Tuning Covalent Organic Frameworks via Nucleophilic Aromatic Substitution

A family of covalent organic frameworks (COFs) with tuned properties has been prepared through nucleophilic aromatic substitution (S_NAr) from a fluorinated imine-linked COF (COF-F). The method has proved useful for O-, S-, and N-based nucleophiles, including bifunctional molecules, leading to average degrees of substitution (DS) ranging from 34% to 87%. COF-EG, COF-Fc, and COF-Cz containing ethylene glycol (DS = 49%), ferrocene (DS = 23%), or carbazole (DS = 34%) moieties, respectively, have been prepared, showcasing the versatility of the methodology. The introduction of these functional molecules on the framework provides unique properties to the organic material, while maintaining the ordered structure. COF-EG is highly crystalline, porous, and hydrophilic, while COF-Fc shows a broad redox profile characteristic of the ferrocene/ferrocenium couple, which is covalently attached to the hexagonal framework. The optical properties of the new family of compounds are also greatly influenced by the substitution, with COF-Cz featuring a remarkably extended absorption profile beyond 700 nm and an optical bandgap of 1.9 eV, significantly reduced compared to the 2.6 eV of the starting fluorinated COF-F material and the rest of the substituted materials in the range of 2.1-2.5 eV.