SYNTHESIS AND CHARACTERIZATION OF NOVEL SUBSTITUTED BENZO[C]THIOPHENES AND POLYBENZO[C]THIOPHENES - TUNING THE POTENTIALS FOR N-DOPING AND P-DOPING IN TRANSPARENT CONDUCTING POLYMERS

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 H-1 NMR spectroscopy and mass spectrometry. 5,6-dichlorobenzo[c]thiophene is quite stable (half-life greater than or equal to 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 IT0-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; electron-withdrawing 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.