Central moiety tuning of building units in hydrazone-linked covalent organic frameworks: Modulating electrochromic color and performance

Covalent organic frameworks (COFs) are crystalline porous organic polymers linked by strong covalent bonds, attracting extensive attention for their diverse, tailorable building units. Non-conjugated linkages endow COFs with improved chemical stability and structural tolerance under electrochemical conditions, but their inherent drawbacks-limited conjugation, sluggish charge transport and low intrinsic redox activity-restrict electrochromic performance. These limitations can be mitigated via molecular skeleton engineering, and the stability advantages outweigh the drawbacks, making non-conjugated COFs promising candidates for electrochemical devices. To explore the regulatory effect of molecular skeleton design on the electrochromic performance of COFs, two hydrazone-linked COFs (COF TFPB-TPH and COF TFPA-TPH ) were synthesized via a solvothermal method, using terephthalohydrazide (TPH) as the core building unit, and two distinct aldehyde monomers-fully aromatic TFPB and redox-active triphenylamine-based TFPA-as co-monomers. Both as-prepared COFs feature porous crystalline frameworks and excellent cycling stability, retaining 83.8% and 92.6% of initial transmittance after 2000 s of continuous switching respectively, ascribed to the outstanding chemical stability of hydrazone bonds. The two materials exhibit markedly distinct electrochromic behaviors: COF TFPB-TPH shows a reversible colorless-to-pale yellow transition with coloring/bleaching times of 11.54 s and 5.47 s; COF TFPA-TPH achieves reversible yellow-to-reddish-brown multicolor switching with faster response times of 7.09 s and 4.15 s. The improved performance originates from the triphenylamine-based donor-acceptor structure, which introduces abundant reversible redox sites and facilitates charge transport. This work elucidates the structure–property relationship of hydrazone-linked electrochromic COFs and offers a feasible molecular design strategy for high-stability, high-performance electrochromic materials.