Fast growth of single-crystal covalent organic frameworks for laboratory x-ray diffraction

Area of Science:

• Materials Science
• Crystallography
• Organic Chemistry

Background:

• Single-crystal growth of covalent organic frameworks (COFs) is crucial for structural determination.
• Existing imine-exchange methods are slow (15-80 days), yield small crystals (<15 microns), and require synchrotron X-ray diffraction.
• Limited success has been achieved, with only six examples reported previously.

Purpose of the Study:

• To develop a faster and more efficient method for growing large single-crystal COFs.
• To enable structural characterization using laboratory X-ray diffraction.
• To expand the scope of single-crystal COF synthesis and analysis.

Main Methods:

• A novel trifluoroacetic acid (CF₃COOH) and 2,2,2-trifluoroethylamine (CF₃CH₂NH₂) protocol was employed.
• Optimized conditions allowed for crystal growth within 1 to 2 days.
• Characterization utilized laboratory single-crystal X-ray diffraction.

Main Results:

• Successfully grew single-crystal COFs up to 150 microns in size within 1-2 days.
• Demonstrated generality by synthesizing 16 high-quality single-crystal COFs.
• Achieved high-resolution structural determination (up to 0.79 Å) using laboratory equipment.
• Observed complex structures including interpenetrated networks and detailed host-guest interactions.

Conclusions:

• The new CF₃COOH/CF₃CH₂NH₂ protocol significantly accelerates single-crystal COF growth.
• This method democratizes structural analysis of COFs by enabling laboratory-based X-ray diffraction.
• The technique facilitates atomic-level understanding of COF structures, including conformational isomerism and guest interactions.