Aggregated Structures of Two-Dimensional Covalent Organic Frameworks

Abstract

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.

The order–disorder conundrum: a trade-off between crystalline and amorphous porous organic polymers for task-specific applications

“…In general, the spatial arrangement of the functional building units in the 3D macromolecular architectures from nano to macroscale governs the application potential of POPs. In this regard, the advanced structural and morphological characterisation tools, such as solid-state 2D NMR, 277,278 small- and wide-angle X-ray diffraction, 47,279 high-resolution electron microscopy, 280 and X-ray tomography 123 techniques, can shed light on the underlying configurational and supramolecular arrangements of the building units in the resultant framework or network (Fig. 31).…”

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

“…Therefore, repulsion forces exist between adjacent methoxy groups, and the extra electron cloud shields the methoxy groups from moving their 1 H SSNMR signal to a higher field (Figure S1). 44 CO 2 cannot trigger the interlayer shifting in TAPB-TA because the interaction between CO 2 and TAPB-TA is weaker than between adjacent COF layers. However, for TAPB-OMeTA, repulsion between adjacent methoxy groups facilitates interlayer shifting.…”

Fluorinated covalent organic frameworks for efficient drug delivery

“…The DF-TATB-COF with more lower BET surface area has less active load sites, and more smaller pore volume of the COF means more weaker ability of accommodating drug molecules into the hole. To prove the importance of fluorine groups for the loading drug capacity of the COFs, we prepared the N-TAPB-OMeTA COF material according to the literature, 35 and its chemical structure and PXRD are shown in Fig. S8.…”