Converting Unstable Imine-Linked Network into Stable Aromatic Benzoxazole-Linked One via Post-oxidative Cyclization

Area of Science:

• Materials Science
• Polymer Chemistry
• Organic Chemistry

Background:

• Covalent organic frameworks (COFs) are crystalline porous polymers.
• Stability and crystallinity are key challenges for COF applications.
• Imine linkages in COFs are often unstable under certain conditions.

Purpose of the Study:

• To develop a method for converting unstable imine-linked COFs into stable COFs.
• To enhance the thermal and chemical stability of COFs.
• To demonstrate a universal strategy for synthesizing stable fused-aromatic COFs.

Main Methods:

• Post-oxidative cyclization of an unstable imine-linked COF.
• Characterization using transmission electron microscopy (TEM), infrared (IR) spectroscopy, solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (PXRD), and nitrogen adsorption-desorption isotherms.
• Application of chemistry used for fused-aromatic ladder-like rigid-rod polymers.

Main Results:

• Successful conversion of an unstable imine-linked COF to a stable benzoxazole-linked COF (BO-COF).
• The resulting BO-COF exhibited significantly improved thermal and chemical stabilities.
• High crystallinity and porosity were maintained and confirmed by multiple characterization techniques.
• The strategy proved effective in creating a stable, irreversible COF structure.

Conclusions:

• Post-oxidative cyclization is an effective strategy for enhancing COF stability.
• The developed method provides a universal route to stable fused-aromatic COFs.
• This approach expands the potential practical applications of COFs in various fields.