Electrosynthesis of 2,7-linked polycarbazole derivatives to realize low-bandgap electroactive polymers

Poly(2,7-carbazole) derivatives have been synthesized because they have more extended conjugation lengths and lower energy bandgaps than poly(3,6-carbazole)s; however, few studies regarding electrochemical synthesis of poly(2,7-carbazole)s have been reported. Here, a series of N-alkylated-2,7-di(2-furyl)carbazoles, N-butyl-2,7-di(2-thienyl)carbazole, and N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole, were synthesized to obtain electroactive films of poly(2,7-carbazole) derivatives by electrochemical polymerization. Cyclic voltammetry revealed that these monomers have excellent polymerization activity, due to their low oxidation potentials (<0.57 V vs. ferrocene (Fc/Fc⁺)), and the corresponding polymers exhibit good redox properties. The energy bandgaps of the polymers obtained from optical absorption spectra range from 2.1 to 2.3 eV. Among the polymers, poly[N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole] shows the lowest bandgap energy of 2.1 eV, which is lower than that of previously reported poly(3,6-carbazole) analogues (2.4 eV). This polymer exhibits a significant color change from red in the oxidized state to blue in the reduced state during an electrochemical redox process. The electrochemical and optical properties of the monomers are dependent on external heteroaromatic rings attached to the 2 and 7 positions of the carbazole unit.