EPR spectroscopy of delocalized and localized charge-transfer excitons in phenanthrene-PMDA single crystals

Detailed EPR investigations of delocalized and localized charge-transfer excitons in phenanthrene-PMDA single crystals are reported. The ionicity of the excited charge-transfer state is investigated by two different methods, by a detailed analysis of the fine structure tensor and by an analysis of the hyperfine tensors of the donor protons. The fine structure analysis is carried out on the basis of a "point charge" model calculation; it yields a degree of charge transfer of 76% ± 5%. This value is more reliable than the number derived from the hyperfine structure analysis due to certainties in available spin densities for the donor and acceptor molecule. The anisotropy of the exciton hopping rate parallel and perpendicular to the crystal stacking axis is at least 100:1 with a hopping rate parallel to the stack axis of ν∥≥108/sec at 130°C. Copyright © 1977 American Institute of Physics.