Light-Driven Tyrosine Radical Formation in a Ruthenium-Tyrosine Complex Attached to Nanoparticle TiO2

We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO2. The complex mimics the function of the tyrosineZ and chlorophyll unit P680 in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO2 via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO2. The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 s with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO2 to the photogenerated Ru(III).