Photoreactions of the Uranyl Ion with Arylaldehydes in Solution

Abstract

Irradiation of benzaldehyde in a deoxygenated aqueous acetone solution in the presence of the uranyl ion with light of λ≥365 nm gives the U(IV) species and benzil (1) as the main photoredox products, and small amounts of 4-hydroxy-4-phenylbutan-2-one (2), trans-4-phenyl-3-buten-2-one (3), and an unidentified product (4). None of them are formed in the absence of the uranyl ion. The products 2 and 3 are assumed to be formed by the uranyl ion-photocatalyzed condensation of benzaldehyde with acetone. Irradiation of acetophenone in the presence of the uranyl ion gives no photoproducts, though the uranyl emission is effectively quenched. p-Methoxybenzaldehyde and p-chlorobenzaldehyde give the corresponding 4,4′-disubstituted benzils respectively by irradiation under similar conditions. The quantum yield for the formation of the U(IV) species decreases whereas the quenching constant increases with the increase in the electron donating power of the substituent. The physical quenching process appears to compete significantly with the chemical (redox) process. The relative importance of the physical quenching to the chemical quenching process has been estimated. As to the photocatalyzed aldol condensation, the excess vibrational energy, which the quencher and/or the quenchee would possess during or immediately after the quenching interactions, may possibly be converted into the energy of the chemical reaction.