Tunable Interlayer Shifting in Two-Dimensional Covalent Organic Frameworks Triggered by CO₂ Sorption

Abstract

Two-dimensional covalent organic frameworks (2D COFs) have been widely viewed as rigid porous materials with smooth and reversible gas sorption isotherms. In the present study, we report an unusual hysteresis step in the CO2 adsorption isotherm of a 2D COF, TAPB-OMeTA. In situ powder X-ray diffraction (PXRD) measurements, computational modeling, and Pawley refinement indicate that TAPB-OMeTA experiences slight interlayer shifting during the CO2 adsorption process, resulting in a new structure that is similar but not identical to the AA stacking structure, namely, a quasi-AA stacking structure. This interlayer shifting is responsible for the step in its CO2 adsorption isotherm. We attribute the interlayer shifting to the interactions between COF and CO2, which weaken the attraction strength between adjacent COF layers. Notably, the repulsion force between the methoxy groups on the backbone of TAPB-OMeTA is essential in facilitating the interlayer shifting process. After further increasing the size of side groups by grafting poly(N-isopropylacrylamide) oligomers to the TAPB-OMeTA backbone via surface-initiated atom transfer radical polymerization (SI-ATRP), we observed a second interlayer shifting and two adsorption steps in the CO2 adsorption isotherm, suggesting tunability of the interlayer shifting process. Density functional theory (DFT) calculations confirm that the quasi-AA stacking structure is energetically preferred over AA stacking under a CO2 atmosphere. These findings demonstrate that 2D COFs can be “soft” porous materials when interacting with gases, providing new opportunities for 2D COFs in gas storage and separation.

Covalent Organic Frameworks as Porous Pigments for Photocatalytic Metal-Free C–H Borylation

“…The field of covalent organic frameworks (COFs)porous structures consisting of organic building blocks reticulated via covalent bondshas gained significant attention in the last decade . COFs have emerged as robust materials for various applications, including gas storage and separation, energy storage, optoelectronics, and catalysis . COFs are perfect hosts for small-molecule catalysts due to their high crystallinity and porosity .…”

Hydrogen Bonding Regulated Flexibility and Disorder in Hydrazone-Linked Covalent Organic Frameworks

“…In recent few years, the dynamic nature of COFs, such as breathing effect and/or subnet sliding, has been observed by external stimuli (guest, light, pressure, etc. ). ,− With further development, more evidences show that most COFs are flexible to some extent due to the widespread noncovalent interactions in COF structures, such as π–π interactions, hydrogen bondings, Lewis acid–base interactions, etc. ,,, The strength of noncovalent interactions is relatively weak, and they are sensitive to external stimuli, which account for the dynamic nature in most COFs. For example, Zhao and co-workers reported that the weak π–π stacking interactions of 2D COFs can be destroyed by solvents, resulting in a reversible interlayer shift from AA-stacking structures in the dried state to quasi-AB-stacking structures in the solvated state, which clearly states the flexibility of COF structures .…”

Recent advances in metal-free covalent organic frameworks for photocatalytic applications in energy and environmental fields

“…Since Yaghi introduced the rst covalent organic framework (COF) in 2005, 12 the preparation of COFs has attracted signicant research interest. COFs have shown great potential for application in gas separation and storage, 13 adsorption of metal ions, 14 sensors, 15,16 optoelectronic devices, 17,18 and heterogeneous catalysis. 19,20 In particular, COFbased electrocatalysts have achieved considerable advances in the water cycle, [21][22][23] the carbon energy cycle, [24][25][26] and supercapacitors; 27,28 the developments of these energy storage and conversion applications have been discussed in detail in some very excellent review articles.…”