Quenching of the luminescent state of the uranyl ion (UO 2+2) by metal ions. Evidence for an electron transfer mechanism

Abstract

The quenching of the luminescence of the uranyl ion by other metal ions has been studied in aqueous solution. The quenching is shown to be a dynamic process, and the correlation of the logarithm of the quenching rate with the metal ion ionization potential suggests that intermolecular electron transfer is the predominant mechanism. Evidence that this involves complete electron transfer comes from flash photolysis of solutions of UO²⁺₂ and manganese(II), where a broad absorption (λ_max= 505 nm) is observed which is assigned to Mn³⁺. Consideration of the energetics of the quenching process suggests that in the quenching of uranyl by silver(I), the products (UO⁺₂ and Ag²⁺) are produced in their electronic ground states. Studies of the effect of temperature on the quenching suggest that if an intermediate complex (exciplex) is involved in the quenching then this must involve only very weak binding. With silver(I), the quenching is sensitive to the ionic strength of the solution. Further studies suggest that the lifetime of the luminescent state of the uranyl ion in aqueous solution varies with both temperature and uranyl ion concentration.