查看文章

Breathing is usually associated with life. However, such a reversible process can also, intriguingly, be associated with solidstate materials that have the ability to ‘breathe’when exposed to an external stimulus (eg, pressure, temperature, light, gas, or solvent adsorption), sometimes with large variations (> 5 Å) in their unit-cell parameters. This phenomenon primarily occurs with porous solids containing a hybrid organic–inorganic framework because of the interaction of guest molecules with the host structure. Furthermore, it is often observed in the subclass of coordination polymers that combine inorganic clusters with organic linkers and in structures where the inorganic moieties are 1D or 2D.[1–6] Kitagawa and co-workers recently proposed categorizing the different behaviors into six classes according to the dimensionality of the inorganic subnetwork and the types of host–guest interactions. However, most of the literature to date has focused on the structure of the initial and final stages of the breathing solid through crystal-to-crystal transformations without seeking a physical explanation for this breathing effect. This Communication focuses on providing an answer to this unresolved issue for the case of CO2 adsorption at room temperature for porous chromium (III) terephthalate MIL-53 [CrIII (OH)(OOC–C6H4–COO)], which was discovered a few years ago.[2, 7]