IONIC INTERACTIONS AND CHARGE-TRANSPORT PROPERTIES OF METALLOPOLYMER FILMS ON ELECTRODES

The effects of supporting electrolyte concentration, redox site loading, and temperature on the electrochemical response of [Os(bpy)(2)(PVI)(n)Cl](+) films, where bpy is 2,2'-dipyridyl and PVI is poly(N-vinylimidazole), are investigated. The cyclic voltammetric, chronoamperometric, and sampled current responses of these modified electrodes are unusually ideal in p-toluenesulfonic acid (pTSA) containing solutions. The dependence of the cell resistance on electrolyte concentration over the range 0.1-1.0 M suggests that the films contain rather large amounts of electrolyte and water, and that Donnan exclusion has failed. The dependence of the formal potential on pTSA concentration suggests that, for high redox site loadings, both the reduced and oxidized forms of the redox couple are ion-paired, while for lower loadings only the oxidized form is ion-paired. This is consistent with measurements of the interfacial capacitance that suggest a lower effective dielectric constant within the polymer film at high redox site loading. The difference in ordering between the reduced and oxidized forms of the redox couples has been probed from the temperature dependence of the formal potential. These data reveal that the local microenvironment of the immobilized redox centers becomes similar to that found in aqueous solution at high supporting electrolyte concentrations. The apparent diffusion coefficient for charge transport D-CT has been evaluated using cyclic voltammetry and chronoamperometry. These two techniques give values of DCT that are indistinguishable within experimental error. In agreement with Dahms-Ruff theory, the rate of charge transport depends linearly on the osmium redox site concentration. These data, together with the low film resistance, and the dependence of D-CT on supporting electrolyte concentration, suggest that electron hopping is the rate-limiting step for these systems. The self-exchange rate constant obtained is (6.0 +/- 0.3) x 10(5) M(-1) s(-1) and is independent of the supporting electrolyte concentration.