Densities of states of the 1L b exciton band and luminescence processes in pyrene

Abstract

Densities of states and bandwidths of 1L b excitons in pyrene microcrystallites embedded in PMMA films are studied, using a technique applied to anthracene microcrystallites. The excitation spectra consist of two components. One referred to as component A shows practically an exponential decrease with increasing energy. Component A is interpreted as owing to the wavevector k = 0 component in the density of states of microcrystallites; the spectral width of component A is equal to the exciton bandwidth. The exciton bandwidth varies with the microcrystallite size, and the exciton bandwidth 2B for bulk crystal is estimated to be at most 330 cm −1. For large microcrystallites, the δ k = 0 selection rule for optical transitions becomes established, causing observed exciton bandwidth tend to zero. On the contrary, component B which overlaps with component A appears as a broad band. Its intensity is relatively weak but it is not negligible for large microcrystallites where component A is weak. With observed exciton bandwidth 2B 330 cm −1 for bulk crystal, the self-trap depth E SF and the lattice relaxation energy E LR of the shallow self-trapped state (V state) are derived to be E SFࣘ 15 cm −1 and E SFࣘ 15 cm −1, respectively, in connection with the 1L b exciton band. Owing to these numerical results the excitonic state and exciton relaxation processes in pyrene crystals are understood quite satisfactorily.