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

Molecular flexibility encoded in low-frequency lattice dynamics is increasingly recognized as a key determinant of the properties of porous materials, particularly metal-organic frameworks, but remains largely unexplored in covalent organic frameworks (COFs). Here, we establish a structure-dynamics-property relationship for chemically related COFs by combining three-dimensional electron diffraction, low-frequency vibrational spectroscopy, and first-principles calculations. Using 3D electron diffraction, we determined the crystal structure of the collapsed reduced framework COF-300-AR, revealing a bent linker conformation. Terahertz and low-frequency Raman spectroscopy, together with solid-state density functional theory and local mode analysis, resolve the lattice dynamics of contracted COF-300-H2O, COF-320A, and collapsed COF-300-AR and quantify linker flexibility through imine/amine torsions. Normal-mode decomposition and torsional potential energy surfaces show that these coordinates are comparatively stiff in the imine-based frameworks, preserving near-planar linker conformations that support guest-responsive structures. In contrast, COF-300-AR exhibits a shallower, multiwell torsional potential that enables bent linker conformations, favoring collapsed packing. Gas-phase local-mode analysis reproduces this relative trend in torsional softness, suggesting that linker flexibility can be screened before a crystal structure is available. The negligible gas uptake of COF-300-AR is therefore linked to the coupling of local amine flexibility with packing constraints in the interpenetrated framework. More broadly, these results demonstrate how low-frequency dynamics can identify molecular coordinates governing the structural integrity and porosity in flexible COFs.