Bottom-Up Synthesis of Covalent Organic Frameworks with Quasi-Three-Dimensional Integrated Architecture via Interlayer Cross-Linking

Developing strategies to enhance the structural robustness
of covalent
organic frameworks (COFs) is of great importance. Here, we rationally
design and synthesize a class of cross-linked COFs (CCOFs),
in which the two-dimensional (2D) COF layers are anchored and connected
by polyethylene glycol (PEG) or alkyl chains through covalent bonds.
The bottom-up fabrication of these CCOFs is achieved by
the condensation of cross-linked aldehyde monomers and tritopic amino
monomers. All the synthesized CCOFs possess high crystallinity
and porosity, and enhanced structural robustness surpassing the typical
2D COFs, which means that they cannot be exfoliated under ultrasonication
and grinding due to the cross-linking effect. Furthermore, the cross-linked
patterns of PEG units are uncovered by experimental results and Monte
Carlo molecular dynamics simulations. It is found that all CCOFs are dominated by vertical cross-layer (interlayer) connections
(clearly observed in high-resolution transmission electron microscopy
images), allowing them to form quasi-three-dimensional (quasi-3D)
structures. This work bridges the gap between 2D COFs and 3D COFs
and provides an efficient way to improve the interlayered stability
of COFs.