β-Phenyl Quenching of Triplet Excited Ketones:  How Critical Is the Geometry for Deactivation?

The phenomenon of β-phenyl quenching has been examined by laser-flash photolysis in a series of α- and/or β-substituted ketones 4−8 with similar excited-state characteristics. It is found that α-substitution markedly increases the triplet lifetimes in contrast to β-substitution. The force field calculations for the various staggered conformers of ketones 4−6 and 8-syn show that the lowest-energy conformation in all these ketones has the carbonyl group and the β-phenyl ring gauche to each other. Despite this geometrical requirement, the longer lifetimes observed are interpreted as being due to the influence of the α-substituent on the rotational freedom of the planar benzoyl moiety as a whole. The experimental results are suggestive of the attainment of what appears to be a critical geometry for quenching. This scenario may be likened to Norrish type II reactions, where the α-substituent has long been known to suppress the elimination pathway and promote Yang cyclization. In addition, we have shown that the diastereomers of α,β-disubstituted ketones exhibit distinct lifetimes.