Crystallographic observation of dynamic Gas adsorption sites and thermal expansion in a breathable fluorous metal-organic framework

Playing accordion: Cooling a single crystal of a microporous fluorous metal-organic framework under ambient atmosphere leads to very large breathing upon gas adsorption, during which multiple N₂ molecules are filled into channels and cages (see picture). While the framework exhibits remarkable positive thermal expansion under vacuum, a gigantic apparent negative thermal expansion takes place when the crystal is exposed to N₂ at ambient pressure.