Nonclassical Crystallization Processes of Single-Crystalline Two-Dimensional Covalent Organic Frameworks

Area of Science:

• Materials Science
• Polymer Chemistry
• Crystallography

Background:

• Controlling two-dimensional covalent organic framework (2D COF) polymerization is crucial for their properties.
• Most 2D COFs are polycrystalline with nanometer domains, limiting applications.
• Understanding factors influencing single-crystal 2D COF formation is needed for controlled synthesis.

Purpose of the Study:

• To investigate the crystallization processes of single-crystalline and polycrystalline 2D COFs.
• To elucidate the guiding principles for controlled two-dimensional polymerization in solution.
• To identify mechanisms leading to micrometer-scale single-crystal 2D COF formation.

Main Methods:

• Utilized *in situ* ultrasmall-angle X-ray scattering (USAXS) to monitor COF formation dynamics.
• Collected scattering data every few seconds to characterize rapid polymerization processes.
• Corroborated findings with *in situ* wide-angle X-ray scattering (WAXS) and scanning electron microscopy (SEM).

Main Results:

• Identified distinct growth mechanisms for single-crystalline and polycrystalline 2D COFs.
• Proposed a nonclassical particle fusion-based growth model for single-crystalline 2D COFs, yielding hexagonal particles.
• Observed polycrystalline COFs forming as spherical aggregates without similar fusion.

Conclusions:

• The study reveals insights into the formation of micrometer-sized crystalline 2D polymers in solution.
• Findings provide a foundation for controlling 2D polymer structure and properties via polymerization.
• Understanding growth mechanisms is key to advancing 2D COF synthesis and applications.