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

Densities of states and bandwidths of 1Lb 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. bserved exciton bandwidth 2B 330 cm−1 for bulk crystal, the self-trap depth ESF and the lattice relaxation energy ELR of the shallow self-trapped state (V state) are derived to be ESFࣘ 15 cm−1 and ESFࣘ 15 cm−1, respectively, in connection with the 1Lb exciton band. Owing to these numerical results the excitonic state and exciton relaxation processes in pyrene crystals are understood quite satisfactorily.