Photochemical charge transfer and hydrogen evolution mediated by oxide semiconductor particles in zeolite-based molecular assemblies

Two integrated systems for light-induced vectorial electron transfer are described. Both utilize photosensitized semiconductor particles grown in linear channel zeolites as components of the electron transfer chain. One system consists of internally platinized zeolites L and mordenite containing TiO2 particles and methylviologen ions, with a size-excluded photosensitizer, tris(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium (RUL(3)(2+)), adsorbed on the external surface of the zeolite/TiO2 composite. In the other system, Nb2O5 replaces TiO2. The kinetics of photochemical electron transfer reactions and charge separation were studied by diffuse reflectance flash photolysis. Despite very efficient initial charge separation, the TiO2 system does not generate hydrogen photochemically in the presence of an electrochemically reversible, anionic electron donor, methoxyaniline N,N'-bis(ethyl sulfonate). Only the Nb2O5-containing composites evolved hydrogen photochemically under these conditions. These results are interpreted in terms of semiconductor band energetics and the irreversibility of electron transfer from Nb2O5 to intrazeolitic platinum particles.