Intramolecular and Lateral Intermolecular Hole Transfer at the Sensitized TiO2 Interface.

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 RuIII/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 (kinj > 108 s–1) and in some cases hole transfer to NAr3 [TiO2(e–)/RuIII–NAr3 → TiO2(e–)/RuII–NAr3•+]. The rate constants for charge recombination [TiO2(e–)/RuIII–NAr3 → TiO2/RuII–NAr3 or TiO2(e–)/RuII–NAr3•+ → TiO2/RuII–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 RuIII centers were oxidized resulted in injection into TiO2 [TiO2/RuIII–NAr3 + hν → TiO2(e–)/RuIII–NAr3•+] followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited-state injection [TiO2(e–)/RuIII–NAr3 → TiO2/RuII–NAr3]. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/RuII–NAr3 + TiO2/RuIII–NAr3•+ → 2TiO2/RuIII–NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)−1.