Tunable Cage-Based Three-Dimensional Covalent Organic Frameworks

It is extremely challenging to construct three-dimen-sional (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 difficul-ties. Herein, we constructed three crystalline isostruc-tural 3D-OC-COFs based on a newly synthesized flexible organic cage (6NH(2)-OC center dot 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 CO2 capture, of which 3D-OC-COF-OH exhibited superior CO2/CH4 separation 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. [GRAPHICS] .