Flicking the switch on a molecular gate.

T he use of external stimuli to manipulate the properties of well-defined materials may find applications in ondemand drug delivery, separation of molecules, sensing, smart coatings, or artificial tissues (1). Most of these applications rely on changes in the shape, stiffness, pore size, or other properties of soft materials in response to external pressure, temperature changes, or electric or magnetic fields. However, the lack of long-range order in polymers makes it difficult to control their porosity and, hence, permeability. On page 347 of this issue, Knebel et al. (2) instead manipulate metalorganic framework (MOF) membranes. They show that the membrane pore size can change upon exposure to an external electric field, enabling precise separation of different gas molecules. MOFs are formed from a combination of organic ligands and metal nodes or clusters. In some of these materials, host-guest interactions respond to physical and chemical stimuli, leading to “breathing” (reversible opening and closing of pores) or swelling. In other cases, linker rotation or net sliding, which are not necessarily accompanied by crystallographic phase transitions, also provide structural flexibility (3). Because of such linker and structural dynamics, sharp molecular sieving—the separation of two molecules with similar kinetic diameters based on size exclusion—has only been demonstrated in a few cases (4, 5). Zeolitic imidazolate frameworks (ZIFs), a subclass of MOFs containing imidazolate building units, are an example of soft MOF crystals in which linker rotation plays an important role. Most ZIFs can accommodate bigger molecules in their pores than one would predict based on structural data. For example, ZIF-7 has a crystallographic window size of ~0.3 nm, yet can accommodate hydrocarbons with kinetic diameters above 0.5 nm. Similarly, ZIF-8, the material studied by Knebel et al., has a crystallographic window size of 0.34 nm but allows diffusion of molecules such as n-hexane, benzene, and 1,3,5-trimethylbenzene (>0.7 nm in kinetic diameter) (6, 7). Many studies have investigated the mechanisms behind MOF flexibility, but