Retractable Cages with Continual Change of Cavity Size and High Iodine Capture

ABSTRACT Molecular cages with an adjustable cavity have great potential in selective separation, adsorption, drug releasing etc. But to construct resizable cages is highly challenging. Here, retractable cages with a continually change of cavity size are found for the first time. The as‐prepared 2 + 3 cages are highly compressed with only 2.83 Å height of cavity. However, the cage is prolonged to 12.88 Å by crystalizing from CHCl 3 /CH 3 OH and encapsulating six methanol molecules while it is transferred into half‐expanded state with 7.50 Å cavity if it is immersed in methanol and includes three methanol molecules. Interestingly, using CHCl 3 /CH 3 CN mixed solvent, compressed cages with both 2.83 and 5.29 Å cavity are obtained by rapid crystallization, whereas expanded cage with 12.12 Å height of cavity is got by slow crystallization. Furthermore, with acid treatment, the compressed cage is expanded to 12.29 Å height. Consequently, the cage's cavity could be continually and precisely adjusted between 12.88, 12.29, 12.12, 7.50, 5.29, and 2.83 Å. This cage change could be easily monitored by 1 H NMR, CPL, and visual color change etc. Due to adaptive adsorption, the scalable cage exhibits very high iodine uptake capacity of up to 6.02 g/g in iodine vapor and 3.70 g/g in water. Exceptionally, by reducing the rigid imine cage into the flexible amine cage, iodine uptake is enhanced by 37% in vapor and 91% in solution, giving an unprecedented example of organic adsorbents with high capacity from flexible cavity.