One of the defining characteristics of the crystalline state is that atoms are generally considered to be frozen in place, each with only a modest ability to vibrate about its well-defined equilibrium position. Although solid-to-solid phase transformations as a result of a variety of physical or chemical factors are well known, it is rare for individual crystals to survive such processes by retaining their mosaicity.[1] Considerable mechanical stress is thought to occur at the boundary of two interconverting phases,[1a] particularly if the two phases are incompatible in packing periodicity. Therefore, in order for a crystal to maintain its monocrystallinity during transformation, it seems reasonable either that the structural changes to the principal framework must be insignificant, or that the molecules must cooperate [1d, 2] in a concerted fashion as they undergo positional and/or topological reorganization. In this regard, a number of recent reports have advocated the phenomenon of cooperativity in order to account for the apparent fluidity of crystalline building blocks during the uptake or release of solvent molecules, with concomitant rearrangement of the host lattice as a single-crystal transformation.[1d, 2, 3] These studies have involved relatively small changes of molecular conformation in systems possessing conceptually infinite rigid assemblies.[1b–g, 2–4] Furthermore, these reports describe structural switching between only two states.

Crystal engineering [5] encompasses both the “synthesis” and modification of structures. In recent years, the principles of this burgeoning field have been applied vigorously to the design of new porous functional materials [6] with a view to mimicking and even surpassing the important properties of zeolites.[7] Targeted applications include catalysis,[8] as well as the storage,[9] separation,[10] and sensing [1a, 11] of molecules. To date, most studies have focused on so-called “soft materials”