PHOTORACEMIZATION OF RUTHENIUM(II) COMPLEXES WITH 2,2'-BIPYRIDINE ANDDI-2-PYRIDYLAMINE

The emission lifetime (tau(obs)) and quantum yield of photoracemizatio n (phi(rac)) 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 60-degrees-C. The values of tau(obs) 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. T he photoracemization reactions of [Ru(bpy)3]2+, [Ru(bpy)2(Hdpa)]2+, [R u(bpy)(Hdpa)2]2+, and [Ru(Hdpa)3]2+ were thermally activated with phi( rac) of (0.1-1.6) x 10(-3), (0.5-6) x 10(-3), (0.04 - 0.22) x 10(-3), and (0.1 - 0.25) x 10(-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-pibpy, 3d- d, and 3n-pi*bpy states. The reactive state, the 3d-d* state, was the rmally yielded from the 3d-pibpy state. The displacement of a bpy lig and by an Hdpa ligand made the quantum yield of the formation of the 3 d-d state from the 3d-pi*bpy state (phi(d-d)) smaller, and the intrin sic quantum yield of photoracemization (phi(rac(d-d))) larger. The fac t that phi(rac) for [Ru(bpy)(Hdpa)2]2+ and [Ru(Hdpa)3]2+ was smaller t han 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-pibpy or 3d-pi*Hdpa states considerably, due to the bypath fr om the 3d-pibpy or 3d-pi*Hdpa states through the 3n(Hdpa)-pi*bpy or 3 n(Hdpa)-piHdpa states to the ground state.