Solvent effects on charge transport through solid deposits of [Os(4,4′-diphenyl-2,2′-dipyridyl)2Cl2]

Abstract

Mechanically attached, solid-state films of [Os(4,4′-diphenyl-2,2′-dipyridyl)₂Cl₂] have been formed on gold macro- and microelectrodes and their voltammetric properties investigated. The voltammetric response of these films associated with the Os^2+/3+ redox reaction is reminiscent of that observed for an ideal reversible, solution phase redox couple only when the contacting electrolyte contains of the order of 40% v/v of acetonitrile (ACN). The origin of this effect appears to involve preferential solvation of the redox centres by acetonitrile which facilitates the incorporation of charge compensating counterions. Scanning electron microscopy reveals that voltammetric cycling in 40 ∶ 60 ACN–H₂O containing 1.0 M LiClO₄ as the electrolyte induces the formation of microcrystals. Voltammetry conducted under semi-infinite linear diffusion conditions has been used to determine the apparent diffusion coefficient, D_app, for homogeneous charge transport through the deposit. The dynamics of charge transport decrease with increasing film thickness but appear to increase with increasing electrolyte concentration. These observations suggest that ion transport rather than the rate of electron self-exchange limit the overall rate of charge transport through these solids. When in contact with 40 ∶ 60 ACN–H₂O containing 1.0 M LiClO₄ as electrolyte, D_app values for oxidation and reduction are identical at 1.7 ± 0.4 × 10⁻¹² cm² s⁻¹. In the same electrolyte, the standard heterogeneous electron transfer rate constant, k°, determined by fitting the full voltammogram using the Butler–Volmer formalism, is 8.3 ± 0.5 × 10⁻⁷ cm s⁻¹. The importance of these results for the rational design of solid state redox active materials for battery, display and sensor applications is considered.