Acid-base equilibria of (2,2 '-bipyridyl-4,4 '-dicarboxylic acid)ruthenium(II) complexes and the effect of protonation on charge-transfer sensitization of nanocrystalline titania

The ruthenium complexes [Ru(dcbpyH(2))(2)(Cl)(2)] (1), [Ru(dcbpyH(2))(2)(NC S)(2)] (2), (Bu4N)(4)[Ru(dcbpy)(2)(NCS)(2)] (3), and (Bu4N)(2)[Ru(dcbpyH)(2 )(NCS)(2)] (4) were synthesized and characterized by cyclic voltammetry, UV - vis absorption, and emission, IR, Raman, and NMR spectroscopy. The absor ption and emission maxima of these complexes red shifted with decreasing pH , and showed pH-dependent excited-state lifetimes. The ground-state pK(a) v alues were determined by spectrophotometeric methods, and the dissociation of protons was found to occur in two steps (pK(a) = 3 and 1.5). The Ru(II)/ (III) couple in the complex (Bu4N)(4)[Ru(dcbpy)(2)(NCS)(2)] is shifted ca. 290 mV negatively with regard Bt that of the complex [Ru(dcbpyH(2))(2)(NCS) (2)] due to the replacement of H+ by tetrabutylammonium cation. The negativ e shift for the dcbpy-based reduction potential is even larger, i.e., about 600 mV compared to that of the complex [Ru(dchpyH(2))(2)(NCS)(2)]. The eff ect of deprotonation on the performance of these complexes as photosensitiz ers for nanocrystalline titania was investigated.