Long-lived photoexcitations in alpha-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 alpha -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 h omega = 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 112 polarons. For the triplet exciton we found a broad PADMR band centered at 1.6 eV with I eV total width. (C) 2001 Elsevier Science B.V. All rights reserved.