Electron Paramagnetic Resonance Absorption in Oriented Phosphorescent Triphenylene-h12 and Triphenylene-d12 from 4 to 250°K

Abstract

Electron paramagnetic resonance absorption by phosphorescent triphenylene‐h 12 and ‐d 12 oriented in single crystals of symmetric dodecahydrotriphenylene has been investigated in the temperature range from 4 to 250°K. It has been shown that both guest molecules are multiply oriented in this host. In addition, two distinct kinds of mixed crystals have been identified on the basis of the zero‐field splittings manifested by the guest molecules. In one kind of mixed crystal, guest splittings were found to change with time until stable, fixed values were established. Since splitting differences are related to orientational differences, guest rearrangement in the solid host is implied, which has not heretofore been reported. The temperature dependence of at least one zero‐field splitting parameter D is complex in that its magnitude does not increase monotonically with decreasing temperature. The temperature dependences of the zero‐field splitting parameters E are even more complex. X‐ray diffraction data indicate that both kinds of mixed crystals have the same over‐all host structure and are free from disorder. The unit cell deviates slightly from the previously reported hexagonal type, indicating lower than threefold site symmetry. Because of this, and also because of the multiple orientation, the E parameters of the triplet guests cannot be expected to vanish. The data provide some evidence that the entire magnitude of E is due to the molecular‐crystal field. The decays of the resonance absorptions of both guests were investigated as functions of temperature, and the angular dependence of the resonance absorption linewidths for triphenylene‐d 12 has been studied near 77°K. The wavefunction of the lowest triplet state of trigonal triphenylene, and its energy relative to the ground state, have been calculated in the Pariser—Parr—Pople approximation. The zero‐field splitting parameter D has been calculated using two‐center integrals evaluated by the Gaussian‐transform method. The calculated values of both the energy and D are in very good agreement with experimental values.