Hierarchical sub-stoichiometric isomeric covalent organic frameworks with diverse topologies derived from the same monomers for efficient atmospheric water harvesting

Abstract

Covalent organic frameworks (COFs) are remarkable porous crystalline materials that have wide usage in various fields. However, the topological isomers of COFs are uncommon, and it is quite infrequent to synthesize COFs with absolutely diverse hierarchical topologies from the same monomers. Herein, a pair of hierarchical topological isomeric COFs (HICOF-mtf and HICOF-bex) designed by adopting a sub-stoichiometric strategy is reported for atmospheric water harvesting. Their structural differences manifest distinctly in their water vapor adsorption: compared to the fully reacted, single-small-pore HICOF-mtf, HICOF-bex features multilevel pores and periodic unreacted amines, exhibiting unique multistage water filling and superior working capacity (0.21 g g⁻¹) in 20–50% relative humidity range. Notably, in contrast to HICOF-mtf with fully condensed frameworks, HICOF-bex has a periodic unreacted functional group arrangement, which exposes more polar sites and enhances the polarity of the channels, thus resulting in better water collection performance. Furthermore, the Grand Canonical Monte Carlo method was used to simulate the distribution of water molecules in COF channels at different pore filling steps, which confirmed the significance of the pore size and channel environment of the COFs for water–vapor filling. This work not only clarified the structure–function relationship between the topology and the vapor adsorption of COFs for the first time, but also broadened the design concept for porous water-collecting materials, offering inspiration for the design and construction of topological isomers.