Temperature dependence of the photochemistry of aryl alkyl ketones

The photochemistry of several phenyl alkyl and p-anisyl alkyl ketones has been examined using laser flash photolysis and conventional quantum yield techniques. The methoxy-substituted ketones show higher activation energies (A£ ~ 3 kcal mol″1) for the Norrish type I and type II processes. It is concluded that both reactions are adiabatic processes occurring from the triplet nπ* surface. In the case of p-methoxy-substituted ketones the upper nπ* surface is reached from the low-lying ππ* triplet, with the energy gap between both states reflected as an increase in the activation energy.