title: 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.
abstract: 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.
authors.affiliation: Laboratory for Photonics and Interfaces, Institute of Physical Chemistry, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland.
authors.forename: Md. K.
authors.initials: MK
authors.lastname: Nazeeruddin
authors.forename: S. M.
authors.initials: SM
authors.lastname: Zakeeruddin
authors.forename: R.
authors.initials: R
authors.lastname: Humphry-Baker
authors.forename: M.
authors.initials: M
authors.lastname: Jirousek
authors.forename: P.
authors.initials: P
authors.lastname: Liska
authors.forename: N.
authors.initials: N
authors.lastname: Vlachopoulos
authors.forename: V.
authors.initials: V
authors.lastname: Shklover
authors.forename: Christian-H.
authors.initials: CH
authors.lastname: Fischer
authors.forename: M.
authors.initials: M
authors.lastname: Grätzel