A Distance Dependence to Lateral Self-Exchange across Nanocrystalline TiO₂. A Comparative Study of Three Homologous Ru^III/II Polypyridyl Compounds

Self-exchange intermolecular Ru III/II electron transfer, a process commonly referred to as “hole-hopping”, is of great interest as it provides a means of charge transport across the surface of nanocrystalline (anatase) TiO 2 mesoporous thin films without the loss of free energy. This process was characterized by cyclic voltammetry and chronoabsorptometry for three homologous Ru diimine compounds of the general form [Ru(LL) 2 (dcbH 2 )](PF 6 ) 2, where LL is 2,2′-bipyridine ( bpy ), 4,4′-dimethyl-2,2′-bipyridine ( dmb ), or 4,4′-di- tert -butyl-2,2′-bipyridine ( dtb ) and dcbH 2 is 2,2′-bipyridyl-4,4′-dicarboxylic acid. Apparent electron diffusion coefficients, D, abstracted from this data increased with dtb < bpy < dmb . Both techniques were consistent with this trend, despite differences in the magnitude of D between the two methods. Temperature dependent measurements revealed an activation barrier for electron self-exchange of 250 ± 50 meV that was within this error the same for all three diimine compounds, suggesting the total reorganization energy, λ, was also the same. Application of Marcus theory, with the assumption that the 900 ± 100 meV total reorganization energy for self-exchange electron transfer was independent of the Ru compound, revealed that the electronic coupling matrix element, H AB, followed the trend dtb (0.02 meV) < bpy (0.07 meV) < dmb (0.10 meV). The results indicate that insulating side groups placed on redox active molecules can be utilized to tune the electronic coupling and hence self-exchange rate constants without significantly altering the reorganization energy for electron transfer on TiO 2 surfaces.