Concentration and Molecular Weight Dependence of the Quenching of Ru(bpy)32+ by Ferricyanide in Aqueous Solutions of Synthetic Hyaluronan

The quenching of electronically excited Ru(bpy) 3 2+ (bpy = 2,2‘-bipyridine) by ferricyanide (FC) in highly viscous aqueous solutions of synthetic hyaluronan (HA) was studied. The quenching reaction was found to occur by direct electron transfer at short times and by diffusional encounter at longer times after the excitation flash. Intrinsic and diffusional quenching rate constants were found to decrease with increasing polymer concentration but not with increasing molecular weight of HA, although the effect of both parameters on macroscopic viscosity was comparable. It was also found, that the lifetime of the sensitizer increased with increasing polymer concentration, indicating retarded quenching by oxygen. Efficient diffusion of the reactants in the aqueous pseudophase occurs even in solutions with high macroscopic viscosity. We compared two kinds of HAs (biosynthetic and animal source) in the present experiments and obtained a sharp contrast in photochemical properties between them while other observable macroscopic parameters showed no difference. Biosynthetic HA is more suitable for the systematic study to elucidate the molecular weight dependence of the present photochemical properties showing microscopic aspects of the HA solution.