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–<i>x</i>(Mebpy-CN)_<i>x</i>]­(PF₆)₂ (<i>x</i> = 1–3) (with bpy = 2,2′-bipyridine). A significant increase in the lifetime and quantum yield of emission of the lowest ³MLCT excited state is disclosed when going from <i>x</i> = 1 to <i>x</i> = 3, evidencing an improvement of the photosensitizing properties with respect to [Ru­(bpy)₃]­(PF₆)₂. Furthermore, quenching by molecular oxygen of ³MLCT excited states of the three complexes produced singlet molecular oxygen (¹O₂) with quantum yield values higher than that of [Ru­(bpy)₃]²⁺ in CH₃CN. The structure of the complex with <i>x</i> = 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]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