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 N(2) 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 N(2) at ambient pressure.