Lateral Electron and Hole Hopping between Dyes on Mesoporous ZrO2: Unexpected Influence of Solvents with a Low Dielectric Constant

Lateral intermolecular charge transfer between photosensitizers on metal oxide substrates is important for the understanding on the overall working principles of dye-sensitized systems. Such studies usually concentrate on either hole or electron transfer separately and are conducted in solvents with a high dielectric constant (epsilon s) that are known, however, to show a drastic decrease of the local dielectric constant close to the metal oxide surface. In the present study, both hole and electron hopping between organic donor-acceptor photosensitizers was experimentally investigated on PB6 dye-sensitized mesoporous ZrO2 films. The donor (close to the surface) and acceptor (away from surface) subunit of the PB6 dye were observed to be involved in hole and electron hopping, respectively. Hole and electron transfer kinetics were found to differ remarkably in high-epsilon s solvents, but similar in solvents with epsilon s < 12. This finding indicates that low-epsilon s solvents maintain similar local dielectric constant values close to, and further away from, the semiconductor surface, which is different from the previously observed behavior of high dielectric constant solvents at a metal oxide interface.