A combined ellipsometric and in situ infrared (SNIFTIRS) study of poly(benzo-[c]-thiophene) films

Abstract

Films of poly(benzo[c]thiophene)[poly(isothianaphthene)] have been grown from a solution of the monomer in acetonitrile by two methods. First, anionic polymerisation onto a platinum electrode in the presence of tetrafluoroborate affords a stable polymer (type I film). Secondly, oxidative polymerisation with Br₂ as an electrochemically generated intermediate gives a rather different material (type II film). Films of type I are found by ellipsometry to be relatively dense, containing very little solvent in the neutral form. On switching from the neutral to the oxidised form, the film swells as solvent and counter anions are absorbed. Type II films are found to have a much more open form and estimates from ellipsometry and coulometry are consistent with a factor of four greater solvent content. Potential cycling of these films appears to have little effect on their thickness. The in situ i.r. measurements, carried out on a type I film, are consistent with absorption of solvent in the oxidised form, but the spectra are dominated by the development of a strong electronic absorption centred above 4000 cm^–1 that shifts towards lower frequency at higher potentials. This electronic band is accompanied by the appearance of strong absorption bands between 1000 and 1400 cm^–1 associated with the five-membered S-containing ring and correlating with the shift in electronic absorption maximum. The enhanced vibrational peaks are due to ‘amplitude-mode’ effects arising from the coupling of the quinonoid vibrations to the mobile charge defect. The evolution of the i.r. spectra with potential can then be associated with the changing character of the charge carriers.