Knowledge Bank

Optically detected ENDOR has been performed on the lowest phosphorescent triplet state of benzil doped into single crystals of $bibenzyl.1$ Analysis of the hyperfine anisotropy unambiguously reveals that the benzil molecule possesses a center of symmetry in the bibenzyl host, thus assuring that the $\alpha$-dicarbonyl fragment in benzil attains a trans-planar configuration. This is in sharp contrast to the geometry of benzil in the neat $crystal,2$ implying that molecular configurations of flexible chromophores are primarily determined by external crystal forces rather than internal molecular ones. ENDOR results are also presented suggested that most of the triplet state spin density is transplanar $\alpha$-dicarbonyls resides on the carbonyl oxygens, with only a small density remaining on the carbon atoms. This is consistent with theoretical calculations on $glyoxal.3$