A Rhodium Centered Supramolecular Complex as a Photoinitiated Electron Collector

The research presented here is focused on photochemical studies of a supramolecular structural motif for photoinitiated electron collection. The complex studied, {(bpy)2Ru(dpp)}2RhCl25, is of the form LA-BL-EC-BL-LA comprising a Ru(II) polyazine light absorber (LA) bearing two bpy (bpy = 2,2'-bipyridine) ligands, two dpp (dpp = 2,3'-bis(2-pyridyl)pyrazine) bridging ligands (BL), and a central rhodium(III) electron collector (EC). Ruthenium-polyazine light absorbers are commonly used in light to energy conversion systems due to the intense metal to ligand charge transfer (MLCT) absorptions observed in the visible spectrum. Electrochemical methods establish rhodium as the site of localization of the lowest lying unoccupied molecular orbital, while phosphorescence measurements are used to study electron transfer within the supramolecular assembly. Electrochemical and photochemical experiments show that the absorption of light in the rhodium centered supramolecular complex, {(bpy)2Ru(dpp)}2RhCl25, can initiate the sequential transfer of multiple electrons to the rhodium metal center, i.e. photoinitiated electron collection. A mechanistic study of photoinitiated electron collection, involving a Stern-Volmer analysis of emission quenching and product formation, is also presented, to determine the rate constants of the possible excited state and ground state reactions. One application of a molecular device for photoinitiated electron collection is fuel production. It has been shown that the complex studied, {(bpy)2Ru(dpp)}2RhCl25, can collect reducing equivalents and use them to catalyze the reduction of water to hydrogen. Photocatalytic hydrogen generation experiments will also be presented including initial quantum yield optimization experiments.