Redox-bipolar mesoporous two-dimensional covalent organic framework for multi-color electrochromism

Two-dimensional covalent organic frameworks (COFs) are promising for electrochromic applications, yet most current systems are limited to microporous, monopolar, imine-linked COFs with inefficient ion utilization and electron transport. Here, we report a mesoporous, hexagonal bipolar COF constructed from triphenylamine (donor) and naphthalene dianhydride (acceptor) units, forming a donor-acceptor (D-A) heterostructure with dual redox-active sites. This architecture facilitates efficient bidirectional ion transport and intramolecular charge transfer, leading to reversible coupling of redox units 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA/NTCDA)^·- and N⁴,N⁴-bis[4'-amino-(1,1'-biphenyl)-4-yl]-(1,1'-biphenyl)-4,4'-diamine (TAAB/TAAB⁺). The resulting COF exhibits multicolor electrochromism (brown-pale-blue-green transitions) with high optical contrasts (e.g., 80% at 850 nanometers and 53% at 485 nanometers) and excellent stability (>91% retention after 500 cycles). Symmetric devices based on this COF show broad spectral tunability (400 to 1100 nanometers) and outstanding cycling stability (<1.5% decay after 1000 cycles), establishing a performance benchmark for COF-based electrochromic systems.