Structure--Dynamics--Property Relationship Reveals the Origin of Porous Integrity in Covalent Organic Frameworks

Flexibility encoded in low-frequency lattice dynamics has been shown to be effective at understanding various properties of porous materials, especially metal--organic frameworks, but remains largely unexplored in structurally similar covalent organic frameworks~(COFs). Here, we establish a structure--dynamics--property relationship for two COFs by combining three-dimensional electron diffraction, low-frequency vibrational spectroscopy, and first-principles calculations. Using 3D electron diffraction, we determine, for the first time, the crystal structure of the closed-pore reduced framework COF-300-AR, revealing a bent linker conformation. Terahertz and low-frequency Raman spectroscopy, together with solid-state DFT and local mode analysis, are then used to resolve the low-frequency lattice dynamics of COF-300 and closed-pore COF-300-AR and to quantify linker flexibility in terms of imine/amine torsions. Normal mode decomposition and torsional potential energy surfaces show that these torsions are roughly twice as stiff in \h\ as in \AR, and that the reduced framework has a shallower, multi-well torsional potential energy landscape, which permits the observed closed-pore structure. In \h, the same torsional coordinate is much stiffer, locking the structure into a porous framework. More broadly, this work demonstrates how combining advanced structure determination with low-frequency vibrational spectroscopy and first-principles calculations can identify, and subsequently tune, the coordinates that control pore opening, guest uptake, and structural integrity in flexible COFs.