Triplet States and Triplet Excitons in Chemically Mixed Crystals of Acridine with Anthracene

Spectral and temporal characteristics and their temperature dependence for the long-lived (phosphorescence and delayed fluorescence) emission of chemically mixed crystals of acridine II (host) with anthracene (guest) were studied under conditions of various spectral resolutions and different modes of excitation. The energy of 0-0 transitions of exciton band and of trap states have been determined and the nature and energy scheme of the triplet states of crystals have been established. The complex arrangement of host molecules into two different pairs in the crystal structure of acridine II is revealed in the energetic "doublet" structure of the trap states for both, the shallow and the deep traps (of acridine and of anthracene origin, respectively). Migration of triplet excitation energy in these chemically mixed crystals is controlled predominantly by the triplet-triplet annihilation of excitons (either heterogeneous or homogeneous) almost in the whole temperature range. The formation of triplet excimers of acridine was definitely ruled out.