Kang Chengjun, Zhang Zhaoqiang, Usadi Adam K, Calabro David C, Baugh Lisa Saunders, Yu Kexin, Wang Yuxiang, Zhao Dan
Department of Chemical and Biomolecular Engineering, National University of Singapore, 117585, Singapore.
ExxonMobil Asia Pacific Pte. Ltd., 1 HarbourFront Place HarbourFront Tower 1, 098633, Singapore.
J Am Chem Soc. 2022 Feb 23;144(7):3192-3199. doi: 10.1021/jacs.1c12708. Epub 2022 Feb 14.
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 C solid-state nuclear magnetic resonance (C SSNMR). We find that C 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 C SSNMR. After analyzing four 2D COFs, we find it particularly useful for C SSNMR to quantitatively distinguish the AA stacking structure from other aggregated structures. Additionally, C 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 C 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.
共价有机框架（COFs）因其晶体结构而得到广泛应用。然而，对COF样品中的晶相进行定量分析仍然具有挑战性。这是因为COFs，尤其是二维COFs，通常以多晶粉末混合物的形式获得。因此，了解二维COFs的聚集结构对于其有效利用具有重要意义。在此，我们报告了使用碳固体核磁共振（C SSNMR）对二维COFs聚集结构的研究。我们发现C SSNMR可以区分二维COF中的不同聚集结构，因为COF层堆叠会形成受限空间，使相邻层的原子/基团之间能够进行紧密相互作用。随后，与非堆叠结构相比，这些原子/基团的化学环境发生了变化。这种化学环境的变化足够显著，能够被C SSNMR检测到。在分析了四种二维COFs后，我们发现C SSNMR对于从其他聚集结构中定量区分AA堆叠结构特别有用。此外，C SSNMR数据表明二维COFs中存在错位堆叠结构。这些错位堆叠结构不是长程有序的，并且无法通过基于X射线的检测方法检测到，因此此前尚未有报道。除了干燥状态，我们还通过C SSNMR研究了溶剂化二维COFs的聚集结构，结果表明二维COFs在干燥状态和溶剂化状态下具有不同的聚集结构。这些结果代表了对二维COFs聚集结构的首次定量研究，加深了我们对二维COFs结构的理解，并进一步推动了它们的应用。
