Effect of Ring Substitution on the Photochemistry of α-(Aryloxy)acetophenones

Article abstract of DOI:10.1021/jo00288a019

The photochemistry of a series of 10 α-(aryloxy)acetophenones has been examined by quantum yield, product studies, and laser flash photolysis techniques.Triplet decay involves a competition of β-cleavage with intramolecular quenching by the β-aryl group, the latter being favored in all systems.Typical triplet lifetimes at room temperature range from 60 to 260 ps and are rather insensitive to the substitution pattern. β-Cleavage rate constants range between 1*10⁷ and 3*10⁹ s^-1 and are strongly favored by electron releasing substituents, such as 4-methoxy.The determination of true quantum yields of β-cleavage requires the addition of thiophenol as a radical trap to prevent the efficient back reaction that takes place otherwise; under these conditions quantum yields range from 10^-3 to 0.14, the highest value being obtained for the 4-methoxy and 4-hydroxy derivatives.