Charge-transfer processes in the quenching of excited uranyl ion by organosulphur, organohalogen and organometal species

Abstract

Strong quenching of the emission of excited uranyl ion, [UO₂²⁺]*, is found on addition of the molecules RHal (Hal = Br, I), R₂S and R₄M (M = Si, Ge, Sn, Pb), both in terms of the intensity and lifetime of the emitting species as determined by 347 nm laser flash photolysis in acetone solution. Kinetic studies have been supplemented by quantum-yield determinations (for U^IV), product analysis and, in certain cases, by e.s.r. investigation of the irradiated system at 77 K.

Second-order rate constants k₂ for the quenching process fall in the region 10⁷–6 × 10⁹ dm³ mol^–1 s^–1 and, in general, correlations can be found between log k₂ and the ionisation potential of the donor for a particular series. Plots for the series R₄Sn and RHal yield gradients of –1.12 ± 0.10 and –1.61 ± 0.07 eV^–1, respectively, while that for the series cyclo-R₂S is markedly different (–0.69 ± 0.07 eV^–1). (None of these gradients conforms to the figure of –16.91 eV^–1 expected from the linear sloping section of a so-called Weller plot.)

ϕ(U^IV) is generally rather low (<0.07) and in the case of RHal extremely so, although U^IV yields could not always be determined because of precipitation of photochemical products. Organic products were identified by gas-chromatography–mass-spectrometry. E.s.r. examination indicated that one-electron transfer from R₂S to [UO₂²⁺]* takes place to yield the species (R₂S)₂^˙+ as the identifiable radical product, although a monomeric radical is found under certain conditions. (R₂S)₂^˙+ was also observed as a transient species during laser flash photolysis of tetrahydrothiophene in the presence of UO₂²⁺ ion, while Me₂S₂ yielded Me₂S₂^˙+.

Mechanisms of the quenching processes are discussed in terms of fast, largely reversible electron transfer and the intermediacy of exciplexes.