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)(NCS)(2) (2), (Bu(4)N)(4)Ru(dcbpy)(2)(NCS)(2) (3), and (Bu(4)N)(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 absorption and emission maxima of these complexes red shifted with decreasing pH, and showed pH-dependent excited-state lifetimes. The ground-state pK(a) values 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 (Bu(4)N)(4)Ru(dcbpy)(2)(NCS)(2) is shifted ca. 290 mV negatively with regard to that of the complex Ru(dcbpyH(2))(2)(NCS)(2) due to the replacement of H(+) by tetrabutylammonium cation. The negative shift for the dcbpy-based reduction potential is even larger, i.e., about 600 mV compared to that of the complex Ru(dcbpyH(2))(2)(NCS)(2). The effect of deprotonation on the performance of these complexes as photosensitizers for nanocrystalline titania was investigated.