Crystal field effects on the ground and lowest excited triplet states of benzene isotopes in a borazine host crystal

Abstract

The phosphorescence spectra of the C 6H 6C 6H 5D 1 p-C 6H 4D 2 symp-C 6H 3D 3, C 6D 6 and 13CC 5D in a borazine host crystal are analyzed at high resolution. The spectral lines are sharp (∼2 cm −1 wide) indicating that the impurity molecules occupy a unique site in the borazine lattice which is probably substitutional. The phonon sidebands are weak,giving clean, well-resolved spectra much like those of isotopic mixed crystals. In contrast, however, the crystal field effects on the ground state vibrational levels are much smaller than those found for isotopic mixed crystals. The gas-to-crystal shifts are very small, the vibrational degeneracies are not removed and orientational splittings are only observable for a few select vibrational levels. For most vibrational levels and for the derivation of selection rules one can asume the effective crystal site symmetry to be D 3d. The data provide the first conclusive evidence that the splitting observed in the benzene phosphorescence spectrum results from a distortion of the molecule when excited to the zeroth vibrational level of the T 1 state. Furthermore, the data suggest that the distortion is intrinsic in nature (i.e.,is not caused by the crystal field).