Experimental Observation of Singlet–Triplet Absorption in Pure Crystalline Benzene

Abstract

The ${}^3{B}_{\text{1u}}{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{A}_{\text{1g}}$ 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 ${\text{0–0\hspace{0.17em}+\hspace{0.17em}ν}}_1$ 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.