Investigating a Seemingly\nSimple Imine-Linked Covalent\nOrganic Framework Structure

The\nstructures of covalent organic frameworks (COFs)\nare typically\ndetermined through modeling based on powder X-ray diffraction. However,\nthe intrinsically limited crystallinity of COFs often results in structural\ndeterminations of low fidelity. Here, we present real-space imaging\nof an extensively studied two-dimensional imine-based COF. Contrary\nto the conventional understanding that this COF features uniform hexagonal\npores, our observations reveal the presence of two distinct sets of\npores with differences in shape and size. Motivated by this finding,\nwe conducted reciprocal-space characterizations, complemented by solid-state\nnuclear magnetic resonance spectroscopy and density functional theory\ncalculations, to reevaluate this seemingly simple structure. The collective\nresults allow for the establishment of a new structural model for\nthis landmark COF and its derivatives, differing from the conventional\nmodel in both intra- and interlayer configurations. Furthermore, we\nidentified various previously unrecognized defective structures through\nreal-space imaging, which have significant implications for COF applications\nin separation and catalysis. Our study demonstrates the complexity\nand heterogeneity of COF structures, while also highlighting the imperative\nfor structural reevaluation using advanced characterization techniques.