Synthesis and characterization of ruthenium(II) molecular assemblies for photosensitization of nanocrystalline TiO2:: Utilization of hydroxyl grafting mode

New Ru polypyridine complexes [(bpy)(2)Ru(L)](2+), where bpy = 2,2'-bipyridine and L = dipyrido[3,2-a:2',3'-c]phenazine -2-carboxylic acid (dppzc), dipyrido[3,2-f:2',3'-h]quinoxaline-2,3-dicarboxylic acid (dpq(COOH)(2)), 3-hydroxydipyrido[3,2-f:2',3'-h]quinoxaline-2-carboxylic acid (dpq(OHCOOH)), 2,3-dihydroxydipyrido[3,2-f:2',3'-h]quinoxaline (dpq(OH)(2)), and [(L')Ru(dppzc)(2)](2+), where L' = bpy and 1,10-phenanthroline (phen), have been synthesized, characterized, and anchored to nanacrystalline TiO2 electrodes for light to electrical energy conversion in regenerative photoelectrochemical cells with I-/I-2 acetonitrile electrolyte. These sensitizers have intense metal-to-ligand charge-transfer (MLCT) bands centered at similar to 450 nm. The effect of pH on the absorption and emission spectra of these complexes consisting of protonatable ligands has been investigated in water by spectrophotometric titration. The excited-state pK(a) values are more basic than the ground-state ones, except the pK(a2) and pK(a2)* in [(bpq)(2)Ru(dpq(OH)(2))](2+), which are equal, suggesting the localization of the lowest-energy MLCT on heteroaromatic bridging ligands, dppzc and dpq. Incident photon-to-current conversion efficiency (IPCE) is sensitive to the structural changes that resulted from introducing different functional groups, used for grafting.