Synthesis and electrochromic properties of polyimides with pendent benzimidazole and triphenylamine units

Five novel near-infrared electrochromic aromatic polyimides (PIs) with pendent benzimidazole group were synthesized from 4,4′-diamino-4″-(1-benzylbenzimidazol-2-yl)triphenylamine (named as DBBT) with five different dianhydrides via two-step polymerization process, respectively. The maximum UV-Vis absorption bands of these PIs locate at about 335 nm for solid films due to the π-π* transitions. A reversible pair of distinct redox peaks, that were associated with a noticeable color change from original yellow to blue, was observed in the cyclic voltammetry (CV) test. A new absorption peak emerged at 847 nm in near-infrared (NIR) region with increasing voltage in UV-Vis-NIR spectrum, which indicates that PI can be used as NIR electrochromic material. These novel PIs have good electrochemical stability, appropriate energy levels for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), in the range of -5.17 eV to -5.20 eV and -2.14 eV to -2.26 eV (versus the vacuum level) determined by cyclic voltammetry method. These values basically consisted with the results of quantum chemical calculation. These polyimides can be used as novel electrochromic and hole transportation materials.