SOLID-STATE ABSORPTION-SPECTROSCOPY OF ALKYL-SUBSTITUTED OLIGOTHIOPHENES

We have investigated the absorption spectra of thin solid films of alk yl-substituted oligothiophenes in both the neutral and doped states ov er a photon energy range of 0.05-5.0 eV. In the neutral state these ol igomers display the pi-pi transition bands in the near-UV to visible region with fine structures on the low-energy side. We found two diffe rent oxidized states for the doped species; the lower state is charact erized by two subgap bands in the near-IR region that are induced upon doping and the higher state by only one band. In the two oxidized spe cies, the pi-pi band exhibits increasing intensity on the lower energ y side at increasing doping levels and, finally, the absorption maximu m is shifted to a low-energy region at the highest doping levels. This is an outstanding feature of the solid films and in sharp contrast wi th the spectroscopic characteristics observed for the materials in sol ution. The origin of this feature is related to the presence of molecu lar associates as charged species such as dimer radical cations. Of th e two subgap modes for the lower oxidized state, the peak position of the lower energy mode varies inversely proportional to the number of t he thiophene rings (i.e., the polymerization degrees of the oligomers) and is extrapolated into zero with increasing polymerization degrees approaching infinity. This feature characterizes the charge-resonance band of the dimer radical cations. We interpret these spectroscopic fe atures observed for the neutral and doped forms on the basis of molecu lar orbital symmetry. Physicochemical implications are discussed in co nnection with the charge transport results that were obtained in FET ( field-effect transistor) configurations. As a result, the charged spec ies are associated with polarons in the solid-state physics terms. Dop ing-induced absorption bands are observed in the mid-IR region and the se results are presented as well.