Nature Commun. 5, 5131 (2014)

Credit: © 2014 NPG

Porosity in supramolecular assemblies has attracted a great deal of attention in recent years, in particular that of metal–organic frameworks (MOFs) and their metal-free counterparts, covalent organic frameworks (COFs). The modular nature of these extended- network materials offers great opportunities in terms of pore engineering, but they are typically difficult to process in solution. Using discrete molecules instead can help circumvent this issue but comes with other challenges: small molecules do not tend to pack into highly porous structures, or do so in a rather unpredictable manner and usually lead to fragile materials.

Ognjen Miljanić at the University of Houston and co-workers in the USA and Taiwan have now prepared a discrete fluorinated trispyrazole molecule that assembles into a non-covalent organic framework featuring large, empty channels. The material is extrinsically porous — that is, its channels arise from the way non-porous molecules are packed, rather than intrinsic porosity from the suitable arrangement of inherently porous building blocks. The discrete molecule devised by the researchers consists of a central phenyl ring from which three 'arms' branch out, each of those composed of a tetrafluorobenzene ring and a terminal pyrazole group. Hydrogen bonding between the terminal pyrazole units of three adjacent molecules promotes their packing in two-dimensional layers. Those sheets are in turn held together through interlayer π–π stacking interactions between the electron-rich pyrazole units and the comparatively electron-poor fluorinated aromatic rings. This arrangement results in a three-dimensional hexagonal network that features one-dimensional channels whose walls are lined with fluorine atoms.

The resulting material retained its structure not only in solution but also upon solvent removal, making the porous domain available for guest uptake. Owing to its fluorinated walls the framework is hydrophobic, and was shown to efficiently capture hydrocarbons and environmentally harmful fluorocarbons. In particular, a quick and reversible uptake — close to 75 weight percent — was observed for perfluorohexane.