Optical Absorption and Exciton Lattice Interaction in α- and β-Perylene

Abstract

The shape of the lowest exciton absorption band in α- and β-perylene due to the B 2u ←A g transition, the halfwidth of the absorption band, and the steepness parameter of the Urbach tail are investigated. From linear temperature dependence of the halfwidth and the Lorentzian line-shape, the transition A u ←A g is well explained by the exciton band picture. The lattice relaxation behavior of excitons after they are formed is described by the strong exciton-phonon coupling scheme. Based on the steepness parameter obtained, 0.93 in α-perylene and 1.38 in β-perylene, and the amount of the Davydov splitting obtained, 2040 cm -1 in α-perylene and 960 cm -1 in β-perylene, the lattice relaxation energy is estimated to be 1530 cm -1 and 523 cm -1 respectively. Also the energy difference between the free and self-trapped exciton states is expected to be 620 cm -1 in α-perylene and 43 cm -1 in β-perylene.