Interlayer Shifting in Two-Dimensional Covalent Organic Frameworks

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.

层堆叠结构在二维共价有机框架（2D COFs）中极为常见。尽管这类材料的结构通常是在无溶剂条件下解析得到的，但溶剂化状态下二维共价有机框架的结构仍未得到充分研究。本文报道了溶剂化二维共价有机框架结构的原位解析结果，其与干燥状态下同一共价有机框架的结构存在显著差异。粉末X射线衍射（PXRD）数据分析、计算建模以及帕利精修（Pawley refinement）结果表明，溶剂化二维共价有机框架发生了显著的层间偏移，形成了类似错位AB堆叠的新型结构，即准AB堆叠结构，而非干燥状态下共价有机框架通常呈现的AA堆叠结构。我们将该层间偏移归因于共价有机框架与溶剂分子之间的相互作用，这类作用可削弱相邻共价有机框架层之间的吸引力。密度泛函理论（DFT）计算证实，在溶剂化的共价有机框架中，准AB堆叠在能量上优于AA堆叠。本研究中考察的四种高结晶度二维共价有机框架在溶剂化过程中均发生了显著的层间偏移，这表明溶剂诱导的二维共价有机框架层堆叠重排具有普遍性。上述发现促使人们重新审视溶剂化状态下二维共价有机框架的结构，并为共价有机框架材料在潮湿环境中的应用提供了新的方向。