Low energy electronic and optical properties of α-sexithiophene single crystals

Low energy electronic transitions in α-sexithiophene (αT6) single crystals are investigated by applying polarized absorption spectroscopy, photoluminescence (PL), time resolved PL and temperature dependent PL spectroscopy. In contrast to thin films, the single crystal optical absorption spectrum gives rise to well-resolved and sharp resonances allowing an electronic and vibrational assignment to be derived. The lowest excited electronic state in αT6 single crystals can be assigned to 2IAg and the narrow resonances in the absorption spectrum can be interpreted as levels arising from vibronic coupling (mixing of electronic states via Herzberg-Teller coupling). Contrary to the absorption spectrum the photoluminescence measured on the same sample shows broad and structureless emission with a temperature dependence pointing to thermally activated defect state emission. The temperature dependence of the photoluminescence and the time-resolved photoluminescence measurements support this scenario.