Delayed Luminescence of Organic Mixed Crystals. VI. Delayed Excimer Fluorescence of Pyrene in Biphenyl

Abstract

Phosphorescence, fluorescence, delayed fluorescence, excimer fluorescence, and delayed excimer fluorescence of pyrene have been studied in a host biphenyl system. The delayed fluorescence and delayed excimer fluorescence are biexcitonic, have identical lifetimes at all temperatures and exhibit similar dependency of emission intensity on temperature; therefore, they both originate in identical kinetic events. These events involve thermal depletion of a trap of energy ∼1000 cm−1 less than the lowest energy host exciton triplet band and of lifetime ∼0.2 sec; this trap species may or may not be, but probably is not, a contaminant detected phosphorimetrically in the purified biphenyl material. It appears that trapping of the host triplet exciton occurs more efficiently at monomer pyrene sites than at nascent excimeric pyrene sites (regions, perhaps, of pyrene microcrystallinity). The excimer and delayed excimer fluorescence exhibit vibrational structure under certain circumstances; this structure is analyzable in terms of ag modes of the ground-state monomeric pyrene. Those crystals which possess broad (i.e., unresolved) excimer spectra exhibit a distinct red shift of this spectrum as the temperature is decreased; this behavior is rationalized herein. The guest phosphorescence intensity and lifetime are more or less temperature-independent; this behavior is quite characteristic of materials with large T1H—T1G bandgaps. Excitation spectra for all emissions are presented; it is shown thereby that trap species other than T1G are the reservoir whence thermal excitation to the host exciton band T1H proceeds. The thesis developed is that delayed excimer fluorescence and delayed fluorescence are produced by annihilation of a trap triplet and a guest triplet.