LOW-FIELD EPR STUDIES OF OPTICALLY EXCITED AROMATIC TRIPLETS ORIENTED IN SINGLE-CRYSTAL HOSTS (LEAST-SQUARES, CRYSTAL-FIELD, ZERO-FIELD SPLITTING)

EPR studies of aromatic hydrocarbons in their phosphorescent state, present as dilute guests in substitutional single-crystals, were conducted at magnetic fields sufficiently low that the Zeeman interaction was only a small perturbation of the intrinsic dipole-dipole interaction between the triplet electrons. This physical situation enabled high precision determinations of the zero-field splittings (ZFS) by direct extrapolation, as well as high resolution measurements of closely spaced resonance absorptions by crystal-field inequivalent triplets.;Four classes of molecular mixed-crystals were examined, each manifesting crystal-field inequivalent triplets due to a different type of guest-host interaction. For p-terphenyl host crystals, the multiplicity of the crystal-field inequivalent triplets exceeded the number of crystallographically inequivalent substitutional host sites both above and below the 193.3(0.2) K phase transition, and was attributed to multiple guest orientations in each case. In contrast, multiplet structure observed with diphenylacetylene host crystals is consistent with isomorphic guest substitution at crystallographically inequivalent sites. Studies of substituent effects on the ZFS of a series of methyl-substituted and peri-bridged naphthalenes uncovered multiplet structure arising from (1) crystal-field interactions of various magnitudes at different sites of certain guests due to crystallographic inequivalence of the adjacent methyl groups of durene, and (2) a double-well potential from the intermolecular effects of the durene methyl-group librations. Finally, preliminary investigations of the composition-dependence of the ZFS in binary host systems forming a continuous series of solid solutions revealed unusually complex multiplet structure with dibenzofuran hosts, and led to the discovery of disorder in the crystal structure of dibenzofuran.;Improvements in the experimental apparatus and statistical methods developed in this research increased the ultimate precision of ZFS determinations to the order of 0.001 MHz, which is generally 2-4 orders of magnitude more precise than achieved by other contemporary magnetic resonance methods. Furthermore, a new iterative weighted least-squares technique, based in part upon recent work at NBS, was used to calculate the fine structure parameters from the ZFS.