Shape memory and self-healing in a molecular crystal with inverse temperature symmetry breaking

Mechanically responsive molecular crystals have attracted increasing attention for their potential as actuators, sensors, and switches. However, their inherent structural rigidity usually makes them vulnerable to external stimuli, limiting their usage in many applications. Here, we present the mechanically compliant single crystals of penciclovir, a first-line antiviral drug, achieved through an unconventional ferroelastic transformation with inverse temperature symmetry breaking. These crystals display a diverse set of self-restorative behaviors well above room temperature (385 K), including ferroelasticity, superelasticity, and shape memory effects, suggesting their promising applications in high-temperature settings. Crystallographic analysis reveals that cooperative molecular displacement within the layered crystal structure is responsible for these unique properties. Most importantly, these ferroelastic crystals manifest a polymer-like self-healing behavior even after severe cracking induced by thermal or mechanical stresses. These findings suggest the potential for similar memory and restorative effects in other molecular crystals featuring layered structures and provide valuable insights for leveraging organic molecules in the development of high-performance, ultra-flexible molecular crystalline materials with promising applications.


Table of Contents
Electronic Supplementary Material (ESI) for Chemical Science.This journal is © The Royal Society of Chemistry 2024

Figure S1 .
Figure S1.Rod-like and plate-like morphology of PCV single crystals cultivated from a mixed alcohol solution.

Figure S2 .
Figure S2.Molecular packing of the LT (A) and HT (B) forms.In both forms, the PCV molecules form an infinite one-dimensional (1D) hydrogen-bonded tape extending along the b-axis.The parallel tapes connect with other tapes via O-H…N and N-H…O hydrogen-bonding interactions, forming a corrugate 2D layer structure.The molecular layers stack further along the [101 ̅ ] and [101] direction in the LT and HT forms, respectively.

Figure S3 .
Figure S3.Molecular mechanism for inverse temperature symmetry breaking of the PCV crystals.(A) Transformation of the space group for the PCV crystals from the LT form (paraelastic phase) to the HT form (ferroelastic phase).(B) Superposition of the molecular packing structures of the LT (in blue) and HT (in red) forms viewed along the b-axis, respectively.

Figure S4 .
Figure S4.Type and distribution of hydrogen bonding interactions in (101 ̅ ) plane of the LT form.Changes along the [010] and [101]directions do not exceed 0.5% within the temperature range of 100 to 300 K.

Figure S5 .
Figure S5.Face indexing of a PCV crystal at 380 and 400 K.The major crystal face is paralleled to the (100) and (001) plane of the LT and HT forms, respectively.

Figure S6 .
Figure S6.The reversible phase transition between PCV LT form and HT form.The position of the phase boundary is indicated with white arrows.The phase boundary line swept across the crystal sample at a rate of 0.1~10.0mm/s.

Figure S7 .
Figure S7.A PLM image of a mechanical twinned PCV crystal.Under shearing stress in the [010] direction, the PCV HT form underwent twinning deformation with a constant bending angle of approximately 7°.

Figure S8 .
Figure S8.Multi-directional superelasticity (SE) of a PCV crystal based on the HT to LT phase transition.The directions in [010]HT and [100]HT in PCV crystal can be effective axes for shear-induced superelastic transition.

Figure S9 .
Figure S9.A PCV crystal fractured at room temperature under loading.(A) Initial state.(B) Cracked state.

Figure S10 .
Figure S10.Conformation of independent molecule in the LT (blue) and HT (Z1: red and Z2: purple) forms.

Figure S11 .
Figure S11.The angle between the centroid connection and the bottom edge of the independent molecule in the LT (A) and HT (B) forms.Two crystallographically independent molecules in HT form (Z1 and Z2) exhibit highly similar molecular conformations.

Figure S12 .
Figure S12.Molecular position in the LT (A) and HT (B) forms.The distances between molecules changed nonuniformly during the phase transition.

Figure S13 .
Figure S13.The morphology of an imperfect healed PCV crystal.(A) Polarized light microscopy images of an imperfect healing process.(B) A Fluorescence image of the imperfect healed PCV crystal at room temperature.The white arrow shows the region partially healed.

Figure S14 .
Figure S14.Diffraction patterns of a PCV crystal under the cracked (A) and healed (B) state.The doubled or split spots at 380 K represent the crystal is in a cracked state, clear diffraction patterns demonstrate the perfect healing of crystals after phase transition (400 K).

Figure S15 .
Figure S15.Measurement of the angle between the simulated basal surface and cleavage surface of the PCV crystals.The basal surfaces for the LT and HT forms are parallel to (100) and (001), respectively.The cleavage surfaces for the LT and HT forms are parallel to (101 ̅ ) and (101), respectively.

Figure S18 .
Figure S18.A SEM image of the cleavage surface in the PCV crystal.The HT form of PCV crystal fractured completely under mechanical stress.The cleavage surface is parallel to the (101 ̅ )LT ((101)HT) plane.

Table S1 .
Phase transition temperatures and enthalpy values of the PCV crystals under different heating-cooling cycles.

Table S2 .
Crystallographic data of the PCV polymorphs.

Table S3 .
Partial crystallographic data of the LT form in different temperatures.

Table S4 .
Unit cell parameters of a twinned PCV crystal at 400 K.