Inorganic photophysics in solution. Part 6.—Isotope effects in the non-radiative deactivation of photo-excited uranyl [UO2]2+ ion in hydroxylic solvents

Abstract

The effect of deuterium substitution on τ(UO²⁺₂)* has been measured in H₂O, mildly acidic H₂O, strong aqueous LiCl and liquid methanol, in most cases over an extended temperature range (77 K to the boiling point). In all media, a strongly temperature-activated term dominates non-radiative processes at room temperature, whilst a temperature-independent term is effective both at 77 K and, in all cases, at considerably higher temperatures. Both effects are sensitive to substitution of the solvent by deuterium atoms: in the case of methanol that of C—H (but not of O—H) is important at room temperature, but that of O—H is especially significant at 77 K. The low-temperature deactivation mechanism is concluded to be due neither to fast, reversible abstraction of a hydroxylic H atom (because of the zero activation energy), nor to fast, reversible electron transfer (because of the isotope effect), but to a physical process.