Structure-Conductivity-Relationship in Current Conducting Polymers of Thiophenes

Abstract

A series of conducting polymers of thiophene and β-substituted thiophenes have been prepared by electrochemical oxidation and polymerization of these monomers in the form of polythiophene films. It has been observed that chemical or electrochemical doping has considerably increases the electrical conductivity of their polythiophenes through manifold of magnitude ranging from insulator (~10^-10 Ω^-1 cm^-1) to semi-conductor and finally to metal (~10⁺² Ω^-1 cm^-1). The electrochemical properties of monomers expressed in terms of peak oxidation potentials (Epa) are found to depend on nature of substituents which is indicated by the linear correlation (r = 0.92) between Epa and Hammett substituent constants (σ). Also, as a result of polymerization, polythiophenes have been found to have relatively less Epa and high conductivity. A linear correlation between Epa and σ (r = 0.96) has also indicated the dependence of electrical properties of the polymers on the nature of substituents. A combination of these two results have proved that a better conducting polymer of desired conductance can be synthesized by suitable introduction of substituent group in thiophene. It is found that free α-position and nature of substituents at β-position, respectively, are responsible for chain growth and variation in conductivity. However, it has also been found that viscous solvent modifies the electrochemical characteristics, which is indicated by the shifts of the oxidation peak towards more positive potential side.