A Locally Flexible Kinetic Valve for Temperature‐Programmed Guest Release and Hydrogen Isotope Separation in Metal–Organic Frameworks

Controlling when an adsorbed guest leaves a porous solid is as important as controlling how much it takes up, yet desorption is almost always dictated by adsorption‐site energetics and the heating profile, leaving little room to program the release temperature. Here, we show that a locally flexible pore aperture can act as a kinetic valve that converts the temperature at which a guest is loaded into the temperature at which it is later released. In a pentoxy‐functionalized isoreticular MOF (C 5 ‐IRMOF), long alkoxy side chains line the aperture to form a strongly confined, thermally responsive gate while preserving a large internal cavity. Unlike its butoxy analogue (C 4 ‐IRMOF), a conventional microporous adsorbent, C 5 ‐IRMOF traps guests until thermally activated chain motions reopen the gate, releasing them abruptly at an unusually high temperature without any strong binding sites. Cryogenic thermal desorption spectroscopy reveals that the release maximum shifts with a near‐unity slope with respect to the initial exposure temperature, defining adsorption‐temperature‐programmed desorption as a distinct mode of pore function, while the same confinement enables kinetic quantum sieving of hydrogen isotopes up to 100 K. These results establish locally flexible side chains as a simple design handle for programming guest release in porous materials.