Bioinspired pore engineering in two-dimensional covalent organic frameworks

Abstract

Covalent organic frameworks (COFs) are permanently porous, molecularly engineered scaffolds that exemplify the intersection of design, chemistry, and function. They are composed of organic monomers stitched together by covalent bonds into highly ordered two- or three-dimensional architectures. The majority of the studies on two dimensional COFs have focused on eclipsed rather than antiparallel stacked structures. Drawing inspiration from non-covalent bonding motifs in biomolecules, we report the design and synthesis of robust two-dimensional (2D) COFs with amide side chains called aCOFamide-1 and -2, which mimic the hydrogen bonding motifs found in β-sheets. These linker-based sidechains adopt an antiparallel stacked configuration to create binding sites that display excellent iodine adsorption capability (5.9 g g−1 and 6.7 g g−1 for I2 in aCOFamide-1 and -2, respectively). The antiparallel stacked layers of aCOFamides form three-dimensional, multi-N, nanotraps, which can also bind and adsorb anions such as iodide (I−) in aqueous environments, a major challenge in molecular recognition. Computational electrostatic maps reveal localized regions of positive electrostatic potential near the amide functional groups in the aCOFamide, which enhances the iodide uptake up to 1.3 g g−1 and 1.7 g g−1 in aCOFamide-1 and -2, respectively. We also show that the crystalline nature of COFs plays a major role in adsorption capability, when compared to a porous organic polymer (POP) composed of monomers containing the same amide sidechains but is polymerized under kinetic control which produces an amorphous network with lower adsorption performance. These results highlight the potential of biomimetic design strategies in engineering COFs with exceptional host–guest performance and reusability.