Intermolecular energy transfer across nanocrystalline semiconductor surfaces.

The yields and dynamics for energy transfer from the metal-to-ligand charge-transfer excited states of Ru(deeb)(bpy)(2)(PF(6))(2), Ru(2+), and Os(deeb)(bpy)(2)(PF(6))(2), Os(2+), where deeb is 4,4'-(CH(3)CH(2)CO(2))(2)-2,2'-bipyridine, anchored to mesoporous nanocrystalline (anatase) TiO(2) thin films were quantified. Lateral energy transfer from Ru(2+)* to Os(2+) was observed, and the yields were measured as a function of the relative surface coverage and the external solvent environment (CH(3)CN, THF, CCl(4), and hexanes). Excited-state decay of Ru(2+)*/TiO(2) was well described by a parallel first- and second-order kinetic model, whereas Os(2+)*/TiO(2) decayed with first-order kinetics within experimental error. The first-order component was assigned to the radiative and nonradiative decay pathways (tau = 1 micros for Ru(2+)*/TiO(2) and tau = 50 ns for Os(2+)*/TiO(2)). The second-order component was attributed to intermolecular energy transfer followed by triplet-triplet annihilation. An analytical model was derived that allowed determination of the fraction of excited-states that follow the two pathways. The fraction of Ru(2+)*/TiO(2) that decayed through the second-order pathway increased with surface coverage and excitation intensity. Monte Carlo simulations were performed to estimate the Ru(2+)* --> Ru(2+) intermolecular energy transfer rate constant of (30 ns)(-1).

Efficient Light-to-Electrical Energy Conversion with Dithiocarbamate-Ruthenium Polypyridyl Sensitizers.

New Ru(dcbH)(dcbH(2))(L) sensitizers, where L is diethyldithiocarbamate, dibenzyldithiocarbamate, or pyrrolidinedithiocarbamate, dcbH is 4-(COOH)-4'-(COO(-))-2,2'-bipyridine, and dcbH(2) is 4,4'-(COOH)(2)-2,2'-bipyridine, have been synthesized, characterized, and anchored to nanocrystalline TiO(2) films for light to electrical energy conversion in regenerative photoelectrochemical cells with I(-)/I(2) acetonitrile electrolyte. The sensitizers have intense metal-to-ligand charge-transfer (MLCT) bands centered approximately 380 and 535 nm that sensitize TiO(2) over a notably broad spectral range. The photophysical and photoelectrochemical studies of these materials are contrasted with cis-Ru(dcbH(2))(2)(NCS)(2), which is one of the most efficient sensitizers reported to date. Photophysical measurements show that the high photocurrent observed for cis-Ru(dcb)(2)(NCS)(2)/TiO(2) is due to efficient and rapid iodide oxidation.