Adaptive Structural Reconfiguration in Ether-Incorporated Covalent Organic Frameworks Enables Efficient Iodine Capture.

The development of efficient adsorbents for radioactive iodine capture is critical for environmental and human safety. Flexible covalent organic frameworks (COFs) are promising candidates due to their structural adaptability, yet how their structural reconfigurations govern iodine adsorption remains unknown. Herein, we report the design and synthesis of two highly crystalline, ether-embedded flexible COFs (F-TEA and F-BEA), along with a rigid, ether-free counterpart (R-TPA) as a control. In the triazine-containing F-TEA, the ether bond reduces steric hindrance between triazine and benzene rings, which improves adsorption-site accessibility and enhances halogen-bond interactions, thereby leading to superior iodine vapor capture (F-TEA>F-BEA>R-TPA). However, the triazine ring also induces an ether-bond locking effect, triggering a water-responsive structural rearrangement that reduces micropore accessibility and water-phase adsorption. In contrast, the triazine-free F-BEA possesses freely rotating ether bonds that enable adaptive framework swelling, which can accommodate more iodine molecules and facilitate iodine uptake through multi-site charge transfer. Consequently, F-BEA achieves a high iodine adsorption capacity of 7.25 g g-1 from aqueous solution, whereas the rigid R-TPA undergoes framework collapse and exhibits the lowest performance. This study establishes conformational control of flexible linkages as a key design principle for high-performance porous iodine adsorbents.