‘Excimer’ fluorescence - IX. Lifetime studies of pyrene crystals

The increase of the excimer fluorescence lifetime Ƭ D (= 1/ k D ) with increasing purity was used to establish a reproducible criterion of purity for crystalline pyrene. Zone-refining, microsublimation and adsorption chromatography yielded material with similar high values of Ƭ D = 113±2 ns at 293 °K, but recrystallization from solution was ineffective. Single crystals of zone-refined pyrene were grown by sublimation, and Ƭ D and the fluorescence quantum efficiency q D were observed from 100° to 350 °K. Values of k fD = 5·55 × 10 6 s -1 , k 0 iD = 0, and k iD = 4·7 × 10 7 exp [ –0·066 (eV)/ kT ] s -1 are obtained for the rate parameters of fluorescence, temperature-independent intersystem crossing and temperature-dependent intersystem crossing, respectively. Ƭ D for microcrystalline layers ca . 0·5 μ m thick is lower than for single crystals, owing to a crystal-size effect which limits the exciton lifetime. The reduction in Ƭ D is accompanied by the appearance of a surface monomer band in the fluorescence spectrum. The exciton migration process in anthracene and pyrene crystals is discussed. From lifetime data it is concluded that singlet excitons in organic molecular crystals are localized and identical with singlet-excited molecules and that they migrate by a hopping mechanism, rather than through a hypothetical exciton band. The pyrene excimer parameters are similar in the crystal and in solution; it is concluded that the excimer molecular configuration is that of the unexcited crystal dimer but with a reduced spacing between the molecular planes.