Winkler Christian, Kamencek Tomas, Zojer Egbert
Institute of Solid State Physics, NAWI Graz, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria.
Nanoscale. 2021 May 27;13(20):9339-9353. doi: 10.1039/d1nr01047f.
Covalent organic frameworks (COFs) have attracted significant attention due to their chemical versatility combined with a significant number of potential applications. Of particular interest are two-dimensional COFs, where the organic building units are linked by covalent bonds within a plane. Most properties of these COFs are determined by the relative arrangement of neighboring layers. These are typically found to be laterally displaced, which, for example, reduces the electronic coupling between the layers. In the present contribution we use dispersion-corrected density-functional theory to elucidate the origin of that displacement, showing that the common notion that the displacement is a consequence of electrostatic repulsions of polar building blocks can be misleading. For the representative case of COF-1 we find that electrostatic and van der Waals interactions would, actually, favor a cofacial arrangement of the layers and that Pauli repulsion is the crucial factor causing the serrated AA-stacking. A more in-depth analysis of the electrostatic contribution reveals that the "classical" Coulomb repulsion between the boroxine building blocks of COF-1 suggested by chemical intuition does exist, but is overcompensated by attractive effects due to charge-penetration in the phenylene units. The situation becomes more involved, when additionally allowing the interlayer distance to relax for each displacement, as then the different distance-dependences of the various types of interactions come into play. The overall behavior calculated for COF-1 is recovered for several additional COFs with differently sized π-systems and topologies, implying that the presented results are of more general relevance.
共价有机框架（COFs）因其化学多功能性以及大量潜在应用而备受关注。特别令人感兴趣的是二维COFs，其中有机构建单元在平面内通过共价键相连。这些COFs的大多数性质由相邻层的相对排列决定。通常发现这些层是横向位移的，例如，这会降低层间的电子耦合。在本论文中，我们使用色散校正的密度泛函理论来阐明这种位移的起源，表明那种认为位移是极性构建块静电排斥结果的普遍观点可能会产生误导。对于COF - 1的典型情况，我们发现静电相互作用和范德华相互作用实际上会有利于层的共面排列，而泡利排斥是导致锯齿状AA堆积的关键因素。对静电贡献的更深入分析表明，化学直觉所暗示的COF - 1的硼氧六元环构建块之间的“经典”库仑排斥确实存在，但由于亚苯基单元中的电荷穿透引起的吸引作用使其被过度补偿。当额外允许层间距离随每次位移而松弛时，情况会变得更加复杂，因为此时各种相互作用的不同距离依赖性开始起作用。对于具有不同大小的π体系和拓扑结构的其他几种COFs，计算得到的COF - 1的整体行为也能得到重现，这意味着所呈现的结果具有更普遍的相关性。
