of solution redox species. This cell, which was first used about a decade ago,13 has seen a number of refinements and improvements.‘4-‘7 Figure 1 shows the design of a reflectance IR spectroelectrochemical cell. I4 An IR transparent window is mounted at one end of the cell; this window may be flat or bevelled (as shown) to minimize reflection losses” The working electrode, which is usually a flat metal disk polished to a mirror finish, is positioned close to the window. For surface IR applications the mirror electrode is pushed up against the window, and a thin solution layer several microns thick is trapped between the electrode surface and the back side of the window. However, for solution applications the electrode is pulled back so that a thicker “thin layer” of electrolyte solution, say 50 microns thick, exists between the metal surface and the IR window. Incident radiation passes through the window and the thin solution layer, strikes the electrode surface and is then reflected back out of the cell to be focussed on a detector. The external reflection geometry serves to minimize absorption of IR radiation

Reference electrode

IR-transphrent window

Fig. 1. Configuration of an external reflectance cell for IR spectroelectrochemistry. Reproduced from Ref. 4.

Solution infrared spectroelectrochemistry: A review 1211

by the solvent, while maxim&g the sampling efficiency of absorbing analyte species present within the diffusion layer near the electrode surface.

The IR window material chosen depends on the spectral region of interest and on the solvent. For mid-IR applications calcium fluoride, silicon, or zinc selenide windows are used most often, while polyethylene or Mylar window materials are used in the far-IR region.

The cell housing can be comprised of inert materials such as g1ass2*16*‘8*‘9, Teflon’7*p, or Kel-F14*2*. It is important that the cell material be inert so that the solution does not become contaminated. The electrode is mounted onto the end of a metal rod which is generally sheathed in glass, Teflon, or Kel-F so that the connecting metal rod is insulated from solution. This rod is then inserted into the cell body so that the polished mirror electrode may be positioned as desired (Fig. 1). A Luggin capillary reference probe or microreference electrode is positioned near the edge ( N 1mM) of the working (mirror) electrode. The secondary (counter) electrode is usually a platinum wire which is placed behind the working electrode in a configuration that minimizes the solution resistance. The geometry of the secondary electrode is circular (see Fig. 1); this gives a symmetric current distribution pattern to the working electrode. Inlets and outlets for de-gassing of the cell must also be provided.

Adaptations to the basic reflectance cell design described above have provided for control of temperature and maintenance of the cell within an evacuated sample chamber.20 Another cell design enables the use of a carbon dioxide laser so~rce.~ Flow cells have also been con- trived so that one may control the flow of electrolyte through the thin layer;‘6s’7 such cells have applications in both surface and solution studies.

Transmittance cells

Just as in the external reflectance cells described above, the design for a transmittance IR spectroelectrochemical cell is based on the re- quirement of a thin solution layer. Such cells are known as optically transparent thin-layer electrochemical (OTTLE) cells, and are based on a “sandwich” design in which an optically transparent electrode (OTE) is positioned between two IR-transparent windows.23 Transmittance cells offer an advantage over external reflection cells in that the cell may be placed directly

within the sample chamber of most conventional IR spectrometers. External reflection cells usually require additional optics that allow for the infrared beam path to be directed onto the face of the working electrode, which is positioned outside the sample chamber or within a specially modified sample chamber. This has three disadvantages. First, throughput is decreased since the IR beam must be reflected off a minimum of at least two additional mirrors, and much light is lost due to reflection losses. Second, alignment of the IR beam is much more difficult if a reflection geometry rather than transmission cell is used. Third, there is added expense since additional optical components must be purchased.

Figures 2 and 3 show two recent designs for thin-layer transmittance cells for use in IR spectroelectrochemistry;a*2s these cells are similar in design to the transmittance cells originally used in UV-visible spectroelcctrochemistry2*3 and in some of the first IR spectroelectrochemistry experiments .23 Other similar designs for solution IR applications have also been re- ported2C29, some requiring the use of O-rings and spacers to sandwich the cell together. Both designs shown in Figs. 2 and 3 avoid the use of adhesives and O-rings, which can cause cell contamination and/or leakage. Besides these advantages, the cells are reusable and robust, and are relatively inexpensive to fabricate. In all cases it is essential to employ a three-electrode design, since it is crucial that solution resistance

2cm

(I) (2)

Fig. 2. (1) Side and (2) end views of a glass (Pyrex) thinlayer IR transmittance spectroelectrochemical cell with silicon windows. (A) Port for Pt gauze OTE; (B) ports for reference (saturated calomel) and secondary (Pt foil) electrodes; (C) Si windows; (D) Teflon stopcock assembly; (E) direction of incident light beam. Reference and secondary electrode ports are omitted from the side view, and the Teflon stopcock from the end view, for the sake of clarity. Repro- duced with permission from the American Chemical Society

from Ref. 24.

1212 KEVIN hUEY

Top view

Front view

Electrode

\( i 1

c”

Fig. 3. Schematic illustration of a transmittance IR spectroelectrochemical cell, fabricated from potassium bromide, with a platinum optically transparent electrode. (i) KBr cell body; (ii) Teflon cell cap; (iii) Tefzel insulating film; (iv) F’t gauze working electrode. W: working electrode lead; C: counter electrode; R: Ag wire reference electrode. Repro- duced with permission from the American Chemical Society

from Ref. 25.

in the thin layer be minimized. The reported percent transmittance attainable with these IR OTTLE cells is as high as 50%,30 representing a very high throughput. The superiority of newer transmission cells partially results from the fact that the measurements were conducted on more favorable systems than those studied early on. For example, organometallic species dissolved in methylene chloride30 are more easily studied by IR than is ninhydrin in wateti3 is, especially when an FTIR instrument is made available.

Instrumental considerations

For the most studies Fourier transform instruments are employed for solution IR spectroelectrochemistry owing to the multiplex and throughput advantages of FTIR.3’ Usually fast scans are required since thin-layer electrochemistry is fraught with inherent instabilities. In many cases electrode reactions within a thin- layer cell can be subject to poisoning by the window material; this is especially a problem with Z&e, which can foul electrolyte solutions rather quickly. Polarization of the electrode at high applied potentials for long periods can also lead to the formation of undesired impurities and, in some instances, films on the electrode surface. Electrolyte depletion within the thin layer may also be a problem if electrodes are held at high potentials for long periods,32,33

although this is less of a concern in solution applications than in surface studies where the thin layer is much thinner. For these reasons it is desirable to obtain IR spectroelectrochemical spectra as quickly as possible. Signal averaging of numerous scans is virtually always necessary in IR spectroelectrochemical experiments.

Both absolute transmission spectra and difference spectra can be obtained by solution IR spectroelectrochemical techniques. Transmit- tance spectra may be obtained at desired applied potentials, and difference spectra may be plotted which show the ratio of absolute spectra taken at different potentials. Difference spectra are often used so that spectral interferences from species whose concentrations do not change with potential (e.g., solvent, electrolyte, atmos- phere, etc.) are effectively cancelled. It is also possible to obtain time-dependent spectra, conduct spectrocoulometric titrations, and perform other experiments as well. In surface IR applications, usually potential-dependent difference spectra are required, although it is possible to obtain absolute spectra of surface species at a single potential by the technique of polarization modulation.* For solution IR applications no light polarization is required. In surface IR spectroscopy of electrodes’ [at least for potential-dependent IR spectroscopy (PDIRS)“], the light must be p-polarized. The lack of the neces- sity for a polarizer is a decided advantage, since polarizers are rather expensive and also serve to attenuate the infrared beam. Solution species can be distinguished from surface species by employing s-polarized radiation.14 However, since in most cases the signal from surface species is much weaker than that from solution species, polarization of the incident radiation is not typically necessary for solution IR applications. It should also be mentioned that in cases where semiconductor electrodes are employed, the field strengths of p- and s-polarized components near the surface are similar, so there is clearly no benefit in using polarized radiation in this situation to determine between surface and solution species.

APPLICATIONS

Solution IR spectroelectrochemistry has been used to study electrode reactions involving inorganic, organic, organometallic, and biological systems. Such investigations have also been done in conjunction with surface IR spectroelectrochemistry in efforts to elucidate the identities


of reactants, intermediates, and products of electrode reactions both on the electrode surface and within the diffusion layer. In this section a few examples of studies in which solution IR spectroelectrochemistry has been employed in the above areas are presented, and the utility of the technique is discussed.

Organic systems

The earliest applications of IR spectroelectrochemistry were in studies of the redox behavior of conjugated and aromatic organic molecules 23,35*36. Early solution studies employed IR- OTTLE cells,23 but the electrochemical characteristics of the thin layer cell were poor and spectral quality was substandard due to a number of factors, not the least of which was the unavailability of FTIR instruments. More recent thinlayer cell designs demonstrate much better cell characteristics than were previously attainable. Thin-layer cells were originally subject to severe potential drop problems caused by high solution resistance, but newer thin-layer cells are designed to alleviate effects of solution resistance within the thin layer as much as possible. Furthermore the advent of FTIR instruments enabled spectra to be obtained much faster than was possible with dispersive IR spectrometers.

An OTTLE cell was employed to study the reduction of tetracyanoquinodimethane (TCNQ) to its radical anion and dianion in the aprotic solvent acetonitrile:24

TCNQ + e- = TCNQ-

TCNQ- + e- = TCNQ2-.

Both electron transfers are nearly reversible, and the radical anion and dianion are stable on the experimental timescale. Figure 4 shows the cyclic voltammetry of this system in the thin- layer transmittance cell illustrated in Fig. 2. The two cathodic waves are well separated and show rather large peak separations compared to the voltammetry that is obtainable in bulk sol- ution3’. However, it is important to emphasize that the quality of the voltamperogram is excel- lent, considering that the thin-layer geometry results in increased solution resistance. The effects of solution resistance in this cell are minimized by careful placement of the working and auxiliary electrodes.

Figure 5 shows potential-difference IR spectra produced by stepping the Pt gauze OTE through the first and second reduction waves.

I I I a5 QO -0.5

Fbtential, V w SCE

Fig. 4. Thin-layer cyclic vohampcrogram of a solution of 5mM TCNQ in acetonitrile (O.lM tetraethylammonium perchlorate supporting electrolyte) obtained in the cell described in Fig. 2. Potentials are us. SCE, and the sweep rate was 5mV/sec. Reproduced with permission from the

American Chemical Society from Ref. 24.

The upper spectra correspond to the first reduction wave (due to the formation of anion radical from neutral TCNQ), while the lower spectra correspond to the second reduction (dianion formation from the radical anion). Nega- tive bands are due to the disappearance of species present at the base (reference) potential,

I

2135

Wavenumbcrs,cm-t

Fig. 5. Potential difference IR spectra obtained during reduction of TCNQ. Upper spectra (referenced to + 0.5V): E = 0.295,0.255,0.210, and 0.130 V us. SCE. Lower spectra (referenced to -0.15 V): E = -0.250, -0.315, -0.350, -0.400, -0.450 V. Reproduced with permission from the


1214 KE-vlNAsHLEY

while positive bands are due to species prevalent at the sample (pulsed) potential. The upper spectrum of Figure 5 shows the disappearance of a weak IR band due to neutral TCNQ, and the appearance of two strong bands due to TCNQ-; the spectral features are due to normal C%N stretching vibrations. The observation of significantly greater extinction coefficients for radical anions than for neutral reactant species has been made previously for aromatic andfor conjugated organic molecules such as an- thracene and tetracyanoethylene (TCNE)3s,39. The lower spectra show the appearance of positive bands due to TCNQ2-, concurrent with the disappearance of bands attributed to TCNQ-. The quality of the different spectra are quite good, and are in agreement with previous stud- ies40. This work illustrates the benefits of the thin-layer IR spectroelectrochemical cell design shown in Fig. 2, which has also been employed in solution IR studies of redox reactions in molten salts4’. This cell can therefore be used in severe solution environments. The novel application to molten salt electrochemical measurements is a demonstration of the applicability of solution infrared spectroelectrochemistry to a variety of diverse and previously unforeseen systems. Other app~cations of these techniques in materials science and engineering may also be forthcoming.

The external reflection technique has been used to study the ion pairing reactions of the dianions of TCNE and TCNQ mentioned above.@ A polished mirror electrode was used, and the electrode surface was pulled back from the window to give a thin layer about 50 microns across. PDIRS experiments were then performed by pulsing between potentials at which different species involved in the reduction of TCNE or TCNQ were predominant. Spectra were collected at pulsed and reference potentials, and ratioed as described previously.

The IR bands due to TCNE2- and TCNQ’- ((3=,N stretching modes) were found to be dependent upon the supporting electrolyte. In acetonitrile the C=N stretching frequencies of the dianions were observed to shift to higher wavenumhers in the presence of alkali metal salts than in the presence of tetraalkyl- ammonium salts. Figure 6 shows representative IR difference spectra (in the C%N stretching region) obtained during the electroreduction of TCNE to TCNE- at a Pt mirror electrode. While there are differences in reflectance intensi- ties for spectra obtained in lithium perchlorate

a 2 a

Y TBAF

L 24ot

W~enurnb~, cni’

Fig. 6. Potential differential spectra in the C-N stretching region for the reduction of TCNE to TCNE at a Pt mirror electrode. Spectra were obtained with an external reflection cell similar to the cell described in Fig. 1. Solutions consisted of 5mM TCNE in CH,CN with either O.lM LiClO, (top spectrum) or 0.1 M TBAF (bottom spectrum) as background electrolyte. The potential was modulated from + 0.5 V to -0.5 V vs. Ag/O.OlM Ag+ reference. Reprinted with per-

mission from Elsevier Science Publishers from Ref. 40.

tetrabutylammonium tetrafluoroborate y&AF) electrolyte, there are no differences in band frequencies for the two bands observed. Figure 7 shows IR difference spectra obtained during the reduction of neutral TCNE to its

a 2 a

2

LiCQ

TBAF

Wavenumbers, cm-’

Fig. 7. PDIRS results (obtained in the C-N stretching region) for the reduction of TCNE to TCNE- at pt; solution ~nditions and reference electrode are the same as in Fig. 6. Top spectrum: 0. I M LiClO, as supporting ehctro- lyte; potential modulated from + 0.5 to - 1.2 V. Bottom spectrum: O.lM TBAF as supporting electrolyte; potential modulated from + 0.5 to -0.2 V. Reproduced with per-

mission from Elsevier Science Publishers from Ref. 40.


dianion. In this case a clear shift in the frequencies of the CN stretching modes is observed, suggesting environmental differences in the two electrolytes. Similar effects were seen in PDIRS results for the TCNQ system.

The observed frequency shifts seen during the formation of the TCNE’- species were attributed to ion pairing between the dianion and the alkali metal cation. No shifts of this kind were observed, however, for the radical anion species. The observed frequency shifts (or ab- sence thereof) correlated well with observed voltammetric behaviour, i.e., if ion pairing was observed, the voltammetry indicated shifts in the value of the half-wave potential for the radical anion ( = = ) dianion redox couple. This work was one of the first applications of solution IR spectroelectrochemistry to studies of ion pairing interactions between solute anions and cations dissolved in solution.

Other studies of organic systems have dealt with features from both the solution within the thin layer as well as the electrode surface.42*43 Reflectance cells are necessary for such investigations, since surface features can only be eluci- dated from a bulk electrode surface and not from an OTE. In one study it was possible to use real-time FTIR spectroscopy to monitor the temporal behavior of electrode reactions on a voltammetric timescale.

Inorganics and organometallics

A number of solution IR studies have focussed on inorganic and organometallic redox systems. Several investigations have focused on the ferro-ferricyanide redox couple.‘7*40,45 This redox couple is used routinely in electroanalysis as an example of a well-behaved, ‘totally reversible’ couple in most solvent and electrolyte systems. Surface IR spectroelectrochemical studies of this system, however, have shown that adsorbed intermediates exist on the electrode surface, suggesting that the electron transfer reaction is not a simple process involving the formation of an activated transition state com- plexU. These results confirmed radiotracer measurements which indicated the presence of irreversibly adsorbed hexacyanoferrate and cyanide ions at platinum electrode surface@. Solution IR spectroelectrochemical studies (employing an external reflectance cell) of the ferro/ferricyanide system in aqueous electrolytes showed that the C = N stretching frequency of the ferrocyanide anion (formed as a result of reduction of the ferricyanide species) was depen-

Table 1. C E N stretching infrared frequencies for various hexacyanoferrate complexes4

Complex Wavenumbers, cm-’

Fe(III)(CN)~- 2116 K,Fe(II)(CN)~‘-“)- K,HFe(II)(CN)g-“‘- z K, H, Fen-“)- 2067 K Li Fe(II)(CN)$4-“-m)“- K: Mim Fe(II)(CN)&‘-“-=“)-

2040 2038

K,, La, Fe(II)(CN#-n-ti)- 2064

dent on the pH and on the identity of the supporting electrolyte cation. A mechanism was proposed for the overall redox process:

M, Fen-“)- + M +

= [M,, , Fe(II)(CN)f-“)-I*

[M, + , Fe(II)(CN)&3-“)-]*

= [M,, , Fe(III)(CN)g-@- ]* + e-

[M,, , Fe(III)(CN)~2-“)-]*

= Fe(III)(CN)i- + (n + l)M +

where M+ is an electrolyte cation, and the superscript * denotes an activated complex. Table 1 lists the various solution complexes that were detected in the IR spectroelectrochemical study, along with their observed C=N stretching frequencies. This study shows how solution IR spectroelectrochemistry is useful for detect- ing dissolved redox species in the diffusion layer which are not otherwise detectable.

Other studies have been concerned with monitoring the redox chemistry of various rhodium organometallic complexes by solution IR spectroscopy. 25,28 Transmittance spectroelectrochemical cells were used for this work. In one study25 the cell shown in Fig. 3 was employed. The cyclic voltammetry of the ferro-ferricene redox couple was first studied in this cell. At slow sweep rates the peak separation between cathodic and anodic peaks was extremely narrow, indicating a small solution resistance across the thin layer. Subsequently, redox systems of interest were investigated in the cell. A case in particular is that of Rh,(ap), (C-=CH) in CH2C12 system, where ap = 2- anilinopyridinate. The neutral compound can either be oxidized or reduced reversibly through one-electron transfers, and the product of the first oxidation can in turn be oxidized to a dication:47

Rh,(ap),(C--CH) + e -

= [Rh2W4(~W1-

1216 KEVIN hHLEY

= [Rh,(ap)&=CH)l + + e -

= [RhZ(ap),&=CH)]2+ + e-

The cation and anion are stable on the experimental timescale, while the dication is unstable and rapidly forms decomposition products.

PDIRS data were obtained for the first oxidation and reduction waves of the neutral rhodium species in CH2C12 containing O.lM tetrabutylammonium perchlorate (TBAP) (Fig. 8). The band at 1954cm-’ is due to the C=C stretching mode of the neutral compound, as indicated by the transmission IR spectrum obtained at O.OV us. Ag/Ag+ [Figure 8(a)]. Difference spectra taken during reduction and oxidation [Figures 8(6) and (c), respectively] indicate a Cc band shift to 1922 cm-’ for the

“1954 cm+

‘1954 cm-’

lr ,I922 cm-’

‘1954 cm-1

L

I I I I I

2100 2Coo lsoo I800 Wavenumber, cm-l

Fig. 8. FTIR spectra of [Rh,(ap)@&H)~ (where x = + 1, 0, or - 1) complexes in 0.1 M TBAPjCH, Cl,. (u) Initial spectrum of the neutral complex; applied potential = 0.0 V us. Ag/Ag+. (b) Difference IR spectrum of neutral and singly reduced species; applied (sample) potential = -0.8 V, reference voltage = 0.0 V. (c) Difference IR spectrum of neutral and single oxidized species; applied potential = + 0.7 V, reference potential = 0.0 V. Repro- duced with permission from the American Chemical Society

from Ref. 25.

anion and a shift to 1972 cm-’ for the cation. While not stated in the original study,25 the shift to lower wavenumber (of the anion) is probably due to higher occupancy of antibonding x* orbitals of the c---=C moiety, which causes an overall decrease in bond order and a corre- sponding decrease in frequency. For the cation, a shift to higher frequency (with respect to neutral species) is attributable to decreased e- donation to the Rh metal center, which results in an increase in bond order of the C%C bond. This work demonstrated the utility of an inexpensive, easily fabricated, and reusable OTTLE cell for solution IR applications. It was deter- mined, however, that use of a thin-layer transmittance cell is not reliable for quantitative purposes in spectral regions where solvent and/or electrolyte absorbance is strong.

Another study concerned the IR spectroelectrochemistry of the Creutz-Taube ion, [Ru(NH,),],pyrazine]’ + , in aqueous solution.@ Near- and mid-IR spectroelectrochemical studies of this ion revealed a low frequency electronic transition, which had been postulated previously based on results from other spectroscopic studies. It was shown that each ruthenium center of the Creutz-Taube ion bears a charge of + 2.5, indicating that on the vibrational timescale the odd electron is fully delocalized between the two metal centers. This work represents the first application of spectroelectrochemical techniques to the study of low energy electronic transitions of mixed- valent compounds.

Several other investigations have been reported20.26,2W’ concerning the redox chemistry of organometallic compounds. Other work has focussed on potential-dependent dynamic processes within the thin layer.W It is clear that from the evidence of these few successes, solution IR spectroelectrochemical methods ought to be applicable to studies of many organometallic and inorganic redox systems.

Biological systems and ‘model’ biosystems

A few IR spectroelectrochemical studies have been concerned with systems of biological interest. In one investigation, an OTTLE cell was used to characterize the electro-oxidation of a series of selected metalloporphyrins.30 Metallo- porphyrins form the central redox sites in a number of biomolecules, including heme pro- teins, chlorophyll, and the cytochromes, to name a few. Analysis of the potential-dependent IR difference spectra of certain metallopor-


phyrins allowed the site of electroxidation on the metalloporphyrin molecule to be deduced.m Furthermore, it was possible to determine the oxidation state of the electrogenerated species and the fate of any axially coordinated NO or CO group. Additionally, it was found that other axial ligands oriented tram to the electrooxidized metalloporphyrin could be spectrally monitored. Such a degree of spectral characterization had not previously been obtained (for electro-oxidized compounds in solution) prior to the advent of solution IR spectroelectrochemistry.

The redox behavior of (TPP)Co(NO) (where TPP is the dianion of tetraphenylporphyrin) has been studied by a number of electrochemical and spectroscopic techniques, including in situ FTIR spectroelectrochemistry in an OTTLE cel150 This metalloporphyrin can undergo two one-electron oxidations and three one-electron reductions in CH,Cl, solvent,” but the third oxidation is observed only at very high potential scan rates or at low temperature. The voltammetry of the (TPP)Co(NO) system, presented in Fig. 9, clearly shows the first two oxidations and reductions, which are reversible at a microelectrode. IR spectroelectrochemistry was employed to study the first two oxidations of

LF (b)

I I I I I I I I I_ 1.5 IO 0.5 00 -0.5 -1.0 -1.5 -20 -25

E,Vw.SCE

Fig. 9. (a) Top: cyclic voltamperogram (scan rate = 10 V/arc); and bottom: steady-state voltamperogram (scan rate = 50 mV/sec) of (TPP)CO(No) at a 25-micron diameter Pt microelectrode in O.lM TBAP/CH,Cl, . (b) Conventional cyclic voltamperogram (scan rate = 100 mV/sec) of (TPP)Co(NO) in 0. IM TBAP/CH,Cl, . Potentials are us. the Ag/AgCl reference. Reproduced with permission from the


i

! 1664

&IL I (a)

I

i 1726

i I

lb)

I c I I

0 I800 1600 1400

Wavenumber,cm-’

Fig. 10. Solution PTIR difference spectra of (fPP)Co(NO) species. (a) Spectrum of neutral species dissolved in 0.M TBAP/CH,Cl,; reference spectrum is that due to blank electrolyte. (b) Spectrum obtained after controlled potential oxidation at + 1.1 V; referenced potential =O.O V. (c) Spectrum obtained after controlled potential oxidation at + 1.4 V. Reproduced with permission from the American

Chemical Society from Ref. 50.

(TPP)Co(NO). Figure 10 shows the in situ IR difference spectra taken (in the N-O stretching region) as a result of pulsing to potentials positive enough to effect the first two oxidations. The spectra indicate that the NO moiety remains associated with the singly oxidized species, but dissociates from the dication at a long time scale. Evidence for this dissociation had not been determinable previously.

IR spectroelectrochemical studies of redox biosystems have appeared. A recent study was concerned with the redox chemistry of bacteri- ochlorophylls and bacteriophytins.‘* IR spectroelectrochemical studies allowed detailed structural analyses of the redox states, and provided for implications concerning the bind- ing of pigments with the reaction centers of photosynthetic bacteria.

CONCLUSION

It is apparent that while surface IR spectroscopy of electrode surfaces is presently a well-developed methodology, solution IR spec-

1218 KEVIN &HLEY

troelectrochemistry needs expansion. Compara- tively few studies have been conducted in which solution IR spectroelectrochemistry was used as a tool. This is rather surprising, considering the wealth of detailed structural, dynamical, and other information that can be gleaned from IR studies of redox species which are dissolved in solution. Numerous advances in this area have been made recently owing to improvements in spectroelectrochemical cell design and instrumental sensitivity. In the next few years it is probable that many redox systems of interest will be investigated with solution IR spectroelectrochemical techniques, and much more elegant experiments than those described in this review are anticipated.

Acknowledgements-This work was supported by the donors of the Petroleum Research Fund (administered by the American Chemical Society) and Research Corporation. The author thanks Dr. Diane Parry for helpful discussions.

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Solution infrared spectroelectrochemistry: A review

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