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)(PF6)(2), Ru2+, and Os(deeb)(bpy)(2)(PF6)(2), OS2+, where deeb is 4,4'-(CH3CH2CO2)(2)-2,2'-bipyridine, anchored to mesoporous nanocrystalline (anatase) TiO2 thin films were quantified. Lateral energy transfer from Ru2+* to Os2+ was observed, and the yields were measured as a function of the relative surface coverage and the external solvent environment (CH3CN, THF, CCl4, and hexanes). Excited-state decay of Ru2+*/TiO2 was well described by a parallel first- and second-order kinetic model, whereas OS2+*/TiO2 decayed with first-order kinetics within experimental error. The first-order component was assigned to the radiative and nonradiative decay pathways (tau = 1 mu s for Ru2+*/TiO2 and tau = 50 ns for OS2+*/TiO2). 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 Ru2+*/TiO2 that decayed through the second-order pathway increased with surface coverage and excitation intensity. Monte Carlo simulations were performed to estimate the RU2+* -> Ru2+ intermolecular energy transfer rate constant of (30 ns)(-1).