Electroactive bilayers employing conducting polymers: Part 5, Electrochemical quartz crystal microbalance studies of the overall switching process

JO~RNAk OF

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Journal of Electroanalytical Chemistry 379 (1994) 365-372

Electroactive bilayers employing conducting polymers: Part 5, Electrochemical quartz crystal microbalance studies of the overall switching process A.

Robert Hillman

*

Department of Chemistry, University of Leicester, Leicester LE1 7RH (UK)

Andrew Glidle Department of Inorganic Chemistry, Glasgow University, Glasgow G12 8QQ (UK) Received 13 January 1994; in revised form I1 March 1994

Abstract

Electrochemical quartz crystal microbalance (EQCM) measurements were used to study redox-induced changes in the ion and solvent content of polybithiophene/polyxylylviologen(PBT/PXV) bilayers. Experimental measurements were taken under slow scan voltammetric conditions in 1 mol dm -3 tetraethylammonium perchlorate + CH3CN. Outer (PXV) layer redox conversion occurs only at potentials where the inner (PBT) layer becomes conducting. On the relatively long time-scales employed, bilayer mass changes are dominated by perchlorate anion transfer (ejection on PXV 2+ reduction and entry on PXV ° oxidation), with relatively little solvent transfer. The mass-charge characteristics of PBT/PXV bilayer and PXV single-layer films are relatively similar, despite their very different mass-potential characteristics.

Keywords: Electrochemical quartz crystal microbalance; Bilayer; Conducting polymer

I. Introduction

1.1. Bilayers Electroactive polymer bilayers were first explored by Murray and coworkers [1,2]. In the ideal case, these structures comprise an underlying electrode, covered sequentially with two uniform, segregated films. Direct electronic communication between the outer layer and the underlying electrode is thus prohibited, and outer layer redox state changes can only be mediated by the inner layer at the polymer Ipolymer interface. The unidirectional character of this polymer Ipolymer interfacial reaction leads to charge trapping in the outer layer. Potential applications of these characteristics include electronic devices (such as diodes [2-5] and "sandwiches" [6]), charge storage [7], display devices [2] and sensors [8].

* Corresponding author. 0022-0728/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 4 ) 0 3 4 3 4 - 5

We are interested in the fundamental processes underlying redox switching of these bilayers. Switching time is a key performance indicator for bilayer-based devices (see above). Analysis of the kinetic and diffusional processes involved in bilayer redox switching showed that inner layer charge transport would most commonly be rate limiting, with interfacial charge transfer at the polymer Ipolymer interface as the most likely alternative [9,10]. In recognition of this, we sought to improve switching time via a conducting polymer inner layer [11-14] (see refs. 11 and 12 for a full bibliography). Of several candidate systems studied [11], we found that sequential deposition onto an electrode of polybithiophene (PBT) then polyxylylviologen (PXV) yielded a structure with the current - voltage (I-E) characteristics of an "ideal" [1] bilayer. For the P B T / P X V system, there are three possible states for each layer: PBT ÷, PBT ° and P B T - for the inner layer, and PXV z+, PXV ÷ and PXV ° for the outer layer. The PXV redox state changes occur only by mediated charge transfer from the inner layer at the relatively positive

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A.R. Hillman,A. Glidle/Journal of Electroanalytical Chemistry379 (1994)365-372

(negative) potentials necessary to generate PBT + ( P B T - ) mediator species, not at potentials characteristic of viologen couples.

( A M / g cm -2) = - 4.426 X 1 0 9 ( A f / H z )

1.2. Techniques The limitation of much of the early work on polymer bilayers was its reliance on current-voltage curve analysis. Electrochemical data provide an excellent measure of the overall charge injection rate, but are at best a very indirect structural probe. We found in situ transmission spectroscopy to be useful in determining the distribution of this electronic charge, both between the inner and outer layers and within the outer layer [12,14]. These speciation data were acquired on both long [12] and short [14] time-scales. In transient situations we identified two features. Firstly, relatively low inner layer doping is sufficient to facilitate outer layer conversion. Secondly, there is localized polymer-polymer interfacial two-electron reduction of PXV 2÷ to PXV ° followed by P X V 2 + / P X V ° conproportionation. The deduction of initial inner layer charge transport control was consistent with current-voltage curve analysis [13], and the switch in the rate-limiting step to outer layer charge transport control was consistent with theoretical predictions [9]. These exclusively "electronic"-based analyses have three fundamental limitations. Firstly, speciation involved fitting absorbance data at multiple wavelengths. For thin films, error accumulation can be appreciable as the populations of all six species are successively extracted. Secondly, for both layers, it became apparent that the rate-limiting step was transfer of an ion, whose population was deduced indirectly via electroneutrality. The problem with this approach is that electroneutrality can be satisfied in more than one way, for example cation ingress or anion egress on electron injection. Furthermore, single-film studies had suggested that the species providing electroneutrality might be time-scale dependent [15]. Thirdly, the ion transfer rate will depend on the film's solvent population, which is generally redox-state dependent (via the activity constraint [16]) and specifically known to be important for PBT single films [17]. In the light of this, a direct measure of the heavy mobile species (ions and solvent) is highly desirable. The electrochemical quartz crystal microbalance (EQCM) [18] offers this capability, via in situ piezoelectric determination of film mass changes. Changes ( A M / n g cm -z) in the mass attached to a quartz crystal oscillator result in changes ( A f / H z ) in the resonant frequency from its base value ( f 0 / H z ) . Provided the attached mass is coupled rigidly to the electrode, these changes are related by the Sauerbrey equation [19]:

Z f = - (2/PoVo) A M f 2

where Po is the crystal density and v o the wave velocity within it. For the 10 MHz AT-cut crystals we employ,

(1)

(2)

so one has nanogram sensitivity. This capability has been exploited by numerous workers to monitor ion and solvent transfer at a variety of single-film polymer-modified electrodes [15,17,20-28] (see ref. 18 for a review). We now apply the technique to the study of an electroactive bilayer film, P B T / P X V .

2. Experimental The EQCM instrumentation and cell configuration were based on those of Bruckenstein and Shay [29] and have been described elsewhere [21]. Measurements were taken under potential control, using an Oxford Electrodes potentiostat. Frequency (mass)- and current-potential data were captured using a Keithley series 570 data acquisition system interfaced to an IBM ATX computer. The quartz crystals (International Crystal Manufacturing Co., Oklahoma City, OK) were 10 MHz AT-cut, coated with Au electrodes of active area 0.23 cm 2. Potentials were measured and are quoted with respect to an aqueous saturated calomel electrode (SCE). The counter electrode was a Pt gauze. Measurements were taken at room temperature, 20(+ 2)°C, under quiescent solution conditions. Reagent purification procedures are given in earlier papers [11,12]. PBT (inner) polymer films were deposited by potentiostatic polymerization of 2,2'-bithiophene (Aldrich, sublimed) at 1.225 V, as described elsewhere [30]. Previously [11-14], we employed a droplet evaporation method for subsequent PXV deposition (as the PXV2+(Br-)2 salt). Outer film uniformity was the limiting feature of this approach. The position-dependent sensitivity of the QCM to attached mass, both ex situ [31] and in situ [32], make it less tolerant to such variations. We therefore arrived at an alternative PXV deposition procedure giving uniform outer (as well as inner) films. This procedure is an electrochemical precipitation. Following PBT deposition, the quartz crystal was rinsed with CH3CN and immersed in a 0.1 mmol dm -3 solution of PXV 2+ in 0.05 mol dm -3 tetraethylammonium hexafluorophosphate (TEAPF) + CH~ CN. The Au working electrode potential was then scanned negatively from 0 V to - 2 . 0 V, back to - 1 . 4 V and held at - 1.4 V for ca. 2 rain. Whilst still under potential control, the electrode was rinsed with PXVfree 0.05 mol dm -3 T E A P F + CH3CN, followed by 1 mol dm -3 tetraethylammonium perchlorate (TEAP) + CH3CN. Following PXV deposition, the bilayer-coated electrodes were transferred to 1 mol dm -3 T E A P +

A.R. Hillman, A. Glidle /Journal of Electroanalytical Chemistry 379 (1994) 365-372

C H 3 C N for the characterization experiments described here. P X V coverage could be varied via the hold time at - 1 . 4 V *. This electrochemical deposition procedure also improved the adherence of P X V films to the PBT inner layer, resulting in a markedly improved stability of electrochemical and E Q C M responses to electrochemical manipulation. Frequency data were converted to mass data using Eq. (2). In addition to the uniformity criterion (above), this equation only holds for rigidly attached films. We have verified this directly (by crystal impedance measurements [33]) for PBT, and indirectly (by invariance of the normalized mass change with film thickness [18]) for the PXV component.

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3. Results and discussion

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We compare the characteristics of P X V (single layer) and P B T / P X V (bilayer) films. The data are presented in parallel style, commencing with the raw data ( I - E and AM-E curves), and followed by derivative data and m a s s - c h a r g e correlations. - 4 0 0

3.1. PXV films Figure 1 shows I - E and AM-E curves for a PXV single film, which may be compared with those for self-assembled viologen films on Au E Q C M electrodes [34,35]. T h e r e is a pair of peaks centred at - 0 . 3 0 V and another at - 0 . 7 4 V, attributable to the P X V 2/÷ and P X V ÷/° couples respectively. These formal potentials are ca. 0.2 V more positive than observed for the polystyrenesulphonate complex of P X V [36]. Each of these reduction steps is accompanied by a mass loss (see Fig. l(b)) from the film, consistent with the expulsion of anion anticipated on electroneutrality grounds. Overall mass changes, represented by a value of AMF/Q = - 1 4 0 g mo1-1 (of univalent species), are in excess of those for anion transfer alone. Consideration of this overall mass change in isolation is misleading, since AM-Q plots (exemplified by Fig. 2) are markedly non-linear. During the initial 25% of the PXV 2+/÷ reduction process, the normalized mass change is ca. - 4 0 0 g mo1-1. This indicates appreciable solvent transfer (here expulsion), a process generally required by a change in film charge state [16]. During the remainder of the first electron reduction process and the entire P X V ÷/° reduction process, the normalized mass change is ca. - 9 5 g mol-1. This is close to that

* All coverages were determined by integration of slow scan voltammetric c u r r e n t - v o l t a g e responses. PBT coverages were determined prior to PXV deposition.

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Fig. 1. Voltammetric EQCM data for a PXV single film: (a) I vs. E, (b) AM vs. E. Fpxv=4.5 nmol cm -2, solution 1.0 mol dm -3 TEAP + CHaCN, scan rate 20 mV s-1, scan commenced at 0 V in a negative direction, arrows indicate the scan direction.

anticipated for anion transfer alone: compare the experimental data with the dashed line in Fig. 2. Although there is some hysteresis in the mass vs. charge plots, it is insufficient to attribute the variation ot solvent transfer with redox state to kinetic effects. The extent of solvent transfer is highlighted in two ways. First, we display the mass change data in differential form, as an M - E plot (Fig. 3(a)), where we use the " d o t " notation to denote time differentials. When normalized with the Faraday constant and molar mas., of anion, this should appear identical with the T - L

A.R. Hillman, A. Grid&/Journal of Electroanalytical Chemistry 379 (1994) 365-372

368

curve of Fig. l(a)), if the anion alone were transferred. Comparison of M (in Fig. 3(a)) and I (in Fig. l(a)) shows that this is approximately the case during the second reduction step, but not the first. In order to extract solvent transfer information explicitly, we have previously [26] introduced the function q~, defined for a selected ion " j " , by

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(4)

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is the net neutral species flux in terms of the mass and charge fluxes, ~/ and I respectively. Throughout, we consider the case j = CIO 4, for which the charge z i =

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j J

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- 1; for presentational clarity we drop the subscript on 4. In Fig. 3(b) we plot the neutral species (.solvent) flux as a function of potential. The value of ~ (Fig. 3(b)), representing the solvent flux, is appreciable during the early stages of PXV 2+ reduction (and final stage of PXV + oxidation, on the reverse scan), but is otherwise close to zero.

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3.2. P B T / P X I / bilayers

c m -2

Fig. 2. Plot of AM vs. Q for the data of Fig. 1. Values are referred to the initial PXV z+ state, with reduction charge increasing to the left. The dashed line has a slope corresponding to A M F / Q = - 9 9 . 5 g m o l - i.

3.2.1. Qualitative observations on raw data Typical slow scan voltammetric EQCM data for a P B T / P X V bilayer are shown in Fig. 4(a). The trace

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A.R. Hillman, A. Glidle /Journal of Electroanalytical Chemistry 379 (1994) 365-372

shows the responses for the first potential cycle and the initial part of the second cycle for a newly fabricated electrode. (The remainder of the second cycle is similar to the first and is omitted for presentational clarity.) It has all the characteristics of an "ideal" bilayer: no current flows when E --- Epxv2+/- or E - Epxv+/0, and oxidation (reduction) current only flows when E approaches E~,BT0/+ (E~,BT0/-). In summary, outer layer redox processes occur exclusively by mediated charge transfer via the inner layer when the latter is in one of its conducting states. These responses show marked improvements in stability and reproducibility over previous work [11]; otherwise they are as expected. Figure 4(b) shows the simultaneously determined mass changes. T h e first (obvious) result is that bilayer mass changes only occur as E approaches E~,BT0/÷ (during positive potential scans) and E~BTO/- (during negative potential scans). This is not a trivial result. The absence of current flow in the region between ca. - 0 . 8 V and +0.2 V obviously implies no net ion flux. However, electrochemical data offer no information concerning the flow of net neutral species (salt and solvent). The absence of significant film mass changes in this region demonstrates the absence of electrostrictive effects. The second result relates to the potentials at which charge and mass fluxes become significant. During positive potential scans, current onset occurs ca. 0.55 V negative of E~,BT0/+. Spectroscopic data [12] showed linear " N e r n s t " plots, with slopes of 180 m V per decade, suggesting that only 0.1% inner layer conversion is required to support mediated oxidation of the outer layer. (This is a consequence of the large driving force for the P B T + / P X V ° cross-reaction.) A similar situation applies to reduction of P X V 2+ by P B T - . Thirdly, we note the direction of the mass flux during charge trapping. Consider the bilayer in an initial P B T ° / P X V 2+ state, subjected to a negativegoing potential scan. On injection of electronic charge (as E approaches E~,BT0/-), the electrons may reside primarily in the inner or outer layer: electrochemical data provide no information as to their fate. Electron injection into the inner layer corresponds to the following process. * :

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(b) E/V Fig. 4. Voltammetric EQCM data for a PBT/PXV bilayer: (a) I vs. E, (b) AM vs. E. FpBT = 5.4 nmol cm -2, FPXV = 8.0 nmol cm 2, solution 1.0 mol dm -3 TEAP+CH3CN, scan rate 5 mV s -l, scan commenced at 0 V from the PBT°/PXV 2÷ state in a positive direction, arrows and numbers indicate scan direction and number respectively.

[PBT°/PXV2+.(A-)2]p + e-+ C[ = [PBT-.C+/PXV2+.(A-)2]p

(5)

where subscripts p and s denote polymer and solution phases respectively, and C + and A - are the cation and

anion present in the electrolyte. Electron injection into the outer layer results in the following overall process: [PBT°/PXV2+.(A-)2]p + e= [ P B T ° / P X V + A - ] p + A~-

(6)

On the relatively long time-scale of the experiment of Fig. 4, this would be followed by * For the moment, we only consider the ionic species, i.e. satisfaction of the electroneutrality condition. Solvent transfer is considered later.

[ P B T ° / P X V + . A - ] p + e - = [ P B T ° / P X V ° ] p + A~-

(7)

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A.R. Hillman, A. Glidle /Journal of Electroanalytical Chemistry 379 (1994) 365-372

The fundamental difference is that reaction (5) would result in a bilayer mass increase, and reactions (6) and (7) a mass decrease. The experimental result (see Fig. 4(b) is a mass decrease. This is consistent with the dominance of reaction (6) (and (7)): anion ejection consequent on the outer layer acting as the primary sink for injected electronic charge. The rather smaller mass increase at the negative end of the negative potential excursion is attributed to cation entry into the inner layer as a consequence of its n-doping:

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[ P B T ° / P X V ° ] p + e - + Cs+ = [ P B T - C + / P X V ° ] p

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These cations are ejected (as evidenced by the decrease in film mass) on scan reversal and undoping of the inner layer. Fourthly, we consider the mass change on charge untrapping. The mass initially increases, as the reverse of reactions (6) and (7) predominate. After the initial current peak, the outer layer is fully oxidized, and can no longer act as a source of electrons. Any further oxidation must therefore be associated with the inner layer. This p-doping-undoping process has been described previously for PBT single films [17,28]. Although ion transfer dominates, there are significant salt and solvent fluxes in opposite directions [17] *. In the experiment of Fig. 4, which involves both p- and n-doping of the PBT inner layer, we observe an additional feature. In the potential region attributed to PBT p-doping, if the PBT has been n doped since the last p-doping half-cycle, the current response shows additional structure in the form of two (incompletely resolved) additional small peaks (compare scans 1 and 2 in Fig. 4(a)). This characteristic was also seen in control experiments on PBT single films. To summarize, all PBT doping-undoping features appear relatively unperturbed by the PXV outer layer. The other primary difference between the first and subsequent scans is that there is no anodic "untrapping" peak. This is simply because, in the absence of a prior negative potential excursion, the outer layer is still in its oxidized form. Thus, the response on the initial positive excursion is solely inner layer related, whilst that for subsequent scans reflects the change in outer layer history. 3.2.2. Quantitative description

We now quantify the mass-charge relationship in two respects: overall and during the charge (un)trap-

* These net neutral species fluxes roughly compensate each other in mass terms. O n the long time-scale of this experiment they are unresolved, giving the illusion of permselectivity, an effect we have termed "apparent permselectivity".

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Fig. 5. Plot of z~M vs. Q for the data of Fig. 4. Values are referred to the initial PBT°/PXV 2+ state, with reduction charge increasing to the left. The dashed line has a slope correspondingto AMF/Q = - 99.5 g mol- 1.

ping process. A plot of AM vs. Q for the experiment of Fig. 4 is shown in Fig. 5. The values of the overall normalized mass changes (with the same caveat as for PXV single films) for the data of Fig. 4 are A M F / Q = - 8 5 g mol - t for the PBT +/° region and A M F / Q = - 6 6 g mol-1 for the PXV 2+/÷/° region. If reaction (6), followed by reaction (7), were a complete description of events in Fig. 4, we would obtain A M F / Q = - 9 9 . 5 g mo1-1 ( A - - C 1 0 ~ - ) . The numerically smaller value observed suggests that anion expulsion is accompanied by the entry of some other species. There are two possibilities: entry of T E A ÷ (as a dopant during n-doping of PBT) or entry of solvent (associated with the general requirement [16] for solvent transfer). In the first instance, we note that n-doping of PBT (which dominates the negative end of the scan) is a prerequisite for PXV 2+ reduction. Pursuing this possibility further, the gravimetric data would require that ca. 17% of the electronic charge injected into the bilayer to achieve PXV 2+ reduction would have to be associated with PBT n-doping. Previous spectroscopic data [12] demonstrated that outer layer reduction requires far less inner layer reduction than this. We therefore reject this hypothesis. In the second instance, we note that solvent transfer has previously been invoked both for PBT [17] and PXV (see Fig. 3) single film redox conversion. This hypothesis appears more reasonable: the quantity of solvent involved for the bilayers is equivalent to ingress,

A.R. Hillman, A. Glidle/Journal of Electroanalytical Chemistry 379 (1994) 365-372

on average, of around one solvent molecule per viologen redox site over the two electron reduction process. At this point we cannot explain why the solvent transfers in a different direction on PXV reduction in single and bilayer films. We can only presume that this is a consequence of the presence of the PBT inner layer: the A u - P X V and P B T - P X V interfaces will be rather different. Although it is convenient, and indeed common (see papers cited in ref. 18), to focus on overall mass changes, these do not tell the whole story. The mass change vs. charge plots (exemplified by Fig. 5) are curved. This indicates that the amount of solvent transferred per redox site converted varies during the conversion process. Typically, we found that there was substantially more solvent transfer associated with the early stages of conversion, and that counter ion transfer dominated the later stages of conversion. This effect is not a feature of the bilayer configuration, since single PXV films also transfer solvent (albeit the other way, as discussed above) in the early stages of their reduction. We do not believe the effect to be kinetic in nature, since neutral species would be expected to move more slowly than charged species [37]. In Fig. 6 we plot the mass (ion + solvent) flux and neutral species (solvent) fluxes as functions of potential. The qualitative similarity of M (in Fig. 6(a)) and I (in Fig. 4(a)) is immediately apparent *. This is a consequence of the fact that the species dominating mass transfer, C 1 0 4 - , is of the same charge type as the electron. The only qualitative difference is at the negative end of the potential interval explored, where the mass and charge fluxes are in opposite directions. This is because at the negative end of the scan, when all the PXV 2÷ has been reduced (with concomitant anion expulsion), the dominant process becomes PBT ° reduction, which is associated with cation entry (reaction (8)). While the species crossing the metal Ipolymer interface is the same in both cases, the species crossing the polymer Isolution interface is of opposite sign. Hence the mass flux changes sign, but the charge flux does not. The value of q)j (Fig. 6(a)), representing the solvent flux, is appreciable during the early stages of bilayer reduction, but falls to small values as outer layer charge trapping proceeds.

3.3. Comparison of single and bilayer films The I-E and A M - E responses for the single film are totally different from those for the bilayer (com-

* T h e numerical similarity of the data (factor of 103 difference in y-scale) is a consequence of the fact that the Faraday constant is close to 105 C mol 1 and the molar mass of the species (perchlorate) dominating the mass flux is close to 102 .

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pare Figs. 1 and 4). This is entirely predictable on the basis of Murray's original model [1]. However, the AM-Q relationships for the single and bilayer films (Figs. 2 and 5) are qualitatively similar. We speculate that the quantitative differences, attributed to differing solvent transfer characteristics, may be due to differing morphologies of PXV films cast on Au and PBT. The key conclusion from the bilayer-single PXV film comparison is that the mass responses are dominated by ion and solvent transfers t o / f r o m the outer (viologen) layer, but these changes occur at potentials dictated by the characteristics (doping potentials) of the inner layer. Appreciable charge and mass transfers t o / f r o m the inner layer only occur when outer layer conversion is largely complete. This is a consequence of two factors: first, the energetics of the inner l a y e r / o u t e r layer mediated reactions are thermodynamically very favourable and, second, we have been successful in making inner layer charge transport more efficient. In the latter case, the result is that only a very small fraction of inner layer sites need to be converted

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for efficient outer layer conversion to proceed. (Alternatively, in the terminology commonly applied to conducting polymers, only low doping levels are required.)

4. Conclusions The EQCM can provide a means of probing the distribution of charge between the inner and outer layers of an electroactive bilayer. In the case of P B T / PXV bilayers, the charge- and mass-potential relationships are determined by the spatial distribution of the two polymers. Outer layer redox conversion can only proceed at potentials where the inner layer becomes conducting. The mass change on medium-long time-scales is dominated by anion transfer: ejection on PXV 2+ reduction and entry on PXV ° oxidation. Consequently, despite their dramatically different currentpotential curves, PBT/PXV bilayer films and PXV single films have relatively similar mass-charge relationships. PXV reduction is also accompanied by a small amount of solvent transfer: exit for PXV single films and entry for PBT/PXV bilayer films. In both cases, solvent transfer is restricted to the initial stages of reduction.

Acknowledgments We thank the SERC ( G R / E 78104) and the NSF (CHE-9115462) for financial support. References [1] H.D. Abruna, P. Denisevich, M. Umana, T.J. Meyer and R.W. Murray, J. Am. Chem. Soc., 103 (1981) 1. [2] P. Denisevich, K.W. Willman and R.W. Murray, J. Am. Chem. Soc., 103 (1981) 4727. [3] M. Aizawa and H. Shinohara, Synth. Met., 18 (1987) 711. [4] H. Nishihara, H. Asai and K. Aramaki, Faraday Trans., 87 (1991) 1771. [5] H. Nishihara, M. Noguchi and K. Aramaki, Chem. Commun., (1987) 628. [6] P.G. Pickup, W. Kutner, C.R. Leidner and R.W. Murray, J. Am. Chem. Soc., 106 (1984) 1991.

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