Spin-quantization and spin-orbit coupling effects on the line shapes of triplet states. II. The "small" exciton problem

This paper presents a detailed study of the effect of spin‐quantization and spin–orbit coupling on the transition energies of triplet state dimers or small excitons. We consider both translationally equivalent (AA) and inequivalent (AB) dimers. For the AA and AB systems, we calculate transition frequency shifts induced by the spin–orbital coupling and by the spin–spin interactions between the plus (+) and minus (−) states of the dimer. As a result of these combined effects the selective coupling between the ± states of the singlet and the ± states of the triplet AA dimer system is not operative in the AB system. Furthermore, the role of the gas‐to‐crystal shifts and the intermolecular spin–spin interactions is to change the observed transition frequencies and hence cause a dispersion in the frequencies of the ± states. The relationship between such a dispersion in the AA and the same AB system is directly related to molecular parameters such as the strength of spin–orbital coupling. These results are applied to three experimental findings obtained for different dimer systems—phenazine, naphthalene, and tetrachlorobenzene dimers isolated in isotopically mixed crystals at T