Rigidity Reinforcement of 3D Covalent Organic Frameworks by Controlling Interpenetration.

Here, we report the synthesis of a 3D covalent organic framework (COF), COF-308, with unique [4 + 2] interpenetration. Rather than inducing unwanted structure dynamics and a drastic drop in surface area, this interpenetration mode enhanced the structural rigidity and achieved a desirable surface area. Such an interpenetration was unveiled by single crystal X-ray diffraction (SCXRD) analysis with a resolution of 0.95 Å. A "face-to-face" packing of tetrahedral shaped building blocks in adjacent frames was observed, locking the slide and twisting the molecular motion. The structure rigidity and permanent porosity were further assessed by in situ small-angle X-ray scattering measured along gas adsorption and desorption processes. The distribution of gases was studied across the entire isotherm, outlining the anisotropy of the pores within COF-308. Such anisotropy, ideally suited for gas storage and separation, was endowed by this new interpenetration mode, where the "face-to-face" packing of building blocks was found to be the critical control.