Tuning Crystallinity and Stacking of Two-Dimensional Covalent Organic Frameworks through Side-Chain Interactions

Abstract

Two-dimensional covalent organic frameworks (2D COFs) form as layered 2D polymers whose sheets stack through high-surface-area, noncovalent interactions that can give rise to different interlayer arrangements. Manipulating the stacking of 2D COFs is crucial since it dictates the effective size and shape of the pores as well as the specific interactions between functional aromatic systems in adjacent layers, both of which will strongly influence the emergent properties of 2D COFs. However, principles for tuning layer stacking are not yet well understood, and many 2D COFs are disordered in the stacking direction. Here, we investigate effects of pendant chain length through a series of 2D imine-linked COFs functionalized with n-alkyloxy chains varying in length from one carbon (C 1 COF) to 11 carbons (C 11 COF). This series reveals previously unrecognized and unanticipated trends in both the stacking geometry and crystallinity. C 1 COF adopts an averaged eclipsed geometry with no apparent offset between layers. In contrast, all subsequent chain lengths lead to some degree of unidirectional slip stacking. As pendant chain length is increased, trends show average layer offset increasing to a maximum of 2.07 Å in C 5 COF and then decreasing as chain length is extended through C 11 COF. Counterintuitively, shorter chains (C 2 –C 4 ) give rise to lower yields of weakly crystalline materials, while longer chains (C 6 –C 9 ) produce greater yields of highly crystalline materials, as confirmed by powder X-ray diffraction and scanning electron microscopy. Molecular dynamics simulations corroborate these observations, suggesting that long alkyl chains can interact favorably to promote the self-assembly of sheets. In situ proton NMR spectroscopy provides insights into the reaction equilibrium as well as the relationship between low COF yields and low crystallinity. These results provide fundamental insights into principles of supramolecular assembly in 2D COFs, demonstrating an opportunity for harnessing favorable side-chain interactions to produce highly crystalline materials.

Rational Regulation of the Exciton Effect of Acrylonitrile-Linked Covalent Organic Framework toward Boosting Visible-Light-Driven Hydrogen Evolution

“…As shown in Figure S5 and Table S5, the crystallinity of TP-BPN COF, TP-PN COF, TP-BPyN COF, and TP-BPyN PCOF was 49.0, 59.4, 46.4, and 35.6%, respectively. The results indicate that the TP-PN COF with the shortest linker length exhibits the highest crystallinity, which may be attributed to the small steric hindrance during the synthesis process …”

Branched-Chain-Induced Host–Guest Assembly in Covalent-Organic Frameworks for Efficient Separation of No-Carrier-Added 177Lu

“…Accordingly, the reduction in the pore size could be observed (Figure S5e, f) with ERCOF-4 (2.1 nm) and ERCOF-8 (2.2 nm) dropping to 1.7 and 1.8 nm, respectively. All these may be triggered by the unidirectional slip-stacking and the disordered oscillation of longer alkyl chains inside the pores . DFT calculations were performed based on characteristic fragments from COFs to quantify the interlayer interaction forces ( E binding ).…”

pyCOFBuilder: A Python Package for Automated Creation of Covalent Organic Framework Models Based on the Reticular Approach

“…However, an aspect that remains relatively unexplored is the intricate interplay of intermolecular degrees of freedom within these frameworks. In two-dimensional COFs, the relative displacements of covalent sheets can give rise to several stacking arrangements, and while the prevailing perception characterizes most COFs with a perfectly eclipsed stacking pattern (AA), deviations may occur in COFs with enlarged pores or bulky functional groups, manifesting as synchronized offsets, − staggered stacking (AB), , or stacking disorder . These structural variations have been shown to profoundly influence COFs electronic properties and gas adsorption capacities. − …”