Structures of the Excited States of Benzil and Related Dicarbonyl Molecules

Abstract

Nonhindered aromatic α-dicarbonyl molecules show abnormal spectroscopic behavior when compared to other molecules containing carbonyl groups. Taking benzil as a typical example, its photoabsorption and emission spectra cannot be accounted for by the normal Jablonski scheme. The spectral data are consistent with a mechanism involving molecular reorientation. It is postulated that photoexcitation of benzil causes it to change from its normal skew configuration into one in which the dicarbonyl system becomes coplanar while it is in the excited singlet state. As a consequence photoemission (both fluorescence and phosphorescence) takes place from this configuration, resulting in a ground-state molecule with the same nuclear configuration as the excited state; relaxation then occurs to the normal configuration. Formation of a trans configuration is most likely to occur on photoexcitation, but a cis configuration cannot be ruled out. The formation of a coplanar dicarbonyl system must involve movement of the phenyl groups into positions out of the dicarbonyl plane. Additional support for this mechanism is obtained from the energetics of the various processes.