Photophysics of the excited uranyl ion in aqueous solutions. Part 4.—Quenching by metal ions

Abstract

The quenching of the excited state of the uranyl ion by metal ions in aqueous solutions has been studied under conditions where (UO²⁺₂)^* decay is biexponential. The effect of metal ions always follows Stern–Volmer behaviour both for real quenching and for the second process, which is suggested to involve reversible crossing via solvent exchange between two energetically close excited states. With Tl⁺, Ag⁺, Fe²⁺, Pb²⁺, Mn²⁺, Ce³⁺ and Ni²⁺ quenching is suggested to occur by electron transfer. Theoretical calculations using a quantum-mechanical tunnelling model support a mechanism involving an inner-sphere exciplex. With Eu³⁺ preferential quenching of the emitting U^* state is suggested to involve enhanced hydrogen-atom abstraction by (UO²⁺₂)^* following complexing and overlap of europium and uranyl orbitals. Initial fluorescence enhancement by metal ions in the uranyl system is observed in both dynamic and static studies. In cases where there is no complexing in the ground state with UO²⁺₂, this initial enhancement is interpreted in terms of an effect of the metal ion on the initial relaxation of the emitting (UO²⁺₂)^* state.

The time-resolved fluorescence spectra of UO²⁺₂, together with other data on the photophysics of (UO²⁺₂)^*, suggest that the UO²⁺₂ ground state and the lowest excited state, X^*, have a slightly bent geometry (ca. 176°), whereas the second excited state, U^*, is linear.