Topology-Derived Construction of Single-Crystal Two-Dimensional Binodal Covalent Organic Frameworks toward Inclined AB Stacking
Covalent organic frameworks (COFs) are crystalline organic porous materials whose atomically precise structures underpin their functional applications. However, atomic-level structural information remains unavailable for most reported COFs, hampering rational design and structure-function studies. For two-dimensional (2D) COFs, synthesizing high-quality single crystals is challenging, and the crystallization mechanism makes it difficult to anticipate stacking arrangements. Lacking direct evidence, researchers often assume AA stacking for [4+4] COFs in powder X-ray diffraction (PXRD) fitting, an assumption now questioned. Here, Zhang et al. report the controlled synthesis of single-crystal 2D binodal COFs via a topological derivation strategy. Using 3D electron diffraction at resolutions of 0.90–1.02 Å, they solved the structures of five COFs (NKCOF-88 to -92) derived from a parent sql framework. The four-connected benzene-core monomers were substituted with extended monomers (pyrene or tetraphenylethylene), decomposable into two three-connected nodes, yielding pseudo-bex and pseudo-hcb networks. Single-crystal analysis revealed that the in-plane chain configuration is determined by imine bond orientation, with cis+cis or trans+trans alignments giving planar layers, while mixed alignments produce undulated layers. Crucially, all five COFs exhibit inclined staggered AB stacking, stabilized by edge-to-face π–π interactions, contradicting the assumed AA stacking. This work provides the first single-crystal evidence of non-AA stacking in [4+4] COFs and establishes a correlation between linkage conformation and layer geometry. The topology-derived method offers a generalizable route to binodal COFs with predefined connectivity, facilitating the fabrication of high-quality single crystals and enabling reticular chemistry to shift from simulated models to real structures.