On the degradation of some thiophene oligomers after doping by ion chloride

FeCl3-doped thiophene oligomers (bi, ter, tetra, penta) in the powder form have been characterized by photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), electron microprobe analysis, visible optical absorption, infrared absorption (FTIR) and electron spin resonance (ESR). The results show that even if the powders are only lightly doped, the chlorine dopant is only partly ionized (80 at.%). The positive charge induced by doping cannot be localized on sulphur, but probably can be localized on carbon. Since the doping is not large, the signal may be too small on sulphur to be detected by XPS, however, the ESR signal cannot be attributed to the S radical and we can imagine that the positive charge induced by doping is localized on carbon. The distribution of the dopant in the powder is not homogeneous. All the experimental results show that while the doping is not strong it has many consequences for the properties of the powder. The main one, corroborated by all the experiments, is that after doping there is a degradation of the bi- and terthiophene. We no longer have any bi- and terthiophene but x-thiophene with x greater than or equal to 5; tetra- and pentathiophene are not modified. (C) 1996 Elsevier Science Limited