PHOSPHORESCENCE STUDIES OF NON-HEXAGONAL BENZENE.

$^{1}$ M. S. de Groot and J. H. van der Waals, Mol. Phys. 6, 545 (1963).Author Institution: Department of Chemistry, University of Rochester; Department of Chemistry, California Institute of TechnologyRecent ESR studies by de Groot and van der $Waals^{1}$ have indicated that benzene in its lowest triplet state does not possess hexagonal symmetry. These findings are confirmed by spectroscopic studies of the phosphorescence of various deuterated benzenes. The phosphorescence spectrum of $C_{6}H_{5}D$ or $p-C_{6}H_{4}D_{2}$ dissolved in $C_{6}D_{6}$ at $4.2^{\circ}K$ arises from two isomers while that of $C_{6}H_{6}$ or sym-$C_{6}H_{2}D_{3}$ arises from only one isomer. The fluorescence spectra of the four above benzenes indicate the presence of only one isomeric form in the ground and first excited singlet states. These result are consistant with the existance of a two-fold, but not a three-fold, axis of symmetry perpendicular to the molecular plane in the lowest triplet state $(^{3}B_{tu})$ of benzene. The configuration lifetime of an isomeric form is longer than the phosphorescence lifetime $(\sim 10 sec)$. Work supported in part by the National Science Foundation