Uranyl Ion-Sensitized Isomerization of the Stilbenes in Solution

By selective excitation of the uranyl ion with light of λ≥365 nm, a deoxygenated acetone solution containing cis- or trans-stilbene and the uranyl ion finally reaches a photostationary state, whose isomer ratio, (t⁄c)pss, varies from 1 to 3 depending on the concentrations of the uranyl ion and the stilbenes. A close correlation is found between the features in the sensitized isomerization and the phosphorescence quenching of the sensitizer. Comparison with the results of halogen radical induced isomerization suggests that catalytic isomerization by free radicals is not an important process. A triplet-triplet energy transfer mechanism is proposed, which is supported by the oxygen effect and considerations on the lowest excited state of the uranyl ion, i.e., its long lifetime and triplet energy level higher than that of the stilbenes.