Key Takeaways & Executive Findings
- •• • Single-crystal structures of five binodal [4+4] 2D COFs (NKCOF-88 to -92) were solved via 3D electron diffraction at resolutions of 0.90–1.02 Å, providing atomic-level evidence that all adopt inclined staggered AB stacking, not the assumed AA stacking. • • The topology-derived synthetic route, based on substituting four-connected nodes with extended monomers decomposable into two three-connected nodes, yields pseudo-bex and pseudo-hcb networks, enabling predictable synthesis of binodal COFs. • • Linkage conformation (cis+cis, trans+trans, or mixed) directly controls layer geometry: planar layers for uniform alignment, undulated layers for mixed alignment, as confirmed by single-crystal analysis. • • The inclined AB stacking modes are categorized into three types based on monomer geometry: horizontal (pyrene-based, NKCOF-90/92), diagonal (tetraphenylethylene or terphenyl, NKCOF-88/89), and vertical (NKCOF-91), with edge-to-face π–π interactions stabilizing the structures.
Abstract
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.
1. Introduction
The field of covalent organic frameworks (COFs) has long been hampered by a critical bottleneck: the lack of atomic-level structural information for the vast majority of reported materials. This deficiency severely limits rational design and the elucidation of structure-property relationships. For two-dimensional (2D) COFs, the challenge is particularly acute because synthesizing high-quality single crystals is notoriously difficult. The crystallization mechanism of 2D polymers makes it hard to predict their crystal morphology and stacking arrangements. Consequently, researchers have relied on a widely adopted simplification: assuming that most [4+4] COFs adopt AA stacking for powder X-ray diffraction (PXRD) fitting. This assumption, however, has never been rigorously validated due to the absence of direct single-crystal evidence.
In this context, Zhang et al. have made a significant breakthrough by developing a topological derivation strategy that enables the controlled synthesis of single-crystal 2D binodal COFs. By substituting four-connected benzene-core nodes with extended monomers that can be decomposed into two three-connected nodes, they generated novel pseudo-bex and pseudo-hcb networks. Using 3D electron diffraction, they solved the structures of five COFs at resolutions of 0.90–1.02 Å, providing the first single-crystal evidence that these [4+4] COFs adopt an inclined staggered AB stacking mode, not the assumed AA stacking. This work not only challenges the long-held view but also establishes a clear correlation between linkage conformation and layer geometry, offering a generalizable route to fabricate high-quality binodal COF single crystals with predictable structures.
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WANG Yong, ZHANG Qiaoqiao (2026). Topology-Derived Construction of Single-Crystal Two-Dimensional Binodal Covalent Organic Frameworks toward Inclined AB Stacking. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4424-5
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Frequently Asked Questions
What is the key innovation in synthesizing single-crystal 2D binodal COFs, and how does it overcome previous limitations?
The innovation lies in the topology-derived synthetic strategy, which predefines binodal connectivity by substituting four-connected nodes with extended monomers that decompose into two three-connected nodes. This approach avoids trial-and-error and ensures predictable framework structures. The resulting COFs (NKCOF-88 to -92) were obtained as high-quality single crystals, with structures solved at resolutions of 0.90–1.02 Å using 3D electron diffraction.
How does the stacking mode in these COFs differ from the commonly assumed AA stacking, and what evidence supports this?
All five COFs exhibit inclined staggered AB stacking, stabilized by edge-to-face π–π interactions, rather than the assumed AA stacking. This is unambiguously confirmed by single-crystal structural analysis using 3D electron diffraction, which provides atomic-level resolution. This is the first single-crystal evidence of non-AA stacking in [4+4] COF systems.
What is the relationship between monomer geometry and the type of inclined stacking observed?
The inclined stacking modes are categorized into three types based on monomer geometry: horizontal inclination for large pyrene-based units (NKCOF-90, -92), diagonal inclination for tetraphenylethylene or terphenyl units (NKCOF-88, -89), and vertical inclination for NKCOF-91. This correlation allows prediction of stacking based on monomer design.
What are the limitations of the topology-derived method, and how might it be extended to other COF systems?
The method requires two coordination units with precisely matched symmetry and bonding angles, limiting its application to single-node or irregular non-sql topologies. However, it is generalizable to other binodal topologies derived from the parent sql framework, and is expected to be applicable to [3+3], [6+3], and other binodal networks in the future.
How does this work impact the field of reticular chemistry and COF design?
By providing atomic-scale structural benchmarks, this work challenges the long-held assumption of AA stacking and demonstrates the importance of direct structural evidence. It enables rational design of binodal 2D COFs with controlled stacking, shifting reticular chemistry from simulated models to real structures. This will accelerate the development of COFs with optimized properties for applications such as gas storage, separation, and catalysis.
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