Photoracemization of ruthenium(II) complexes with 2,2'-bipyridine and di-2-pyridylamine

The emission lifetime (τobs) and quantum yield of photoracemization (φrac) for a series of complexes, [Ru(bpy)n(Hdpa)3-n]2+ (n=0-3), were studied in aqueous solution at temperatures of between 20 and 60degC. The values of τobs of [Ru(bpy)2(Hdpa)]2+ were found to vary around 80-200 ns, depending upon temperature, while [Ru(bpy)(Hdpa)2]2+ and [Ru(Hdpa)3]2+ did not emit at the temperature studied. The photoracemization reactions of [Ru(bpy)3]2+, [Ru(bpy)2(Hdpa)]2+, [Ru(bpy)(Hdpa)2]2+, and [Ru(Hdpa)3]2+ were thermally activated with φrac of (0.1-1.6)x10-3, (0.5-6)x10-3, (0.04-0.22)x10-3, and (0.1-0.25)x10-3, respectively, depending upon the conditions. The photochemical behavior of [Ru(bpy)2(Hdpa)]2+ was explained by a set of excited states, which was assumed to include the 3d-π*bpy. 3d-d*, and 3n-π*bpy states. The reactive state, the 3d-d* state, was thermally yielded from the 3d-π*bpy state. The displacement of a bpy ligand by an Hdpa ligand made the quantum yield of the formation of the 3d-d* state from the 3d-π*bpy state (φd-d) smaller, and the intrinsic quantum yield of photoracemization (φrac(d-d)) larger. The fact that φrac for [Ru(bpy)(Hdpa)2]2+ and [Ru(Hdpa)3]2+ was smaller than that for [Ru(bpy)3]2+ and [Ru(bpy)2(Hdpa)]2+ could be interpreted by assuming that the displacement of the bpy ligand by the Hdpa ligand diminished the quantum yield of the formation of the 3d-d* state from the 3d-π*bpy or 3d-π*Hbpa states considerably, due to the bypath from the 3d-π*bpy or 3d-π*Hdpa states through the 3nHdpa-π*bpy or 3nHdpa-π*Hdpa states to the ground state. (author)