Efficient charge separation in TiO2 films sensitized with ruthenium(II)-polypyridyl complexes: hole stabilization by ligand-localized charge-transfer states

We have studied the interfacial electron-transfer dynamics on TiO(2) film sensitized with synthesized ruthenium(II)-polypyridyl complexes--Ru(II)(bpy)(2)(L(1))] (1) and Ru(II)(bpy)(L(1))(L(2))] (2), in which bpy=2,2'-bipyridyl, L(1)=4-2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol, and L(2)=4-(N,N-dimethylaminophenyl)-2,2'-bipyridine-by using femtosecond transient absorption spectroscopy. The presence of electron-donor L(2) and electron-acceptor L(1) ligands in complex 2 introduces lower energetic ligand-to-ligand charge-transfer (LLCT) excited states in addition to metal-to-ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse-width-limited (<100 fs) electron injection from populating LLCT and MLCT states are observed on account of strong catecholate binding on the TiO(2) surface. The hole is transferred directly or stepwise to the electron-donor ligand (L(2)) as a consequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron-donor (L(2)) ligand and the electron injected in TiO(2) nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) process in the 2/TiO(2) system relative to the 1/TiO(2) system. Our results suggest that Ru(II) -polypyridyl complexes comprising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with Ru(II)-polypyridyl complexes comprising MLCT states only.