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

Polyimide (PI) exhibits good thermal performance and high stability due to its highly rigid main chain and excellent mechanical properties, making it an ideal electrochromic material. However, its applicability is limited by poor solvent solubility, which results from the rigid backbone, and film yellowing, caused by charge-transfer complexes (CTC). 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 CTC formation, 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 colorless and 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 Td5% above 406 °C and Tg of 347 °C. It also exhibited good electrochemical cycling stability, with redox potentials of 1.09 and 0.88 V, as well as excellent electrochromic performance, achieving an electrochromic efficiency of 222 cm2 C–1. These results show the application potential of THZ-PI in smart windows and adaptive camouflage.