Covalent organic frameworks (COFs) offer a high degree of chemical and structural flexibility. There is a large family of COFs built from 2D sheets that are stacked to form extended crystals. While it has been common to represent the stacking as eclipsed with one repeating layer (“AA”), there is growing evidence that a more diverse range of stacking sequences is accessible. Herein, we report a computational study using density functional theory of layer stacking in two prototypical COFs, Tp-Azo and DAAQ-TFP, which have shown high performance as Li-ion battery electrodes. We find a striking preference for slipped structures with horizontal offsets between layers ranging from 1.7 Å to 3.5 Å in a potential energy minimum that forms a low energy ring. The associated symmetry breaking results in a pronounced change in the underlying electronic structure. A band gap opening of 0.8–1.4 eV is found due to modifications of the underlying valence and conduction band dispersion as explained from changes in the π orbital overlap. The implications for the screening and selection of COF for energy applications are discussed.

中文翻译：

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层状共价有机框架中的位移不稳定性导致带隙打开

共价有机框架 (COF) 提供了高度的化学和结构灵活性。有一大类 COF 由 2D 片材制成，这些片材堆叠形成扩展晶体。虽然将堆叠表示为与一个重复层 (“AA”) 相重叠是很常见的，但越来越多的证据表明，可以使用更多样化的堆叠序列。在此，我们报告了一项使用密度泛函理论在两个原型 COF（Tp-Azo 和 DAAQ-TFP）中的层堆叠的计算研究，它们已显示出作为锂离子电池电极的高性能。我们发现在形成低能量环的势能最小值中，层间水平偏移范围为 1.7 至 3.5 的滑动结构具有惊人的偏好。相关的对称性破坏导致底层电子结构发生显着变化。由于基础价和导带色散的修改，如 π 轨道重叠的变化所解释的那样，发现了 0.8-1.4 eV 的带隙开口。讨论了用于能源应用的 COF 筛选和选择的意义。