title: Beta-phenyl quenching of triplet excited ketones: how critical is the geometry for deactivation?
abstract: The phenomenon of beta-phenyl quenching has been examined by laser-flash photolysis in a series of alpha- and/or beta-substituted ketones 4-8 with similar excited-state characteristics. It is found that alpha-substitution markedly increases the triplet lifetimes in contrast to beta-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 beta-phenyl ring gauche to each other. Despite this geometrical requirement, the longer lifetimes observed are interpreted as being due to the influence of the alpha-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 alpha-substituent has long been known to suppress the elimination pathway and promote Yang cyclization. In addition, we have shown that the diastereomers of alpha,beta-disubstituted ketones exhibit distinct lifetimes.
authors.affiliation: Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India.
authors.forename: Subhas
authors.initials: S
authors.lastname: Samanta
authors.forename: Brijesh Kumar
authors.initials: BK
authors.lastname: Mishra
authors.forename: Tamara C S
authors.initials: TC
authors.lastname: Pace
authors.forename: Narayanasami
authors.initials: N
authors.lastname: Sathyamurthy
authors.forename: Cornelia
authors.initials: C
authors.lastname: Bohne
authors.forename: Jarugu Narasimha
authors.initials: JN
authors.lastname: Moorthy