Synthesis and photophysical characterization of highly luminescent complexes of Ru(II) containing 4,4′-di-( p-carboxyphenyl)-2,2′-bipyridine

Abstract

The synthesis and photophysical characterization of several complexes of Ru(II) containing 4,4′-di(carboxyphenyl)-2,2′-bipyridine (DCPB) and related bipyridine ligands are reported. In all complexes studied the lowest excited state is associated with Ru→DCPB charge transfer (CT) transition. The introduction of phenyl groups between the peripheral carboxyl group and the bipyridine framework caused a red-shifted, more intense absorption but the excited state properties (luminescence and transient absorption) remain nearly the same. The acid-base properties in the ground and excited state are different. With respect to 4,4′-carboxy-2,2′-bipyridine (dcbpy) derivative, the ground state p K a of Ru(DCPB) 3 increases by about 2.5 pH units and a further extremely small increase in the p K a is observed upon visible light excitation. The results suggest that the charge density is largely localized in the diimine unit and does not carry through the phenyl group. This could account for inefficient sensitization (⩽8% monochromatic photon-to-current conversion efficiency) obtained on TiO 2 electrodes. Possible implication of the present results on the design of photosensitizers is also discussed.