Synthesis and Visualization of Entangled 3D Covalent Organic Frameworks with High-Valency Stereoscopic Molecular Nodes for Gas Separation

The structural diversity of three-dimensional (3D) covalent organic frameworks (COFs) are limited as there are only a few choices of building units with multiple symmetrically distributed connection sites. To date, 4 and 6-connected stereoscopic nodes with T-d, D-3h, D-3d and C-3 symmetries have been mostly reported, delivering limited 3D topologies. We propose an efficient approach to expand the 3D COF repertoire by introducing a high-valency quadrangular prism (D-4h) stereoscopic node with a connectivity of eight, based on which two isoreticular 3D imine-linked COFs can be created. Low-dose electron microscopy allows the direct visualization of their 2-fold interpenetrated bcu networks. These 3D COFs are endowed with unique pore architectures and strong molecular binding sites, and exhibit excellent performance in separating C2H2/CO2 and C2H2/CH4 gas pairs. The introduction of high-valency stereoscopic nodes would lead to an outburst of new topologies for 3D COFs.