Photochemistry of α-Cycloalkoxyacetophenones. Type II and Vibronic Pathways for Radiationless Triplet Decay

Medium and small ring α-cycloalkoxyacetophenones other than α-cyclopropoxyacetophenones undergo normal photochemical type II reactions with high efficiency. The products obtained are a novel family of 1-oxaspiro[3.n]alkanes as well as the corresponding cycloalkanones. The photochemical behavior of α-cyclopropoxyacetophenones deviates markedly from that of their higher homologs and is characterized by: (1) low efficiency of product formation, (2) unprecedented cis-trans isomerization of the cyclopropane ring for the α-(2,3-dimethylcyclopropoxy) acetophenones, and (3) triplet lifetimes which are shortened by remote ring methyl substitution. Replacement of the γ hydrogen with deuterium increases the triplet lifetime and the quantum yields for the type II and ring isomerization processes. Both the ring isomerization and the rapid radiationless decay of the triplet state are attributed to vibronic energy transfer between the excited carbonyl chromophore and the γ hydrogencyclopropyl unit.