Mechanism of the Quenching of the Phosphorescence of Tris(2,2′-bipyridine)ruthenium(II) by Some Cobalt(III) and Ruthenium(III) Complexes

The quenching of the Ru(bipy)₃²⁺ phosphorescence by Co(NH₃)₅X³⁺ (X = Br^-, Cl^-, H₂O and NH₃) and by Ru(NH₃)₅X³⁺ (X = Cl^- and NH₃) complexes has been studied by a spectrofluorimetric technique. Stern-Volmer constants were determined and the kinetics of the quenching reactions investigated in 0.5 M sulfuric acid at 25°. Some measurements were also performed in 50% 2-propanol. The Co (NH₃)₅X³⁺ quenched rates increase in the order NH₃ < H₂O < Cl^- < Br^- with the latter reaction being diffusion controlled. In the absence of 2-propanol the ratio k₂/k_Q, where k₂ is the second-order rate constant for the formation of Ru(bipy)₃³⁺ and k_Q is the second-order quenching rate constant, was close to unity for all of the cobalt(III) complexes studied. This ratio was somewhat lower for Co(NH₃)₅Cl²⁺ in 50% 2-propanol. The Ru-(NH₃)₅X³⁺ complexes quenched much more efficiently than the corresponding Co(NH₃)₅X³⁺ complexes; however, in contrast to the cobalt(III) reactions, the ruthenium(III) reactions were not accompanied by the net oxidation of Ru(bipy)₃²⁺. The results are discussed in terms of an electron transfer quenching mechanism, and a value of 0.9 V is estimated for the oxidation potential of ³Ru(bipy)₃²⁺.