Soluble polyimide-based colorless-to-orange–red switching electrochromic film by incorporating a universal joint-like structure†

Polyimides (PIs) exhibit good thermal performance and high stability due to their highly rigid main chain and excellent mechanical properties, making them ideal electrochromic materials. However, the applicability of PIs is limited by poor solvent solubility, which results from the rigid backbone, and film yellowing, caused by charge-transfer complexes (CTCs). Herein, a novel phenothiazine-based diamine (THZ-DA) was synthesized, where a universal joint-like structure was constructed during its formation. The introduction of the non-coplanar universal joint reduced intermolecular interactions and suppressed the formation of CTCs, improving the solubility and enabling high transmittance of THZ-PIs in the neutral state. Moreover, the lone pair of electrons on the nitrogen atom of THZ allowed for the formation of a cationic radical (THZ˙+) under electrochemical oxidation, which was stabilized by the resonance effect, enabling reversible color switching between the colorless and the orange–red state. Among the three systems, THZ-b, composed of THZ-DA and 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, showed the best thermal stability, with a T_d5% above 406 °C and a T_g of 347 °C. It also exhibited good electrochemical cycling stability, with redox potentials of 1.09 and 0.88 V, and excellent electrochromic performance, achieving an electrochromic efficiency of 222 cm² C⁻¹. These results show the application potential of THZ-PIs in smart windows and adaptive camouflage.