Simultaneous synthesis and establishment of a covalent organic framework

(Nanowerk News) Electrogenerated acids can catalyze the synthesis and simultaneous immobilization of imine-based covalent organic frameworks (COFs) to electrodes, reported scientists from Tokyo Tech. Starting with amine and aldehyde monomers, they obtain suitable COF films, including three-dimensional films, with high crystallinity, porosity and controlled thickness. This may facilitate COF synthesis with applications in functional electrodes and sensing materials.

Covalent organic frameworks (COFs) are versatile materials consisting of interconnected organic molecules held together by covalent bonds. These frameworks can be constructed in two-dimensional or three-dimensional (3D) forms which have a unique combination of low density, high surface area, and manageable properties. Among the various types of COFs, imine-linked COFs have received much attention due to their excellent thermal and chemical stability and wide range of monomer starting materials.

However, traditional bulk synthetic methods for COFs often produce powders that are insoluble in common organic solvents, posing challenges during printing and subsequent mounting on substrates. While alternative fabrication approaches, such as exfoliating bulk COF into nanosheets, use of novel interfaces, and use of templates for freestanding films, overcome this limitation, they usually involve several steps and produce low quality COF structures.

Recently, a research team from Japan, led by Professor Shinsuke Inagi of the Tokyo Institute of Technology (Tokyo Tech), has developed a new method to synthesize and refine high-quality imine-based COFs. Their work was published as “Hot Paper” in Journal of the International Edition of Angewandte Chemie (“Site Selective Synthesis and Simultaneous Immobilization of Imine-Based Covalent Organic Framework on Electrodes Using Electrogenerated Acids”).

“The proposed method uses electrogenerated acid (EGA), generated via the electrochemical oxidation of 1,2-diphenylhydrazine (DPH) in an organic electrolytic solution, as a catalyst for COF synthesis and immobilization onto the electrode surface. It acts as a strong Brønsted acid, driving the condensation reaction between the amine monomers and aldehydes—the building blocks of imine-based COFs—to form a network of covalent bonds,” explained Prof. Inagi.

The team chose DPH as the EGA source because of its low oxidizing potential and acid releasing properties and used 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA). Using the potential-sweep method for electrolysis, they managed to obtain film-like deposits of COF on indium tin oxide electrodes immersed in nitromethane. TAPB-PDA COF film has high crystallinity and porosity. Moreover, the thickness can be controlled by modulating the electrolysis time.

The researchers are also extending the electrochemical-based approach to the synthesis of other structures, including triazine-based and 3D COFs.

In closing, Prof. Inagi highlighted the future potential of the proposed method. “This eliminates the need for long reaction times, high temperatures and Lewis acid catalysts normally required in conventional COF synthesis, making it environmentally friendly,” he explains. “In addition, direct attachment of COF films to electrodes is promising for COF-based applications, especially in functionally modified electrodes and sensing materials.”