Linear and nonlinear spectroscopy of highly oriented thin films of α-sexithienyl: a model for polythiophene

Abstract

α-sexithienyl (T₆) is a large oligomer of polythiophene with twelve conjugated double bonds forming highly ordered thin films. The chemical definition and the ordered solid state aggregation allows to investigate the energetics of the molecule as a model for polythiophene. Linear spectroscopy including single crystal and ordered thin film absorptiion and one-photon excitation allows to locate and to assign the lowest Frenkel 1 ¹B_u exciton at 17450 cm^-1. Nonlinear two-photon absorption spectroscopy on the film allows to locate the lowest 2 ¹AA_g Frenkel exciton at about 900 cm^-1 above the 1 ¹B_u e-e correlation is therefore crucial in determining the energetics of T₆. The level crossing of the 1¹Bu and 2 ¹A_g states provides an efficient radiationlless relaxation pathway. We suggest that the energitics of T₆ provides a good model for the understanding of polythiophene.