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 tetraphenylmethane, respectively), have been synthesized and characterized. The tripodal sensitizers consist of a rigid-rod arm linked to a RuII-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 (kcs > 108 s-1) excited-state electron injection is observed for 1?4/TiO2. The recombination of the injected electron with the oxidized RuIII 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 1 × 10-11 cm2 s-1.