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)x]­(PF6)2 (x = 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 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 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 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 delocalization effect
of a nitrile group in the 4′ position of the bpy ring.