Bimodal versus Unimodal Pore Architectures in Diimine-Linked Two-Dimensional Covalent Organic Frameworks.

Imine-linked two-dimensional covalent organic frameworks (2D COFs) are commonly considered structurally simple materials, yet precise structure determination by X-ray diffraction remains challenging due to the difficulty of obtaining large single crystals. Here, we show that a single-atom change in the aldehyde substituent is sufficient to switch both pore architecture and lattice symmetry in a prototypical 2D COF system. Comparing the widely studied TAPB-DMPDA (COF-OMe) with its -SMe analogue (COF-SMe), we establish a bimodal mesoporous framework for COF-OMe and a unimodal one for COF-SMe through a combination of advanced imaging and diffraction techniques alongside finely sampled gas/vapor physisorption, which resolves two-step capillary processes exclusively in COF-OMe. Electron ptychography reveals previously unrecognized structural features in COF-OMe and enables refinement of its model to propeller-like 1,3,5-tris(4-aminophenyl)benzene nodes with unusually large dihedral angles. Simulated electrostatic potential and X-ray diffraction pattern based on the refined model reproduce the experimental data with high fidelity. COF-SMe undergoes a symmetry reduction from hexagonal to triclinic during kinetic-to-thermodynamic phase evolution, driven by subtle interlayer slippage and intralayer distortion while retaining a single pore type. Together, these results uncover unexpected structural diversity and substituent-governed flexibility in 2D COFs, underscoring the need for state-of-the-art characterization to reassess long-accepted structural models.