Phosphorescence Decay of Benzene and Methylbenzene Derivatives

Phosphorescence lifetimes of benzene and methylbenzene derivatives, deuterated and undeuterated, have been measured in glassy matrices at 4.2 and 77°K. Spin–orbit coupling calculations have been performed for benzene ${\mathrm{(D}}_{\text{6h}})$ , distorted benzene ${\mathrm{(D}}_{\text{2h}})$ , and toluene; these calculations included spin–own‐orbit, spin–other‐orbit, and vibronic considerations and were extended to all the methylbenzenes by a vector‐sum semiempirical method. Franck–Condon calculations were performed for all methylated, deuterated, and distorted benzene derivatives. The results of these calculations lead to a general understanding of phosphorescence lifetimes, at least insofar as these are affected by either methylation or deuteration or both. Matrix effects and temperature effects on the phosphorescence lifetimes are largest in the case of benzene and toluene and decrease with increasing methylation of benzene and with increasing size of the polynuclear hydrocarbon. We attribute this sensitivity to the small size of the space available to the $\pi$ subsystem in benzene, to the forbiddenness of the electric‐dipole $T_1{\mathrm{\hspace{0.17em}\to \hspace{0.17em}S}}_0$ process in this system, and to the ease with which matrix effects may perturb the rather “naked” skeleton of this molecule and its less‐methylated derivatives. A concept of solvent‐assisted distortions is introduced in order to explain the direction of solvent and temperature effects and the means whereby these effects become operative. This last is a very tenuous suggestion, its primary merit being that it provides some rationalization of the observed phenomenology.