A comparative study of redox-active, ambipolar electrochromic triphenylamine-based polyimides prepared by electrochemical polymerization and conventional polycondensation methods†

N,N’-Bis(4-diphenylaminophenyl)pyromellitimide (TPA-PMDI) was synthesized from the condensation reaction of 4-aminotriphenylamine with pyromellitic dianhydride (PMDA). The triphenylamine-diimide TPA-PMDI could be electrochemically polymerized into ambipolar, redox-active polyimide films on the electrode surface in an electrolyte solution via the coupling reactions between triphenylamine radical cations. The electro-generated polyimide films (coded as PI-E) exhibit reversible redox processes and stable color changes upon electro-oxidation, changing from a colorless neutral form to orange-yellowish and blue oxidized states. The film could also display pale green and pink cathodic coloring upon electroreduction. Electrochromic devices using the electropolymerized film as an active layer were also fabricated as preliminary investigations for electrochromic applications. For a comparative study, a structurally similar polyimide (PI-C) was prepared by a conventional two-step condensation from N,N’-bis(4-aminophenyl)-N,N’-diphenyl-4,4’-biphenyldiamine with PMDA, and its spectroscopic, electrochemical and electrochromic properties were compared with those of the electro-synthesized PI-E.