Abstract

Electrogenerated films of polybenzo[c]thiophene (PBCT) and poly-5-fluorobenzo[c]thiophene (PFBCT) have been studied in situ under potential control using reflectance FTIR spectroscopy. For PBCT, the onset of oxidation (p-doping) results in the appearance of infrared activated vibrations (IRAVs), which are very similar to those observed earlier during photoinduced IRAV experiments. However, at more positive potentials, the spectra become more complex, as a new charge carrier evolves, and positive of +0.5 V (vs. SCE), the features due to the first charge carrier are lost. This change in behaviour is compared with the results of earlier in situ data obtained using electronic and EPR spectroscopies. Although peak potentials for the p-doping process are only slightly more positive for PFBCT than for PBCT, it is clear from the IR data that the onset of p-doping occurs at a potential ca. 0.4 V more positive for PFBCT than for PBCT. In other respects, the evolution of charge carriers in PFBCT films is similar to that in PBCT. Some unusual bands at ca. 1700 cm⁻¹ in the spectra of both polymers are attributed to chain-terminating carbonyl groups in this quinoid polymer. The reduction of PBCT and PFBCT to the n-doped conducting state has also been examined. Interestingly, the IRAVs which develop as the film becomes n-doped are very similar to those of the first stage of p-doping. The onset of n-doping, deduced from the appearance of the characteristic electronic band in the IR spectrum, occurs at a potential 0.5 V more positive for PFBCT than for PBCT. Thus, the relatively trivial substitution of one F at C5 has a remarkable effect on the redox potentials for n- and p-doping in PBCTs.