Stabilization of 2D Imine‐Linked Covalent Organic Frameworks: From Linkage Chemistry to Interlayer Interaction

Abstract

Imine‐linked covalent organic frameworks (imine‐COFs) represent the most sought‐after class of COFs due to their broad monomer scope and ease of synthesis. Owing to the reversible nature of imine linkages, however, the chemical stability of most imine‐COFs is still far from adequate. In this context, emerging strategies, ranging from linkage chemistry to interlayer interaction, have been employed to construct stable imine‐COFs for their applications in electronics, sensing, and energy storage devices. This Concept article summarizes the latest advances aimed at tuning the structural stability of imine‐COFs. Furthermore, this Concept provides a prospective for the precise design of stable imine‐COFs based on the characteristics of structure, physical properties, and chemical functions, as well as the mechanism of structure locking and stabilization during crystal growth.

Fully Conjugated Covalent Organic Frameworks: Synthesis, Structures and Applications†

“…In this approach, Imine‐ linked COFs are converted into ultra‐stable porous aromatic frameworks. [ 85‐86 ] For instance, Lotsch and co‐workers have reported the topochemical conversion of triphenyl triazine‐based Imine‐linked COFs ( COF‐96 ) into thiazole‐linked COFs ( COF‐97 ) through a post‐synthesis locking strategy involving elemental sulfur (Figure 27). [ 87 ] This method leads to a change in the symmetry of the COF crystals while maintaining their original topology, thereby improving their chemical stability.…”

Expanding Olefin-Linked Covalent Organic Frameworks toward Selective Photocatalytic Oxidation of Organic Sulfides

“…As a kind of crystalline porous material constructed by assembling predesigned organic blocks in order, covalent organic frameworks (COFs) are attracting widespread attention for their potential applications. − Due to their outstanding properties, COFs have displayed various prospects in photocatalysis, , such as hydrogen evolution, − carbon dioxide reduction, − hydrogen peroxide generation, , and selective organic transformation. − Usually, COFs have been synthesized through highly polarized linkages like imines until now, which result in inadequate π-electron delocalization and impeded charge transfer. , Furthermore, imine linkages also bring a negative impact on the chemical stability of COFs . Compared with those highly reversible polar bonds, olefin linkages possess a higher bond energy and planar conjugated structures, which result in excellent chemical stability and full in-plane π-electron delocalization of COFs. − Fully conjugated skeletons endow olefin-linked COFs with extended visible-light absorption and smooth charge carrier migration. , The formation of olefin linkages in COFs always requires a low-reversibility reaction between active methylene/methyl and aldehyde groups, which brings many challenges in the construction of highly crystalline COFs. − Therefore, expanding the family of olefin-linked COFs toward robust photocatalysis is hugely desirable …”

Single-Atom Catalysts on Covalent Organic Frameworks for Energy Applications

“…53 The reversibility, on the other hand, may diminish the COF stability, so investigation into irreversible linkages, yet forming a crystalline network, such as vinylene or dioxin, is essential to search for robust COFs. 54 Both topologies and linkages, which determine COF lattice sizes, shapes, and inherent backbones, further affect their electronic, conjugated, and crystalline properties and applications. 39 For example, COFs using different monomers may result in different packing densities when anchoring atomic metals, which is critical for catalytic performance.…”

“…Linkage engineering. 54,58 Copyright 2021 John Wiley and Sons and copyright 2023 John Wiley and Sons. metal binding sites enable uniform distribution of the active centers and high loading of metals.…”

“…For example, boroxines, boronate ester, or imine bonds are reversible, resulting in COFs with high crystallinity due to error correction during crystallization . The reversibility, on the other hand, may diminish the COF stability, so investigation into irreversible linkages, yet forming a crystalline network, such as vinylene or dioxin, is essential to search for robust COFs …”