Intrinsic paramagnetism of doped polypyrroles and polythiophenes: Electron spin resonance of the polymers prepared by the use of copper(II) compounds as oxidative coupling agents

Electron spin resonance studies were carried out on the perchlorate and trifluoromethanesulfonate (triflate) of polypyrrole and polythiophene, which were prepared by polymerizing an appropriate monomer by the use of copper(II) perchlorate or triflate as an oxidative coupling agent. The linewidth was largely influenced by the nature of the counter anions as well as by the adsorbed oxygen. The signal intensity observed for all the materials including both the oxygen-free and the oxygen-adsorbing polymers was very weakly temperature dependent, in contrast to the Curie-type paramagnetism already reported for electrochemically prepared polypyrrole: the Weiss constant was 225 - 573 K depending on the nature of the polymers. This suggests the presence of a strong antiferromagnetic interaction between unpaired electrons involved in the polymer chains; polarons as well as defect spins are possible paramagnetic species.