Structure Evolution of 2D Covalent Organic Frameworks Unveiled by Single-Crystal X-ray Diffraction

Area of Science:

• Materials Science
• Crystallography
• Supramolecular Chemistry

Background:

• Covalent organic frameworks (COFs) are crystalline porous polymers with tunable structures.
• Understanding the structural dynamics of 2D COFs is crucial for designing functional materials.
• Previous studies often lack detailed structural information on COFs during dynamic processes.

Purpose of the Study:

• To elucidate the detailed crystal structures of a 2D COF and its derivatives.
• To investigate the structural evolution of the 2D COF during chemical reactions and solvent exchange.
• To provide insights into the relationship between molecular conformation and framework ordering.

Main Methods:

• Single-crystal X-ray diffraction (SCXRD) was employed to determine 9 crystal structures.
• High-resolution SCXRD data (up to 0.85 Å) allowed detailed analysis of structural parameters.
• In situ or ex situ analyses captured dynamic states during solvent exchange/loss and chemical derivatization.

Main Results:

• Detailed structural parameters including stacking mode, interlayer distance, pore aperture, and molecular incline angle were determined for the parent COF, 5 derivatives, and 3 dynamic states.
• The study observed stepwise conformational transformations of molecular building blocks driving structural evolution.
• Despite significant structural changes, the long-range ordering of the 2D COF was preserved.

Conclusions:

• The study provides unprecedented structural insights into the dynamic behavior of 2D COFs.
• Understanding structure evolution is key to controlling COF properties for applications.
• Preservation of long-range order during transformation highlights the robustness of the COF framework.