Synthesis of Single-Crystal Two-Dimensional Binodal Covalent Organic Frameworks through a Topology Derivation Strategy

Growing single-crystal two-dimensional covalent organic frameworks (2D COFs) for precise atomic-level structural determination presents a formidable yet crucial challenge. Here, we develop a topology derivation strategy to design and synthesize five single-crystal 2D binodal [4 + 4] COFs, transforming the parent edge-transitive sql topology network into two distinct derivative networks with pseudo-bex and pseudo-hcb topologies by strategically varying the symmetry of the organic building blocks. Their precise structures are unambiguously resolved by three-dimensional electron diffraction with a resolution of up to 0.90 Å, and all non-hydrogen atoms are directly located. A systematic structure analysis reveals that the imine bond orientation dictates intralayer arrangements, yielding both planar and unconventional wavy layers. We also observe that all COFs prefer an inclined, staggered AB stacking pattern, with layers inclined along three distinct directions, breaking the diagram that [4 + 4] COFs exhibit AA stacking. Furthermore, we demonstrate that, compared with traditional low-crystallinity powdery phases, single-crystal COFs significantly boost C2H2/CO2 separation performance, underscoring the critical importance of constructing advanced materials with highly ordered structures. Atomically precise structure determination provides a definitive foundation for linking structural order to emergent properties in 2D COFs, pushing the boundaries of reticular chemistry.