RESUMEN
Covalent organic frameworks (COFs) are emerging crystalline porous polymers, showing great potential for applications but lacking gas-triggered flexibility. Atropisomerism was experimentally discovered in 1922 but has rarely been found in crystals with infinite framework structures. Here we report atropisomerism in COF single crystals. The obtained COF atropisomers, namely COF-320 and COF-320-A, have identical chemical and interpenetrated structures but differ in the spatial arrangement of repeating units. In contrast to the rigid COF-320 structure, its atropisomer (COF-320-A) exhibits unconventional gas sorption behaviours with one or more sorption steps in isotherms at different temperatures. Single-crystal structures determined from continuous rotation electron diffraction and in situ powder X-ray diffraction demonstrate that these adsorption steps originate from internal pore expansion with or without changing the crystal space group. COF-320-A recognizes different gases by expanding its internal pores continuously (crystal-to-amorphous transition) or discontinuously (crystal-to-crystal transition) or having mixed transition styles, distinguishing COF-320-A from existing soft/flexible porous crystals. These findings extend atropisomerism from molecules to crystals and propel COFs into the covalently linked soft porous crystal regime, further advancing applications of soft porous crystals in gas sorption, separation and storage.
RESUMEN
Carbon capture is one of the essential low-carbon technologies required to achieve societal climate goals at the lowest cost. Covalent organic frameworks (COFs) are promising adsorbents for CO2 capture because of their well-defined porosity, large surface area, and high stability. Current COF-based CO2 capture is mainly based on a physisorption mechanism, exhibiting smooth and reversible sorption isotherms. In the present study, we report unusual CO2 sorption isotherms featuring one or more tunable hysteresis steps with metal ion (Fe3+, Cr3+, or In3+)-doped Schiff-base two-dimensional (2D) COFs (Py-1P, Py-TT, and Py-Py) as adsorbents. Synchrotron X-ray diffraction, spectroscopic and computational studies indicate that the sharp adsorption steps in the isotherm originate from the insertion of CO2 between the metal ion and the N atom of the imine bond on the inner pore surface of the COFs as the CO2 pressure reaches threshold values. As a result, the CO2 adsorption capacity of the ion-doped Py-1P COF is increased by 89.5% compared with that of the undoped Py-1P COF. This CO2 sorption mechanism provides an efficient and straightforward approach to enhancing the CO2 capture capacity of COF-based adsorbents, yielding insights into developing chemistry for CO2 capture and conversion.
RESUMEN
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.
RESUMEN
Resolving single-crystal structures of two-dimensional covalent organic frameworks (2D COFs) is a great challenge, hindered in part by limited strategies for growing high-quality crystals. A better understanding of the growth mechanism facilitates development of methods to grow high-quality 2D COF single crystals. Here, we take a different perspective to explore the 2D COF growth process by tracing growth intermediates. We discover two different growth mechanisms, nucleation and self-healing, in which self-assembly and pre-arrangement of monomers and oligomers are important factors for obtaining highly crystalline 2D COFs. These findings enable us to grow micron-sized 2D single crystalline COF Py-1P. The crystal structure of Py-1P is successfully characterized by three-dimensional electron diffraction (3DED), which confirms that Py-1P does, in part, adopt the widely predicted AA stacking structure. In addition, we find the majority of Py-1P crystals (>90%) have a previously unknown structure, containing 6 stacking layers within one unit cell.
RESUMEN
Covalent organic frameworks (COFs) have found wide applications due to their crystalline structures. However, it is still challenging to quantify crystalline phases in a COF sample. This is because COFs, especially 2D ones, are usually obtained as mixtures of polycrystalline powders. Therefore, the understanding of the aggregated structures of 2D COFs is of significant importance for their efficient utilization. Here we report the study of the aggregated structures of 2D COFs using 13C solid-state nuclear magnetic resonance (13C SSNMR). We find that 13C SSNMR can distinguish different aggregated structures in a 2D COF because COF layer stacking creates confined spaces that enable intimate interactions between atoms/groups from adjacent layers. Subsequently, the chemical environments of these atoms/groups are changed compared with those of the nonstacking structures. Such a change in the chemical environment is significant enough to be captured by 13C SSNMR. After analyzing four 2D COFs, we find it particularly useful for 13C SSNMR to quantitatively distinguish the AA stacking structure from other aggregated structures. Additionally, 13C SSNMR data suggest the existence of offset stacking structures in 2D COFs. These offset stacking structures are not long-range-ordered and are eluded from X-ray-based detections, and thus they have not been reported before. In addition to the dried state, the aggregated structures of solvated 2D COFs are also studied by 13C SSNMR, showing that 2D COFs have different aggregated structures in dried versus solvated states. These results represent the first quantitative study on the aggregated structures of 2D COFs, deepen our understanding of the structures of 2D COFs, and further their applications.
RESUMEN
Layer-stacking structures are very common in two-dimensional covalent organic frameworks (2D COFs). While their structures are normally determined under solvent-free conditions, the structures of solvated 2D COFs are largely unexplored. We report herein the in situ determination of solvated 2D COF structures, which exhibit an obvious difference as compared to that of the same COF under dried state. Powder X-ray diffraction (PXRD) data analyses, computational modeling, and Pawley refinement indicate that the solvated 2D COFs experience considerable interlayer shifting, resulting in new structures similar to the staggered AB stacking, namely, quasi-AB-stacking structures, instead of the AA-stacking structures that are usually observed in the dried COFs. We attribute this interlayer shifting to the interactions between COFs and solvent molecules, which may weaken the attraction strength between adjacent COF layers. Density functional theory (DFT) calculations confirm that the quasi-AB stacking is energetically preferred over the AA stacking in solvated COFs. All four highly crystalline 2D COFs examined in the present study exhibit considerable interlayer shifting upon solvation, implying the universality of the solvent-induced interlayer stacking rearrangement in 2D COFs. These findings prompt re-examination of the 2D COF structures in solvated state and suggest new opportunities for the applications of COF materials under wet conditions.
RESUMEN
The rectangular crystal packing of methyl-branched paraffins in their orthorhombic forms is studied systematically by electron diffraction to show that, irrespective of branch position on the chain, a close interaction of chain double rows occurs, placing the branch in the space between two chain ends. If the chain branches occur near the ends, the structure can slowly rearrange into a true bilayer. If the branch occurs near the center, then there are a large number of intermediate "nematocrystalline" disordered forms that are possible before the final ordered layered packing.
