Bottom-Up Synthesis of Crystalline Covalent Organic Framework Nanosheets, Nanotubes, and Kippah Vesicles: An Odd-Even Effect Induction

Few-layer organic nanosheets are becoming increasinglyattractiveas two-dimensional (2D) materials due to their precise atomic connectivityand tailor-made pores. However, most strategies for synthesizing nanosheetsrely on surface-assisted methods or top-down exfoliation of stackedmaterials. A bottom-up approach with well-designed building blockswould be the convenient pathway to achieve the bulk-scale synthesisof 2D nanosheets with uniform size and crystallinity. Herein, we havesynthesized crystalline covalent organic framework nanosheets (CONs)by reacting tetratopic thianthrene tetraaldehyde (THT) and aliphaticdiamines. The bent geometry of thianthrene in THT retards the out-of-planestacking, while the flexible diamines introduce dynamic characteristicsinto the framework, facilitating nanosheet formation. Successful isoreticulationwith five diamines with two to six carbon chain lengths generalizesthe design strategy. Microscopic imaging reveals that the odd andeven diamine-based CONs transmute to different nanostructures, suchas nanotubes and hollow spheres. The single-crystal X-ray diffractionstructure of repeating units indicates that the odd-even linkerunits of diamines introduce irregular-regular curvature inthe backbone, aiding such dimensionality conversion. Theoretical calculationsshed more light on nanosheet stacking and rolling behavior with respectto the odd-even effects.

Automated summary

Few-layer organic nanosheets are gaining attention as 2D materials due to their precise atomic connectivity and tailor-made pores. Most synthesis strategies rely on surface-assisted methods or top-down exfoliation, but a bottom-up approach with well-designed building blocks is more convenient for bulk-scale synthesis of 2D nanosheets. Crystalline covalent organic framework nanosheets were successfully synthesized by reacting tetratopic thianthrene tetraaldehyde (THT) and aliphatic diamines, with the bent geometry of thianthrene and flexible diamines contributing to nanosheet formation.