Inorganic Carcerand: Kinetically Persistent Confinement Through Asymmetric Gating

In this study, we introduce persistent molecular confinement, a kinetically enforced encapsulation state in which guest release is suppressed by asymmetric gating. Using a Keplerate-type polyoxometalate, {Mo₁₃₂}, we demonstrate that it functions as an inorganic carcerand cage capable of confining bulky organic molecules. Despite narrow pore apertures, bulky polar organic guests can enter the inner cavity under thermal or mechanochemical activation. Systematic guest screening implies that dipole-driven attractive interactions facilitate ingress of bulky polar guests, whereas nonpolar analogues are excluded. Once encapsulated, rigid bulky guests are not released even upon exposure to organic media and remain confined under ambient conditions for weeks. This effectively irreversible retention suggests an intrinsic asymmetry between the uptake and release pathways rather than exceptionally strong binding. Interaction between the macrocyclic pore and an incoming guest may induce transient "induced-fit"-like pore adaptation, whereas the reverse process likely involves a substantially higher kinetic barrier, possibly due to steric constraints at the interior side. Kinetic analysis indicates that the irreversible uptake of rigid bulky guests arises from a large, entropically dominated large activation barrier for release, consistent with asymmetric gating behavior. These findings highlight asymmetric gating as a principle for achieving kinetically persistent molecular confinement.