Bimodal versus Unimodal Pore Architectures in Diimine-Linked Two-Dimensional Covalent Organic Frameworks

Area of Science:

• Materials Science
• Nanotechnology
• Chemistry

Background:

• Two-dimensional covalent organic frameworks (2D COFs) are typically viewed as structurally simple.
• Precise structure determination of 2D COFs via X-ray diffraction is hindered by challenges in obtaining large single crystals.

Purpose of the Study:

• To investigate how subtle structural modifications, specifically a single-atom change in aldehyde substituents, affect the pore architecture and lattice symmetry of 2D COFs.
• To re-evaluate existing structural models of 2D COFs using advanced characterization techniques.

Main Methods:

• Comparison of a widely studied 2D COF (COF-OMe) with its analogue (COF-SMe) featuring a single-atom substituent difference.
• Advanced imaging and diffraction techniques, including electron ptychography.
• Finely sampled gas/vapor physisorption to analyze pore structure.
• Simulations of electrostatic potential and X-ray diffraction patterns.

Main Results:

• A single-atom change switched the pore architecture from bimodal (COF-OMe) to unimodal (COF-SMe).
• Electron ptychography refined the COF-OMe model, revealing propeller-like nodes with large dihedral angles.
• COF-SMe exhibited a symmetry reduction from hexagonal to triclinic during phase evolution, driven by interlayer slippage and intralayer distortion.
• Gas/vapor physisorption confirmed distinct capillary processes in COF-OMe, indicative of its bimodal porosity.

Conclusions:

• Substituent variations in 2D COFs can lead to significant and unexpected structural diversity and flexibility.
• Advanced characterization methods are crucial for accurately determining the complex structures of 2D COFs.
• Existing structural models for some 2D COFs may require reassessment based on these findings.