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(dcbpyH2)2(Cl)2] (1), [Ru(dcbpyH2)2(NCS)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 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 (Bu4N)4[Ru(dcbpy)2(NCS)2] is shifted ca. 290 mV negatively with regard to that of the complex [Ru(dcbpyH2)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(dcbpyH2)2(NCS)2]. The effect of deprotonation on the performance of these complexes as photosensitizers for nanocrystalline titania was investigated.