Vibronic perturbation of the lowest triplet state and phosphorescence of aromatic ketones

The polarization of phosphorescence of acetophenone and its derivatives reveal the extent to which the lowest triplet state is perturbed by a nearby triplet state, and it leads to a conclusive assignment cf the lowest triplet state. In acetophenone, the lowest triplet state is of n -> pi* origin in 3-methylpentane glass and of pi -> pi* origin in EPA glass. All common parasubstituted acetophenones possess the lowest triplet state of pi -> pi* origin both in hydrocarbon and hydroxyl solvents. The Franck-Condon factor dependence of the vibronic coupling is suggested by the observation that low frequency vibrational modes appear to become less efficient in coupling (3)pi pi* and (3)pi pi* states as the energy gap between these states increases.