Transient spectroscopy of Frenkel excitons in α-hexathiophene single crystals

Photoexcitation dynamics in pure single molecular crystals of α-hexathiophene are studied using ultrafast photomodulation spectroscopy in the spectral range from 0.15 to 2.3 eV. Frenkel excitons are uniquely described by transient photoinduced absorption and stimulated emission bands at 1.3 and ~2 eV, respectively. The transient spectra are interpreted using exciton states identified by one- and two-photon absorption. Exciton dynamics in pure crystals dramatically differs from that in polycrystalline films, which is attributed to a much lower defect density in single crystals. Intramolecular internal conversion rate is found to be ~150 fs for excitation energies up to 3.6 eV.