Absorption spectrum, lifetime and photoreactivity towards alcohols of the excited state of the aqueous uranyl ion (UO2+2)

Abstract

Microsecond flash photolysis of solutions of uranyl salts in water, H3PO4, H2S04 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-'. The decay of the transient in water was determined by laser flash photolysis to be first order with kl = 8.02 x lo5 s-I, but with a considerable isotope effect ( k l p , 0 ) = 4 . 2 0 ~ lo5 s-l). kl accords with the lifetime of the luminescence of aqueous UO;' ion determined by single photon counting (k, = 3.85 x loJ s-I). 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 UOz+ is responsible for (i) emission to the ground state, (ii) the absorption at 570 nrn 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 -CH20H or =CHOH group.

A Study of U(VI) Hydrolysis and Carbonate Complexation by Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLFS)

“…This has been interpreted in terms of the availability of nonradiative decay paths, e.g., energy transfer from the excited electronic state of the metal ion to vibronic modes of ligands like water [14]. Such extensive work as that for the +3 valence state is not available for uranyl ion, whereas experimental work on the fluorescence of U(VI) shows that the lifetime, τ, of the U0 2+ (aq) ion is in the range of 1 μβ [15][16][17]. It has also been reported [18] that a ligand coordination strongly affects the lifetime as well as the emission spectrum of the excited uranyl state.…”

Stokes Relationship in Absorption and Fluorescence Spectra of U(VI) Species

“…+ fluorescence is about 1 μβ at 25°C [4]. Dependence of the fluorescence life times and the emission spectra on temperature and presence of quenchers like inorganic and organic substances, e.g.…”

“…fluorescence behaviour on pH has been discussed in terms of discrepant explanations. While in acidic solutions luminescence decay is single-exponential with an emission lifetime of about 1 μβ [4], a double-exponential decay has been reported when pH was shifted to about pH 3 [5] with a longer life time of the second decay. Double-exponential decay is interpreted by formation of an exciplex H(U0 2 ) 2 + [5], by a reversible crossing mechanism between two electronically excited states *U and *X [6] or by formation of additional solution species, e.g.…”

Photooxidation of cellulose acetate and cellobiose by the uranyl ion

“…To obtain further information on the mechanism of the quenching process, model studies were carried out using cellobiose as substrate. These were carried out in aqueous species, 30, 31 and the observed spectrum is presented as an inset in Fig. 5.…”