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

Abstract

Few-layer organic nanosheets are becoming increasingly attractive as two-dimensional (2D) materials due to their precise atomic connectivity and tailor-made pores. However, most strategies for synthesizing nanosheets rely on surface-assisted methods or top-down exfoliation of stacked materials. A bottomup approach with well-designed building blocks would be the convenient pathway to achieve the bulk-scale synthesis of 2D nanosheets with uniform size and crystallinity. Herein, we have synthesized crystalline covalent organic framework nanosheets (CONs) by reacting tetratopic thianthrene tetraaldehyde (THT) and aliphatic diamines. The bent geometry of thianthrene in THT retards the out-ofplane stacking, while the flexible diamines introduce dynamic characteristics into the framework, facilitating nanosheet formation. Successful isoreticulation with five diamines with two to six carbon chain lengths generalizes the design strategy. Microscopic imaging reveals that the odd and even diamine-based CONs transmute to different nanostructures, such as nanotubes and hollow spheres. The single-crystal X-ray diffraction structure of repeating units indicates that the odd−even linker units of diamines introduce irregular−regular curvature in the backbone, aiding such dimensionality conversion. Theoretical calculations shed more light on nanosheet stacking and rolling behavior with respect to the odd−even effects.

Single-Crystal One-Dimensional Porous Ladder Covalent Polymers

“…Dynamic covalent chemistry has attracted significant attention in the solution synthesis of crystalline organic materials and has achieved great success in preparing 2-dimensional (2D) and 3-dimensional (3D) covalent organic frameworks (COFs) − Under the condition of dynamic covalent chemistry, the covalent bonds between building blocks can reversibly form, break, and reform (i.e., self-repair), for which the structural error can be corrected through thermodynamic control. For comparison, the preparation of crystalline 1D polymers, especially single-crystal 1D polymers, has rarely been reported.…”

Alkene-Bridged Ionic Covalent Organic Nanosheets (iCONs) Based on D-π-A for Photocatalytic Hydrogen Evolution

“…Covalent organic nanosheets (CONs) are porous materials with excellent stability that are connected by strong covalent bonds and symmetrically arranged along zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) directions. − However, the recollapse between layers may result in the incomplete exfoliation of CONs. Ionic covalent organic nanosheets (iCONs), which can self-peel into ultrathin nanosheets, are prepared using ionic building blocks .…”

Improving the Bioactivity and Stability of Embedded Enzymes by Covalent Organic Frameworks

“…Immobilizing enzymes on solid supports is an effective strategy to enhance their stability, reusability, and easy operation, which are desired for the development of enzyme catalysts. − Due to the advantages of reticular chemistry materials (such as high porosity, stability, and tunability), metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have recently been regarded as excellent supports for enzyme immobilization. − To date, a series of MOFs, especially ZIFs, have been widely explored for de novo immobilizing enzymes; however, the bioactivity of immobilized enzymes is sometimes far from satisfactory, which is caused by the diffusion barriers of ZIFs (pore size of the commonly used ZIF-8: 0.34 nm) and the unavoidable activity loss during the immobilizing process, such as harmful metal ions. − Meanwhile, a lot of research has successfully enhanced the activity of the immobilized enzyme, such as modulating the microenvironment, shortening diffusion paths, and enabling biocatalytic cascades. − However, there is still an essential problem that has not been solved in enzyme@MOF biocomposites, which is the unavoidable influence of metal ions on enzymatic activity. Some reports have proved that metal ions can coordinate with enzymes, which may perturb their conformation or cover their active sites, leading to unpredictable changes in activity and even the complete inactivation of enzymes. − …”