THE CRYSTAL SPECTRUM OF PERYLENE

Abstract

The electronic absorption spectrum of crystalline perylene has been examined in the spectral region 3000–4700 Å. The lowest energy electronic state of perylene has a large oscillator strength and the crystal spectrum deviates considerably from that calculated from a weak coupling (Davydov) model.The sublimation flakes were examined at room temperature and at the temperature of boiling nitrogen. The lowest energy crystal state was polarized along the crystallographic a-axis (Bu) and the factor group splitting of lowest vibrational envelope of the electronic band was 800 cm−1. The whole spectrum was more intense along the a- than along the b-axis of the crystal. These results are consistent with the notion that the crystal spectrum is derived from dipole-induced dipole interactions between molecular B2u levels of perylene and neighboring unexcited molecules. This assignment of the lowest excited level of perylene is in agreement with theory.The molecular vibrational structure is severely altered in the crystal and the observed crystal shift is much smaller than that predicted by the Davydov theory.