9 951 R-USALE THIN-L AYR SPECTROELECTROC E NCAL CELL FOR IllNOW-AQUEOUS SOLVENT SYSTEMS(U) YORK UNIV TORONTO(ONTARIO) DEPT OF CHEMISTRY U A NEVIN ET AL. NOV *?

UNCASSIFiE TR-19 N@99 14-4-6-021F/G?7/4 NL

7LoEmohhohho..ohEEl.omomoo

IIA

I MICROCOPY RESOLUTION TFSI CHART

qb I ..

% %~ ~ W~5#~ -- ~.-,-- 0%

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|n OFFICE OF NAVAL RESEARCH

0)to Contract N00014-84-G-0201

Task No. 0051-865

Technical Report #19

Re-Usable Thin-Layer Spectroelectrochemical Cell forNon-Aqueous Solvent Systems

By

W. Andrew Nevin and A.B.P. Lever*

in

Analytical Chemistry

York University DTICDepartment of Chemistry, 4700 Keele St., North York ELECTE

Ontario, Canada M3J IP3 S DEC 1 4 1987 0

UReproduction In whole, or in part, is permitted for any purpose of the UnitedStates Government

*This document has been approved for public release and sale: its distribution isunlimited

*This statement should also appear in Item 10 of the Document Control Data-DD form1473. Copies of the form available from cognizant contract administrator

87 12. 3 0o9

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2b. DECLASSIFICATIONDOWNGRADING SCHEDULE As it appears on the report

4. PERFORMING ORGANIZATION REPORT NUMBER(S) S MONITORING ORGANIZATION REPORT NUMBER(S)

Report # 19

6a. NAME OF PERFORMING ORGANIZATION 6b OFFICE SYMBOL 7a NAME OF MONITORING ORGANIZATION

A.B.P. Lever, York University (If applicable) Office of Naval ResearchChemis try Department I

6c. ADDRESS (City, State, and ZIP Code) 7b. ADDRESS (City, State, and ZIP Code)4700 Keele St., North York, Ontario M3J 1P3 Chemistry Division

Canada 800 N. Quincy StreetArlington, VA 22217 U.S.A.

Sa. NAME OF FUNDING /SPONSORING 8b. OFFICE SYMBOL 9 PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERORGANIZATION (If applicable) N00014-84-G-0201

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PROGRAM PROJECT TASK WORK UNITELEMENT NO NO. NO ACCESSION NO

11 TITLE (include Securtty Claw ficatton)

Re-Usable Thin-Layer Spectroelectrochemical Cell for Non-Aqueous Solvent Systems

12 PERSONAL AUTHOR(S)W. Andrew Nevin and A.B.P. Lever*

13a. TYPE OF REPORT 13b. TIME COVERED 14 DATE OF REPORT (Year, Month. Day) 1 PAGE COUNT

Technical FROM AupL8_L, TO November 87 , 20

16. SUPPLEMENTARY NOTATION

17 COSATI CODES 18 SUBJECT TERMS (Continue on reverse ,f'necessary and identify by block number)

FIELD GROUP SUB-GROUPSpectroelectrochemistry Thin Layer

Cell

19 ABSTRACT (Continue on reverse if necessary and identify by block number) NTIS RA&I or

DTIC TAB FjUnanounced

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Dr. Robert K. Grasselli

DD FORM 1473, 34 VAR dl APR eR O'y raf De Aild .nr .,ur'J'!ed SE " Y CLASS,F CAT ON OF ', S , 0AdI'Ot-er eat Ores are OOoet-

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14/10/87 Anal. Chem. AC870454I --1--

Re-Usable Thin-Layer Spectroelectrochemical Cell for Non-Aqueous Solvent

Systems

W. Andrew Nevin and A.B.P. Lever*

Department of Chemistry, York University, 4700 Keele Street, North York

(Toronto), Ontario M3J 1P3, Canada.

V. p.

'f

PS

4.

4.. ° . • . • • p • . i

14/1/87 Anal. Chem. AC8'04541 ---

Abstract

A thin-layer spectroelectrochemical cell for use with aqueous or organic

solvent systems is described and evaluated. The cell design, comprising a

working chamber formed by sandwiching Teflon spacers between two optical

windows and a Teflon body, allows versatility or tne working electrode

material, and is easily dismantled and reassembled. Variation of the

window material permits measurements to be made over a wide spectroscopic

range, from the uvr/isible to the infrared regions. The performance of

the cell is assessed for a gold minigrid working electrode in the

uv-visible region using the oxidation of [2,9,16,23-tetra~neopentoxy)-

phthalocyaninatolzinc to its n-cation radical in o-dichlorobenzene, and

in the infrared region by investigating the reductions of bis(2,2'-bi-

pyridine)tZ,2'-azodipyridinejruthenium(II) bis~hexailuoropnosphate) in

deuterated dimethylsulfoxide.

SN,

11/I0/87 Anal h+m+ A'"'..,,

Thin-laver specr ro++-itctroheomistry r S f'.,-^ v

technique in many labor-itories tor tne chara,:tr .- r r- ...

spectroscopic propertie ,-, o t1 ect ,itrtiA v- . , . '-,

designs of optically transparent than-javer I

been reported, based on a variety (,: worKlg ,, k2 ,, ,-

gold mirigrid, platinum gauze, vitreous carr)on, In, T..ai . . I ,. ,'

thin metai or carbon Iirms. ketativeiy Slmpie ':'+. I m [r l.s av. '

successfully with aqueous solutions; however, dutaicuit2,-. w, -

organic solvents, since these attack the adhesives commoni," usea in : £

construction, so that cell lifetime is severely iimitea. s res-ait.

most ceils designed for use with organic soivents nav- ,[. +tea ox

complex assembiies, withq inconvenient construction and in wn:.',4 cit -arinr, .

or replacement of the working electrode is otten dtiacu~t. partl,-u~ariy

with the fragile gold minigrid (2-6). An improved ceii i r,'enLu:v t ,

reported by Lin and Kadish (6) which shows excellent i:ctrcchmica

behavioiir; however, it requires a fairly rooust platinum gauze woriarIg .-

electrode. As yet, no design has been reported tor organi, sunvnt c wnhz .

allows convenient assembly, cleaning and versatility ,-i w,-+rs a , ,ctidei

material.

We report here a re-usable UTTLE ceil in which a pDL-i miriip ri,3

working electrode is sanowiched between fetlon Dr, . :n .- tin )y

without the use of epoxies or other adhesive materi-i;. . ... :A-e j., n&,

a problem over tLie normai ttxperimentai t, Ime p.irl,:. C, I

easily dismantled ror -teaning or repla,?erner, ,t t ftht ,,. I rill r i i tilI

rapidly reassembied. It can be used under 1-gassei .I I. wIII

aquleous solution or a variety of comon rg-an Ic 1 ,+.t r . -, 1 1

soivenrts , u( h as o-dicnrloroberizene, N, N-dimethyf 1 )rmam' I . ;t ,

carbonate and dimethyl:ulphoxide. A t i u, f-- tIt, ur. -i :. '- . ,',

.

4 - r U W - .rr, A 4" C -C--

vfr~at i ity ,jv r ; w j-it r Dstr _-su, rp' c range ty vrv iri i !ne w lnauw

'n:tr, i 1 - ?r,m The ,j-v sI D 1 t- near [r >uart- 'vrex to tne

Ifn r rre v CM 1 r *gIn is a a 2 3 1 j Li t t i st n a s De tn m.-Ac. cr TTL Fs

r,., ;iren re -, ri i*' i n t rest i Tearc, To ii:v,i re etn growing

r,--r.t

e;. r t (e 'eii 1i1n te 1v-vis1ibIe reg1on 1s 1iiustraLeol by tie

,r. t ;iri rI [ Z, 16,WI -tetra(neopentoxyjpnthalocyaninatoJzinc

,nINPci-.-. in o-dichlorobenzene, and in the infrared region by the

reduction of fbis({2,2' -bipyrrdine)(2,2'-azodilpyridine ruthenium(II)

bis(hexarluorophosphate) (ku(bpy)2 (Azdpy(r 1'6 )V I in deuterated

dimethyisul foxide.

E'xperimental Section

e e s t onstr uct 1ln

The design or the thin-iayer cell is shown in rigure I. The working

criamber is formed by sanawiching two Teflon spacers oetween two windows,

and -ontains a semi-transparent gold minigrid (bO0 wires/inch, 60%

transmittance, buckbee Mears Co., St. Paul, MN) as the working electrode,

platinum foil as the counter, and silver foil (Aldrich, u.025 mm

thick) or AgCI coated silver foil as the reference electrocie. Ihis last was

prepared electrochemicaily by passing an anodic current of ca. 15 I'A/cmz

for M min through t~m., silver roil immersed in U.1 M HCI (15). The pure

silver roil reterence will drift up to lOOmV which can be an inconvenience.

The AgCI coated foil is much more staole, and is preferred, especially if

the fnoiution can tclerate chloride ion as the supporting electrolyte anion.

The c ounter and reference electrodes are separated from each other aria

trf)m tri- worKing ,-iect.rode by two rolls or l'eflon tape. [he assembly is

h,[id between two Tef ori holders which are tightenea to1'eltler to give a

pr-ssurf, . a ,a between tht! Ft fI ,n ;p.ctrs and w in n,:,w!,. It-e Ii ensions3 (,t

14/10/87 Anal. Chem. AC8704541 --5--

the working chamber are defined by the size and thickness of the Teflon

spacers. in this study the cell thickness was 0.45 mm, with a chamber

volume of 100 ul. Electrical contact to the gold minigrid is made outside

the working chamber by attaching a tinned copper wire with molten indium

metal using a soldering technique. Contacts to the counter and reference

electrodes are made using microgator clips. Further information on the cell

design may be obtained from the Authors.

The cell is filled via hypodermic needles inserted between the

Teflon spacers, and in order to eliminate oxygen, cell filling may be

carried out under an inert atmosphere using degassed solutions. An

assembled cell can usually be used for several experiments, by cleaning

each time with a suitable solvent, via the hypodermic needles.

Materials

ZnTNPc(-2) (16) and Ru(bpy)2 (Azdpy)(PFs)2 (1Y) were prepared as

described elsewhere. o-Dichlorobenzene (DCB; Aldrich, Gold Label) and

deuterated dimethylsulfoxide (DMSO-ds; MSD Isotopes) were used as supplied.

Tetrabutylammonium perchlorate (TBAP; Kodak) was recrystalilsed from

absolute ethanol and dried in a vacuum oven at )UOC for 1 days. Lithium

chloride (Anachemia, reagent grade) was dried at 12ooC for 24 h. Water was

purified by double distillation over KMnO4, foiluwed by passage through a

Barnstead organic removal cartridge and two barnstead mixed resin Ultrapure

cartridges. All other chemicals used were of anaiytical ,Irade.

ftethods

Electronic spectra were recorded with a Hlitachi kPerkin-Elmtr

Microprocessor Model 340 spectrometer. [n lrared spectra wfre recorded

using a Nicolet Model SX20 Fourier Transform Infrared t"li< :-pectrnnirter.

I

14/10/87 Anal. Chem. AC8704b41 -

A gain of 16 and 64 scans per spectrum were used for the i "FIR

measurements. Electrochemical measurements were made with a erinceton

Applied Research (PAR) Model 174A polarographic analyzer coupied to a PAR

Model 175 universal programmer. The thin-layer cell was tilled under an

atmosphere of nitrogen in a Vacuum Atmospheres Drilab (organic solutions)

or a glove bag (aqueous solutions). DMO-ds solutions were prepared in

the dry box. DCB solutions were prepared in air. degassed by repeated

freeze-pump-thaw cycles, and then transferred to the dry box. Aqueous

solutions were degassed by purging with nitrogen gas.

The optical window material may be varied according to the

spectroscopic range required. In this study, eyrex windows were used tor

the uv-visible region, and sodium chloride windows tor tne intrarea

measurements.

Results and Discussion

The electrochemical characteristics of the cell were assessed ior

aqueous solution using the [Fe(CN)SG]-/[Fe(CN)s64 - couple in I il xNW3

solution, as shown in Figure '. The cyciic voitammetric waves -,v_ aIAU.n.

symmetrical, and the current returns approximately to rfte oaseilrne atter

each peak maximum, indicating that the cell closely folows tr,,- "peted

ideal thin-layer behaviour 1 18, and that edge effects are bmall. .One

effects of the resistance of the thin solutlnm £aver -ire w.'t-rved, *

evidenced by an increase in peak to peaK 3eparatiun witn ;r-'ra.in scan

rate from Ib mV at lmV/s to lUU mV at lUU mv/. r-eaK IC, ,:.ik ;eparatiurts

measured at several scan rates are given in 'i'abe! i

organic solutions give a greater devlation trram dl ;I1 hf-ihavi ur

Peak to peak separations are larger than for aqueouz c .Qn, retlec ing

a higher i[t drop across the thin layer. Fi uro-s in: . ,n. ,'. it.

I% -I . ' '. ' ' . * * '* * * *

' % .-,

14/10/87 Anal. Chem. AC8704541 -- 7--

obtained for the first oxidation of ZnTNPc(-Z) to its ii-cation radical,

[ZnTNPc(-l)] , in DCB solution with 0.3 M TBAP as supporting electrolyte.

The cyclic voitammogram (Fig. 3) shows well defined rairly symmetrical

waves, having a peak to peak separation of 240 mV at a scan rate of 1

mV/s. Figure 4 shows the formation of the -cation radical species upon

oxidation across this couple. The final spectrum is very similar to those

recently reported (19) for the electrochemical and photochemical oxidation

of the unsubstituted zinc phthalocyanine derivatives, ZnPctim, (im =

imidazole) and ZnPc(py) (py = pyridine), respectively, in dichloromethane

solution. The spectroscopic changes are fully reversible to the initial

species upon re-reduction. Time for equilibrium at each potential is ca.

15 min in DCB solution; however, in more conducting solvents such as

N,N-dimethylformamide, equilibrium is reached within 5 min.

The UTTLE cell can easily be adapted for the infrared region by

using NaCI or another suitable window material, and by choosing a

solvent/electrolyte combination whose absorption is low in the region of

interest for the compound under investigation. Use of the FTIR technique

enables one to subtract out the solvent/electrolyte absorption from the

sample soLution to leave the pure sample spectrum.

As an example, we report the reduction of Ru(bpy)2(Azdpy)(iFS )2 in

DMSU-de with LiCI as supporting electrolyte. This solvent/electrolyte

combination gives a zpectroscopic window in the region of 1101) 1700

cm-i, where important infrared bands of this complex occur. bpectra were

obtained by subtraction of the spectrum of a 'blank' solution or LiCI in

DMS;O-dS in the (JTTLE cell taken through the gold minig4rici. from that.

containing the sample. The subtracted spectrum or the sample in soLution

war- found to he almost identical to that measured in a mtrr jisc, with

little shifting of the band trequencies. Table ii iist-; r-h, frequencits

i

14/10/87 Anal. Chem. AC870454I --8--

of the main absorptions in the region of 1200 - 1600 cm- 1 . The principal

features (17) are the occurrence of a strong band at 1334 cm-' due to the

azo N=N stretching mode, and several strong overlapping bands in the

regions of 1430 - 1500 cm-L and 1560 - 1600 cm-1 , assigned to azopyridine

ring and bipyridine modes. A shoulder at 1317 cm-1 is also associated

with the azo group, and is probably an (Naz-C) mode (17,20). The cyclic

voltammogram (17) of Ru(bpy)2(Azdpy)(PFs)2 in DMSO/LiCI shows two

reversible one-electron reductions, at -0.76 V and -1.29 V versus the

ferrocenium/ferrocene couple, corresponding to the consecutive addition of

electrons to the azo N=N bond (17,21).

Figures 5(a) and 5(b) show the spectroscopic changes observed on

reduction of a solution of 0.011 M Ru(bpy)2(Azdpy)(PF6)2 in DMSO-d6 with

0.09 M LiCl, across the first and second reduction couples, respectively.

The main bands of the reduction products are given in Table II. Reduction

across the first reduction wave results in several changes in the peaks

between 1400 and 1600 cm-1 . The band at 1605 cm-1 shows a large decrease

in intensity, while small shifts of 3-5 cm-1 to lower energy are seen for

the bands at 1583 and 1568 cm-i. In the region of 1400 cm-1 , the 1467 and

1447 cm-1 bands disappear, while new bands appear at 1457 and 1418 cm-1

Change in specific bands, however, are difficult to follow due to the

overlapping nature of the azopyridine and bipyridine absorptions. The

band assigned to the azo group shows a reduction in intensity relative to

those at 1400 cm-1 , and is shifted by 20 cm-1 to lower ,:nergy, consistent

with the addition of an u'[ctron to the N=N bond (O, )n forming the

second reduction product, the azo peak at 1304 cm-1 disappears and a

single relatively more intense band appears to Lower energy, at 129S cm-1

Several changes also occur in the region of 14UU-1600 ,!m-L as shown in

Fig. 5(b), notably a merging of the bands near 1600 cm-1 in-!i one broad

14/10/87 Anal. Chem. AC8704541 -9-

-J

multiple band at 1602 cm . These spectroscopic changes are reversible

upon re-oxidation to the initial species.

The results reported here have been obtained using a gold minigrid

working electrode. However, an attractive feature of the cell is that

many other electrode materials can be easily incorporated. We have

obtained sarisfactory spectroelectrochemical performance using windows

coated with a conducting layer of tin oxide or thin carbon film as the

working electrode. Similarly, the cell may readily be used with semi-

transparent metal film electrodes, or other electrode materials, such as

platinum gauze.

In summary, the thin-layer cell described here has several advantages

over many previous cell designs. These include versatility of the

spectroscopic range and electrode material, good electrochemical

behaviour, ease of cleaning and assembly, and suitability for use with

aqueous or organic solutions. In particular, the cell allows the

convenient and rapid acquisition of infrared data on redox species, an

area which has been neglected in the past, but which can provide important

information concerning the nature of the redox species and the

electrochemical processes taking place.

No problems were experienced in recording the spectra of air

sensitive materials even though there is evidence that Teflon Is

somewhat permeable to oxygen. Where extreme air sensitivity Is a problem,

Kel-F might be used for the spacer material.

Acknowledgements

We are grateful to Shafi Greenberg and Wei Liu for help with the vnthesis

of complexes. I0

14/10/87 Anal. Chem. AC870454I -J-

Credit: We thank the Natural Sciences and Engineering Research Council

(Ottawa, Canada) and the Office of Naval Research (Washington, DC) for

financial support.

14/10/87 Anal. Chem. AC870454I 11

References

1. Kuwana, T; Winograd, N. In Electroanalytical Chemistry; Bard, A.J.,

Ed.; Marcel Dekker: New York, 1974; Vol.7, p.) .

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14/)0/87 Anal. Chem. AC870454I - -

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14/)0/87 Anal. Chem. AC8704541 13

Figure Legends

Figure 1. Schematic diagram of the thin-layer spectroelectrochemical

cell: (a) front view; (b) side view; (c) assembly of the cell.

Figure 2. Thin-layer cyclic voltammograms of I nM K3Fe(CN)6 in I M KNO3,

recorded at potential scan rates of (a) 100 mV/s (__), and (b) JmV/s

(----). The reference electrode is AgCl-coated Ag foil.

Figure 3. Thin-layer cyclic voltammogram of 4.5 x 10- 4 M ZnTNPc in DCB

with 0.3 M TBAP. Scan rate - I mV/s. The reference electrode is AgCI-

coated Ag foil.

Figure 4. Development of the thin-layer spectra with time for the

oxidation of 1.6 x 10- 4 M ZnTNPc(-2) In DCB with 0.3 M TBAP. The

reference electrode is Ag foil. Reading from the top spectrum down, the

percentage of un-oxidised material is 100, 95, 69, 56, 39, 21, 12, 6,

2, 0%. The experiment is complete in about 15 minutes.

Figure 5. Development of the thin-layer FTIR spectra with time for the

reduction of 0.011 M Ru(bpy) 2 (Azdpy)(PF6 )2 in DMSO-d6 with 0.09 M LICI:

(a) first reduction; Reading from the top spectrum down, the percentage

of un-reduced material Is 100, 71, 54, 34, 0%. (b) second reduction.

The reference electrode is Ag foil. THe experiment is complete in about

15 minutes.

-3.6-

Table I. Peak to Peak Separations ( E) of the Fe(CN)63-/Fe(CN)64-

Couple in 1 M KN03 Solution.

Scan rate (mV/s) 1 10 20 50 IOU

E (mV) 15 20 25 45 lUO

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