Highly Crystalline, Free-Standing Covalent Organic Framework Films Produced Directly from Monomer Solutions

Films made from covalent organic frameworks (COFs) are promising materials for a variety of applications, such as membranes for chemical separation, mechanically robust engineering materials, and components of energy storage devices. However, current techniques for synthesizing imine-based COF films use methods that are difficult to scale, produce films of only nanometer-scale thickness, or yield films with modest surface areas and low crystallinity. Here, a facile, scalable process for casting free-standing, imine-based COF films tens of microns in thickness directly from a solution of monomers in trifluoroacetic acid and water is reported for the first time. Highly crystalline films with surface areas (2200–2400 m2/g) approaching the theoretical maximum could be achieved by controlling the initial film formation conditions and by heating the produced films in ethanol with a small amount of residual acid.