Directional dispersion of exciton levels in α-oligothiophenes

Angle dependence of the upper Davydov transition of alpha -quinquethiophene (5T) is presented in terms of macroscopic dielectric theory. Optical constants are derived from ellipsometry measurements and used in the model to analyse the directional dispersion of excitons with incidence angle. The observed transition around 3.6 eV lies between the frequencies corresponding to the transverse optic (TO) at 3.35 eV and longitudinal optic (LO) mode at 3.8 eV.