THE LOWEST TRIPLET STATES OF SOME AROMATIC $HYDROCARBONS^{*}$

Abstract

Recent theoretical work on the energy levels of aromatic hydrocarbons in the polyacene series has focused attention particularly on the experimental assignments of transitions in this series. In the case of anthracene, the lowest triplet level was reported at $14,700 cm^{-1}$ (Lewis and Kasha, 1944). Recently, C. Reid in three papers has offered three different lines of indirect evidence against the earlier assignment. We have reexamined the anthracene lowest triplet level by two independent direct approaches: (a) the phosphorescence spectra at $77^{\circ} K$ of anthracene and seven variously halogenated anthracenes. (b) the singlet-triplet absorption of 9,10-dichloro- and 9,10-dibromoanthracene using long optical paths. These experiments have led to the certain confirmation of the earlier assignment of $14,700 cm^{-1}$ as the height above the ground state of the lowest triplet level of unsubstituted anthracene. The results are unambiguously confirmed by the perturbation experiments reported by McGlynn and Kasha.