Direct Observation of Photodriven Intermolecular Hole Transfer across TiO2 Nanocrystallites: Lateral Self-Exchange Reactions and Catalyst Oxidation

Photoselection of Ru(II)-polypyridyl sensitizers with polarized pulsed-light excitation, when anchored to TiO2 nanocrystallites interconnected in a mesoporous thin film, results in an anisotropic distribution of excited sensitizers. Under conditions where excited-state sensitizers efficiently inject electrons into TiO2, the resulting oxidized sensitizers exhibit an initial anisotropy in their absorption difference spectra. Over the course of the charge-separated lifetime for many sensitizers, the transient absorption anisotropy signal decays to nearly zero indicative of lateral self-exchange hole-transfer reactions at the interface. When a cobalt metalloporphyrin catalyst was coanchored to the sensitized nanocrystalline TiO2 film, excited-state injection was followed by lateral hole transfer to oxidize the surface-bound catalyst.