Study of the lowest electronic states in a polythiophene oligomer, α -sexithienyl, by one and two-photon spectroscopy

α -Sexithienyl (T<SUB>6</SUB>) is a large rigid rod molecule with twelve conjugated double bonds. The electronic structure of T<SUB>6</SUB> has been investigates by one-photon and two-photon spectroscopy as well as by photoconductivity. From the single crystal polarized spectra at 4.2K we observe that the lowest allowed π-π transition (λ <SUB>max</SUB> = 478 nm) is polarized parallel to the long molecular axis and a second intense transition at 370 nm is polarized parallel to the short in plane axis. The two-photon excitation spectra in polycristalline thin films have been investigated at 4.2 K in the spectral range between 910 and 1180 nm of the fundamental laser radiation. The intense two-photon absorption band at 544.9 nm is assigned to the 2<SUP>l</SUP>Ag exciton band origin. The lowest "gerade" exciton level lies therefore at 898 cm<SUP>-1</SUP> above the lowest one-photon allowed 1<SUP>1</SUP>B<SUB>u</SUB> exciton level. The photophysics of T<SUB>6</SUB> is studied by photoconductivity and one-photon excitation spectroscopy. An efficient pathway for carrier generation open up at energies in the proximity of the 2<SUP>1</SUP>A<SUB>g</SUB> exciton. We suggest that T<SUB>6</SUB> is a good model compound for polythiophene (PT) and therefore the energetics of PT should be very similar to T<SUB>6