LINEAR AND NONLINEAR SPECTROSCOPY OF HIGHLY ORIENTED THIN-FILMS OF ALPHA-SEXITHIENYL - A MODEL FOR POLYTHIOPHENE

Alpha-sexithienyl (T6) is a large oligomer of polythiophene with twelv e conjugated double bonds forming highly ordered thin films. The chemi cal definition and the ordered solid state aggregation allows to inves tigate the energetics of the molecule as a model for polythiophene. Li near spectroscopy including single crystal and ordered thin film absor ption and one-photon excitation allows to locate and to assign the low est Frenkel 1(1)B(u) exciton at 17450 cm-1. Nonlinear two-photon absor ption spectroscopy on the thin film allows to locate the lowest 2(1)A( g) Frenkel exciton at about 900 cm-1 above the 1(1)B(u). e-e correlati on is therefore crucial in determining the energetics of T6. The level crossing of the 1(1)B(u) and 2(1)A(g) states provides an efficient ra diationless relaxation pathway. We suggest that the energetics of T6 p rovides a good model for the understanding of polythiophene.