Tunable Cage-Based Three-Dimensional Covalent Organic Frameworks

Abstract

It is extremely challenging to construct three-dimensional (3D) crystalline covalent organic frameworks (COFs) with flexible building blocks and to further explore their tunable or adaptive characteristics due to crystallization and structure determination difficulties. Herein, we constructed three crystalline isostructural 3D-OC-COFs based on a newly synthesized flexible organic cage (6NH2-OC.4HCl) through a novel in situ acid-base neutralization strategy. Interestingly, network conformations can be fine-tuned by rationally introducing different substituents into the starting dialdehydes during the assembly process, resulting in one expanded structure (3D-OC-COF-H) and two different contracted structures (3D-OC-COF-OH and 3D-OC-COF-Cl), which can be ascribed to the hinge-like motions of organic cage building blocks. By virtue of the unique pore environments and different functional groups, we further employed these COFs for CO2capture, of which 3D-OC-COF-OH exhibited superior CO2/CH4separation performance. This study not only demonstrates a new strategy for constructing 3D cage-based COFs, but also identifies a novel factor that affects flexible building blocks for realizing structural diversity.