Investigation of triplet state energy transfer in organic single crystals at low guest concentrations and low temperatures by magnetic resonance methods

The transfer of triplet excitation from phenanthrene guest molecules to naphthalene guest molecules in diphenyl single host crystals was investigated by magnetic resonance methods. Mole fractions of guest molecules in the diphenyl crystals were from 0.014 to 0.003 for phenanthrene and from 0.0077 to 0.0005 for naphthalene. The temperature of the diphenyl crystals was ∼1.7°K. Several types of experiments were employed both to prove the existence of the transfer and to elucidate the nature of the mechanisms and processes which were responsible for it. We were able by use of the results of these experiments, to eliminate several a priori possibilities for the mechanism of transfer. These results also showed that the main energy transfer processes were initiated either by intramolecular intersystem crossing of singlet excited phenanthrene donor molecules to triplet states as high or higher in energy than the lowest triplet exciton band of the host crystal, or by guest host intermolecular intersystem crossing which created host molecules in triplet states. These initial processes were then followed by migration of the triplet excitation through the crystal to the lower lying naphthalene triplet state traps.