Addition of the carbonyl oxygen to the ipso- or ortho-carbon atoms of the beta-phenyl ring followed by intersystem crossing and rapid relaxation to the ground-state ketones: a mechanism for beta-phenyl quenching of the first triplet excited states of derivatives of beta-phenylpropiophenone

The first triplet excited states of beta-phenylpropiophenone 1a and derivatives are known to have unusually short triplet lifetimes. On the basis of pronounced substituent and solvent effects observed in the case of 4-methoxy-beta-phenylpropiophenone 1b, a mechanism involving substantial electron transfer has been assumed to be operative. This contribution outlines an alternative mechanism involving addition of the excited carbonyl moiety at the ipso (preferred) or ortho positions of the beta-phenyl ring. The triplet biradicals thus formed may undergo rapid intersystem crossing to the singlet manifold. On the singlet hypersurface, the biradicals are not predicted to be minima, relaxing to the singlet ground-state ketones. Overall, this addition, intersystem crossing, elimination sequence provides a plausible reaction pathway for beta-phenyl quenching. Calculated activation enthalpies and substituent effects are in agreement with experimental data published in the literature.