Solvent shift and excited state geometries of benzil

The room temperature and low temperature absorption and emission spectra corresponding to the “relaxed” and “unrelaxed” excited states of benzil are reported. It is observed that while the spectra corresponding to the unrelaxed state exhibit an appreciable blue shift in an alcoholic solvent compared with those in a hydrocarbon solvent, there is no such shift for emissions originating from the relaxed state. The lack of a solvent shift for relaxed emission furnishes additional evidence that the relaxed state has a trans-planar geometry with a zero dipole moment, while the unrelaxed form must be skew in both singlet and triplet excited states.