In-situ grown benzotrithiophene-containing covalent organic framework film for dual-responsive visible-to-near infrared electrochromic and bright-to-quenched electrofluorochromic smart windows

Covalent organic frameworks (COFs) have attracted massive interest because of their exceptional mechanical robustness, high surface area, tunable porosity, and intrinsic crystallinity, making them ideal for electrochemical devices. Precisely, low bandgap redox active COFs with π-conjugation and a donor-acceptor (D-A) nature are suitable for electrochromism across the visible to near-infrared (NIR) range. Herein, a conjugated D-A type COF, BTTh-Tz-COF, was synthesized via a one-pot condensation reaction and grown in-situ thin film on indium tin oxide (ITO)-coated glass, overcoming the typical limitations of ex-situ COF film formation. Powder X-ray diffraction (PXRD) exposed high crystallinity of BTTh-Tz-COF with a sharp peak at 2θ = 4.32°, and Brunauer–Emmett–Teller (BET) analysis confirmed a surface area of 819 m²/g with a microporous nature. The fabricated solid-state electrochromic device (ECD) exhibited reversible multicolor EC response, covering yellow to orange to reddish-brown under +1.7 V and +2.5 V, attributed to oxidation of imine and BTTh moieties. The ECD revealed high coloration efficiencies of 768.4 cm²/C at 700 nm and 491.7 cm²/C at 1000 nm, and fast switching (∼5 s) in both visible and NIR ranges. Furthermore, the BTTh-Tz-COF film efficiently blocked 67 % of solar irradiation and disclosed low power consumption, making it appropriate for smart window applications. The device also showed reversible electrofluorochromism (EFC), where bright yellow fluorescence of the film was quenched at +2.5 V and recovered at −0.9 V. This study demonstrates the development of a robust, dual-functional COF film for next-generation energy-efficient EC and EFC smart windows.