Unusual room temperature afterglow in some crystalline organic compounds

Abstract

STRONG room temperature phosphorescence studies of certain doped solid systems^1–4 have shown that the characteristic phosphorescence peak wavelengths agree with those established for the corresponding dopants dissolved in 77K systems; the room temperature lifetimes being smaller by a factor of 1.5–2, depending upon the nature of the matrix. We have recently observed an unusual new phenomenon—a weak afterglow from certain of the dopant compounds including carbazole, dibenzothiophene, dibenzofuran and triphenylene when examined as pure crystalline solids. This weak afterglow has different peak wavelengths and lifetimes, but in dilute solid solution form in room temperature resin systems there is no trace of the weak afterglow properties. Melting and resolidification of these pure aromatic compounds do not affect the weak afterglow, and, after vacuum sublimation of carbazole, the afterglow is still observed from the purified product. We believe that we can rule out an impurity effect and this view is reinforced as the afterglow intensifies after recrystallisation from ethanol. As the crystals probably improve on recrystallising, the crystal quality may play a role in this phenomenon.