sln-Topological Covalent Organic Frameworks with Shape Dimorphism and Dipolar Rotors

Abstract

We report herein the first (i) covalent organic frameworks (COFs) with sln topology, (ii) drastic control of the shape of COF crystals with the same topology and chemical composition, (iii) insight that these different shapes are accompanied by conformational isomerism, and (iv) installation of dipolar rotors into three-dimensional (3D) COFs. We used a new building block, hexaarylbenzene with three dipolar 1,2-difluorophenyls. Depending on the solution composition, we generated two types of COF crystals with different shapes: hexagonal prism (TK-COF-P) and membrane (TK-COF-M). Although they exhibit distinctly different powder X-ray diffraction patterns, they are chemically identical. The structural determinations revealed that they have a low-symmetric, low-density sln topology that is yet to be reported for COFs. The two distinct shapes─shape dimorphism─is found to accompany conformational isomerism. Temperature-dependent dielectric and ¹⁹F NMR relaxation-time measurements reveal that the rotor motion is suppressed at room temperature, but the rotors respond to an external electric field at elevated temperatures owing to the high activation energy for rotation (∼20 kcal mol^–1), which is desired for room-temperature applications including molecular memories. These outcomes have not only expanded the diversity of COFs but have also provided a foundation for installing external-field-responsive functions into COFs that have high thermal stability, which is expected to invoke broad applications.