Photophysics of small α-dicarbonyls

The ODMR spectra of biacetyl in neat crystals and in 3-methylpentane glass have been analysed. Several discrete sites exist in the glass medium but one x-trap dominates in neat crystals. Triplet kinetic studies have been made on the biacetyl x-trap. The z-level is pre-eminent in radiative and total decay, but the y-level receives a substantial portion of the triplet population, resulting in a spectacular spin polarization on ‖y>. The orientations of the principal magnetic fine-structure axes relative to the molecular framework have been determined using optically detected high-field E.P.R. The y-axis is found to be out-of-plane and the z-axis lies approximately along the C=O direction. This observation is in disagreement with a recent calculation by Leclercq et al. A case is made establishing the origin of the ‘strong green’ emission and our ODMR signals as a biacetyl x-trap instead of impurities, as previously suggested by Brand and Mau. The implications of these findings to the photophysics of biacetyl and other α-dicarbonyls are discussed.