J. Phys. Chem. 1981, 85, 818-822
818
between first and second order. Possibly these changes reflect the increase in the rate of reaction of MB2+. with solvent as the mole fraction of organic component inso that the Order Of decay in [MB2’*1 declines from second to (pseudo) first when the organic content is high enough. At present, very little is known about the nature of the chemical reactions by which MB2+. decays under any conditions although the reported production of leucothionine and HzOzin photolysis of aqueous MB+ in weakly alkaline solutionla suggests that oxidation
of solvent is one end result of such reactions. Acknowledgment. This work was sponsored by the US. Department of Energy under contract EY-764-02-2889. Model c~culationsWere performed by Dr, J. S. Connolly at the Solar Energy Research Institute, Golden, Co. 1048-1056. (18) Usui, Y.; Obata, H.; Koizumi, M. Bull. Chem. SOC. Jpn. 1961, (19) Kosower, E. M. “An Introduction to Physical Organic Chemistry”; Wiley: New York, 1968; pp 296-304.
Electrochemical and Spectroelectrochemical Properties of Polyviologen Complex Modifled Electrodes Haruo Akahoshl, Shlnobu Toshlma, Faculty of Engineering, Deparfment of Applied Chemistry, Tohoku University, Sendei 980, Japan
and Klngo Itaya’ Institute of Electric Communication, Tohoku University, Sendai 980, Japan (Received: August 7, 1980; In Fhai Form: November 21, 1980)
The electrochemistry and the spectroelectrochemistry of the polymer complex polyviologens-poly(styrenesulfonate) modified electrodes were examined in an aqueous solution. The surface waves observed at -0.65 and -1.20 V vs. SCE were due to reductions of the electrochemical active centers (viologen moieties) in the polymer layer. Excellent stability of the polymer complex modified electrodeswas obtained on repeated scanning over the first wave, between +0.5 and -0.8 V vs. SCE, causing only a 5% decrease in the peak height after 100 cycles at a scan rate of 50 mV/s. The redox behavior of Fe(CN)a-/&was examined at the modified electrodes, demonstrating a mediated electron-transferreaction through the redox centers in the polymer film. The change of the color of the polymer film on electrodes could be seen as red-purple. The absorption coefficient (CY) at 560 nm of the polymer film was obtained as 1.7 X lo4 cm-l.
Introduction Numerous investigations of chemically modified electrodes and schemes for modification of electrode surfaces have been reported by several group^.^-^ The applications of these chemically modified electrodes have been proposed for electrocatalytic properties,s as chiral electrodes: for stabilization of semicond~ctors,~ and for sensitization of solar energy conversion.6 Instead of chemical modification techniques, it has been shown that either electrodeposition or dip-coating techniques of polymer with electroactive centers can give similar behavior of the chemically modified electrodes.’a The polymer-coated electrode has been getting considerable interest“14 since electrodes coated with polymeric mole(1) Itaya, K.; Bard, A. J. Anal. Chem. 1978,50, 1487. (2) A list of the relevant literature can be found in ref 1. (3) Evans, J. F.; Kuwana, T.; Henne, M. T.; Royer, G. P. J. Electroanul. Chem. Interfacial Electrochem. 1977, 80, 409. (4) Watkins, B. F.; Behling, J. R.; Kariv, E.; Miller, L. L. J.Am. Chem. SOC. 1975, 97, 3549. (5) Wrighton, M. S.; Austin, R. G.; Bocarsly, A. B.; Bolts, J. M.; Haas, 0.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Am. Chem. SOC.1978,100, 1602. (6) Fujihira, M.; Ohnishi, N.; Osa, T. Nature (London) 1977,268,226. (7) Miller, L. L.; Van de Mark, M. R. J.Am. Chem. SOC.1978,100,639. (8) Merz, A.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 3222. 0022-3654/81/2085-0818$01.25/0
cules containing electroactive groups were first deA few new applications of polymer-coated ~cribed.lJ*~ (9) Oyama, N.; Anson, F. C. J. Am. Chem. SOC. 1979,101, 739. (IO) Oyama, N.; Anson, F. C. J. Am. Chem. SOC.1979, 101, 3450. 1980, 127, 640. (11) Oyama, N.; Anson, F. C. J. Electrochem. SOC. (12) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochen. 1980, 108, 121. (13) Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Patel, V. V. Appl. Phys. Lett. 1980, 36, 422. (14) Kaufman, F. B.; Schroeder, A. H.; Engler, E. M.; Kramer, S. R.; Chambers, J. Q J. Am. Chem. SOC.1980,102,483. (15) Factor, A.; Heisohn, G. E. Polym. Lett. 1971, 9, 289. (16) Furue, M.; Sumi, K.; Nozakura, S. Polym. P r e p . , Jpn. 1980,29, 280. (17) It is well understood that poly(vinylferrocene), for example, cannot be soluble in acetonitrile, but still it is not certain that the completely oxidized form of the polymer is insoluble in acetonitrile in spite of having electrical changes on the Dolvmer chain. (18) Remba;, A.; B a m i A n e r , W.; Eisenberg, A. J. Polym. Sei., Part B 1968. 6. 159. (19)’M’ichaels,A. S. Ind. Eng. Chem. 1965,57,32. (20) Mackey, L. N.; Kuwana, T. Bioelectrochem. Bioenerg. 1976, 3, 596. (21) Fan, F. F.; Reichman, B.; Bard, A. J. J. Am. Chem. SOC.1980,102, 1488. (22) Jasinski, R. J. J. Electrochem. SOC. 1977, 124, 637. (23) Van Dam,H. T.; Ponjee, J. J. J. Electrochem. SOC.1974, 121, ~~
1555.
(24) Chang, I. F.; Howard, W. E. ZEEE Trans. Electron Deuices 1975, ED-22, 749.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 7, 1981 819
Polyviologen Complex Modified Electrodes
electrodes have recently been proposed including use as a reference electrode,12as an electrochromic display device,13and as a potentiometric sensor.3o An apparent advantage of the polymer-coated electrode seems to be that the surface coverage of electroactive centers can be greatly increased compared with the chemical modification techniques. Such a high coverage gives promise for successful application of polymer-coated electrodes. In the present report, we address a new way of coating electrodes with polyelectrolyte complexes between polyanions and polycations. The solubility of the polymer on the electrodes is required to be zero or to be negligibly small in a certain solvent in order to get successful results for the electrochemistry of polymer-coated electrodes. I t is not usually difficult to find solvents where the polymer on the electrodes never dissolves. For example, polymers with no charge on a polymer chain, like polystyrene and poly(vinylferrocene)18J2cannot be soluble in polar solvents such as water and acetonitrile. On the other hand, polymers with a charge on them, like polyvi~logens~~ and poly(viny1bipyridine)-Ruz+ complex,16 are soluble in water.l' Although the viologen polymers poly(xyly1 viologen dibromides) are soluble in water to the extent of 1-2% at room temperature,15 they form polymer salts (polymer complexes) with polyanions such as sodium poly(styrenesulfonate) (PSS) and sodium polyacrylates which are totally insoluble in common so1vents.l5J8J9Therefore, it is expected that the polyviologen complex modified electrodes can be stable even in water. Poly(xyly1 viologen dibromides) (para and ortho, p-PXV-Br2 and o-PXV-Br2) and potassium poly(styrenesu1fonate) were chosen as the redox polymer and as the polyanion, respectively.
An example of electrochemical deionization using ion adsorption electrodes based on the above redox polymers has been recently demonstrated by Factor and Rouse.36 The viologens have been known as electron mediators in biological systems20and in hydrogen evolution of solar cells.21 The reduced forms of viologens, radical cations, have such intense color that the longer-chain alkyl derivatives and the viologen polymers have been considered for (25) Wiley, R. H.; Smith, N. R.; Ketterev, C. C. J. Am. Chem. SOC. 1954, 76, 720. (26) Hawkridge, F. M.; Kuwana, T. Anal. Chem. 1973, 45, 1021. (27) Kawata, T.; Yamamoto, M.; Yamana, M. Jpn. J. Appl. Phys. 1975. 14. 725. (28) Peerce, P. J.; Bard, A. J., submitted for publication. (29) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. (30) Heineman, W. R.; Wieck, H. J.; Yacynych, A. M. Anal. Chem. 1980, 52, 345. (31) Kaufman, F. B.; Engler, E. M. J. Am. Chem. SOC.1979,101,547. (32) Bruinink, J.; Kregting, C. G. A.; Ponjee, J. J. J.Electrochem. SOC. 1977,124, 1854. (33) Jasinski, R. J. J. Electrochem. SOC.1978, 125, 1619. (34) Schoot, C. J.; Ponjee, J. J.; Van Dam, H. T.; Van Doom, R. A.; Bolwijn, P. T. Appl. Phys. Lett. 1973, 23, 64. (35) Reichman, B.; Fan, F. F.; Bard, A. J. J. Electrochem. SOC.1980, 127,333. (36) Factor, A.; Rouse, T. 0. J . Electrochem. SOC.1980, 127, 1313.
7 0.05mA IC mz
I
+0.5
I
0
-0.5
E (Vs.SCE)
-1.0
-1.5
Figure 1. Cyclic voltammetric behavior of the polyvlologen complex (p-PXV-PSS) modified electrode (SnO,) at different scan rates. The electrode area was -0.33 cm2and the solution was 0.15 M phosphate buffer (pH 7.2). The thickness of the polymer film was -400 A. (A) The potential was scanned from +0.5 to -0.8 V: (1) 5, (2) 10, (3) 20, and (4) 50 mV/s. (9) The potential was scanned from + O S to -1.4 V at 100 mV/s.
electrochromic The spectroelectrochemistry of the o- and p-PXV-PSS complex coated electrodes is also described in the present paper.
Experimental Section Materials. Synthesis of poly(xyly1 viologen dibromides), p-PXV-Br2and o-PXV-Br2,was accomplished by reactions of equimolar 4,4'-bipyridyl and cY,cu'-dibromo-p-xylene or a,d-dibromo-o-xylene in dry acetonitrile following a literature method.15 The elemental analysis and the ultraviolet spectra indicated pure materials. Potassium poly(styrenesulfonate) was made by a literature method.26 Equimolar aqueous solutions of the purified p-PXV-Br2 or o-PXV-Br2 and of potassium poly(styrenesu1fonate) were mixed together, yielding solids of the polyelectrolyte complexes (PXV-PSS).15 The composition of the complexes was confirmed as a molar ratio of 1:2 by elemental analysis. The polyelectrolyte complexes are soluble in a ternary solvent of concentrated HC1-H20-dioxane (45550 ~ 0 1 % but ) are totally insoluble in water. Electrode Preparation. Sn02 electrodes were used for optical measurements and for spin-coating techniques. Pt disk electrodes were used for dip-coating techniques. The solutions of the polyelectrolyte complexes (concentration 20-50 mg/ 10 cm3) were used for dip-coating and spincoating techniques. Spin coating was carried out by using a Mikasa spinner 1H-2 operating at 1000-3000 rpm for 60 s. The film of the polymer layer was allowed to air dry at 40-50 "C for 30 min, and then it was washed repeatedly with triple-distilled water. The thickness of the polymer layer was measured by a Nikon surface finish Microscope. The accuracy of the measurement was --f50 A. Measurements. The electrochemistry of the polymer complex coated electrode was examined in aqueous phosphate buffered solution (pH 7.2; 0.15 M). The oxygen-free nitrogen was bubbled through the solutions in order to prevent reactions of the reduced polymers (radical cations) and o ~ y g e n . A ~ ~saturated ,~~ calomel electrode (SCE) was used as a reference electrode. The cell used for measurements of the electrochemistry and of the spectroelectrochemistry of the modified electrodes of SnO?was similar to one developed by KuwanaF6 The W a n d wsible spectra of the modified electrodes under electrolysis were N
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The Journal of Physical Chemistry, Vol. 85, No. 7, 1981
0
0.1
0.2
scan rate ( V l s )
Flgure 2. Peak current (i,) of the first reduction wave for the electrode of Figure 1 as a function of scan rate.
taken with a Cary-14 recording spectrophotometer using a quartz cell. The absorption change at a f i e d wavelength was measured by a silicon photodiode (Hamamatsu TV, S-780 5BK) mounted just behind the SnOz electrode. Results and Discussion Electrochemistry of the Modified Electrodes. Figure 1shows a typical cyclic voltammogram of a p-PXV-PSS modified Sn02electrode in a phosphate buffered solution (pH 7.2). The polymer layer was made by spin coating, yielding -400 f 50 A thickness. The peak potentials of the first and second reduction waves were found at -0.65 and -1.20 V vs. SCE, respectively. Many viologen derivatives show two reduction waves whose potentials have almost the same values obtained here.21-23*27 The cathodic and anodic peaks of the first reduction wave were found at -0.65 and -0.5 V, respectively, and showed a slight dependence on scan rates. Half-widths of -250 mV were observed in both the reduction and the oxidation waves. Exactly the same result was obtained at 0-PXV-PSS modified electrodes. Such an asymmetric behavior and a half-width greater than 90 mV are indicative of the kinetic limitation of either slow electron transfer or slow ion transport of counterions in the complexes. A new model has been proposed recently by Peerce and Bard28for the cyclic voltammetric behavior of polymeric films on electrode surfaces. The stability of the polymer complex modified SnOz electrode was excellent provided that the electrode potential remained above -0.9 V vs. SCE. Repeated scanning over the first wave, between +0.5 and -0.8 V, caused only a 5% decrease in the peak height after 100 cycles at a scan rate of 50 mV/s. No more decrease was observed for 1000 cycles for preliminary life tests. In the case of similar electrodes coated with ferrocene-type polymers, the currents decreased more during the initial scans, perhaps because of dissolution of the oxidized forms of the polymers.1*8The structure of the present polyelectrolyte complexes is highly crosslinked by the formation of complexes between the polymer chains of the polycation and the polyanion. This crosslinking strongly reduces the solubilities of both the oxidized and the reduced polymer films. Figure 2 shows the peak current dependence on the scan rate. At lower scan rates, less than 50 mV/s, the current was directly proportional to the scan rate. A deviation was observed at the scan rate above 100 mV/s. Oyama and Anson observed proportionality to the square root of the scan rate (u1l2) rather than u with sufficiently heavily We coated electrodes (thickness more than 10000 also observed similar behavior with the polyelectrolyte complex modified electrodes (thickness more than 5000
A).
Akahoshi et al.
Figure 3 shows that the total charge consumed for the reduction of the bounded polymer (QJ was almost independent of the scan rate (u). From the Q, value, assuming n = 1, an average surface coverage equal to 5.2 X lo-’ mol/cm2 was found. The density of the polymer layer
O’
f
10
io
scan rate ( mWs4
5’0 100
’
Figure 3. Surface coverage (Q,) for the electrode of Flgure 1 as a function of scan rate.
(thickness 400 A) can be estimated from the surface coverage by assuming the molecular composition of the yielding 1.1g/cm3. polymer layer as (C1J116N2).(CJ17S03)2, Oyama and Anson reported that the effective density of the layer of poly(4-vinylpyridine) was only -0.08 g/cm3, indicating that the polypyridine layer was quite porous.l’ On the other hand, the complex polymer layer presented here is quite dense. The difference may be either in the method of coating or in the nature of the polymers. The polymer thickness, just after spin coating, was almost the same as that after heat annealing above 80 “C for 10 min. And also, the same value was obtained after an electrochemical experiment. The final oxidation state of the polymer film was the same as the original one (dication) for thickness measurement. Note that it was not found that a dip-coating technique made a difference. The above discussion suggests that the polyviologen complex layer might be a pinhole-free film. The electron transfer between dissolved reactants and electrode surfaces covered with polymer films is receiving considerable attention for two reasons. One is that the electron-transfer reaction may be understood simply by diffusion of reactants through “pinholes” of the porous layer. The other possible path of electron transfers is that electroactive centers in films may mediate the electron-transfer reaction. Merz and Bard, and Itaya and Bard, observed the reversible waves of 9,lO-diphenylanthracene and of 10methylphenothiazine at poly(viny1ferrocene)-coatedelectrodes (thickness of -200 A)s and at poly[(hydroxymethylferrocene)methacrylate]-coatedelectrodes(400A),l respectively. Oyama and Anson found that the Fe(CN)63-/4-couple showed almost reversible behavior even at a rather thickly coated electrode of attached Ru3+ (edta)poly(vinylpyridine)having a thickness of 10 Fm.l’ In previous studies of electron transfer mediated by surface-bounded species, the diffusion of reactant in solutions through “pinholes” of the porous layer has not been completely eliminated. Kaufman et al. have recently demonstrated a polymer-mediated electron-transfer reaction taking a rather complicated system, an electrochemistry of 1,2-(1’,9’-dioxophena1enium)benzenetetrafluoroborate at TTF polymer e1e~trodes.l~ As discussed above, coated electrodes of the polyviologen complex have very dense layers which suggests that reactants in solutions will show little or no transport through the film. In order to distinguish between the two mechanisms, we took the reduction of Fe(CN)63-as a solution oxidant which has more positive redox potential than that of the polymer complexes. Note that the reduction potentials of reactant in solution should be more positive than that of anchored redox centers in films because the above is the only thermodynamically possible path for electrons via anchored redox centers to the oxidant in the solution. Figure 4 shows the reduction behavior of Fe(CN):- at a p-PXV-PSS-coated Pt electrode with a thickness of
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The Journal of Physical Chemistty, Vol. 85, No. 7, 1981 821
Polyviologen Complex Modified Electrodes
0
U ‘
+0.5
0 -0.5 E (V=SCE )
-1.0
Flgure 4. Cyclic voitammetric behavior of the polyviologen complex (p-PXV-PSS) modified Pt electrode: (A) in the absence of Fe(CN)?and (6)in a solution of 8 mM Fe(CN),& at a scan rate of 50 mVls; the electrode area was -0.18 cm2; the thickness of the polymer film was -900 A; (C) at a bared Pt electrode with the same electrode area.
01 0 2 03 0 4 05 (scan rateY ( v / s Y 5
Figure 6. Peak current (Aipc)of the reduction of Fe(CN):for the electrode of Figure 4 as a function of scan rate. The concentratlon of Fe(CN),& was 8 mM.
Flgure 7. Schematic representation of the mediation of electrontransfer reactions.
C (mM) Figure 5. Peak currents (Ai,) for the electrode of Figure 4 as a function of concentration of Fe(CN)6& at a scan rate of 50 mV/s; (1) at a bared Pt electrode ; (2) at the modified Pt electrode.
-900 A. Many interesting results are found in Figure 4, (1)The reduction of Fe(CN)63-did not in practice occur a t its redox potential. (2) The electron transfer to Fe(CN)63-was only observed at the potential where the polyviologen centers were reduced. (3) The onset of the reduction of the polymer film on the Pt electrode was ---0.35 V vs. SCE so that a large overpotential for the reduction of Fe(CN)63-was obtained as 0.55 V. (4)The reoxidation of Fe(CN):- was again not observed at its redox potential. (5) The wave height of the reoxidation of the polymer (at - 4 . 4 5 V) was decreased by increasing the concentration of Fe(CN):-. All of these results are strong evidence for the mediated electron-transferreaction. Note that small cathodic and anodic plateaus were observed commencing at the redox potential of Fe(CN)2-/4-, This may be caused by nonuniformity of the film. Exactly the same results were obtained at a rather thinly coated Pt electrode with a thickness of -400 A. Figure 5 shows the dependence of the peak current (Ai ; subtracted by the current due to the reduction of tR“e polymer on the electrode) on the concentration of Fe(CN):-. Linear dependence of the peak current on the concentration was observed. This strongly suggests that the cathodic current, commencing at -0.35 V, is actually due to the reduction of Fe(CN&&. The same dependence was observed with a bared Pt electrode, but the slope was almost 2 times than that at the modified electrode (see Figure 5). There are two possible reasons which may explain the difference. One is that nonuniformity of the thickness of the polymer layer may determine “an effective electrode area”. The other reason is that reduction of Fe(CN)63-started at the onset of the reduction of the viologen center in the film (-0.35 V) and then showed the peak at -0.65 V. Such a broad wave is surely due to the kinetic limitation stated above, which makes it possible to differ from the theory for usual cyclic voltammograms.2B
Figure 8. Absorption spectra: (A) p-PXV-PSS modified SnO, electrode reduced at -0.8 V vs. SCE; the thickness of the polymer layer was 2500 A; (6)a partlally reduced solution of p-PXV-Br, (- 10% reduction) by Na,S,O,.
Figure 6 shows the dependence of the peak current (Ai on the scan rate of the electrode potential. A linear ependence of the peak current on the square root of the scan rate was observed. The observation of no reoxidation of Fe(CN)6” at more positive potential than the redox potential of Fe(CN)&I4indicates that electron transfer from the reductant of Fe(CN)t- is possible thermodynamically to neither the electroactive center (viologens)in the film on the electrode nor the electrode itself. The above result clearly proves that the electron-transfer reaction is actually mediated by the redox center in the film on the electrodes. A schematic representation of the mediation of the electron-transfer reaction is shown in Figure 7. Exactly the same results discussed above were obtained in 0.1 M KC1 solution. Studies of the effects of the solvent and the supporting electrolytell on the electrochemical behavior should be undertaken. One of heme proteins, cytochrome c, has been investigated electrochemically by many workers.20 It is well known that methyl viologen is a good mediator of the reduction of cytochrome c in solution. It is interesting that similar behavior is expected with the viologen complex
d
822
Akahoshi et ai.
The Journal of Physical Chemisfty, Vol. 85, No. 7, 1981 I
'
I i
0.2
I
x
.-
L
O'*
I
3 0.1 0
. / 0
10
20
Qc (rnClcrn2)
'-
*1
1
Figure 9, Optical density change at 560 nm for the polyviologen complex (p-PXV-PSS) modified SnO, electrode as a function of charge consumed during electrolysls. The thlckness of the polymer layer was 1200 A.
modified electrodes. The electrochemistry of other redox couples on the present modified electrodes is currently under investigation. Spectroelectrochemistry of the Modified Electrodes. Figure 8 shows the absorption spectra of -2500 A of pPXV-PSS film on the SnOz electrode reduced at -0.8 V (Figure 8A) and of the partially reduced solution of ppoly(xyly1viologen dibromide) (p-PXV-Brz)by Na2Sz04. The good correspondence between the two absorption spectra suggests that the viologen centers in the polymer film are not very much affected by the formation of the complex with the polyanion, potassium poly(styrenesulfonate). Figure 9 shows the dependence of the changes of the optical density on the charge consumed for the reduction of the p-PXV-PSS layer on SnOP The thickness of the polymer film was 1200 A. The total charge of the complete reduction of the polymer film at -0.8 V was -2.0 mC/cm2. Up to 2.0 mC/cm2, the optical density at 560 nm was directly proportional to the charge consumed during the electrolysis, indicating that the current efficiency is almost unity. The color at the electrode surface can be seen as red-purple. Kaufman et al. proposed a new class of electrochromic materials using polymer-coated e l e ~ t r o d e s . l ~ The J~~~~ viologen derivatives have been considered as an electrochromic display, because a strongly colored salt (radical cation) is formed on electrodes during the electrochemical redu~tion.22-24,32,34 The electrochromic efficiency at 560 nm,24 which is defined as a ratio of the change of optical density to the consumed charge, was -0.105 (mC/cmz)-l. The viologen derivatives have similar efficiency values; 0.12 was reported for benzyl viologen.24The absorption coefficient (a)at 560
-
I
0
5 Time ( sec )
10
Flgure 10. Current 0 and optical density (OD) at 560 nm for transients detected in the stepping potential ( E ) for the polyvioiogen complex modified electrode of Figure 9.
nm of the present polymer film was calculated as 1.7 X lo4 cm-l. Heptyl viologen has shown a value of 2.6 X lo4 cm-' at 560 nm for the absorption ~oefficient.~~ Figure 10 demonstrates the responses of the current and of the change of the optical density, stepping the electrode potential between +0.5 and -0.8V vs. SCE. The rise time of the coloration and the decoloration at 560 nm was 1 s. Repeated stepping over the first wave, between +0.5 and -0.8 V, showed only less than 5 % decrease in the absorption change after 100 cycles. No substantial changes in the rise times were observed. Most of the viologen radical salts have shown aging effects caused by the crystallizationof the radical cations (reduced form).32 The result of the crystallization is a poor erasure behavior in electrochromic displays, because the crystallized radical cation is not completely reoxidized during an electrochemical oxidation step. No similar behavior was observed on the modified electrodes. This may be very important in the application of the present electrodes for display devices. Reichman et al. recently proposed a photoelectrochromic display system of l,l-diheptyl-4,4'-bipyridylbromide on ~ - G ~ AThe S . systems ~ ~ presented here can also be used for a similar application.
-
Acknowledgment. The financial support of the Institute of Electric Communication Foundation (Tohoku University, 1980) is gratefully acknowledged.
