Tuning Crystallinity and Stacking of Two-Dimensional Covalent Organic Frameworks through Side-Chain Interactions

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Polymer Chemistry

Background:

• Two-dimensional covalent organic frameworks (2D COFs) are layered polymers with tunable properties influenced by interlayer stacking.
• Controlling the stacking arrangement of 2D COFs is critical for their emergent properties, but principles remain poorly understood.
• Many 2D COFs exhibit disordered stacking, limiting their potential applications.

Purpose of the Study:

• To investigate the impact of pendant n-alkyloxy chain length on the stacking geometry and crystallinity of 2D imine-linked COFs.
• To elucidate the fundamental principles governing supramolecular assembly in 2D COFs through systematic variation of side-chain length.
• To identify strategies for producing highly crystalline 2D COFs.

Main Methods:

• Synthesis of a series of 2D imine-linked COFs with n-alkyloxy chains ranging from C1 to C11.
• Characterization using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) to assess crystallinity and morphology.
• Molecular dynamics (MD) simulations and in situ proton NMR spectroscopy to understand self-assembly mechanisms and reaction equilibrium.

Main Results:

• A C1 chain COF showed eclipsed stacking, while longer chains (C2-C11) exhibited unidirectional slip stacking with increasing offset up to C6.
• Counterintuitively, shorter chains (C1-C4) yielded low quantities of weakly crystalline materials, whereas longer chains (C5-C11) produced high yields of highly crystalline COFs.
• MD simulations indicated favorable long alkyl chain interactions promote sheet self-assembly, corroborated by NMR insights into reaction equilibrium and crystallinity.

Conclusions:

• Pendant chain length significantly influences both the stacking geometry and crystallinity of 2D COFs.
• Longer alkyl chains can be strategically employed to enhance the self-assembly and promote the formation of highly crystalline 2D COFs.
• This study provides fundamental insights into supramolecular assembly principles for designing advanced crystalline COF materials.