Temperature dependence of the delayed fluorescence lifetime in anthracene crystals with triplet exciton traps

The lifetime of delayed fluorescence of anthracene crystals excited by the chopped-light of a He-Ne laser is investigated as a function of temperature over the range 110480 K. As compared with the fluorescence intensity, the lifetime is not influenced much by the temperature dependence of triplet-triplet annihilation rate and singlet-triplet absorption coefficient. Considering a trap saturation effect, an analysis has been made of the temperature dependence of the lifetime. In defect-induced crystals, three kinds of defects are observed by X-ray irradiation. Two are 0.33 and 0.16 eV in triplet trap depth, and the trapped triplet decay rate is 5×10⁶ s⁻¹ for both; the other is 0.29 eV and 1 s⁻¹. The latter is of a different type from the former, namely, it is almost a pure trapping center which hardly quenches triplet excitons.