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Precise measurement of bending kinematics induced by a photochemical reaction in a single crystal can be used to extract the kinetic parameters of the underlying reaction with high accuracy.

The capability of cellulose microfibrils to elicit directionality by anisotropically restricting the deformation of amorphous biogenic matrices is central to the motility of many plants as motoric and shape‐restoring elements. Herein, an approach is described to control directionality of artificial composite actuators that mimic the hygroinduced motion of composite plant tissues such as the opening of seed pods, winding of plant tendrils, and burial of seed awns. The actuators are designed as bilayer structures where single or double networks of buried parallel glass fibers reinforce the composite. By anisotropically restricting the expansion along certain directions they also effectively direct the mechanical reconfiguration, thereby determining the mechanical effect. A mathematical model is developed to quantify the kinematic response of fiber‐reinforced actuators. Within a broader context, the results of this study provide means for control over mechanical deformation of artificial dynamic elements that mimic the oriented fibrous architectures in biogenic motoric elements.

The effect of thermal strain on the rate of photoisomerization is qualitatively different from the effect of strain generated by elastic loading.

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.

For martensitic transformations the macroscopic crystal strain is directly related to the corresponding structural rearrangement at the microscopic level. In situ optical microscopy observations of the interface migration and the change in crystal shape during a displacive single crystal to single crystal transformation can contribute significantly to understanding the mechanism of the process at the atomic scale. This is illustrated for the dehydration of samarium oxalate decahydrate in a study combining optical microscopy and single-crystal X-ray diffraction.

Exposure of a photoreactive single crystal to light with a wavelength offset from its absorption maximum can have two distinct effects. The first is the “direct” effect, wherein the excited state generated in individual chemical species is influenced. The second is the “indirect” effect, which describes the penetration of light into the crystal and hence the spatial propagation and completeness of transformation. We illustrate using the nitro–nitrito isomerization of [Co­(NH3)5NO2]­Cl­(NO3) as an example that the direct and indirect effects can be independently determined. This is achieved by comparing the dynamics of macroscopic crystal deformation (bending curvature and crystal elongation) induced by the photochemical reaction when irradiating a crystal at the absorption maximum and at different band edges (above or below the maximum) of the same band. Quantitative description of the macroscopic strain dynamics in comparison with experiments allowed us to suggest that irradiation at different tails of the same absorption band causes isomerization to proceed via different excited states and an additional photochemical reaction (presumably, reverse nitrito–nitro isomerization) can occur on irradiation at the ligand-field band edges.

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