Long-range electron transfer across molecule-nanocrystalline semiconductor interfaces using tripodal sensitizers

Four tripodal sensitizers, Ru(bpy)(2)(Ad-tripod-phen)(2+) (1), Ru(bpy)(2)(Ad-tripod-bpy)(2+) (2), Ru(bpy)(2)-(C-tripod-phen)(2+) (3), and Ru(bpy)(2)(C-tripod-bpy)(2+) (4) (where bpy is 2,2'-bipyridine, phen is 1,10-phenanthroline, and Ad-tripod-bpy (phen) and C-tripod-bpy (phen) are tripod-shaped bpy (phen) ligands based on 1,3,5,7-tetraphenyladamantane and tetraphenyl methane, respectively), have been synthesized and characterized. The tripodal sensitizers consist of a rigid-rod arm linked to a Ru-II-polypyridine complex at one end and three COOR groups on the other end that bind to metal oxide nanoparticle surfaces. The excited-state and redox properties of solvated and surface-bound 1-4 have been studied at room temperature. The absorption spectra, emission spectra, and electrochemical properties of 1-4 in acetonitrile solution are preserved when 1-4 are bound to nanocrystalline (anatase) TiO2 or colloidal ZrO2 mesoporous films. This behavior is indicative of weak electronic coupling between TiO2 and the sensitizer. The kinetics for excited-state decay are exponential for 1-4 in solution and are nonexponential when 1-4 are bound to ZrO2 or TiO2. Efficient and rapid (k(cs) > 10(8) s(-1)) excited-state electron injection is observed for 1-4/TiO2. The recombination of the injected electron with the oxidized Ru-III center is well described by a second-order kinetic model with rate constants that are independent of the sensitizer. The sensitizers bound to TiO2 were reversibly oxidized electrochemically with an apparent diffusion coefficient similar to1 x 10(-11) cm(2) s(-1).