Laser-Excited Delayed Fluorescence of Crystalline p-Terphenyl

Abstract

The steady state and time-dependent laser excited delayed fluorescence from high purity p-terphenyl single crystals is discussed. Both are shown to be highly dependent on the presence of deep trapping centers at a concentration ≤ 10⁻⁸ mole/mole. General equations for the decay of the host delayed fluorescence in the presence of traps under conditions of weak excitation directly into the triplet state are employed to yield the trapping parameters. In addition the study of delayed fluorescence spectra and the total intensity of delayed fluorescence with temperature yields information about singlet and triplet exciton localization by the traps. ΔE₁ ^s and ΔE₁ ^T, the singlet and triplet trap depths respectively, are 4,600 ± 100 cm⁻¹ and 7,550 ± 500 cm⁻¹ and at 190 K, β_p the trap decay rate is 44 ± 6 s⁻¹. These values are consistent with trapping centers formed by the substitution of anthracene molecules in the p-terphenyl lattice. The reported phase transition in p-terphenyl at ca 190 K has no marked effect on the triplet trapping behavior.