The photophysical and photoelectrochemical properties of Ru(deeb)(bpy)2(PF6)2, where bpy is 2,2′bipyridine and deeb is 4,4′-(COOEt)2-2,2′-bipyridine, anchored to nanocrystalline TiO2 (anatase) or ZrO2 films are reported. In neat acetonitrile (or 0.1 M tetrabutylammonium perchlorate) long-lived metal-toligand charge transfer (MLCT) excited states are observed on both TiO2 and ZrO2. Addition of LiClO4 results in a red shift in the MLCT absorption and photoluminescence, PL, spectra on both TiO2 and ZrO2, and a concentration-dependent quenching of the PL intensity on TiO2. The Li + -induced spectroscopic changes were found to be reversible by varying the electrolyte composition. Time-resolved absorption measurements demonstrate that the presence of lithium cations increases the quantum yield for interfacial charge separation with no discernible influence on the rate of charge recombination. A second-order kinetic model quantified charge recombination transients. A model is proposed wherein Li + ion adsorption stabilizes TiO2 acceptor states resulting in energetically more favorable interfacial electron transfer. The generality of this model was explored with different electrolytes and sensitizers. In regenerative solar cells, the addition of Li + increases both the efficiency and long wavelength sensitivity of the cell.

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Pulse radiolysis and laser flash photolysis have been used to generate and characterize Ni(cyclam)+, where cyclam is 1,4,8,11-tetraazacyclotetradecane, in aqueous solution. Reduction of Ni(cyclam)2+ was carried out at ambient temperature (22 ± 2 °C) by solvated electrons, eaq", hydrogen atoms, H", and carbon dioxide radical anions, C02-. Reduction by eaq~o ccurs with a rate constant of (4.1 ± 0.2) x 1010 M_1 s_1 (7 < 2 x -4 M). Reduction by C02*_ and H' proceeds via an inner-sphere mechanism to yield [Ni(cyclam)(C02)]+ and [Ni(cyclam)(H)]2+, with rate constants of (6.7 ± 0.2) x 109 and (5 ± 2) x 109 M_l s_1, respectively. Decomposition of the adducts, to yield Ni(cyclam)+ and C02 or H+, proceeds with rate constants of (2.0 ± 0.2) x 106 and (5.3 ± 0.7) x 105 s_l, respectively.Carbon dioxide and proton dissociation constants for [Ni(cyclam)(C02)]+ and [Ni(cyclam)(H)]2+ were found to be (6.2 ± 0.3) x 10-2 and (1.6 ± 0.4) x 10-2 M, respectively. The rate constants for the addition of C02 and H+ to Ni(cyclam)+ were calculated to be (3.2 ± 0.4) x 107 and (3 ± 1) x 107 M-1 s_1, respectively. The selective reduction of C02 by Ni(cyclam)+ in the presence of protons at pH 4 is rationalized.

Excitation of Ru(deeb)(bpy)2 2+, bis(2,2‘-bipyridine)(2,2‘-bipyridine-4,4‘-diethylester)ruthenium(II) hexafluorophosphate, bound to nanocrystalline TiO2 thin films and immersed in an acetonitrile bath at 25 °C under an argon atmosphere, results in the formation of a species, the transient spectral characteristics for which are, consistent with a metal-to-ligand charge transfer, MLCT, excited state. The spectrum decays by kinetics that are inconsistent with a simple first-order process. Modeling of the data as a function of irradiance has been accomplished assuming parallel unimolecular and bimolecular excited-state deactivation processes. The quantum yield for excited-state formation depends on the excitation irradiance, consistent with triplet−triplet annihilation processes that occur with k ≥ 1 × 108 s-1.