Trapping of Triplet Excitons and the Temperature Dependence of Delayed Fluorescence in Anthracene Crystals

Abstract

Singh and Lipsett have observed that the intensity of fluorescence due to triplet—triplet annihilation in anthracene crystals exhibits an anomalous temperature dependence involving several distinct maxima. It is shown that this behavior can be understood on the basis of trapping of triplet excitons. A simple kinetic model leads to the conclusion that every trapping level with a triplet lifetime long compared to the lifetime of a ``free'' triplet exciton gives rise to such a maximum. In favorable cases these observations can be analyzed completely in terms of depths, concentrations, and triplet lifetimes of traps. An analysis of Singh and Lipsett's data is presented. It leads to a broad continuum of trapping levels with depths ranging from about 3000 cm⁻¹ to very small values, and two more or less discrete traps with depths of about 650 and 130 cm⁻¹, respectively. The shallower traps are shown to have the higher concentrations and the longer triplet lifetimes. Indirect evidence is presented for the existence of still shallower traps with a very short triplet lifetime. The nature of these traps is discussed.