Sonochemiluminescence of Ru(bpy)33+ in aqueous solutions. Evidence of the formation of hydrated electrons during the single-bubble sonolysis in a neutral aqueous medium.

Abstract

Consideration is given to single and multibubble sonochemiluminescence (SCL) in acidic Ru(bpy)33+ aqueous solutions due to the light emission of the excited ion *Ru(bpy)32+ at 613 nm. Spectrophotometrically controlled reduction of Ru(bpy)33+ to Ru(bpy)32+ occurs in both regimes of sonolysis. The kinetics parameters of reduction under SCL during the multibubble sonolysis do not depend on the H+ concentration in the solution (0.005 and 0.5 M H2SO4), but differ for the single-bubble sonolysis. The intensity of single-bubble SCL is maximal at the beginning of sonolysis and monotously decreases to the level of steady-state SCL after complete reduction of Ru(bpy)33+ to Ru(bpy)32+. The intensity of multibubble SCL at the beginning of sonolysis is below the recognition threshold. Then it grows, passes through the maximum and also approaches the steady-state level after the reduction of ruthenium. By considering possible chemiluminescent reactions Ru(bpy)33+ + eaq → *Ru(bpy)32+, Ru(bpy)33+ + H → *Ru(bpy)32+ + H+, Ru(bpy)33+ + Ru(bpy)3+ → *Ru(bpy)32+ + Ru(bpy)32+ as well as the reactions of sonochemical formation of Ru(bpy)33+ and Ru(bpy)3+ from Ru(bpy)32+ and the reaction H+ + eaq → H, the conclusion is made about the generation of eaq during the single-bubble sonolysis of water molecules. The presence and absence of eaq during the single and multibubble sonolysis respectively explain the distinctions in the SCL kinetics. The evidence for the generation of eaq lies also in the reduction of the Fe(CN)63- ion observed during the single-bubble sonolysis in the neutral solution, which is impossible without eaq. The yield of eaq is comparable to that of the hydrogen atom.