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Mobile triplet excitons in A/TCPA have been detected from 1.2 to 300 K via ESR and zero-field ODMR. Their features such as linewidth and zero-field-splitting (zfs) parameters are discussed in terms of the temperature-dependent crystal structure. Between 1.2 and 4.2 K the excitons are caged by barriers. When raising the temperature a thermally activated hopping takes place between two triplet bands separated by 40 cm−1, having different zfs parameters | DA| = 2126.5, |EA| = 241.0, | DB| = 2112.5, | EB| = 238.0 MHz and different zfs tensor orientations. Above 100 K an increase of the librations of the anthracene molecules broadens the homogeneous excitonic lines. The homogeneous linewidth is measured by the free induction decay. The CT character of the triplet state is estimated as c2CT<̳4%.

The longitudinal proton relaxation time T1in methyl naphthalene crystals, differing in the arrangement and number of the substituted CH3groups, was measured as a function of the temperature above 77 °K and the magnetic field between 0.9 and 20 kOe. The results can be described by hindered rotation of the methyl groups with the jumping times and activation energies strongly dependent on the group arrangement. In the β-position the rotational barrier of 0.8 kcal/mol is predominantly determined by the infermolecular interaction, whereas in the case of the a-position and for adjacent CH3’s the hindering potential of 2.4 kcal/mol arises largely from the intramolecular term.