Shape-Memory and Self-Healing Effects in Mechano­salient Molecular Crystals

The
thermo­salient crystals of terephthalic acid are extraordinarily
mechanically compliant and reversibly shape-shift between two forms
with different crystal habits. While the transition of form II to
form I is spontaneous, the transition of form I to form II is latent
and can be triggered by applying local mechanical stress, whereby
crystals leap several centimeters in air. This mechano­salient
effect (mechanically stimulated motility) is due to sudden release
of strain that has accrued in the crystal of form I, which is a metastable
structure at ambient conditions. High-speed optical analysis and serial
scanning electron microscopy reveal that the mechanical effect is
due to rapid reshaping of crystal domains on a millisecond time scale
triggered by mechanical stimulation. Mechanically pre-deformed crystals
taken over the thermal phase transition exhibit memory effects and
partially regain their shape, while cracked, sliced, or otherwise
damaged crystals tend to recover their macroscopic integrity by restorative
action of intermolecular π–π interactions in a
manner which resembles the behavior of shape-memory and self-healing
polymers. These observations provide additional evidence that the
thermo-/photo-/mechano­salient effects are macroscopic manifestations
of martensitic-type transitions in molecular solids.