Polythiophenes via thiophene, bithiophene and terthiophene in propylene carbonate: an electrochemical and in-situ FTIR study

Polythiophene films obtained by the galvanostatic polymerization of thiophene (Th), bithiophene (BTh) and terthiophene (TTh) in 0.5 M LiClO₄ + propylene carbonate have been investigated by cyclic voltammetry and in-situ Fourier transform IR (FTIR) spectroscopy. The polymerization potential in the propylene carbonate electrolyte decreases in the order Th BTh TTh. Both the charge capacity and the doping level of the resulting polymers increase in the order polythiophene (PTh) < polybithiophene (PBTH) polyterthiophene (PTTh). For PTTh, a doping level of 37% is obtained during cycle 10 and 31% during cycle 1000.In-situ FTIR spectra of the neutral PBTh and PTTh films show a single band at 3063 cm⁻¹ (corresponding to aromatic β-(C---H)-groups) which suggests a regular α,α′-linking of the monomeric units. For thin PTh films the intensity of this band is very weak, indicating a disordered cross-linked polymer structure. During the electrochemical oxidation of the three polythiophenes investigated from 3.3 V up to 4.4 V vs. Li/Li⁺ the intensity of the electronic absorption band (above 2000 cm⁻¹) increases with increasing potential, but it strongly decreases at potentials above about 4.4 V, indicating a strong irreversible oxidation of the polymer films. Furthermore, strong CO₂ evolution is observed at potentials above 4.2 V.