Molecular structure engineering of COF films via β-Ketoenamine linkages and hydroxyl groups: Scalable fabrication for...

To elucidate the structure-activity relationship between molecular architecture and properties in electrochromic covalent organic framework (COF) materials, this study employs electrochemically active azobenzene as the primary building block. Through strategic modification of the molecular structure in a complementary COF component, we achieved significant enhancements in optical transmittance and cycling stability. Specifically, by using nitrogen-containing azobenzene as the core structural unit and leveraging dynamic covalent chemistry, hydroxyl functional groups were incorporated into the secondary molecular component - this modification facilitated the formation of β-ketoenamine linkages. The electrochromic COF materials were subsequently synthesized via a sequential two-step method: emulsion polymerization followed by electrophoretic deposition. Comparative analyses of material composition, structural characteristics, and morphological features were conducted to correlate molecular design with performance. Compared to imine-linked COF_AZO-TB films, the β-ketoenamine-linked COF_AZO-TP films, modified with hydroxyl groups, exhibit superior electrochemical kinetics (faster coloring/bleached times), higher optical modulation (42.5 % vs. 25.6 %), and exceptional cycling stability (98.2 % vs. 91.4 % transmittance retention after 1000 s), directly attributing to the enhanced chemical stability of β-ketoenamine linkages and improved precursor solubility/film uniformity from hydroxyl groups. This systematic investigation further elucidated the mechanistic basis for improvements in electrochemical behavior and electrochromic performance, providing critical insights for the rational design of high-performance electrochromic materials.