Permanent porosity is a property that is always associated with substances in the solid state. Engineering permanent microporosity into the liquid state to obtain porous liquids is a novel concept. By combining the properties of porous solids and fluidity of the liquid, porous liquids may show promising applications such as shape- and size-selective sorption, loading and delivering gaseous guests in a flowing system, etc. Three generic types of porous liquids were put forward. Type I porous liquids are neat liquids consisting of empty hosts. Type II porous liquids consist of empty hosts dissolved in solvents which are sterically prevented from entering the host cavity. Type II porous liquids are similar to Type III in containing solvents which are sterically prevented from entering the pores, but differ in containing particles of microporous materials rather than discrete molecular hosts. Given the technical challenges associated with synthesizing porous liquids, the successful construction of porous liquids was not reported until 2015, which do exhibit some remarkable properties, such as very fast gas diffusion and high gas solubilities. However, the porous liquids reported to date are mostly type II and type III. This is because the porous solids used to construct type II and type III porous liquids do not need to melt at room temperature. By contrast, Type I porous liquids may be the most challenging to prepare since most host species which are large enough to encapsulate other molecules have high melting points.
To tackle the issue, we aimed to lower the melting point of a porous host material using ionic liquid strategy to develop the first ever porous ionic liquid at the molecular level and to explore their properties as Type I permanently porous liquids. Self-assembled metal coordination cages, in which organic ligands coordinate to metal cations to form a molecular capsule, were chosen as scaffolds for permanently PIL. In the target PIL, the cations are coordination cages and thus contain an empty cavity. The strategy employed to synthesize permanently PIL is functionalizing a task-specific coordination cages with imidazole cations together with the selection of proper anions in order to liquefy coordination cages at room temperature (Figure 1b). Successfully, by careful condition searching, we were able to develop a porous ionic liquid, that was a bit different from our previous molecular design but was conceptually the same. Both the ionic liquid property and porosity were confirmed by experimental results (listed below). We have also explored the guest encapsulation properties of these new liquid capsules by monitoring guest uptake in neat permanently porous liquid. This porous liquid sequesters trichlorofluoromethane (CFCl3), dichlorodifluoromethane (CF2Cl2) and chlorotrifluoromethane (CF3Cl), which are ozone-depleting and climate-warming chlorofluorocarbons (CFCs) among the most abundant and longlived in the atmosphere.
