Intermolecular Energy Transfer across Nanocrystalline Semiconductor Surfaces

• Published in: The journal of physical chemistry. B 2006-02, Vol.110 (6), p.2598-2605
• Main Authors: Higgins, Gerard T, Bergeron, Bryan V, Hasselmann, Georg M, Farzad, Fereshteh, Meyer, Gerald J
• Format: Article
• Language: English
• Subjects: Crystallization; Energy Transfer; Monte Carlo Method; Nanostructures - chemistry; Organometallic Compounds - chemistry; Osmium - chemistry; Oxidation-Reduction; Ruthenium - chemistry; Semiconductors; Surface Properties; Time Factors
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp0543680

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 (τ = 1 μs for Ru2+*/TiO2 and τ = 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.