Synthesis and characterisation of novel substituted benzo[c]thiophenes and polybenzo[c]thiophenes: tuning the potentials for n- and p-doping in transparent conducting polymers

Abstract

The novel benzo[c]thiophenes 4- and 5-fluorobenzo[c]thiophene, 5-methylbenzo[c]thiophene and 5,6-dichlorobenzo[c]thiophene have been synthesized in a classical five-step route from the corresponding substituted phthalic acid or anhydride, and have been characterized by ¹H NMR spectroscopy and mass spectrometry. 5,6-dichlorobenzo[c]thiophene is quite stable (half-life 12 h in CDCl₃ at room temperature). Electropolymerisation of these compounds in dry 0.2 mol dm^–3 tetraethylammonium tetrafluoroborate-acetonitrile solution using repetitive cyclic voltammetry (CV) yields the corresponding polybenzo[c]thiophenes as adherent films on platinum disc or ITO-coated glass electrodes. Like the parent polybenzo[c]thiophene, the polymers show two superimposed redox waves and are electrochromic. The 5- and 5,6-substituted polymers are transparent in the p-doped (conducting) state and blue in the neutral (insulating) state. The substituent(s) have only a small effect on the peak potentials for p-doping; electronwithdrawing groups shift these in the positive direction. However, they have a dramatic effect on the potential at which n-doping occurs, so that poly(5,6-dichlorobenzo[c]thiophene) is n-doped beginning at –0.2 V, more than 0.8 V positive of the corresponding potential for polybenzo[c]thiophene. Comparison of electrochemical and in situ electronic spectroscopy data suggests that the change in the optical bandgap does not match the decrease in the electrochemical bandgap caused by the substituents.