Luminescence and ODMR studies of X-traps for triplet excitons in organic crystals

Abstract

The excited triplet states of shallow X-traps in neat single crystals of 4,4-dichlorobenzophenone were studied at liquid helium temperature by luminescence spectroscopy and magnetic resonance with optical detection. The vibrational phosphorescence bands, the polarization of phosphorescence spectra, and the triplet zero-field splitting parameters of these traps have been analyzed. A specific model of the structure defect (a molecule is turned by 180° on its long axis) has been considered in organic crystals of benzophenone type which consist of molecules whose dipole moments compensate each other in a unit cell. The calculated depth of such a dipole X-trap for triplet excitons in 4,4-dichlorobenzophenone crystals at the account of a defect molecule relaxation upon its slewing was found to be comparable with the experimental one.