X-ray photoelectron spectroscopic studies of poly(2,2’-bithiophene) and its complexes

The charge distributions in chemically synthesized poly(2,2'-bithiophene) (PBT) complexes have been studied by x-ray photoelectron spectroscopy (XPS). XPS results suggest the presence of multiple chemical states for the ``dopants'' in perchlorate, iodide, and ${\mathrm{FeCl}}_{4}^{\mathrm{\ensuremath{-}}}$ complexes. Properly curve-fitted C 1s and S 2p core-level spectra reveal the simultaneous presence of neutral and polarized (or partially charged) species in both carbon and sulfur. The relative amounts of the neutral and polarized species vary in accordance with the oxidation level of the polymer. These results suggest that each dopant anion is associated with a thiophenium ion in the polymer chain. Similar results are also observed for the poly(3-methylthiophene) complexes. The present XPS results differ from most of the earlier XPS studies that have reported overall binding-energy shifts for the C 1s and/or S 2p core-level spectra of the oxidized thiophene polymer complexes. The undoped PBT is susceptible to reoxidation by iodine, ferric chloride, and strong organic acceptors, such as 2,3-dichloro-5,6-dicyano-p-benzoquinone and tetracyanoethylene.