Triplet Exciton Phosphorescence in Crystalline Anthracene

Abstract

Long-lived triplet excitons in anthracene crystals were detected by simultaneous observation of both their weak red phosphorescence and the delayed blue fluorescence which results from exciton-exciton interaction. Absorption and emission spectra at room temperature show that the interacting excitons are free, i.e., they correspond to the electronic excitation levels of the pure crystal. Decay curves obey the anticipated bimolecular-interaction kinetic equations. The triplet radiative lifetime is about 60 sec. In contrast to the results at room temperature, the low-temperature behavior is dominated by exciton-tapping effects. The interaction between free and shallowly trapped excitons is much more efficient in yielding delayed blue fluorescence than is the free-free interaction which is dominant at room temperature.