Electrochemical synthesis of electrochromic polycarbazole films from N-phenyl-3,6-bis(N-carbazolyl)carbazoles

Two compounds with a tri-carbazole (3Cz) structure, namely 3,6-di(carbazol-9-yl)-N-(4-nitrophenyl)carbazole (NO2-3Cz) and 3,6-di(carbazol-9-yl)-N-(4-aminophenyl)carbazole (NH2-3Cz), were synthesized and electropolymerized into robust polymer films on the electrode surface in an electrolyte solution via the oxidative coupling reactions. The electro-generated polymer films exhibited reversible electrochemical oxidation processes, with significant electrochromic behaviour. The colour of P(NO2-3Cz) films changed from the pale yellow neutral state to yellow-green as a radical cation and then to blue when fully oxidized. Upon oxidation, the colour of P(NH2-3Cz) films changed from colourless to pale green and finally to blue. Furthermore, we also synthesized and characterized some N-phenylcarbazoles with different substituents on the phenyl group. Based on a comparative study, the possible electropolymerization mechanisms of the NO2-3Cz and NH2-3Cz monomers are suggested; that of the former is through carbazole–carbazole coupling, and the latter is through both carbazole–carbazole and NH2–carbazole coupling.