Subpicosecond photoinduced charge injection from "molecular tripods" into mesoporous TiO2 over the distance of 24 angstroms

Extended rigid tripodal sensitizers were used to investigate the rate of long-distance photoinduced charge transfer from the MLCT excited states of RuII-based chromophores into mesoporous TiO2 films. The distance between the RuII center and the surface of the semiconductor was 24 A. Rapid biexponential charge injection with a major subpicosecond component as fast as 240 fs was observed upon femtosecond laser excitation of the tripods bound to the TiO2 surface. This rate exceeds the typical rates of vibrational cooling and thus strongly supports the possibility of "hot electron injection" occurring at very large donor-to-semiconductor distances.