Science 332, 228–231 (2011)

Creating extended porous networks from molecular building blocks is typically achieved with the help of noncovalent interactions such as hydrogen bonding or metal–ligand interactions. However, much more robust structures can be achieved if organic building blocks are held together by strong covalent bonds. Such covalent organic frameworks (COFs) have been synthesized before as microcrystalline powders or as submonolayers on single-crystal metal surfaces, but these can be difficult to process into thin films, which limits their use in applications such as optoelectronic devices. William Dichtel and colleagues at Cornell University have now synthesized two-dimensional COF films on single-layer graphene surfaces.

The researchers created a framework known as COF-5, which has a hexagonal lattice, using a solvothermal condensation reaction between PBBA (a compound composed of a benzene ring with two boronic acid groups (R–B(OH)2) attached at opposite ends) and HTTP (a flat trigonal building block composed of four fused benzene rings with two OH groups at each corner). The reaction is carried out in the presence of substrate-supported graphene and forms a layered film of COF-5 on the graphene. The layers are vertically aligned and have a crystallinity superior to the equivalent powdered samples.

The Cornell team show that the synthesis can be carried out using graphene supported on a range of substrates such as copper and transparent fused silica. Moreover, they fabricate two other COFs just as effectively with the technique, including one with a two-dimensional square lattice.