ABSTRACT
Mechanical response of single crystals to light, temperature, and/or force-an emerging platform for the development of new organic actuating materials for soft robotics-has recently been quantitatively described by a general and robust mathematical model ( Chem. Rev . 2015 , 115 , 12440 - 12490 ). The model can be used to extract accurate activation energies and kinetics of solid-state chemical reactions simply by tracking the time-dependent bending of the crystal. Here we illustrate that deviations of the macroscopic strain in the crystal from that predicted by the model reveal the existence of additional, "hidden" chemical or physical processes, such as sustained structural relaxation between the chemical transformation and the resulting macroscopic deformation of the crystal. This is illustrated with photobendable single crystals of 4-hydroxy-2-(2-pyridinylmethylene)hydrazide, a photochemical switch that undergoes E-to-Z isomerization. The irreversible isomerization in these crystals results in amorphization and plastic deformation that are observed as poor correlation between the transformation extent and the induced strains. The occurrence of these processes was independently confirmed by X-ray diffraction and differential scanning calorimetry. An extended mathematical model is proposed to account for this complex mechanical response.
ABSTRACT
When exposed to external stimuli such as heat or light, certain single crystals can acquire momentum and undergo motion. On a molecular scale, the motility of such dynamic single crystals is triggered by a phase transition or chemical reaction without gaseous products, and macroscopically manifests as either slow (reversible or irreversible) deformation, or as rapid, almost instantaneous propulsion of the crystals that is oftentimes accompanied by disintegration. While the elastic energy of the slow reconfiguration processes such as bending, twisting and coiling can be utilized for actuation of other objects, the fast disintegrative processes could be exploited to initiate pressure-sensitive applications. This short review intends to summarize recent developments in the growing research on dynamic crystals, especially aspects of the mechanism of rapid motion of thermosalient and photosalient (leaping) crystals. The collective evidence indicates that these solids are organic-based analogues of the inorganic martensitic materials. While qualitative explanation of the molecular processes that lead to the related dynamic phenomena can be provided, quantification of their kinematics, estimation of the useful work that can be extracted, and prediction of their occurrence are yet to be established. Harnessing the potential of these materials to rapidly and efficiently perform the fundamentally important process of transduction of heat or light into kinetic energy appears as a prospective basis for their application in motion gears and devices.
ABSTRACT
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.
