Improving the Photosensitizing Properties of Ruthenium Polypyridyl Complexes Using 4-Methyl-2,2'-bipyridine-4'<arbonitrile 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&#39;-bipyridine-4&#39;-carbonitrile (Mebpy-CN) as an auxiliary ligand of general formula [Ru(bpy)3-x(Mebpy-CN)x(PF6)2 (x = 1-3) (with bpy = 2,2&#39;-bipyridine). A significant increase in the lifetime and quantum yield of emission of the lowest &#179;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)3]-(PF6)2. Furthermore, quenching by molecular oxygen of &#179;MLCT excited states of the three complexes produced singlet molecular oxygen (&#185;O2) with quantum yield values higher than that of [Ru(bpy)3]2+ 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)2RuII(Mebpy-CN)RuIII(NH3)5]5+ displays a considerable metal--metal electronic coupling due to the derealization effect of a nitrile group in the 4&#39; position of the bpy ring. [ABSTRACT FROM AUTHOR]