Linker-Flexibility-Induced Triangular Antiprism Nodes in a Three-Dimensional Covalent Organic Framework

The rational design of polyhedral nodes is crucial for constructing three-dimensional covalent organic frameworks (3D COFs), yet such nodes remain scarce. Here we show that linker flexibility during framework assembly can induce polyhedral node geometries in building blocks that typically favor two-dimensional connectivity, thereby enabling the construction of 3D COFs. By reacting a six-connected building block commonly used for two-dimensional COFs with a flexible precursor containing C-O single bonds, we synthesized a new highly crystalline 3D COF (3D-TAPB-COF). Continuous rotation electron diffraction analysis with a resolution up to 1.0 Å reveals that the six-connected nodes adopt a triangular antiprism geometry and that 3D-TAPB-COF exhibits an 18-fold interpenetrated pcu topology, representing the highest degree of interpenetration reported for COFs to date. Furthermore, the flexible C-O single bonds endow the framework with structural adaptability, giving rise to reversible swelling during solvent vapor adsorption and desorption. This work demonstrates a strategy wherein linker flexibility during framework assembly can be exploited to induce polyhedral node geometries, thereby expanding the structural diversity of 3D COFs and providing new insights into the design of dynamic frameworks.