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Intramolecular and Lateral Intermolecular Hole Transfer at the Sensitized TiO2 Interface

TitleIntramolecular and Lateral Intermolecular Hole Transfer at the Sensitized TiO2 Interface
Publication TypeJournal Article
Year of Publication2014
AuthorsHu, K, Robson, KCD, Beauvilliers, EE, Schott, E, Zarate, X, Arratia-Perez, R, Berlinguette, CP, Meyer, GJ
Date PublishedJAN 22

Characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds containing both a terpyridyl ligand with three carboxylic acid/carboxylate or methyl ester groups for surface binding and a tridentate cyclometalated ligand with a conjugated triarylamine (NAr3) donor group is described. Spectroelectrochemical studies revealed non-Nernstian behavior with nonideality factors of 1.37 +/- 0.08 for the Ru-III/II couple and 1.15 +/- 0.09 for the NAr3 center dot+/0 couple. Pulsed light excitation of the sensitized thin films resulted in rapid excited-state injection (k(inj) > 10(8) s(-1)) and in some cases hole transfer to NAr3 {[}TiO2(e(-))/Ru-III-NAr3 -> TiO2(e(-))/Ru-II-NAr3 center dot+]. The rate constants for charge recombination {[}TiO2(e(-))/Ru-III-NAr3 -> TiO2/Ru-II-NAr3 or TiO2(e(-))/Ru-II-NAr3 center dot+ -> TiO2/Ru-II-NAr3] were insensitive to the identity of the cyclometalated compound, while the open-circuit photovoltage was significantly larger for the compound with the highest quantum yield for hole transfer, behavior attributed to a larger dipole moment change (Delta mu = 7.7 D). Visible-light excitation under conditions where the Ru-III centers were oxidized resulted in injection into TiO2 {[}TiO2/Ru-III-NAr3 + h nu -> TiO2(e(-))/Ru-III-NAr3 center dot+] followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited-state injection {[}TiO2(e(-))/Ru-III-NAr3 -> TiO2/Ru-II-NAr3]. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/Ru-II-NAr3 + TiO2/Ru-III-NAr3 center dot+ -> 2TiO(2)/Ru-III-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).