High-temperature in situ crystallographic observation of reversible gas sorption in impermeable organic cages

Crystallographic observation of adsorbed gas molecules is a highly difficult task due to their rapid motion. Here, we report the in situ single-crystal and synchrotron powder X-ray observations of reversible CO₂ sorption processes in an apparently nonporous organic crystal under varying pressures at high temperatures. The host material is formed by hydrogen bond network between 1,3,5-tris-(4-carboxyphenyl)benzene (H₃ BTB) and N,N-dimethylformamide (DMF) and by π-π stacking between the H₃ BTB moieties. The material can be viewed as a well-ordered array of cages, which are tight packed with each other so that the cages are inaccessible from outside. Thus, the host is practically nonporous. Despite the absence of permanent pathways connecting the empty cages, they are permeable to CO₂ at high temperatures due to thermally activated molecular gating, and the weakly confined CO₂ molecules in the cages allow direct detection by in situ single-crystal X-ray diffraction at 323 K. Variable-temperature in situ synchrotron powder X-ray diffraction studies also show that the CO₂ sorption is reversible and driven by temperature increase. Solid-state magic angle spinning NMR defines the interactions of CO₂ with the organic framework and dynamic motion of CO₂ in cages. The reversible sorption is attributed to the dynamic motion of the DMF molecules combined with the axial motions/angular fluctuations of CO₂ (a series of transient opening/closing of compartments enabling CO₂ molecule passage), as revealed from NMR and simulations. This temperature-driven transient molecular gating can store gaseous molecules in ordered arrays toward unique collective properties and release them for ready use.