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

Abstract

We have studied the interfacial electron-transfer dynamics on TiO₂ film sensitized with synthesized ruthenium(II)–polypyridyl complexes—[Ru^II(bpy)₂(L¹)] (1) and [Ru^II(bpy)(L¹)(L²)] (2), in which bpy=2,2′-bipyridyl, L¹ = 4-[2-(4′-methyl-2,2′-bipyridinyl-4-yl)vinyl]benzene-1,2-diol, and L²=4-(N,N-dimethylaminophenyl)-2,2′-bipyridine—by using femtosecond transient absorption spectroscopy. The presence of electron-donor L² and electron-acceptor L¹ 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 TiO₂ surface. The hole is transferred directly or stepwise to the electron-donor ligand (L²) 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 (L²) ligand and the electron injected in TiO₂ nanoparticles (NPs). Thus, we observed a significant slow back-electron-transfer (BET) process in the 2/TiO₂ system relative to the 1/TiO₂ 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 RuII–polypyridyl complexes comprising MLCT states only.