On the photolysis of hydroxy alkylphenones and o‐substituted derivatives. Laser flash photolysis and photocuring studies

Abstract 1‐Phenyl‐2‐acetoxy‐2‐methyl‐propanone‐1 (III) and 1‐phenyl‐2‐methoxy‐2‐methyl‐propanone‐1 (IV) were irradiated at room temperature with 15 or 25 ns flashes of 265 nm or 347 nm light in Ar‐saturated cyclohexane or benzene solution. Transient absorptions and emissions were recorded as a function of time between 300 and 550 nm. Triplet lifetimes of III in cyclohexane and benzene and of IV in p‐dioxane were determined as ca. 2 μs. In contrast to the photolysis of 1‐phenyl‐2‐hydroxy‐2‐methyl‐propanone‐1 (compound I) and of 1‐[4‐(2‐propyl)‐phenyl]‐2‐hydroxy‐2‐methyl‐propanone‐1 (compound II), α‐cleavage occurs upon UV irradiation of III and IV to a minor or negligible extent. Triplets of III were found to react effectively with N‐methyldiethanol amine, but a reaction with cyclohexane did not become noticeable. They react with O2 with k(T + O2) = 1.6 · 109 l/mol s. Triplets of IV react with cyclohexane with k(T + CH) = 8.6 · 105 l/mol s and with O2 with k(T + O2) = 1.1 · 109 l/mol s. They are inert towards p‐dioxane. Photocuring of a transparent acrylate coating system yielded the following results on the basis of pendulum hardness measurements: I and II exhibit excellent initiator effectivity, III and IV are not capable of initiating curing if applied without coinitiator, e.g. N‐methyldiethanol amine. In the presence of the latter III and IV exhibit satisfactory curing capability. In conclusion: the high curing effectivity of I and II is due to the high yield and the high rate of α‐cleavage. III and IV do not undergo α‐cleavage and therefore do not initiate curing. The curing capability of III and IV in the presence of a coinitiator is due to the formation of ketyl radicals by a hydrogen transfer mechanism.