Using sound to synthesize covalent organic frameworks in water

Covalent organic frameworks (COFs) are typically synthesized using solvothermal conditions (>120 C-degrees, >72 hours) in harmful organic solvents. Here we report a strategy to rapidly (<60 minutes) synthesize imine-linked COFs in aqueous acetic acid using sonochemistry and thus avoid most of the disadvantages of solvothermal methods. Using the sonochemical method, we synthesized to our knowledge previously unreported COFs. The crystallinity and porosity of these COFs is comparable to or better than those of the same materials made by established solvothermal routes. The sonochemical method even works in sustainable solvents, such as food-grade vinegar. The generality of the method is shown in the preparation of a 2D COF with pendant functionalization and of a COF with 3D connectivity. Finally, a COF synthesized sonochemically acts as an excellent photocatalyst for the sacrificial hydrogen evolution from water, showing a more sustained catalytic performance compared with that of its solvothermal analogue. The speed, ease and generality of this sonochemical method together with improved material quality makes the use of sound an enabling methodology for the rapid discovery of functional COFs.