Photoenolization of ortho-alkyl-substituted acetophenones: evidence for the enol triplet state

Abstract

The photoenolization of 2-methyl-, 2,4-dimethyl- and 2,5-dimethylacetophenone has been investigated using conventional flash photolysis. It was found that initial concentration of photoenol in aprotic solvents increased with the addition of oxygen. The results are interpreted in terms of the formation of an oxygen quenchable enol triplet which can decay either to the enol or the ketone ground state. The data are used to calculate relative quantum yields of enol formation of ethanol (1.0), diethyl ether (0.5), and cyclohexane (0.1). A model is proposed which leads to the conclusion that the enol triplet is quenched at a near diffusion-controlled rate. No photoenolization was detected in 2-isopropyl- and 2,4-di-isopropylacetophenone. These results are interpreted in terms of the above model.