A Case Study on the Influence of Substitutes on Interlayer Stacking of 2D Covalent Organic Frameworks

Interlayer stacking of 2D covalent organic frameworks (COFs) plays a crucial role in determining not only the geometry of channels inside COFs but also the mobility of carrier transport between COF layers. However, though topological structures of 2D COFs monolayers can be precisely predicted through the structures of building blocks, factors affecting their interlayer stacking remain poorly understood. In this work, a truxene-based building block on which six methyl groups are introduced was designed. The condensation of it with 1,4-diaminobenzene or benzidine afforded 2D COFs with the methyl groups extending out-of-plane of the layers. A significant influence of the methyl groups on interlayer stacking of the COFs was revealed by the adoption of inclined packing of monolayers, which has never been experimentally observed before. This unprecedented stacking manner was confirmed by powder X-ray diffraction analysis, pore-size distribution analysis, and TEM investigation.