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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In 1974, the metal-to-ligand charge transfer (MLCT) excited state, [Ru(bpy)3]2+*, was shown to undergo electron transfer quenching by methylviologen dication (MV2+), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a “modular approach”. In the intervening years, application of synthesis, excited-state measurements, and theory to [Ru(bpy)3]2+* and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assemblies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated.

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K(4)[Fe(CN)(6)] and Na(2)[Fe(LL)(CN)(4)], where LL = bpy (2,2'-bipyridine), dmb (4,4'-dimethyl-2,2'-bipyridine), or dpb (4,4'-diphenyl-2,2'-bipyridine). Coordination of Fe(CN)(6)(4-) to the TiO(2) surface results in the appearance of a broad absorption band (fwhm approximately 8200 cm(-1)) centered at 23800 +/- 400 cm(-1) assigned to an Fe(II)-->TiO(2) metal-to-particle charge-transfer (MPCT) band. The absorption spectra of Fe(LL)(CN)(4)(2-) compounds anchored to TiO(2) are well modeled by a sum of metal-to-ligand charge-transfer (MLCT) bands and a MPCT band. Pulsed light excitation (417 or 532 nm, approximately 8 ns fwhm, approximately 2-15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO(2)(e(-)), Fe(III)], k(inj) > 10(8) s(-1). Charge recombination is well described by a second-order equal concentration kinetic model and requires milliseconds for completion. A model is proposed wherein sensitization of Fe(LL)(CN)(4)(2-)/TiO(2) occurs by MPCT and MLCT pathways, the quantum yield for the latter being dependent on environment. The solvatochromism of the materials allows the reorganization energies associated with charge transfer to be quantified. The photocurrent efficiencies of the sensitized materials are also reported.