title: High extinction coefficient Ru-sensitizers that promote hole transfer on nanocrystalline TiO₂.
abstract: Two series of Ru(II) polypyridyl compounds with formulas [(bpy)2RuL](PF6)2 and [(deeb)2RuL](PF6)2, where bpy is 2,2'-bipyridine, deeb is 4,4'-diethylester-2,2'-bpy, and L is one of several substituted 9'-(1,3-dithiole-2-ylidene)-4',5'-diazafluorene ligands, were studied as potential photosensitizers for TiO2. These compounds possess notably high extinction coefficients (≥40,000 M(-1) cm(-1) @470 nm) which are shown by time-dependent density functional theory (TD-DFT) calculations to result from overlapping metal-to-ligand charge transfer (MLCT) and ligand-localized transitions. Low-temperature absorption and photoluminescence measurements were suggestive of a short-lived MLCT excited state. When adsorbed onto TiO2 thin films, both the free ligands (L) and their corresponding [(deeb)2RuL](2+) complexes exhibited rapid excited-state electron injection into TiO2; in the case of the complexes, this was followed by rapid (k>10(8) s(-1)) hole transfer from Ru(III) to the 1,3-dithiole ring of the L ligand. Observation of diffusion-limited reductive quenching of the [Ru(bpz)3](2+)* (bpz is 2,2'-bipyrazine) excited state by the L ligands in solution supported the occurrence of intramolecular hole transfer following electron injection by the TiO2-anchored complexes.
authors.affiliation: Department of Chemical and Biological Engineering, Chalmers University of Technology, 412 96 Gothenburg (Sweden). abmaria@chalmers.se.
authors.forename: Maria
authors.initials: M
authors.lastname: Abrahamsson
authors.forename: Joachim H J
authors.initials: JH
authors.lastname: Hedberg
authors.forename: Hans-Christian
authors.initials: HC
authors.lastname: Becker
authors.forename: Aaron
authors.initials: A
authors.lastname: Staniszewski
authors.forename: Wayne H
authors.initials: WH
authors.lastname: Pearson
authors.forename: William B
authors.initials: WB
authors.lastname: Heuer
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Meyer
keywords: Ruthenium; charge transfer; dye-sensitized solar cells; extinction coefficients; hole transfer