The structure of low temperature triplet energy traps in organic solids from pmdr spectroscopy 11. 1,3,5-Trichloro and hexachlorobenzenes

Abstract

The new technique of polarized microwave PMDR spectroscopy is used to determine the structure of the low temperature (1.6 – 4.2°K) triplet energy traps. The structure of 1.3,5-trichlorobenzene (Tri-CB) and hexachlorobenzene (HCB) molecules in hexamethylbenzene (HMB) host as well as in their own neat crystals (present as x-traps) is determined from the linear polarization characteristics of their optically detected microwave zerofield (zf) transitions as well as from the analysis of their phosphorescence emission. The former technique gives the direction of the principal magnetic axes of these trap molecules in the crystal as well as the relative order of their zf levels. ln HMB host, deviation from trigonal symmetry is found to be only slight for HCB and absent for Tri-CB. ln the neat crystals, large deviations from trigonal symmetry are observed for the traps of both HCB and Tri-CB. In HCB x- traps, the HCB molecule is found to be slightly contracted along the CCl axis near parallel to the c′ crystal axes. Deviation From planarity is also strongly suggested by the large value of the zf parameter D. ln addition, the principal magnetic axis of the 2| E| moment for HCB x-traps is found to lie only 5° off the molecular N axis. The | D| + | E| and | D| — | E| moments, however, are 15° away from the A and B molecular axis, respectively. In Tri-CB neat crystal. two traps are observed optically with their phosphorescence origins 10 cm −1 apart. The zf parameter E is found to have a non-zero value and is opposite in sign for the two traps. If the distortion is to be blamed on pseudo-Jahn-Teller forces, the results lead to the conclusion that the x-(shallow) trap is contracted while the y- (deep) trap is expanded along an in-plane axis going through the CCl bonds near parallel to the a axis of the Tri-CB crystal. The plane containing the | D| + | E| and | D| — | E| moments of the x-trap suffers a rotation around the N-molecular axis, which is almost parallel to the 2| E| moment. Th | D| + | E| moment is 10° off the A axis and the | D| — | E| moment is 10° off the B axis. The 2| E| transition moment of the y-trap lies off the molecular N axis and the plane containing the | D| + | E| and | D| — | E| moments moves upward from the The results of these and other studies suggest that low temperature trapping in neat crystals of this type results from crystal induced geometrical and orientational changes in the molecules at point defects. The observed traps are those molecules for which the crystal field induced deformation leads to a lowering in their singlet-triplet transition energy as compared with that for the host lattice.