Experimental Observation of Singlet–Triplet Absorption in Pure Crystalline Benzene

Abstract

The B1u3 ← A1g1 intrinsic absorption spectra of C6H6 and C6D6 at 4.2°K have been obtained for the first time at high resolution using the phosphorescence–photoexcitation method. The spectrum exhibits very clear evidence of a pseudo-Jahn–Teller distortion of the normally hexagonal benzene molecule upon excitation to the triplet state. Factor-group splitting of the 0–0 and 0–0 + ν1 exciton bands have also been observed. The position of the mean of the 0–0 exciton band of C6H6 when compared to the phosphorescence origin of a C6H6 guest in a C6D6 crystal, indicates that the “static” intermolecular interactions between guest and host are different for C6H6 and C6D6 hosts. This result has important implications for the applicability of mixed crystal theories to benzene.