Long-lived photoexcitations in α-hexathiophene single crystals

We employed various steady-state spectroscopic techniques, such as absorption, photoluminescence, photoinduced absorption (PA) and PA-detected magnetic resonance (PADMR), to study long-lived photoexcitations in α-hexathiophene single crystals. The lowest lying absorption band appears at 2.4 eV and is polarized along the oligomer chains (ac-polarized), whereas the photoluminescence is largely unpolarized. The PA spectrum, recorded from ℏω=0.6 to 1.8 eV, is relatively weak and shows three bands at 0.75, 1.25 and 1.6 eV, respectively, where the 1.6 eV band is by far the most intense. All three PA bands are ac-polarized. Using PADMR we identify the 1.25 eV band as due to bipolarons, and show that the 0.75 eV band is due to spin 1/2 polarons. For the triplet exciton we found a broad PADMR band centered at 1.6 eV with 1 eV total width.