Level-anticrossing and cross-relaxation effects in oriented molecular triplet states. 3(n,π*) benzophenones in 4,4'-dibromodiphenylether

This paper reports the observation and analysis of magnetically induced changes in the phosphorescence intensities of triplet state benzophenone, carbonyl-13C-benzophenone, and three 4,4'-dihalobenzophenones in single crystals of 4,4'-dibromodiphenylether at 1.6degreeK. The signs, magnitudes, field positions, and widths of these changes can be interpreted in terms of two effects, anticrossings of two different electron--nuclear spin manifolds of the guest molecule and cross relaxation between the spin system of the guest and that of the host. At fields where anticrossings occur, the mixing between the spin states is strong and no hyperfine structure is observed. However, by comparing the results for the different molecules in the same host, it is shown that secular terms of the hyperfine interaction play a key role in determining the minimum spectral widths of the ''transitions''. At fields where cross relaxation predominates, the mixing between the spin states is weak and hyperfine and quadrupole splittings of both the guest and host can be resolved. The magnetic and optical parameters obtained by fitting the ''spectra'' are in excellent agreement with those determined by ODMR techniques [J. A. Mucha and D. W. Pratt J. Chem. Phys. 66, 5339 (1977), preceding paper]. Thus, experiments of this type can be utilized to provide information about the fine-structure, g, hyperfine, and quadrupole tensors of emitting paramagnetic species in the complete absence of perturbing radiofrequency or microwave fields. Moreover, the systems described herein offer considerable promise in future studies of spin relaxation mechanisms in molecular solids by optical detection methods