Triplet Excitons in Morpholinium TCNQ

Certain coincidences are noted between (a) the magnetic axes of triplet excitons in morpholinium TCNQ and (b) crystallographic axes determined by x-ray diffraction. These coincidences can be understood quantitatively in terms of an assumed crystal structure in which pairs of TCNQ− molecules (``dimers'') have D2h point-group symmetry and dimer symmetry axes that (nearly) coincide with certain crystallographic axes of the triclinic crystal. Observed fine-structure splitting parameters, | D | = 450 Mc/sec, | E | = 53.8 Mc/sec are in excellent agreement with theoretical values D=−464 Mc/sec, E=44 Mc/sec calculated using Hückel spin densities on TCNQ molecules together with the assumption that there is no intermolecular charge (and spin) transfer in the triplet-exciton state.