J. Phys. Chem. 1994, 98, 917-923
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Electron and Counterion Diffusion Constants in Mixed-Valent Polymeric Osmium Bipyridine Films Nigel A. Surridge,+Connie S. Sosnoff,* R. Schmehl,g John S. Facci,l and Royce W. Murray’ Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received: June 24, 1993; In Final Form: November 2, 1993’
Recent theory has shown that, in transient electrochemical oxidation or reduction of a redox polymer film, electroneutrality coupling between concurrently transported electrons and counterions in the polymer film can enhance the rate of electron-hopping transport as expressed by the apparent electron diffusion coefficient De,app. We have investigated this problem in the redox polymers poly[Os(2,2’-bipyridine)z(N-(4-pyridyl)cinnamamide)z12+ (poly-I), poly[Os(2,2’-bipyridine)2(4-~inylpyridine)2] 2+ (poly-11), and poly[Os(4-viny1-4’-methyl2,2’-bipyridine)312+ (poly-111) by measuring and comparing the diffusion coefficients (DcJ of C1- counterions in the cationic polymer films to the transient chronoamperometric (De,app) and steady-state (De) electron diffusion coefficients. At room temperature, the ratio u = Dion/Dcis 1.25,0.16, and 0.076 for the three redox polymers, respectively. According to the electroneutrality coupling theory, the coupling effect is insignificant in poly-I films a t room temperature, with and without dilution of the Os centers by copolymerized R u complexes, but at lowered temperature, assuming u 0, coupling in poly-I has a substantial effect. Electroneutrality coupling in poly-I11 films enhances the room temperature chronoamperometric over the (“true”) steadystate De value by cu. 2-fold. The lack of agreement between experiment and bimolecular electron self-exchange theory for Ru-diluted poly-I films is proposed to lie in the differing barrier heights for electron transfer and for microscopic (bounded) diffusion of Os sites, as reflected in an Os site concentration-dependent activation barrier for De,app.
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Electron transport in polymeric solids has seen extensive investigation over the past decade on theoretical and experimental fronts. These polymers include electronically delocalized materials (e.g., the polyheterocyclesl polypyrrole and polythiophene and certain organometallic assembliesz) and redox polymers containing electronically localized (e.g., polymeric metal bipyridines, viologens, quinones, and ferrocenes). Electron transport in redox polymers occurs by site-to-siteelectron hopping (donor/acceptor electron self-exchange), and its rate becomes importad-5 in their uses for electrocatalysis and electroanalysis. This paper deals with electron self-exchangedynamics in redox polymer films electropolymerized6onto electrodes from solutions of the metal polybipyridine complexes shown in Figure 1. In particular we address the extent of electroneutrality coupling7 between the electron self-exchange rate and that of the concurrent diffusive transport of charge-compensating counterions that is required during transient (i.e., chronoamperometric) charging of such films. Electropolymerization of the metal complexes 1-111 (Figure 1) gives spatially well-defined, structurally dense films in the 10-300-nm-thickness range, which have been investigated with respect to their electrocatalytic activity,6c,d,8 molecular sieving ~ermeability,~ and electron transport dynamics.6J0J In an early experiment,IO I was copolymerized with its Ru analog, and the OS^+/^+ electron self-exchange rates in the co-poly-I(Os,Ru) copolymer measured as a function of the mole fraction (xa) of Os sites in the film. The self-exchange rates were measured by the transient technique, chronoamperometry. Expressed as apparent electron diffusion coefficients3 (De,app), lowering xa from 1.0 to ca. 0.5 caused the rates to decrease as would be expected for biomolecular, nearest-neighbor Os3+/2+ electron exchange. A plateau in and an increase in the activation Current address: Boehringer-Mannheim Corp., Indianapolis, IN 462500100.
t Current address: Centers for Disease Control and Prevention, Atlanta,
GA 30333.
I Current address: Tulane University, New Orleans, LA 701 18. I Current address: Xerox Corp., Webster, NY 14580. *Abstract published in Advance ACS Abstracts, December 15, 1993.
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CH3
2+
111 ~oly-[Os(vb~y)~I~+
Figure 1. Metal complex monomers electropolymerized to prepare the films in this study.
barrier E A at lower xa was rationalized*0aas an increasing role of microscopic Os site diffusivity. Although it was an early example of a redox polymer electron self-exchange rate measurement,the co-poly-I(Os,Ru) copolymer dataloa remain the only results available for a redox polymer in which the electron-hopping sites have been diluted isostructurally.lZ Isostructural dilution ensures that effects of donor/ acceptor site concentration on can be examined apart from the potential effects of changing structural aspects of the polymer phase. That such an ostensibly ideal system produced unexpected behavior invites further analysis and explanation. In a recent theoretical study7 of coupling between the rates of electron self-
0022-365419412098-0911%04.50/O @ 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 3, 1994
exchange and concurrent, electroneutrality-driven counterion diffusion during transient redox film charging, it was suggested that the De,app valuesloa in co-poly-I(Os,Ru) films might have been elevated by such coupling. The suggestion was based on an assumption7that thecounterion diffusivity Di,, was much smaller than the true electron diffusivity De. This paper examines the validity of that assumption' by developing a measurement of the actual counterion diffusivity. The Dion measurement is based on permeability9 of the electrooxidizable counterion C1- through redox polymer films prepared from complexes 1-111 to the underlying Pt electrode. Assuming that C1- and C104- diffusivities should be similar, the Dionvalues are compared to electron diffusivities obtained by chronoamperometric (De,aw) and steady-state methods. Steady-state methods" avoid concurrent, macroscopic counterion transport and produce more reliable electron diffusivities (Devalues). We additionally present and analyze, according to the theory,' previously unpublishedlObDe,app data obtained in co-poly-I(Os,Ru) films a t lowered temperature, where it is more certainlld-' that Dionvalues are small.
Experimental Section ChemicalsandMonomers. Acetonitrile (CHJCN, Burdickand Jackson) was used as received and stored over 4-A molecular sieves. Tetraethylammonium perchlorate (Et4NClOd) was recrystallized twice from water. Preparations of [Os(bpy)~(vpy)~](C104)z (IO, [Os(vbpy)~I(C104)~ (IW, and [(Os or Ru)(bpy)z(pcinn)~](C104)2(I) were as reported before.6 (bpy = 2,2'bipyridine, vpy = 4-vinylpyridine, vbpy = 4-methyl-4'-vinyl-2,2'bipyridine, p-cinn = N-(4-pyridyl)cinnamamide). Preparation of Films. Thin films of the redox polymers were formed on Pt disk electrodes by reductive electropolymerization under an inert atmosphere by scanning the electrode potential between ca. -1 .Oand -1.7 V us SSCE in 0.06-0.3 M Et4NC104/ CHJCN solutions containing between 0.5 and 2 mM of the metal complex monomers.6 Electrochemistry. Permeation Measurements. Rotated disk voltammetry of C1- oxidation at redox polymer-coated electrodes was performed in 0.1 M Et4NC104/CH$N solutions containing Et4NCI with a Pine Instruments MSR rotator using previously described procedures.9 Permeation-limited currents (e.g., Figures 3 and 4) were taken by stepping the potential from 0.65 to 1.3 V us Ag/AgCl, measuring iLim when the current had settled to a steady value. Minor correction for a slow film degradation (probably due to the C12 electrode reaction product), depressing permeation currents by 5-1 5% over a series of the measurements, was made by assuming a time-linear decay. Cyclic voltammetry was performed with conventional homebuilt potentiostats. Chronoamperometry. Current-time responses were obtained at redox polymer-coated Pt electrodes in 0.3 M Et4NC104/CH3C N solutions in a cell fitted with a Luggin reference electrode probe placed
