Mechanically Responsive Crystals: Analysis of Macroscopic Strain Reveals “Hidden” Processes

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. Rev20151151244012490). 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.

Using light intensity to control reaction kinetics and reversibility in photomechanical crystals

“…This discrepancy between the reaction and mechanical response times has been observed before in other photomechanical crystal systems 32,34,54 and probably reects the nonlinear relation between population and mechanical action. Finally, complete recovery to q(0) could be prevented by the presence of a weak 405 nm hold beam, as illustrated in Fig.…”

“…Quantitative models for photomechanical bending have typically assumed standard rst-order chemical kinetics, 2,22,32,33 although it has been noted that such models cannot explain all experimental observations. 34 In order to examine the connection between complex kinetics and photomechanical behavior more closely, we chose 4-uoro-9-anthracenecarboxylic acid (4F-9AC) as a model photomechanical crystal system. In the crystal, this molecule arranges into parallel head-to-head stacks in which the anthracene cores are well-positioned to undergo a [4 + 4] photodimerization reaction.…”

Organic acids under pressure: elastic properties, negative mechanical phenomena and pressure induced phase transitions in the lactic, maleic, succinic and citric acids

“…35,[119][120][121][122][123][124][125][126][127][128][129][130][131][132][133] Mechanically responsible organic crystals showing macroscopic motion as a response to pressure or deformation are very interesting due to their potential applications in nanotechnology as organic actuators. 98,[130][131][132][133] The mechanical response of molecular crystals, polymers and organic materials related to living organism physiology is of direct and enormous interest for the human being. The interest in the research of the effect of high pressure on living systems was originated by the discovery of piezophile organisms in the deep marine environments.…”

Multifunctional Deformable Organic Semiconductor Single Crystals

“…The light weight, determined by their chemical composition, as well as the mechanical robustness and compliance, rooted in their non‐covalent interactions, stand among the most important aspects of the performance of organic crystals. Along this line of pursuit, their mechanical attributes, which are critically important for durability and robustness of the ensuing devices, are becoming a popular subject of exploration within the emerging field of crystal adaptronics [16–30] . The prospects of using dynamic crystals have already been demonstrated with lab‐scale applications that include prototypical fuses, [31] transistors, [32] and switches [33] .…”