Improving the photosensitizing properties of ruthenium polypyridyl complexes using 4-methyl-2,2'-bipyridine-4'-carbonitrile as an auxiliary ligand.

We report in this work the synthesis and spectroscopic, electrochemical, spectroelectrochemical, and photophysical characterization of a novel series of ruthenium polypyridyl complexes with 4-methyl-2,2'-bipyridine-4'-carbonitrile (Mebpy-CN) as an auxiliary ligand of general formula Ru(bpy)3-x(Mebpy-CN)x2 (x = 1-3) (with bpy = 2,2'-bipyridine). A significant increase in the lifetime and quantum yield of emission of the lowest (3)MLCT excited state is disclosed when going from x = 1 to x = 3, evidencing an improvement of the photosensitizing properties with respect to Ru(bpy)32. Furthermore, quenching by molecular oxygen of (3)MLCT excited states of the three complexes produced singlet molecular oxygen ((1)O2) with quantum yield values higher than that of Ru(bpy)3 in CH3CN. The structure of the complex with x = 1 has been determined by X-ray diffraction. The photoconductivity of ZnO nanowires covered with this same complex is increased by an order of magnitude, pointing to its feasibility as a component of a DSSC. A new dinuclear complex with Mebpy-CN as a bridging ligand has also been prepared and characterized by physicochemical techniques. The derived mixed-valent species of formula (bpy)2Ru(II)(Mebpy-CN)Ru(III)(NH3)5 displays a considerable metal-metal electronic coupling due to the delocalization effect of a nitrile group in the 4' position of the bpy ring.