New Mechanistic Insights into the Formation of Imine-Linked Two-Dimensional Covalent Organic Frameworks

Abstract

A more robust mechanistic understanding of imine-linked two-dimensional covalent organic frameworks (2D COFs) is needed to improve their crystalline domain sizes and to control their morphology, both of which are necessary to fully realize their application potential. Here, we present evidence that 2D imine-linked COFs rapidly polymerize as crystalline sheets that subsequently reorganize to form stacked structures. Primarily, this study focuses on the first few minutes of 1,3,5-tris­(4-aminophenyl)­benzene and terephthaldehyde polymerization, which yields an imine-linked 2D COF. In situ X-ray diffraction and thorough characterization of solids obtained using gentler isolation and activation methods than have typically been used in the literature indicate that periodic imine-linked 2D structures form within 60 s, which then form more ordered stacked structures over the course of several hours. This stacking process imparts improved stability toward the isolation process relative to that of the early stage materials, which likely obfuscated previous mechanistic conclusions regarding 2D polymerization that were based on products isolated using harsh activation methods. This revised mechanistic picture has useful implications; the 2D COF layers isolated at very short reaction times are easily exfoliated, as observed in this work using high-resolution transmission electron microscopy and atomic force microscopy. These results suggest improved control of imine-linked 2D COF formation can be obtained through manipulation of the polymerization conditions and interlayer interactions. Qualitatively similar results were obtained for analogous materials obtained from 2,5-di­(alkoxy)­terephthaldehyde derivatives, except for the COF with the longest alkoxy chains examined (OC12H25), which, although shown by in situ X-ray diffraction to be highly crystalline in the reaction mixture, is much less crystalline when isolated than the other COFs examined, likely due to the more severe steric impact of the dodecyloxy functionality on the stacking process.

Conquering the crystallinity conundrum: efforts to increase quality of covalent organic frameworks

“…As the observed sheets were very thin, it was suggested that COFs might initially crystallize as disordered sheets, followed by a recrystallization to ordered and porous powder. 118 Moreover, equilibration of these 60 seconds samples in supercritical CO 2 (scCO 2 ) for 72 hours led to crystalline peaks as observed by PXRD. The difference between both proposed mechanism is illustrated in Fig.…”

Strategies for Improving the Catalytic Performance of 2D Covalent Organic Frameworks for Hydrogen Evolution and Oxygen Evolution Reactions

“…And the large number of amine and aldehyde linkers made imine-linked COFs to be the most popular strategy to synthesize COF. [41][42][43] Wang et al reported a 2D COF (COF-LZU1) linked by condensation reaction of 1,4-diaminobenzene and 1,3,5-tricarbonylbenzene (Figure 2b). [44] The as-synthesized COF-LZU1 showed extremely high stability in common organic solvents and water.…”

“…Research showed these imine‐bonded 2D COFs possessed higher stability in water. And the large number of amine and aldehyde linkers made imine‐linked COFs to be the most popular strategy to synthesize COF [41–43] . Wang et al reported a 2D COF (COF‐LZU1) linked by condensation reaction of 1,4‐diaminobenzene and 1,3,5‐tricarbonylbenzene (Figure 2b) [44] .…”

Synthesis of Side-Chain-Free Hydrazone-Linked Covalent Organic Frameworks through Supercritical Carbon Dioxide Activation

“…28 The COF polymerization process has been studied by Marder and co-workers, and they found that the disordered thin, crystalline COF layers form within the aggregates at the very early stage of the COF polymerization. 16 Though this work was performed with imine COFs, these principles appear to be relevant in the case of hydrazone COFs as well as hydrazone COFs, when carefully activated, retain their crystalline structure. The nature of the loss of crystallinity upon activation is less clear, especially since we have observed it with hydrazone COFs that are similar to those known to readily delaminate in polar solvent 18 (TFPT-DHz and TFPB-DHz COFs) as well as Py-DHz COF which is made with a tetraphenylpyrene monomer that is well-known to form highly correlated stacking interactions in 2D-COFs.…”

“…These materials have been employed for various applications including catalysis, 8 gas storage, 9 gas separations, 10 energy storage, 11 and sensing 12 due to their promising properties such as well-defined structures, 3,13 high surface area, 9 tunable pore size, 14 and low density. 9 The general mechanisms of COF polymerization and assembly have received considerable attention over the past few years 15,16 in order to help researchers understand why certain combinations of monomers form crystalline COFs over others. Hydrazone-based COFs have been studied extensively due to their physicochemical stability compared to imine, or boronate ester linked COFs.…”