A Crystalline\nThree-Dimensional Covalent Organic Framework\nwith Flexible Building Blocks

Abstract

The construction of three-dimensional\ncovalent organic frameworks\n(3D COFs) has proven to be very challenging, as their synthetic driving\nforce mainly comes from the formation of covalent bonds. To facilitate\nthe synthesis, rigid building blocks are always the first choice for\ndesigning 3D COFs. In principle, it should be very appealing to construct\n3D COFs from flexible building blocks, but there are some obstacles\nblocking the development of such systems, especially for the designed\nsynthesis and structure determination. Herein, we reported a novel\nhighly crystalline 3D COF (FCOF-5) with flexible C–O single\nbonds in the building block backbone. By merging 17 continuous rotation\nelectron diffraction data sets, we successfully determined the crystal\nstructure of FCOF-5 to be a 6-fold interpenetrated <b>pts</b> topology. Interestingly, FCOF-5 is flexible and can undergo reversible\nexpansion/contraction upon vapor adsorption/desorption, indicating\na breathing motion. Moreover, a smart soft polymer composite film\nwith FCOF-5 was fabricated, which can show a reversible vapor-triggered\nshape transformation. Therefore, 3D COFs constructed from flexible\nbuilding blocks can exhibit interesting breathing behavior, and finally,\na totally new type of soft porous crystals made of pure organic framework\nwas announced.