Dependence of the photophysical and photochemical properties of the photosensitizer tris(4,4′-dicarboxy-2,2′-bipyridine)ruthenium(II) on pH

Abstract

The pH dependence of the photoluminescent properties and the emission quenching of tris(4,4′-dicarboxy-2,2′-bipyridine)ruthenium(II) (Ru(dhch) 3) by ferricyanide ion (Fe(CN) 6 3−), methyl viologen (MV 2+), cupric ion (Cu 2+), N, N′-bis(3-sulphopropyl)-4,4′-bipyridinium inner salt (V 2+ (PS −) 2), nitrobenzene and O 2 were investigated. The emission intensity and excited state lifetime of the complex are at a minimum and the wavelength of the emission peak is at a maximum near pH 3. The solubility of the complex is at a minimum near pH 2. These results indicate that two carboxylic acid groups are present in the dissociated state near pH 2. The p K a values of the conjugated acid of the two dissociated carboxylate groups were estimated from luminescence titration and excited state lifetime data to be 0.7 and 3.8 for the ground state and excited state of the complex respectively. The corresponding values for the four less acidic carboxylic acid groups are 3.7 and 8.2. The rate of quenching of ∗Ru(dhcb) 3 by Fe(CN) 6 3− is greater at lower pH, whereas the opposite trend is observed for reaction with the other quenchers. The rate of quenching by Cu 2+ is unusually fast, presumably due to the chelation of Cu 2+ to the carboxylate groups of the complex. The order of the oxidation potentials of the excited state Ru(II) complexes is ∗ Ru(dhcb) 3 2+ < ∗ Ru(dhcb) 3 4− < ∗ Ru(bpy) 3 2+ . This order is the same as that of the quenching rate constants for the complexes by neutral or zwitterionic quenchers.