Conducting polymers from anodic coupling of some regiochemically defined dialkoxy-substituted thiophene oligomers

Electrochemical polymerization of thiophene oligomers (n = 2−4) bearing pentoxy groups in regiochemically defined positions (3 or 4 of the terminal thiophene rings) was performed in acetonitrile. Cyclic voltammetry and UV-Vis spectroscopy showed that the degree of polymerization decreases as the monomer length increases and this result is accounted for by the corresponding decrease of the coupling rate constant, measured by cyclic voltammetry for the less reactive 3-substituted series. Compared with alkyl substitution, alkoxy substitution induces a smaller bandgap and lowers the oxidation potential, therefore giving the polymer a higher stability of the doped state. However, no variation in the effective conjugation length was found when alkoxy groups were present in head-to-head or tail-to-tail linkages. It is suggested that the presence of the oxygen atom not only minimizes the steric hindrance but also plays a role in planarizing the polymer chain.