Interlayer Interactions as Design Tool for Large-Pore COFs

Abstract

Covalent organic frameworks (COFs) with a pore size beyond 5 nm are still rarely seen in this emerging field. Besides obvious complications such as the elaborated synthesis of large linkers with sufficient solubility, more subtle challenges regarding large-pore COF synthesis, including pore occlusion and collapse, prevail. Here we present two isoreticular series of large-pore imine COFs with pore sizes up to 5.8 nm and correlate the interlayer interactions with the structure and thermal behavior of the COFs. By adjusting interlayer interactions through the incorporation of methoxy groups acting as pore-directing “anchors”, different stacking modes can be accessed, resulting in modified stacking polytypes and, hence, effective pore sizes. A strong correlation between stacking energy toward highly ordered, nearly eclipsed structures, higher structural integrity during thermal stress, and a novel, thermally induced phase transition of stacking modes in COFs was found, which sheds light on viable design strategies for increased structural control and stability in large-pore COFs.

NO as a Reagent for Topochemical Framework Transformation and Controlled Nitric Oxide Release in Covalent Organic Frameworks

“…Crystallinity and therefore enhanced accessibility of the NONOate functional groups in the COF pores could be achieved by implementing readily available strategies to direct and lock the layer stacking, or by using 3D COFs. [51][52][53][54] Taken together, our NMR analysis explains the different linkage stabilities and transformations, and rationalizes the observed initial irreversible adsorption behavior, which is largely due to the chemical reaction of NO with terminal amine groups or linkages of the frameworks.…”

Understanding fragility and engineering activation stability in two-dimensional covalent organic frameworks

“…However, these results are consistent with studies of the thermal stability of COFs by Lotsch and coworkers. 33 They found that pore substituents increased the strength of interlayer interactions and improved COF thermal stability. Interestingly, a prior report by Dichtel and coworkers reported the opposite trend in the thermal stability of COFs.…”

“… 34 Lotsch and coworkers introduced side chains in large pore COFs to improve thermal stability. 33 Other approaches include the introduction of fluorine atoms 35 and intralayer hydrogen bonding interactions 36 to produce COFs with improved chemical stability. However, these studies did not specifically focus on stability during solvent drying or activation.…”

“…Recent work probed the thermal degradation of COFs. 32,33 These studies were focused on understanding structural and chemical changes in COFs at elevated temperatures and revealed two distinct degradation mechanisms. The rst degradation involves structural distortion or a phase transition from an eclipsed to staggered conformation and is reected in a loss or reduction of crystallinity, and at higher temperatures the COF will degrade chemically.…”

Film Nanoarchitectonics of Pillar[5]arene for High-Performance Fluorescent Sensing: a Proof-of-Concept Study

“…We infer that the above-mentioned excellent characteristics of the nanofilm, such as uniformity, integrity, and flexibility, may partly come from the fact that the nanofilm could adapt its structure via reversible bond formation/dissociation of the dynamic covalent bonds. Compared to covalent organic framework (COF) films, the building blocks in the nanofilm as prepared may not be so orderly aligned owing to the flexibility of the linkage between them. Nevertheless, abundant intermolecular hydrogen bonds are evidenced by the wideness and shift of the FTIR absorption peaks for −NH 2 , −NH, and −CHO exist in the nanofilm.…”