Influence of 4 and 4′ Substituents on RuIII/IIBipyridyl Self-Exchange Electron Transfer Across Nanocrystalline TiO2Surfaces

Abstract

Lateral self-exchange electron transfer across oxide surfaces is important to many solar energy capture and conversion schemes. Substituent effects on lateral RuIII/II self-exchange electron transfer were studied using a series of RuII polypyridyl compounds of the type [Ru(R2bpy)2(P)]2+, where P is 2,2′-bipyridyl-4,4′-diphosphonic acid and R2bpy was a 4,4′-substituted-2,2′-bipyridine with six different R groups: −OCH3, −C(CH3)3, −CH3, −H, −Br, and −CF3. These functional groups were chosen mainly for their electron-withdrawing or -donating ability. Chronoabsorptometry was used to probe the apparent diffusion coefficient, DCA, that was proportional to the self-exchange rate constants, and these values were found to be between 2.8 × 10–11 and 7.9 × 10–9 cm2/s. The measured DCA values showed no correlation with the electron-withdrawing or -donating ability of the functional groups, but were instead correlated with the steric size of the substituents that also influenced the saturation surface coverage and thu...