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

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.