Delayed Luminescence of Organic Mixed Crystals

Abstract

The luminescence spectra of the following mixed-crystal systems have been investigated: naphthalene-h8 guest—biphenyl host; naphthalene-d8 guest—biphenyl host; phenanthrene guest—biphenyl host; chrysene guest—biphenyl host; naphthalene-h8 guest—durene host; naphthalene-d8 guest—durene host; phenanthrene guest—durene host; chrysene guest—phenanthrene host; dibenzanthracene guest—phenanthrene host; phenanthrene-d10 guest—biphenyl host. The quantities on which experimental information is provided are: temperature and excitation intensity dependence of phosphorescence and delayed fluorescence intensities; concentration dependence of phosphorescence and delayed fluorescence intensities; and decay data at different temperatures for delayed fluorescence. Delayed fluorescence of the guest is observed from all mixed-crystal systems for which S1H > S1G > T1H > T1G, where, for example, S1H denotes the first excited singlet state of the host as well as the energy of this state. If the gap T1H↔T1G is sufficiently large, the delayed fluorescence may be very weak, or absent. Two types of triplet—triplet annihilation processes have been substantiated: guest—guest annihilation which is of significance at temperatures above 100°K, and guest—trap annihilation which is dominant in the region 100°↔77°K. A third type of triplet—triplet annihilation, namely trap—trap annihilation, is probably causative of the delayed fluorescence of so-called ``pure'' or ``one-component'' crystals. The nature of the traps is not understood; they may be surface host species, defect host species or adventitious trace impurities.