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Efficient Charge Separation in TiO 2 Films Sensitized with Ruthenium(II)–Polypyridyl Complexes: Hole Stabilization by Ligand‐Localized Charge‐Transfer States
Author(s) -
Verma Sandeep,
Kar Prasenjit,
Das Amitava,
Ghosh Hirendra N.
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201001798
Subject(s) - chemistry , ruthenium , electron transfer , electron donor , ligand (biochemistry) , photochemistry , electron acceptor , photoexcitation , femtosecond , excited state , crystallography , catalysis , organic chemistry , atomic physics , laser , biochemistry , physics , receptor , optics
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.