Facile One-Pot Synthesis of Ruthenium(II) Quaterpyridine-Based Photosensitizers for Photocatalyzed Hydrogen Production.

We present here the efficient microwave-assisted synthesis and photophysical study of a family of ruthenium(II) complexes of the general formula [Ru(bpy)(qpy)] (where bpy = 2,2'-bipyridine, qpy = 4,4':2',2″:4″,4‴-quaterpyridine, and x = 0, 1, 2 giving compounds 1 = [Ru(bpy)(qpy)], 2 = [Ru(bpy)(qpy)], and 3 = [Ru(qpy)]). Compared to the standard reference, [Ru(bpy)] (τ = 870 ns, Φ = 9.5%), the complexes display longer-lived excited state lifetimes at room temperature (τ: 1 = 1440 ns, 2 = 1640 ns, 3 = 1780 ns) and improved quantum yields (Φ: 1 = 14%, 2 = 19%, 3 = 23%). Theoretical calculations were performed to support the interpretation of these photophysical properties. These complexes are excellent photosensitizers as they absorb light throughout the visible spectrum, have excellent excited state lifetimes at room temperature, and have high quantum yields. In combination with a cobalt dimethylglyoxime catalyst, they exhibit remarkable hydrogen evolution with blue light, and they are far more efficient than the reference in the field, [Ru(bpy)].