Synthesis of Single-Crystal Two-Dimensional Covalent Organic Frameworks and Uncovering Their Hidden Structural...

Area of Science:

• Materials Science
• Nanotechnology
• Crystallography

Background:

• Two-dimensional covalent organic frameworks (2D COFs) are 2D polymers with potential applications in various fields.
• Synthesizing single-crystal 2D COFs is crucial for understanding their properties but remains a significant challenge.
• Previous studies often resulted in polycrystalline materials, limiting structural characterization and property elucidation.

Purpose of the Study:

• To develop a reproducible method for synthesizing single-crystal 2D COFs.
• To identify and control key synthetic parameters influencing 2D COF crystallization.
• To characterize the detailed structure of single-crystal 2D COFs using advanced techniques.

Main Methods:

• Systematic variation of synthetic conditions: glassware, degassing, solvent, temperature, modulator, and reaction time.
• Focus on imine-linked TPB-DMTP-COF as a representative model system.
• Characterization of single-crystal structures using three-dimensional electron diffraction (3DED).

Main Results:

• Optimized conditions (homogeneous system, suitable modulator, 50-70 °C) yielded single-crystal TPB-DMTP-COF as isolated rods (200 nm–3 μm diameter, 1–20 μm length).
• 3DED revealed two linker conformations (trans and cis) within the 2D polymer sheets.
• Antiparallel stacking of these sheets resulted in frameworks with double-sized unit cells, uncovering overlooked structural details.

Conclusions:

• Key synthetic parameters significantly control the polymerization and crystallization of 2D COFs, enabling high-quality single crystal growth.
• The study provides unprecedented insights into the intricate structural features of 2D COFs, including linker conformations and stacking modes.
• This work lays the foundation for the rational design and synthesis of high-quality single-crystal 2D COFs for advanced applications.