Mobile and Trapped Triplet States in Single Crystals of Charge Transfer Complexes

Homogeneously doped crystals of charge transfer(CT-)complexes were grown by incorporating aromatic guest donors in host CT-cryslals. The host crystals contained 1, 2.4, 5-tetracyanobenzene (TCNB) as acceptor and deuterated aromatic electron donors. By using such doped crystals CT-complexes in a well defined configuration may be studied. The triplet states of the guest complexes were used as ESK spectroscopic probes in order to determine the molecular arrangement in the host lattice. The zero-field-splitting (ZFS) parameters, D and E, of the triplet energy traps were determined and the degree of electron delocalization in the triplet state was calculated from these values. In some cases a very strong guest ho>t interaction (multicomplex formation) was established, A method for the determination of CT-tripIet energies is described (accuracy 200 cm^-1). The phosphorescence spectrum of the anthraeene-TCNB complex was obtained from the delayed emission spectra of different anthracene doped CT-cryslals. The vibronic structure is identical to that of anthracene, while the O-O-band of the complex is blue shifted by 600 cm^-1. It is shown that the undoped anthracene-TCNB crystal exhibits P-type delayed fluorescence and that the triplet exciton diffusion in this crystal is nearly temperature independent. In the undoped biphenyl-TCNB crystal E-type delayed fluorescence originating from the thermal depopulation of the mobile triplet excitons is established. The remarkable differences of the two types of triplet excitons are interpreted in terms of the different polarity in the triplet states of the two CT-crystals.