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 (RuL₃²⁺), 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.