title: Intramolecular and lateral intermolecular hole transfer at the sensitized TiO2 interface.
abstract: 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(•+/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(•+)]. The rate constants for charge recombination [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3 or TiO2(e(-))/Ru(II)-NAr3(•+) → 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 (Δμ = 7.7 D). Visible-light excitation under conditions where the Ru(III) centers were oxidized resulted in injection into TiO2 [TiO2/Ru(III)-NAr3 + hν → TiO2(e(-))/Ru(III)-NAr3(•+)] 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(•+) → 2TiO2/Ru(III)-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).
authors.affiliation: Departments of Chemistry and Materials Science and Engineering, Johns Hopkins University , 3400 North Charles Street, Baltimore, Maryland 21218, United States.
authors.forename: Ke
authors.initials: K
authors.lastname: Hu
authors.forename: Kiyoshi C D
authors.initials: KC
authors.lastname: Robson
authors.forename: Evan E
authors.initials: EE
authors.lastname: Beauvilliers
authors.forename: Eduardo
authors.initials: E
authors.lastname: Schott
authors.forename: Ximena
authors.initials: X
authors.lastname: Zarate
authors.forename: Ramiro
authors.initials: R
authors.lastname: Arratia-Perez
authors.forename: Curtis P
authors.initials: CP
authors.lastname: Berlinguette
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Meyer
mesh_terms: D000042:Absorption; D000588:Amines / Q000737:chemistry; D004579:Electron Transport; D009010:Monte Carlo Method; D011789:Quantum Theory; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*
chemical_list: D000588:Amines; C009495:titanium dioxide; D014025:Titanium