Beta-phenyl quenching of triplet excited ketones: how critical is the geometry for deactivation?


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.

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@article{Samanta2006BetaphenylQO, title={Beta-phenyl quenching of triplet excited ketones: how critical is the geometry for deactivation?}, author={Subhas Samanta and Brijesh Kumar Mishra and Tamara C. S. Pace and Narayanasami Sathyamurthy and Cornelia Bohne and Jarugu Narasimha Moorthy}, journal={The Journal of organic chemistry}, year={2006}, volume={71 12}, pages={4453-9} }