Stepwise Assembly of Tris-Heteroleptic Polypyridyl Complexes of Ruthenium(II)

A general method for the synthesis of tris-heteroleptic ruthenium(II) complexes of the type Ru(dcbpy)(dmbpy)(ddtc) ( 3 ) and Ru(dcbpy)(dmbpy)(NCS) 2 ( 4 ) is reported, where the ligands (dcbpy = 4,4‘-dicarboxy-2,2‘-bipyridine, dmbpy = 4,4‘-dimethyl-2,2‘-bipyridine, and ddtc = diethyldithiocarbamate) are introduced sequentially. The complexes have been characterized by UV/visible, emission, IR, Raman and NMR spectroscopies and cyclic voltammetry. The effect of pH on the absorption spectra and luminescence behavior of these complexes consisting of protonatable ligands has been investigated in a water/ethanol solvent mixture by pH titration. The dissociation of protons is in sequential steps (p K a = 3.5 and 1.8). The excited-state p K a values are more basic than the ground-state p K a values. Resonance Raman spectra of these complexes strongly suggest that the lowest-energy metal-to-ligand charge-transfer transition bands are localized on the dcbpy ligand. The molecular crystal structure of 3 and the performance of these complexes as charge-transfer photosensitizers in a nanocrystalline TiO 2 -based solar cell is discussed.