The structure of layered covalent-organic frameworks.

Covalent-Organic Frameworks (COFs) are a new family of 2D and 3D highly porous and crystalline materials built of light elements, such as boron, oxygen and carbon. For all 2D COFs, an AA stacking arrangement has been reported on the basis of experimental powder XRD patterns, with the exception of COF-1 (AB stacking). In this work, we show that the stacking of 2D COFs is different as originally suggested: COF-1, COF-5, COF-6 and COF-8 are considerably more stable if their stacking arrangement is either serrated or inclined, and layers are shifted with respect to each other by ~1.4 Å compared with perfect AA stacking. These structures are in agreement with to date experimental data, including the XRD patterns, and lead to a larger surface area and stronger polarisation of the pore surface.