Crystal Field Effects on Triplet States of Molecular Solids: The Electronic States of Quinoxaline

Abstract

A description of the effects of the crystalline environment on the paramagnetic excited states of a guest molecule substitutionally dissolved in a molecular solid is presented in the deep-trap limit. Perturbations by host electronic states on the individual spin substates of the guest are explored and shown to make contributions to detailed experimental studies of the triplet state in all mixed crystal systems to a degree dependent on both the geometry and electronic structure of the host crystal. The effect of the crystal field on such experimental measurements as the polarization of singlet–triplet transitions, zero field splittings, and the Zeeman effect in mixed crystal systems is considered. Finally, the results of this study are applied to the reported anomalous polarization measurements of the phosphorescence of quinoxaline in durene. Within the framework of our description of the crystal field, it is shown that there are no special intermolecular effects in the durene mixed crystal, but that the differences in polarization properties of quinoxaline in durene and in naphthalene (in which quinoxaline exhibits a behavior quite close to that predicted from the oriented gas ratios) reduce simply to differences in crystal geometry.