Charge transport through osmium-containing redox polymers in nitrophenyl-based solvents: Effect of solvent size

Most studies on osmium-containing redox polymers have been carried out in aqueous media. The rate of charge transport through the osmium-containing redox polymer as a function of solvent properties has not been investigated. This paper reports studies of osmium-containing redox polymers in five nitrophenyl-based solvents: nitrobenzene, 2-nitrotoluene, 3-nitro-o-xylene, 2-nitrophenyl pentyl ether, and 2-nitrophenyl octyl ether. These solvents differ from solvents conventionally used for studying osmium-containing redox polymers, such as water and methanol, in that they have both high dipolarities and larger molecular sizes. Both chronoamperometry and cyclic voltammetry studies on those solvents indicate that it is the solvent size rather than the solvent polarity that determines the rate of charge transport through the osmium-containing redox polymer. The larger the solvent size, the slower the charge transport. We also found that the type of counterion has a strong effect on the cyclic voltammetry of the osmium-containing redox polymers in nitrophenyl-based solvents. No cyclic voltammetry waves were observed when using tetraphenylborate-based supporting electrolyte. Charge transport activation enthalpy-entropy compensation was observed for the redox reactions of the osmium-containing polymer in methanol, nitrobenzene, and 2-nitrotoluene. This suggests that the effect of solvent on the rate of charge transport is likely to involve ion pairing and the associated solvation and polymer rearrangements accompanying the making and breaking of the ion pairs.