We have studied the interfacial electron-transfer dynamics on TiO2 film sensitized with synthesized ruthenium(II)–polypyridyl complexes—[RuII(bpy)2(L1)] (1) and [RuII(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 RuII–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.
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