Stimuli-responsive conjugated copolymers having electro-active azulene and bithiophene units in the polymer skeleton: Effect of protonation and p-doping on conducting properties

Abstract

A series of stimuli-responsive conjugated copolymers containing alkylated bithiophene and electroactive azulene in the polymer backbone was synthesized by oxidative polymerization. These copolymers were formed in good yields with good solubility in common organic solvents such as chloroform, toluene, and THF. They also possess high molecular weight (M n) ranging from 16 000 to 41 000, and their structures are consistent with those expected as indicated by NMR, FTIR, and elemental analysis. These copolymers are highly thermally and environmentally stable. Like polyanilines, these azulenethiophene copolymers could be rendered highly conductive (1-50 S cm -1) via two routes: iodine doping (p-doping) or protonation by TFA. The effects of p-doping and protonation on their conducting properties could be observed by UV-vis-NIR, EPR, SEM, and CV measurements, revealing different conducting mechanisms in p-doping and protonation. The EPR studies in particular confirmed the reversible protonation-deprotonation processes and high stability of the formed cation radicals at a p-doped or protonated state, indicating their potential application in electronic switcher or antioxidant materials. SEM studies revealed the formation of nanodoping centers upon iodine-doping and formation of conducting channels upon TFA protonation that contributed to the high conductivities observed. CV studies showed a reversible p-doping process, and the electrochemical band gap is in good agreement with their optical band gap. Protonation was found to affect the redox process by greatly decreasing the electrical band gap. Electrochemical impedance spectroscopy (EIS) measurements showed that these polymers became conducting upon protonation with inorganic acids (such as HCl), thus indicating the potential application in corrosion coating.