Tuning Covalent Organic Frameworks via Nucleophilic Aromatic Substitution.

Area of Science:

• Materials Science
• Organic Chemistry
• Nanotechnology

Background:

• Covalent organic frameworks (COFs) are crystalline porous polymers with tunable structures.
• Functionalizing COFs is crucial for tailoring their properties for specific applications.
• Existing methods for COF modification can be limited in scope and efficiency.

Purpose of the Study:

• To develop a versatile post-synthetic modification strategy for imine-linked COFs.
• To introduce diverse functional groups (O-, S-, N-based) onto a fluorinated COF (COF-F) via nucleophilic aromatic substitution (S_NAr).
• To investigate the impact of functionalization on the structural, electronic, and optical properties of the resulting COFs.

Main Methods:

• Preparation of a fluorinated imine-linked COF (COF-F).
• Post-synthetic modification of COF-F using various nucleophiles (alcohols, thiols, amines, bifunctional molecules) through S_NAr reactions.
• Characterization of the modified COFs (COF-EG, COF-Fc, COF-Cz) using techniques to assess crystallinity, porosity, redox behavior, and optical properties.

Main Results:

• Successful functionalization of COF-F with O-, S-, and N-based nucleophiles, achieving degrees of substitution (DS) from 34% to 87%.
• Synthesized COF-EG (49% DS), COF-Fc (23% DS), and COF-Cz (34% DS), demonstrating the method's versatility.
• COF-EG exhibited high crystallinity, porosity, and hydrophilicity.
• COF-Fc displayed a characteristic redox profile due to covalently attached ferrocene.
• COF-Cz showed significantly extended optical absorption (>700 nm) and a reduced optical bandgap (1.9 eV) compared to COF-F (2.6 eV).

Conclusions:

• Nucleophilic aromatic substitution (S_NAr) is an effective and versatile strategy for post-synthetic modification of imine-linked COFs.
• Functionalization allows for precise tuning of COF properties, including hydrophilicity, redox activity, and optical characteristics.
• The developed methodology enables the creation of novel COF materials with tailored functionalities for advanced applications.