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.
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - 2015 Nov 17|
Bibliographical noteFunding Information:
ACKNOWLEDGMENTS. This work was supported by National Research Foundation of Korea (National Honor Scientist Program: 2010-0020414) and KISTI (KSC-2014-C3-019, KSC-2014-C3-020). We acknowledge the Pohang Accelerator Laboratory for the use of the synchrotron 2D (SMC) beamline (2014-3rd-2D-004 and 2015-1st-2D-003). Experiments
at the Pohang Light Source were supported in part by Ministry of Science, ICT and Future Planning of Korea and Pohang University of Science and Technology. We acknowledge the Korea Basic Science Institute Daegu Center for the use of 600-MHz NMR spectrometer for deuterium NMR experiments.
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