A Three-Dimensional Covalent Organic Framework Enables Guest-Triggered Reversible Disorder-Order Structural...

Achieving high crystallinity and controlled framework flexibility in covalent organic frameworks (COFs) remains a significant challenge for dynamic gas adsorption. Here we report a three-dimensional COF obtained through a single-crystal-to-single-crystal imine-to-amine linkage transformation that exhibits flexible yet robust adsorption behavior. A rigid imine-linked single-crystal COF (39-i) was converted into its amine-linked analogue (39-a) by postsynthetic Leuckart-Wallach reduction with formic acid, while preserving single crystallinity. Despite their identical topologies, 39-i and 39-a exhibit distinct structural responses during guest adsorption-desorption processes: 39-i retains its framework rigidity, whereas polar molecules (e.g., SO₂, H₂O, MeOH) induce pore-opening behavior in 39-a. PXRD indicates that 39-a undergoes a reversible transformation from a guest-stabilized crystalline phase to a distorted phase with reduced long-range order upon guest removal. Notably, 39-a achieves an exceptionally high SO₂ uptake of 10.4 mmol g^-1 at 298 K and 1 bar, representing one of the highest values reported for COFs under ambient conditions, despite exhibiting negligible N₂ uptake at 77 K. Experimental and computational analyses reveal that activation induces a folded state in 39-a, retaining guest-accessible volume prior to further pore expansion, with framework compression accommodated through cooperative folding and torsional distortion. In addition, the secondary amine linkages reduce the energetic penalty for framework deformation, allowing strong host-guest interactions with SO₂ to drive pore adaptation while preserving structural integrity during the reversible disorder-order-disorder transformation observed in SO₂ sorption. These findings establish linkage-controlled structural adaptation as a design strategy for dynamic gas adsorption in chemically aggressive environments.