Physical and chemical quenching of excited uranyl ion by organic molecules studied by fluorimetric and laser flash photolysis methods

Abstract

The quenching by a large number of carboxylic acids of the luminescence of excited uranyl ion has been examined. Acetic and propionic acids display weak quenching as measured by the Stern–Volmer constant K_SV, but branching or lengthening of R in RCO₂H increases K_SV to figures of up to 15 dm³ mol^–1. Cycloalkanecarboxylic acids show considerably higher K_SV. The introduction of halogen atoms introduces diverse effects: CF₃CO₂H and CCl₃CO₂H increase the luminescence intensity very strongly, ClCH₂ CO₂H and BrCH₂CO₂H weaken it very slightly. ICH₂CO₂H powerfully quenches it (K_SVca. 2 000 dm³ mol^–1). Introduction of Cl or Br at the 2- or 3-positions of propionic acid exerts moderate to strong quenching which is ascribed to a physical, rather than a chemical mechanism, a result also indicated by studies of quenching by various alkyl halides. The introduction of alkoxy-groups increases K_SV by a factor of 200, but this has been found to be due to a remarkably high sensitivity of ether groups to (UO₂²⁺)* as further evinced by a high quantum yield for U^IV production. The introduction of CC bonds in strategic positions also enormously increases K_SV(up to 1 600 dm³ mol^–1) but this was also found to be due to the CC bond itself, and a number of alkenes were found to exhibit quenching rates approaching diffusion control, presumably via an exciplex mechanism. Substituted benzoic acids quench (UO₂²⁺)* most effectively (K_SV up to 6 000 dm³ mol^–1) and the rates follow a good Hammett correlation, (ρ–0.88 ± 0.04). In some instances, the quenching data have been augmented by quantum yield measurements, and absolute constants for the reaction between (UO₂²⁺)* and substrate have been obtained directly by monitoring the absorption of (UO₂²⁺)* at 590 nm using ns flash photolysis.