An electrochemical study of quenching reactions of polypyridine complexes of Ru(II) and Os(II) in excited states

Abstract

The quenching of the excited states of the complexes, Ru(bpy) 3 2+, Ru (4-dmbpy) 3 2+, Ru(5-dmbpy) 3 2+, Ru(bpy) 2(CN) 2, Os(bpy) 3 2+, Os(5-dmby) 3 2+, Os(phen) 3 2+ and Os(5-Clphen) 3 2+ (bpy=2,2-bipyridine; 4-dmbpy=4,4′-dimethyl-2,2′-bipyridine; 5-dmbpy=5,5′-dimethyl-2,2′-bipyridine; phen=1,10-phenanthroline; 5-Clphen=5-chloro-1,10-phenanthroline), by Fe 3+ aquaions in aqueous solution is studied by an electrochemical transient method. An electrolyte containing one of the complexes, M(II), and iron aquaions, Fe 3+ and Fe 2+, in sulphuric acid is irradiated by a flash of 3 μs width from a xenon lamp. A cathodic current flows at a platinum grid electrode which is placed in the solution and kept at the formal, standard potential of the Fe 3+/Fe 2+ redox couple. The current vs. time curve is recorded after the irradiation is complete. A theoretical expression for this curve is derived by taking both the electron-and energy-transfer quenching paths into consideration and solving Fick's diffusion equation. Analysis of the experimental results is conducted, on this theoretical basis and yields the Stern-Volmer constant K sv and the second-order rate constant k t for the electron-transfer reaction between M(III) and Fe 2+. Agreement in k t and K sv between this and previous works is satisfactory. No cathodic photocurrent is observed for the iron complexes studied, Fe(bpy) 3 2+, Fe(bpy) 2(CN) 2 and Fe(phen) 3 2+.