Molecules and molecular assemblies of effectively closedshell topologies have been the subject of much contemporary interest because of their ability to encapsulate smaller molecular substrates. [1, 2] The cavity interiors have been used for a number of remarkable applications, including for enantioselective recognition, [3] for stabilizing and characterizing reactive species, [4] as microreaction chambers, [5] and to demonstrate new forms of stereoisomerism. [6] The unique properties of the so-called container molecules or capsules stem directly from their ability to bind guests constrictively. [7] That is, the nearly closed surfaces of molecular containers provide comparatively large steric barriers to the ingress and egress of guests.The resulting complexes experience a corresponding enhancement in their kinetic stabilities, thus allowing encapsulation phenomena to be studied in detail. We have initiated a program aimed at tuning the thermal stabilities of materials generally derived from container molecules, one future goal of which is the synthesis of novel microporous materials with potential uses for gas storage or smallmolecule separations. Our hypothesis is that, relative to traditional clathrates and solid-state inclusion compounds, those materials constructed from container-like molecules ought to exhibit appreciable kinetic stabilities with respect to the thermal loss of encapsulated guests. This feature augurs well for gas separation or storage applications, and organic materials possessing empty voids for such purposes are emerging. [8] Collets elegant cryptophanes, [1] constructed by the covalent bridging of two cuplike C 3 -substituted cyclotribenzylenes (CTBs), are quintessential molecular containers. They typically encapsulate guests constrictively, with constrictive binding energies (that is, the activation energies of complexation, DG °c ) in excess of about 40 kJ mol À1 . [1, 9] Furthermore, they display an impressive ability to bind and discriminate between guests on the basis of size, shape, and electronic characteristics in both hydrophilic and lipophilic solvents. We report