Covalent organic frameworks (COFs) are porous crystalline networks made from organic building blocks. Linking these building blocks via C=C bonds leads to stable structures with efficient π-electron delocalization. There are several examples of cyanovinylene (−CH═C(CN)−) or vinylene-bridged COFs. They can be synthesized, e.g., by Knoevenagel condensations. However, there still is a lack of available building blocks for the formation of other COFs of this type. COFs with vinylene-type bridges could have applications, e.g., in photocatalysis or electrochemical energy storage.

Fan Zhang, Shanghai Jiao Tong University, China, and colleagues have synthesized three vinylene-bridged 2D COFs via Knoevenagel condensations of tricyanomesitylene with aromatic di- or trialdehydes, i.e., 4,4”-diformyl-p-terphenyl (DFPTP), 4,4′-diformyl-1,1′-biphenyl (DFBP), or 1,3,5-tris(4-formylphenyl)benzene (TFPB). The reactions were performed in the presence of a secondary amine (piperidine or dimethylamine) as a catalyst in a mixture of N,N-dimethylformamide (DMF) and ortho-dichlorobenzene (o-DCB) at 180 °C. The resulting COFs with DFPTP, DFBP, or TFPB as the aldehyde component are called COF-p-3Ph, COF-p-2Ph, or COF-m-3Ph, respectively. The COFs have honeycomb-like porous structures.

The team found that the products are highly crystalline, show efficient π-conjugation, have large surface areas, and are good semiconductors. They can be used as light-harvesting systems. As an example, the researchers used COF-p-3Ph as the photosensitizer for a photocatalytic oxidative hydroxylation that converts arylboronic acids to phenols under visible light. They found that the COF promotes the reaction with high efficiency for a range of substrates. The COF can be easily recovered by filtration and reused for further reactions.

• Vinylene-Bridged Two-Dimensional Covalent Organic Frameworks via Knoevenagel Condensation of Tricyanomesitylene,
  Shuai Bi, Palani Thiruvengadam, Shice Wei, Wenbei Zhang, Fan Zhang, Lusha Gao, Junsong Xu, Dongqing Wu, Jie-Sheng Chen, Fan Zhang,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.0c04594