Perpetually Self-Propelling Chiral Single Crystals

When heated, single crystals of enantiomerically
pure d- and l-pyroglutamic acid (PGA) are capable
of recurring
self-actuation due to rapid release of latent strain during a structural
phase transition, while the racemate is mechanically inactive. Contrary
to other thermosalient materials, where the effect is accompanied
by crystal explosion due to ejection of debris or splintering, the
chiral PGA crystals respond to internal strain with unprecedented
robustness and can be actuated repeatedly without deterioration. It
is demonstrated that this superelasticity is attained due to the low-dimensional
hydrogen-bonding network which effectively accrues internal strain
to elicit propulsion solely by elastic deformation without disintegration.
One of the two polymorphs (β) associated with the thermosalient
phase transition undergoes biaxial negative thermal
expansion (αa = −54.8(8)
× 10–6 K–1, αc = −3.62(8) × 10–6 K–1) and exceptionally large uniaxial thermal expansion
(αb = 303(1) × 10–6 K–1). This second example of a thermosalient solid
with anomalous expansion indicates that the thermosalient effect can
be expected for first-order phase transitions in soft crystals devoid
of an extended 3D hydrogen-bonding network that undergo strongly anisotropic
thermal expansion around the phase transition.