Absorption spectrum, lifetime and photoreactivity towards alcohols of the excited state of the aqueous uranyl ion (UO 2+2)

Abstract

Microsecond flash photolysis of solutions of uranyl salts in water, H₃PO₄, H₂SO₄ and other media yields an intense, but short lived transient absorption spectrum in the region of 570 nm which exhibits vibronic structure with an average band separation of 580 cm^–1. The decay of the transient in water was determined by laser flash photolysis to be first order with k₁= 8.02 × 10⁵ s^–1, but with a considerable isotope effect (k_1(D2O)= 4.20 × 10⁵ s^–1). k₁ accords with the lifetime of the luminescence of aqueous UO²⁺₂ ion determined by single photon counting (k₁= 3.85 × 10⁵ s^–1). Both the 570 nm absorption and the emission (as determined by both single photon counting and conventional fluorimetry) are quenched on addition of various alcohols, the Stern–Volmer quenching constants closely correlating with the absolute quenching constants for the transient determined by laser flash photolysis. Large isotope effects were found for quenching by deuterated methanol, isopropanol and cyclohexanol both of the transient absorption and the emission. We conclude that the same electronic state of UO²⁺₂ is responsible for (i) emission to the ground state, (ii) the absorption at 570 nm to a higher excited state and (iii) the photo-oxidation of aliphatic alcohols, principally by a process of abstraction of an H atom bonded to carbon from a —CH₂OH or CHOH group.