Influence of 4 and 4′ Substituents on RuIII/II Bipyridyl Self-Exchange Electron Transfer Across Nanocrystalline TiO2 Surfaces

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, D CA, 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 D CA 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 thus the intermolecular distance. With some assumptions to estimate the intermolecular distance, the self-exchange rates were found to possess an exponential dependence with the distance, β = 1.2 ± 0.2 Å–1. Independent tests of the were carried out by varying the surface coverages from which β = 1.18 ± 0.09 Å–1 was found. The results indicate that the substituent’s steric size is the dominant factor that controls lateral RuIII/II self-exchange electron-transfer rates at these interfaces.