Substituent effects in the quenching of acetophenone and benzophenone triplets by oxygen and the di- tert-butylnitroxy radical, and the efficiency of singlet oxygen photogeneration

Abstract

The behavior of the triplets of a number of p-substituted acetophenones and benzophenones with respect to quenching by oxygen and the di- tert-butylnitroxy radical (DTBN) has been examined in benzene and acetonitrile using 337.1 nm laser flash photolysis. In several cases the efficiency φ Δ of singlet oxygen ( 1O 2 *) generation in the course of triplet quenching by oxygen has been determined using 1,3-diphenylisobenzofuran as the trapping agent for 1O 2 *. The variation of the rate constants for triplet quenching with respect to the substituent nature suggests that the aromatic carbonyl triplets act as donors and acceptors in the quenching interaction with oxygen and DTBN respectively. Relative to the rate constant for quenching by oxygen, the substituent effect on φ Δ is less pronounced, particularly in the case of benzophenones; the φ Δ are limited to the range 0.3 − 0.7 in the two solvents. Fast equilibration among the various spin configurations of the encounter complex responsible for the nearly substituent-independent φ Δ for benzophenones and back electron transfer in charge-transfer-derived ion pairs leading to 1O 2 * formation are suggested.