Effects of Traps on Migration and Annihilation of Triplet Excitation in Phenanthrene Crystals

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Abstract
Delayed fluorescence originating from triplet–triplet annihilation is reported from a mixed‐crystal system characterized by an added impurity in low concentrations and large triplet trap depth. Temperature and impurity (guest) concentration dependences are extensively investigated in the anthracene in phenanthrene system. It is observed that for concentrations below 10−4 moles/mole, and at temperatures below 50°K, host‐delayed fluorescence is independent of added anthracene and is best described as originating from triplet–triplet annihilation between host triplet excitons and shallow triplet traps, whose phosphorescence emissions are found to correlate with the phenanthrene‐delayed fluorescence as the temperature is changed in the range 4.2–60°K. Near 77°K, these shallow traps become ineffective acceptors of host triplet excitation and delayed fluorescence arises mainly from the interaction between the anthracene and phenanthrene triplet excitations. The temperature dependence of the delayed fluorescence strongly supports the trapping and the detrapping mechanism formulated by Siebrand [J. Chem. Phys. 42, 395 (1965)]. In this present system, two discernible shallow trap phosphorescences are observed. A kinetic model is proposed and supported by the observed dependence of the delayed fluorescence intensity on guest concentration, temperature, exciting light intensity, as well as the observed activation energy.