Synthesis, characterization and electropolymerization of ferrocene monomers with aniline and phenol substituents

79

1. Chem., 324 79-91 Elsevier Sequoia S.A., Lausanne

JEC 01874

Synthesis, characterization and electropolymerization of ferrocene monomers with aniline and phenol substituents Cohn P. Horwitz, Norman Y. Suhu, and Gregory C. Dailey Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 (USA) (Received 17 June 1991; in revised form 17 September 19911

AbStraCt

The synthesis and electrochemical properties of the ferrocene compounds (CsH,lFe(C,H, CH,NHR,), where R,=&H, (11,C,H,3-NH, (21, C,H,-3-OH (31 and (CH,l,NHC,H5 (41, are described. All compounds polymerize upon electrochemical oxidation of the phenyl portion of the complex in a CH,CN solution containing Bu,NClO, as supporting electrolyte. The phenol-substituted complex shows unique polymerization behavior: adding NEt, to the electrolysis solution greatly enhances the rate of film deposition. Electrochemical characterization of the films shows well-defined couples for the ferrocene but no indication that the polymer backbones are electroactive. Spectroelectrochemical measurements show changes in the visible region of the spectrum characteristic for formation of the ferricinium cation, A,, = 620 nm, upon oxidation. In the case of the polymer from 2 a moderately intense absorption at 480 nm is also recorded, which suggests that the polymer backbone may be electrochemically active.

INTRODUCTION

The functionalization of electrode surfaces with transition metal complex polymers continues to be an active research area [l-4]. Considerable effort is being invested in finding novel means for preparing these polymers and examining new metal complexes. Surfaces modified with ferrocene subunits have a special place in the chemically modified electrode arena [1,5-81. A multitude of deposition methodologies have been developed for tethering ferrocene to an electrode surface. Chemical and electrochemical polymerization of functional groups on the cyclopentadienyl rings [g-151 and reaction of modified ferrocenes with surface-active moieties on the electrode 116-181 are two examples. In addition, because of the high electron self-exchange rate for ferrocene, it has played a significant role in our understanding of charge conduction in immobilized transition metal complexes [19-241. More recently, ferrocene-modified electrodes have been used for electrochemical glucose sensors 125-291. One approach used for fabricating thin films of the 0022-0728/92/$05.00

8 1992 - Elsevier Sequoia S.A. All rights reserved

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sensor is electropolymerization of a solution containing ferrocene, glucose oxidase and pyrrole [30]. Reproducibility between electrode preparations is likely to be problematic. Despite this drawback, electropolymerization is the best route for preparing ultrathin films with rapid response times. The reproducibility problem is partially overcome by covalently linking the ferrocene to an electropolymerizable functionality. This approach is used by a number of investigators [12-G]. In most instances, pyrrole is attached to the cyclopentadiene ring through an alkyl spacer group. However, it is difficult to prepare homopolymers from these pyrrole-substituted ferrocene monomers. One often has to resort to copolymerizations with pyrrole itself in order to prepare the films. In a recent communication we presented the synthesis and electrochemical properties of some ferrocene monomers containing pendant aniline-like groups (1 and 2) [31]. These compounds readily form polymer films upon electrochemical oxidation of the aromatic amine. From synthetic and electrochemical perspectives, compounds 1 and 2 appear to be the best systems described to date for depositing both thick and thin polyferrocene films. We report here additional details regarding these compounds. Also described are the electrochemical properties of compound 3, a new monomer displaying novel electropolymerization characteristics. Compound 4 was prepared by an analogous metathesis route used for l-3. However, multiple substitutions of ferrocene on the ethylenediamine chain complicate the chemistry for this compound. EXPERIMENTAL

Physical measurements

IR spectra of solids (KBr disks) were taken on a Perkin-Elmer 283B IR spectrometer. Specular reflectance IR spectra were taken on a Perkin-Elmer 1600

81

or 1800 FTIR spectrometer. UV-vis spectra were measured with a HewlettPackard HP8452A diode array spectrometer controlled by a Zenith 159 personal computer. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian XL200 or IBM WP-100 spectrometer. The CD&N (Cambridge Isotopes) was used as received. Elemental analyses were performed by Quantitative Technologies, Inc., Bound Brook, NJ. Electrochemistry

Electrochemical measurements were obtained using an EG & G PAR Model 273 potentiostat-galvanostat controlled by the Zenith 159 computer. Voltammograms were obtained in a three-compartment cell with platinum disk working electrodes (A = 0.15 cm’; polished with 1 pm alumina (Buehler) prior to use), a platinum wire counterelectrode and an SSCE (sodium chloride calomel electrode) reference electrode. The supporting electrolyte was Bu,NClO, (TBAP) (Baker analyzed) and CH,CN (Burdick and Jackson) dried over molecular sieves was the solvent. The NESA@ glass was a gift from PPG Industries, Inc., Pittsburgh, PA. All experiments were performed under N, unless otherwise noted. Materials

The N,N-dimethylaminomethylferrocene (Strem Chemicals), triethylamine, methyl iodide, aniline, 1,3-aminophenol and N-phenylethylenediamine (Aldrich) were used as received. 1,3-phenylenediamine was recrystallized from chloroform and stored under N, at -10°C. Syntheses for (C,H,XC,H,CH,NHC,H,)Fe (1) and (C,H,XC,H,CH,NHC,H,-3-NH,)Fe (2) have already been described [31331. [(C,H,XC,H,CH,N(CH,),Fe]I was prepared by standard procedures. Synthesis of (C,H,)(C,H,CH,NHC,H,3-0H)Fe

(3)

0.5 g (1.3 mmol) of N,N-dimethylaminomethylferrocene methiodide and 0.25 g (2.3 mmol) of 3-aminophenol were combined in 30 ml of water and the solution refluxed for 3 h. During the reflux period a red-orange oil formed. After cooling to room temperature the mixture was extracted with C&Cl,, dried with Na,SO, (the oil obtained after evaporation often becomes dark brown upon standing in air) and then chromatographed on Florisil using 1: 1 ethyl acetate : hexane as eluent. A dark orange band was collected. After removal of the solvent 0.25 g (above 60% yield) of a sticky dark orange solid was recovered. Dissolution of the solid in Et,0 followed by slow addition of petroleum ether afforded a powdery orange solid. ‘H NMR (CD&N): 6 6.96-6.83 (m, 1H); 6.64 (br, 1H [OH]); 6.19-6.05 (m, 3H); 4.26-4.04 (m, 9H); 3.95 (s, 1H).

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Synthesis of (CsHs)(CsH4CH2NH(CHz)zNHC, Hs)Fe (4) 0.9 g (2.3 mmol) of N,N-dimethylaminomethylferrocene methiodide and 350 /zL (2.6 retool) of N-phenylethylenediamine were combined in 30 mL of water and the solution refluxed for 5 h. During reflux a red-orange oil formed. After cooling to room temperature the mixture was extracted with CHCI3, dried with Na2SO 4 and then chromatographed on silica gel using ethyl acetate as eluent. Three bands were collected. After removal of the solvent all formed tacky orange oils. Band 1 (165 mg recovered) 1H NMR (CD3CN): 8 7.09 (tr, 2H, J 8 Hz); 6.6-6.5 (m, 3H); 4.3-4.0 (m, 22H); 3.07 (tr, 2H, J 6 Hz); 2.6 (tr, 2H, J 6 Hz). Band 2 (250 mg recovered) ~H NMR (CDaCN): 8 7.15 (tr, 2H, J 7 Hz); 6.8 (d, 2H, J 8 Hz); 6.6 (tr, 1H, J 8 Hz); 4.4-4.0 (m, 22H); 3.4 (tr, 2H, J 7 Hz); 2.8 (tr, 2H, J 7 Hz). Band 3 (65 mg recovered) 1H NMR (CD3CN): 6 7.1 (m, 2H); 6.6-6.4 (m, 3H); 4.4-3.9 (m, llH); 3.5 (tr, 2H, J 7 I-Iz); 2.8 (tr, 2H, J 7 Hz).

Electropolymerization of 1 and 2 The monomers were dissolved in CH3CN containing 0.1 M TBAP with no other additives and deaerated with N 2 prior to polymerization. Monomer 1 was polymerized by cycling the electrode potential between 0 and 1.0 V while monomer 2 polymerized upon cycling the electrode potential between 0 and 0.75 V. Polymerizations were usually performed at 50 or 100 mV s -1. The color of the film depended on its thickness, ranging from golden for thin films to blue for thick ones.

Electropolymerization of 3 with NEt3 present NEt 3 was added to a known concentration of 3 in CH3CN via syringe, solution was deaerated with N 2 for about 3 rain and then the material deposited by cycling the electrode between 0.75 and 0 V at 50 mV s -1. estimate at least a 10% error in the experimental data owing to NEt3 addition its volatilization during deoxygenation of the acetonitrile.

the was We and

Spectroelectrochemistry Compounds 1-3 were electropolymerized on NESA ® glass under conditions similar to those used for preparing films on platinum electrodes. Thick films were prepared by depositing a thin layer of the film with five to ten cyclic scans followed by application of a constant potential appropriate for the monomer under investigation. The CH3CN solution was stirred to maintain a high monomer concentration in the vicinity of the electrode surface. The solution darkened during the deposition process, probably because of the presence of soluble low molecular weight oligomers.

83 Visible spectra were obtained in a CH 3CN solution containing 0.1 M TBAP by stepping the potential in 0.1 V increments starting at least 0.3 V positive of the ferrocene oxidation, allowing the current to decay to background levels and then taking the visible spectrum. The original spectrum is regained by reducing the oxidized f'dm at 0 V. RESULTS

Synthesis All compounds with the exception of 4 are prepared in high yield using a simple metathesis reaction: ®

~CH2N{CH3)3 lee + I

~ CH2NHRn I:~= 1 NHCsHs H20 H2NFIn ~ Fe 2 NHC,ItH4-3-NH= reflux I 3 NHC'eH4"3-OH ~ 4 NH(CH,)2NI"ICeHs

(1)

Isolation of the phenol derivative is more complicated than either 1 or 2 [31]. A side reaction produces a black tarry material following the initial extraction for 3. The isolated yield of 3 is still 60%. Compound 4 was synthesized to ascertain whether moving the aniline away from the ferrocene affected the electropolymerization and film properties. We have observed that polymerization depends on the alkyl chain length for some substituted pyrroles [34]. Three products are isolated from the reaction mixture by chromatographic means. The first band collected is 5 and the second is 6, as determined by IH NMR spectroscopy:

0

NH

iCH92 I

Fe I

Fe !

5

Fe i

6

Similar multiple substitutions of ferrocene on amino-alkyl pyrroles such as 1-ethylaminopyrrole have been observed [35]. The third product, formed in the lowest isolated yield, is 4. No efforts to improve the yield of 4 are contemplated because its tacky texture makes it difficult to handle. It does eleetropolyrnerize.

Electropotymerizatbn Compound 1 The deposition of this monomer from acetonitrile solution has already been detailed [31]. Polymer film formation is relatively rapid when the potential is cycled partially through the aniline oxidation, E,,Ox= 0.975 V and Epolym= 1 V vs. SSCE at 20 mV s- r. 0, must be excluded from the solution. We tried the polymerization in aqueous solution containing a variety of acids to form a conducting polyaniline backbone [36,37]. However, following initial dissolution of the compound in the acid solution, a yellow solid precipitates and no electrochemical response is detected for the ferrocene. Apparently the ammonium salt of 1 is insoluble in an aqueous acid solution. Compound 2 A preliminary description of the electropolymerization of 2 has already been presented [31]. The electron-donating NH, makes the monomer easier to oxidize and polymerize than compound 1. The aniline portion is oxidized at approximately 0.41 V [311. Most polymerizations of this compound are performed by cycling the nntential r______-_ between ___.. ____ 0 and OJs

v

at lQ!j m_v s-la

Si_m_ilar -__--_I results -_-1 are nhtained -- _--___-

hv _,

repetitive potential excursions to 0.45 V at the same scan rate, but surface coverages are lower. Low ferrocene concentrations and rapid scan rates are needed to avoid depositing films too quickly, since distorted current-voltage curves are observed for the polymer. Compound 3 The polymerization characteristics of this monomer are substantially different from those of 1 or 2 when the electrochemistry is performed under analogous conditions. Oxidation of phenol in non-aqueous solution usually causes passivation of the electrode surface following a few repetitive potential sweeps [38-411. A ,.:-:I”.. ..,,,:..,.c:,, 1s :, ““JF;I”F;” ,t..“,.,.,,a F,.. “..___....rl J. 1 72a ..,“I..+:,, .WQVG . .. . ..a F..c ,l.,..,.i s11111161 t.&lsJl”drl”U 1”1 L”,lly”UUU LUG“AlUQLI”,, I”I CL, L11G pIIGII”I, E p,ox= 0.95 V, shows a monotonic decrease during eight continuous potential cycles to 1.2 V at 50 mV s-r (Fig. la> and cessation of film growth occurs. A golden-colored film is observed on the electrode surface upon its removal from solution. Clear evidence for surface-immobilized ferrocene is obtained when the electrode is placed in a solution containing only supporting electrolyte, but the quantity of material is small. Thick polymer films of compound 3 form if a base such as N(CH,CHs), (NEt,) is included in the electrolysis solution. The presence of the base results in the electropolymerization occurring at 0.75 V or less. Figure lb shows electrochemical polymerization of the monomer in a solution containing 1.2~ of NEt,, where c is the concentration of monomer. The phenoi oxidation is neariy coincident with ferricinium cation formation. This makes the observed Z,,OXa composite of the ferrocene and phenol oxidations, Z,,,,,. An Z,,oGtot/Z,,red ratio of approximately 2.5 is measured when 2.2~ of base is present (Fig. 2a). This indicates that oxidation

8.5 a

I

1.2

I

d.4 o!g ’ W vs SSCE

’

b

Fig. 1. Oxidative electropolymerization of 3 by repeated potential cycling: a, 4.5 mM 3 and 0 mM NEt,; b, 3.4 mM 3 and 4.1 mM N&s. The first scan followed by every second scan is shown. CVs were taken at a platinum working electrode in 0.1 M Bu,NClO, + CH,CN at u = 50 mV s-r. a

b

160 140 120 4, -100 +ii

I

.

.

. .

4 1

140

l

_v

“-\: l ;., .,.; 0.0

160

.

0.5

1.0

1.5

2.0

[NEd/bl

/ 120

100 :'6Oe

i

5

6 i

i

i

10

no. polymenzation scans

Fig. 2. Dependence of electropolymerization of 3.25 mM 3 on added NEts: a, absolute values of I,,Orqt,,t (0) and IPred ( n ) vs. [NBt,]/[3] for the first potential sweep; b, Ir,,,t,, vs. number of repetitive potential scans for [NEt,]/[3] 0.44 (ml, 0.66 (A), 0.89 f+) and 1.33 (0); c, surface coverage I& vs. [NEt,]/[3]. Experimental conditions are analogous to those in Fig. 1.

86

becomes an overall two-electron process, while reduction remains a one-electron event. The polymerization rate shows some dependence on NEt 3 concentration (Fig. 2b), as determined by comparing the slopes of the lines at the various NEt, concentrations. Currents were measured following the fourth cyclic scan, because at low NEt, concentrations fii growth is slow and only small current changes are observed at the start of the polymerization. Without attempting to optimize polymerization conditions, a 1: 1 NEt, : ferrocene ratio gives good polymerization rates and only a few scans are needed to form reasonably thick films. A large excess of NEt, is undesirable since demetallation of the ferricinium will occur [42]. Compound 4

Owing to the difficulties in isolating and manipulating this compound, a limited number of electrochemical studies have been performed. A ferrocene redox process at E” = 0.415 V and two irreversible oxidation waves are observed at 0.87 and 1.06 V vs. SSCE in acetonitrile solution. The most positive oxidation process occurs at a potential similar to aniline oxidation in compound 1, E,, = 0.975 V. While the origin of the 0.87 V oxidation is unknown, a polymer film grows when cycling through this wave. The resulting polymer exhibits a low surface coverage. Reasonably rapid polymerization occurs by cycling the potential through the more positive redox process. However, after only a few scans the polymerization stops and the ferrocene waves become less defined. No other polymerization conditions were investigated. Polymer film characterization Ferrocene portion The polymer films were characterized

electrochemically by transferring a coated electrode to an acetonitrile solution containing only 0.1 M TRAP, then measuring the current-voltage curves at various scan rates. All polymers show redox processes attributable to the ferrocene portion of the polymer. The anodic and cathodic peak currents are linearly related to the scan rate up to 200 mV s-l for thin films. At slow rates, 5 mV s -l, thin films exhibit a peak separation approaching the theoretical 0 V [1,43]; the actual AEP = 0.01 V. The separation increases at faster scan rates. For thick films the peak current at high scan rates is lower than predicted from extrapolation of slow scan rate data. Surface coverages IY,, in/m01 cm-’ were ascertained in the usual fashion: I’,, = Q/nFA, where Q is the charge under the oxidation or reduction potential-current envelope, n is the number of electrons (n = 0, F is the Faraday constant and A is the electrode area [l]. In general, r,, is linearly related to the number of repetitive cyclic potential scans used in preparing the polymer film. IT,,, is dependent on NEt, concentration for compound 3 (Fig. 2c). Some thick films have slightly lower than predicted surface coverages, presumably arising from ion transport limitations.

87

4bo

560

660

760.

660

hhm

Fig. 3. Spectroelectrochemistry of poly-2 on NESA@ glass at the applied potentials indicated on the figure. Inset: spectra of poly-3 at 0 V and 0.7 V.

Polymer backbone

There is no unambiguous electrochemical evidence that the polymer backbone in any of the films is electronically conducting. Recent reports have indicated that a polyaniline f&m prepared in acetonitrile solution with no acid present [44,45] becomes electronically conducting upon transferring the coated electrode to an aqueous acid solution. Electrochemical measurements in aqueous acid show only the modified voltammetry expected for the ferrocene-ferricinium couple [20] for poly-1. Poly-2 “dissolves” from the electrode surface in a similar solution. However, in aqueous 0.1 M HClO, there is some indication in the cyclic voltammogram that the polymer backbone is electroactive for poly-2 [46]. Spectroelectrochemistry

Polymers from 1 and 3 oxidized to 0.7 V show a weak absorption at approximately 620 nm from the ferricinium cation [46] (inset of Fig. 3). Similar behavior is observed for a polymer from 2 in the 620 nm region, but a broad, medium intensity absorption centered at 480 nm is also observed (Fig. 3). The intensity of the high energy band is much greater than expected from the organometallic cation alone. Removal of the iron (greater than 80%) by oxidizing poly-2 in the presence of 2,2’-bipyridine [31,42] results in loss of both absorption bands. A polymer film remains on the electrode surface. Identical results are obtained with poly3. These experiments suggest that the polymer backbone does not make a contribution to

the absorption spectrum. However, we cannot exclude irreversible degradation of the polymer backbone resulting from the method used to remove the iron. Further studies on related compounds are in progress to define better the absorption spectrum. DISCUSSION

The electrochemical polymerization of these ferrocene monomers undoubtedly occurs through the pendant aniline or phenol groups. No polymer forms if these moieties are not oxidized. Attack of the aromatic amine on the cyclopentadiene rings is unlikely since the E” for each polymer is shifted only slightly positive of its respective monomer. Analogous behavior is observed for aniline-substituted porphyrin [48-501, Schiff base [51] and metal polypyridine [521 complexes. The electrochemical oxidation of N-substituted anilines usually favors tail-to-tail dimerization rather than head-to-tail coupling, particularly in acetonitrile [53-551. No definitive assignment for a structure to the polymer backbone in the ferrocene polymers is possible with the current information. However, substitutions on the aromatic amine show that both the amine nitrogen and the para position are involved in film formation. Ferrocene monomers 7 and 8 do not electropolymerize:

7

8

FTIR specular reflectance measurements on poly-1 show a very weak N-H stretching mode, indicating reactivity at this site. For poly-2 the primary amine stretches (doublet at 3453 and 3418 cm-’ in the monomer) are significantly reduced compared to the monomer, while a broad, relatively intense stretching mode for a secondary amine is still present (3370 cm-‘) 1311.These observations for poly-2 strongly support the primary amine as the dominant nucleophile for attack on an adjacent aromatic ring. In the case of the 1,3-substituted anilines the exact coupling site(s) has not been determined. Roth NH, and OH groups are ortho, para directors favoring attack at the 4 and 6 positions on an adjacent aniline ring. Electronically, the least desirable coupling site is the resonance destabilized 5 position. The aromatic overtone bands in the IR spectrum are too weak to be observed in the polymers. The NEt, deprotonates the phenol in compound 4 to form the more easily oxidized phenoxide ion, Em = = 0.4 V. Irreversible oxidation of substituted phenoxide ion is a one-electron process 141,561. The dependence of IP,,,t,t on NEt, concentration in the ferrocene complex (Fig. 2a) also reveals a one-electron oxidation for the phenoxide ion. The coupled product is electroinactive in the

89

potential regime for polymerization since Zr+., remains independent of NEt, concentration. Oxidation of phenol or phenoxide generally leads to rapid passivation of the electrode surface. Similar behavior is observed for poly-2 films grown in the absence of base. We surmise that sustained film growth with NEt, present occurs by the ferrocinium-cation-mediated oxidation of the phenoxide ion at the film surface. Thus the redox hopping characteristics of the ferrocene are essential for continued oxidation of the monomer and deposition of polymer. In the absence of base the phenol oxidation is too far positive to be mediated by the metal complex, so film growth stops. When 2,6dimethylphenol is oxidized in CH,CN containing a sixfold excess of polymer is reported to form [41]: NEt *HP &,0X = 0.3 V, a polyphenyleneoxide-like

+

BH’

(2b) At present we have no evidence that a similar backbone forms in poly3. A scenario involving tail-to-tail coupling to form a quinone-like structure, a second known coupling mode for oxidized phenoxide, is deemed unlikely in the present case because the dependence on base concentration would be different [411. Furthermore, it is improbable that the dimer would be sufficiently insoluble to precipitate on the electrode surface. The electronic spectra for polyaniline and short-chain oligomers often exhibit absorption bands in the 700-800 nm region upon partial oxidation [57-631. However, the absorption spectra can depend on the polymerization conditions and the medium in which the film is examined. No band in this low energy region is observed in any of the ferrocene polymers we prepared. The oxidized polymer of compound 2 shows a strong feature at 450 nm that does not appear in the other polymers. This absorption may arise from oxidation of the polymer backbone, but when the metal is stripped from the film, this absorption is no longer observed. Any further speculations regarding the nature of the polymer backbone are unsupportable with the currently available information. CONCLUSIONS

The various substituted ferrocenes described here exhibit a number of desirable properties for eventual use in sensors. These include ease of monomer synthesis, mild positive electropolymerization potentials, polymer stability to continuous potential cycling and stability to storage at ambient conditions. Unfortunately, the nature of the polymer backbone cannot be assigned definitively. Nevertheless, the

90

utility of pendant phenol and aniline groups for anchoring metal complexes to electrode surfaces is a method worthy of further investigation. Studies are continuing to determine the generality of the present approach to poiymer formation. ACKNOWLEDGMENTS

We thank the donors of American Chemical Society, Defense Advanced Research tered by the Office of Naval

the Petroleum Research Fund, administered by the for partial financial support of this work as well as the Projects Agency (DARPA) through a grant adminisResearch.

REFERENCES 1 R.W. Murray, Electroanal. Chem., 13 (1989) 191. 2 J.D. Swalen, D.L. Allara, J.D. Andrade, E.A. Chandross, S. Garoff, J. Israelachvili, T.J. McCarthy, R. Murray, R.F. Pease, J.F. Rabolt, KJ. Wynne and H. Yu, Langmuir, 3 (1987) 932. 3 AJ. Bard, J. Chem. Educ., 60 (1983) 302. 4 H.D. Abruna, Coord. Chem. Rev., 86 (1988) 135. 5 A.B. Fischer, M.S. Wrighton, M. Umana and R.W. Murray, J. Am. Chem. See., 101 (1979) 3442. 6 K.W. Willman, R.D. Rocklin, R. Nowak, K.-N. Kuo, F.A. Schultz and R.W. Murray, J. Am. Chem. Sot., 102 (1980) 7629. 7 T. Inagaki, H.S. Lee, T.A. Skotheim and Y. Okamoto, J. Chem. Sot., Chem. Commun., (1989) 1181. 8 P. Daum and R.W. Murray, J. Phys. Chem., 85 (1981) 389. 9 H. Nishihara and K Aramaki, J. Chem. Sot., Chem. Commun., (1985) 709. 10 T. Kawai, C. Iwakura and H. Yoneyama, J. Electmchem. Sot., 137 (1990) 2667. 11 H. Nishihara, M. Noguchi and K. Aramaki, Inorg. Chem., 26 (1987) 2862. 12 A. Haimerl and A. Merz, Angew. Chem., Int. Edn. Engl., 25 (1986) 180. 13 A. Merz, A. Haimerl and A.J. Gwen, Synth. Met., 25 (1988) 89. 14 T. Inagaki, M. Hunter, X.Q. Yang, T.A. Skotheim and Y. Okamoto, J. Chem. Sot., Chem. Commun., (1988) 126. 15 J.G. Eaves, H.S. Munro and D. Parker, Synth. Met., 16 (1986) 123. 16 A.H. Schroeder, F.B. Kaufman, V. Pate1 and E.M. EngIer, J. Electroanal. Chem., 113 (1980) 193. 17 P. Daum, J.R. Lenhard, D. Rohson and R.W. Murray, J. Am. Chem. Sot., 102 (1980) 4649. 18 C. Zou and M.S. Wrighton, J. Am. Chem. Sot., 112 (1990) 7578. 19 S. Nakabama and R.W. Murray, J. Electroanal. Chem., 158 (1983) 303. 20 P.A. Peerce and A.J. Bard, J. Electroanal. Chem., 114 (1980) 89. 21 H.S. White, J. Leddy and A.J. Bard, J. Am. Chem. Sot., 104 (1982) 4811. 22 A.R. Hillman, D.C. Loveday and S. Bruckenstein, J. Electroanal. Chem., 274 (1989) 157. 23 M.D. Ward, J. Electtoanal. Chem., 236 (1987) 139. 24 M. Watanabe, M.L. Longmire and R.W. Murray, J. Phys. Chem., 94 (1990) 2614. 25 P.D. Hale, T. Inagaki, HI. Karan, Y. Okamoto and T.A. Skotheim, J. Am. Chem. Sot., 111 (1989) 3482. 26 A.R. Hillman, D.A. Taylor, A. Hamnett and S.J. Higgins, J. Electroanal. Chem., 266 (1989) 423. 27 M.J. Green and H.A.O. Hill, J. Chem. Sot., Faraday Trans., 82 (1986) 1237. 28 A.E. Cass, G. Davis, G.D. Francis, H.A.O. Hill, W.J. Aston, I.J. Higgins, E.V. Plotkin, L.D.L. Scott and A.P.F. Turner, Anal. Chem., 56 (1984) 667. 29 J.E. Frew and H.A.O. Hill, Anal. Chem., 59 (1987) 933. 30 C. Iwakura, Y. Kajiya and H. Yoneyama, J. Chem. Sot., Chem. Commun., (1988) 1019. 31 C.P. Horwitz and G.C. Dailey, Chem. Mater., 2 (1990) 343. 32 J.T. Pennie and T.I. Bieber, Tetrahedron I&t., 34 (1972) 3538.

91 33 A. Lombard0 and T.I. Bieber, J. Chem. Educ., 60 (1983) 1080. 34 C.P. Horwitz, Chem. Mater., 1 (1989) 463. 35 N. Suhu and C.P. Horwitz, unpublished results, 1990. 36 Y. Wei, W.W. Focke, G.E. Wnek, A. Ray and A.G. MacDiarmid, J. Phys. Chem., 93 (1989) 495. 37 G.E. Wnek, Synth. Met., 15 (1986) 213. 38 N. Oyama, T. Ohsaka, T. Hirokawa and T. Suzuki, J. Chem. Sot., Chem. Commun., (1987) 1133. 39 T. Ohsaka, M. Ohba, M. Sato and N. Oyama, J. Electroanal. Chem., 300 (1991) 51. 40 S. Kunimura, T. Ohsaka and N. Oyama, Macromolecules, 21 (1988) 894. 41 N. Oyama, T. Oh&a, Y. Ohnuki and T. Suzuki, J. Electrochem. Sot., 134 (1987) 3868. 42 R. Prins, A.R. Korswagen and A.G.T.G. Kortbeek, J. Organomet. Chem., 39 (1972) 335. 43 A.J. Bard and L.F. Faulkner, Electrochemical Methods, Wiley, New York, 1980. 44 M.C. Miras, C. Barbero, R. Katz and 0. Haas, J. Electrochem. Sot., 138 (1991) 335. 45 T. Osaka, T. Nakajima, K. Naoi and B.B. Owen, J. Electrochem. Sot., 137 (1990) 2139. 46 W.E. Rudzinski, M. Walker, C.P. Horwitz and N.Y. Suhu, J. Electroanal. Chem., submitted. 47 Y.S. Sohn, D.N. Hendrickson and H.B. Gray, J. Am. Chem. Sot., 93 (1971) 3693. 48 A. Bettelheim, B.A. White, S.A. Raybuck and R.W. Murray, Inorg. Chem., 26 (1987) 1009. 49 A. Bettelheim, D. Ozer and R. Harth, J. Electroanal. Chem., 266 (1989) 93. 50 B.A. White and R.W. Murray, J. Electroanal Chem., 189 (1985) 345. 51 C.P. Horwitz and R.W. Murray, Mol. Cryst. Liq. Cryst., 160 (1988) 389. 52 C.P. Horwitz and Q. Zuo, Inorg. Chem., 31 (1992) in press. 53 K.B. Prater, J. Electrochem. Sot., 120 (1973) 365. 54 R.L. Hand and R.F. Nelson, J. Am. Chem. Sot., 96 (1974) 850. 55 T. Mizoguchi and R.N. Adams, J. Am. Chem. Sot., 84 (1962) 2058. 56 A. Richards, P.E. Whitson and D.H. Evans, J. Electroanal. Chem., 63 (1975) 311. 57 G. Zotti and G. Schiavon, Synth. Met., 30 (1989) 151. 58 D.E. Stilwell and S.-M. Park, J. Electroanal. Chem., 136 (1989) 427. 59 F. Wolf, C.E. Forbes, S. Gould and L.W. Shacklette, J. Electrochem. Sot., 136 (1989) 2887. 60 B. Shim, M.-S. Won and S.-M. Park, J. Electrochem. Sot., 137 (1990) 538. 61 M. Inoue, R.E. Navarro and M.B. Inoue, Synth. Met., 30 (1989) 199. 62 S. Rao and E. Hayon, J. Phys. Chem., 79 (1975) 1063. 63 T. Kobayashi, H. Yoneyama and H. Tamura, J. Electroanal. Chem., 177 (1984) 281.