Figure 1. Schematic of the shape-memory effect in the flexible porous HOF.

(a) Structure of as-prepared compound 1o·0.33DMF at 123 K; (b) structure of compound 1o at 123 K; (c) structure of compound 1o at 393 K; and (d) structure of compound 1t at 438 K (red and blue frameworks represent the pair of sextuple interlocked frameworks with opposite handedness). To understand this cycle more easily, we can set compound 1t as the initial state. Guest incorporation induces ferroelastic transition, during which the host–guest interactions act as stress, inducing the structural strain and resulting in the transition of compound 1t to compound 1o·0.33DMF. At lower temperatures, compound 1o is energetically preferred; therefore, the strain can be preserved after the stress is removed and the framework remains unchanged upon desolvation under vacuum. However, compound 1t can be recovered under thermal treatment via a reverse ferroelastic transition (transition from a ferroelastic phase to a paraelastic phase); therefore, shape-memory behaviour is observed.