Total scattering reveals the hidden stacking disorder in a 2D covalent organic framework

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Interactions between extended π-systems drive stacking in 2D organic polymers and covalent organic frameworks (COFs).
• Layer stacking in COFs significantly impacts material properties like pore accessibility and surface states.
• Eclipsed stacking is often assumed due to high symmetry diffraction, but offset stacking is energetically favored.

Purpose of the Study:

• To investigate the nature of interlayer interactions and stacking modes in imine-linked TTI-COFs.
• To analyze the influence of synthesis temperature on COF structure and stacking.
• To explore the prevalence of stacking disorder in COFs using advanced scattering techniques.

Main Methods:

• Preparation of TTI-COF modifications via high- and low-temperature reactions.
• Local structure investigation using pair distribution function (PDF) analysis.
• Simulations of stacking disorder and analysis using total scattering methods.

Main Results:

• Lower and higher apparent symmetry modifications of TTI-COF were synthesized.
• Random local layer offsets were observed in the low-temperature COF modification.
• Stacking disorder can be overlooked due to apparent crystallographic symmetry, but total scattering reveals it.

Conclusions:

• Defective local structures, specifically random layer offsets, may be more common in COFs than previously recognized.
• Total scattering methods are crucial for accurately characterizing COF structures and uncovering subtle disorder.
• Understanding local structure and stacking disorder is key for designing highly porous, tailor-made COFs.