Dynamic Two-Dimensional\nCovalent Organic Frameworks\nwith Large Solvent-Induced Lattice Expansion

Dynamic covalent\norganic frameworks (COFs) can switch\nreversibly\nbetween crystalline phases with different unit cell parameters and\nporosities upon physisorption of guest molecules. While impressive\nchanges in unit cell volumes have been realized for three-dimensional\nframeworks, the solvent-induced volume changes of two-dimensional\n(2D) COFs have remained comparably small. We have now developed a\nseries of 2D COFs where we systematically varied the length of the\ninterconnecting bridge units. The optimized materials achieve solvent-induced\nexpansions of up to 85% relative to the volume of the solvent-free\ncontracted COFs. These structural changes are fully reversible and\nthe frameworks fully retain their high degree of crystalline order.\nThis study introduces a versatile design strategy for dynamic 2D COFs,\nwhich could enable future applications of these materials in gas separation\nand sensing.