Geometry of the Lowest Triplet State of Benzene

Abstract

The phosphorescence, fluorescence, and absorption spectra of seven isotopic benzenes in a C₆D₆ host crystal were obtained at 4.2°K. The isotopes having less than D_3h symmetry show splittings in their phosphorescence and singlet absorption spectra. Expected splittings in the fluorescence spectrum have not yet been observed because of the greater difficulty in obtaining the required high‐resolution fluorescence spectra. The splittings are interpreted in terms of a distortion, which may arise either from extrinsic or intrinsic perturbations, in the ¹B_1u and ³B_1u states of crystalline benzene. This distortion causes conformers of isotopic benzenes with less than trigonal symmetry to have different zero‐point energies and leads to the observed line multiplicity. The vibronic structure of the phosphorescence and fluorescence emissions implies that the magnitude of the distortion is small. Benzene molecules in the low‐symmetry crystalline field are not expected to retain a hexagonal shape. However, the splittings in the fluorescence are only about ⅛ as large as those in the phosphorescence, implying that the distortion in the ³B_1u state is larger than that in the ¹B_2u state. There is therefore some cause to believe that the distortion in the ³B_1u state is partly intrinsic and thus has the significance attached to it by theorists. Our experiments, however, cannot actually distinguish between intrinsic and extrinsic distortions, but certainly give the impression that the distortion of the ³B_1u state of benzene in the crystal is not very substantial and is either wholly or partly caused by the effect of the crystal field. The relative intensities of the components of the multiplet structure in the phosphorescence are strongly concentration dependent because of trap‐to‐trap excitation migration in the crystal. At low guest concentrations, where little migration can occur, the intensity ratio gives the statistical weights of the components; at high concentrations a Boltzmann intensity ratio is obtained. The interpretation of the available optical data can be made to agree with the interpretation of ESR results by de Groot, Hesselmann, and van der Waals that a distorted ³B_1u benzene is preferentially oriented in the site cavity of the solid phase, but the resulting picture is not a wholly satisfying one.