cis-Azo-Promoted Construction of Vinylene-Linked Covalent Organic Frameworks with Exceptional Capability of Stepwise Water Harvesting

sp2-nitrogen atom holds a promising role either in promoting the construction of covalent organic frameworks (COFs) or tailoring their properties and functions. Herein, starting from 3,6-dimethylpyridazine as the linear ditopic monomer embedded with two adjacent sp2-nitrogen atoms, we successfully built up two novel vinylene-linked COFs upon Knoevenagel condensation with triformyl substituted aromatic derivatives. The finely-resolved powder X-ray diffraction (PXRD) patterns demonstrated their high crystalline structures with a hexagonal lattice in AA mode stacking along vertical direction. The resultant one-dimensional channels possess fruitful strong hydrogen-bond accepting sites arising from the decorated cis-azo units with two pair of the fully exposed lone pair electrons. In combination with their finely tailored micro-/nano-pore sizes, high surface areas and stable nonpolar vinylene linkages, as-prepared COFs enable exceptionally stepwise water harvesting from air, as shown by their water sorption isotherms consisting of successive steep water uptake steps even starting from a very low humidity (~10%), and reaching the largest water uptake capacity up to 1.26 g/g at P/PSTA = 0.95 (25 °C), representing the record values among the reported COF materials so far. Dynamic vapor sorption measurements confidently revealed the fast kinetics of such kinds of COFs, even in the cluster formation process. Water uptake and release cycling test manifested their outstanding hydrolytic stability, durability and adsorption-desorption retention ability.