Covalent organic frameworks (COFs) are porous crystalline networks of organic compounds. Similar to metal-organic frameworks (MOFs), they have potential for applications such as gas storage, catalysis, and optoelectronics. Commonly used techniques for linking the framework’s building blocks are the condensation of boronic acids or the trimerization of nitriles. However, there have been few examples of thermally and chemically stable COFs until now.

Xiaoming Liu, Jilin University, Changchun, China, and colleagues have developed a stable, crystalline, porous and luminescent COF which can be used to detect copper ions. The team used azine groups to link the COFs aromatic building blocks, supported by hydrogen bonds between the azine’s nitrogen atoms and neighboring OH-groups. The framework, named COF-JLU3, was synthesized under solvothermal conditions by co-condensation of 1,3,5-tris(3′-tert-butyl-4′-hydroxy-5′-formylphenyl)benzene and hydrazine hydrate in the presence of acetic acid as a catalyst.

The product was characterized using infrared (IR), UV-Vis, and ¹³C NMR spectroscopy, thermogravimetric analysis, and nitrogen sorption. COF-JLU3 exhibited a strong orange-red luminescence. The researchers tested the material’s capability for metal detection by immersing it in solutions of a variety of metal nitrates (Li, Na, K, Mg, Ca, Ba, Zn, Cd, Ni, Pb, Co, Cu, Ag, Fe, and Al), and found that the COFs red emission under UV light depended heavily on the metal. The transition metals, especially copper, quenched the emission. Thus, the framework could have applications in ion-sensing.

• A robust and luminescent covalent organic framework as a highly sensitive and selective sensor for detection of Cu²⁺ ion,
  Xiaoming Liu, Zhongping Li, Yuwei Zhang, Hong Xia, Ying Mu,
  Chem. Commun. 2016.
  DOI: 10.1039/c6cc01476c