Absorption and Fluorescence Spectra of Crystalline Pyrene

The absorption and fluorescence spectra of crystalline pyrene have been measured at room temperature and 77°K. Only the first absorption region could be studied because of crystal thickness but a Davydov splitting of about 18 cm—1 was detected with the a polarization at higher energy than the b polarization. Calculations of the splitting (dipole-dipole approximation for the intermolecular potential) give 23 and 13 cm—1, respectively, for the B2u and B3u levels with the observed sense, and so it is not possible to decide between two possible assignments from theory. The polarization ratio shows that mixing with a higher level of opposite polarization is present and this is consistent with the only theoretical prediction of energy levels in the molecule. The fluorescence is displaced to long wavelengths from the absorption spectrum and is a broad band with a maximum which is further displaced on cooling the crystal. The band has been interpreted as a charge-transfer transition involving the molecules grouped in pairs in the crystal.