now-aqueous solvent systems(u) thin-l ayr … · non-aqueous solvent systems by w. andrew nevin and...
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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
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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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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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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
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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.
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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.,
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2. Heineman, W.R.; Hawkridge, F.M.; Blount, H.N. In Electroanalytical
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p.l. .
3. Robinson, J. In Electrochemistry - Specialist Periodical Reports;
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Inorg.Chim.Acta 1987, 129, LI; Bullock, J.P.; Boyd, D.C.; Mann, K.R.
Inorg.Chem. 1987, 26, 3084.
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14/)0/87 Anal. Chem. AC870454I - -
129, LI; Bullock, J. P.; Boyd, D. C.; Mann, K. R. Inorg.Chem., 1987,
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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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