Beyond a Plausible Model: Lessons from Revisiting the Structure of a Covalent Organic Framework

Precise structural determination is essential for understanding and controlling the properties of covalent organic frameworks (COFs), yet most COF structures are not solved from single-crystal diffraction data but inferred from chemically plausible models constrained primarily by powder X-ray diffraction. Although indispensable, such models can remain underdetermined when reflections are broad, weak superstructure features are overlooked, or stacking geometries and molecular conformations are imposed as prior assumptions. Here, using a prototypical two-dimensional imine-linked COF as a case study, we illustrate how structural models can be progressively refined through advances in characterization. Its description evolved from a unimodal, AA-stacked model to a bimodal, AA′-stacked model and subsequently to a lower-symmetry model featuring propeller-like nodes with large torsion angles. This progression was enabled by low-dose transmission electron microscopy and electron ptychography, low-angle X-ray diffraction, three-dimensional electron diffraction, solid-state NMR, and physisorption measurements sampled at fine pressure intervals, each constraining a different aspect of the structure, including lattice periodicity, stacking arrangement, local molecular conformation, and pore architecture. This case emphasizes that a satisfactory fit does not establish structural uniqueness and that a revised model is not necessarily final. Achieving increasingly precise structural understanding in COF chemistry therefore requires iterative model testing and refinement against independent and complementary evidence.