The surface morphology of a diarylethene single crystal [1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene] determined by atomic force microscopy changed reversibly upon photoirradiation. The crystal underwent a thermally irreversible but photochemically reversible color change (colorless to blue) upon alternate irradiation with ultraviolet (wavelength λ = 366 nm) and visible (λ > 500 nm) light that drove reversible photocyclization reactions. Upon irradiation with 366-nm light, new steps appeared on the (100) single-crystalline surface that disappeared upon irradiation with visible light (λ > 500 nm). The step height, about 1 nm, corresponds to one molecular layer. Irradiation with 366-nm light formed valleys on the (010) surface that also disappeared by bleaching upon irradiation with visible light (λ > 500 nm). The surface morphological changes can be explained by the molecular structural changes of diarylethenes regularly packed in the single crystal. These crystals could potentially be used as photodriven nanometer-scale actuators.

In the last century, molecular crystals functioned predominantly as a means for determining the molecular structures via X-ray diffraction, albeit as the century came to a close the response of molecular crystals to electric, magnetic, and light fields revealed that the physical properties of molecular crystals were as rich as the diversity of molecules themselves. In this century, the mechanical properties of molecular crystals have continued to enhance our understanding of the colligative responses of weakly bound molecules to internal frustration and applied forces. Here, the authors review the main themes of research that have developed in recent decades, prefaced by an overview of the particular considerations that distinguish molecular crystals from traditional materials such as metals and ceramics. Many molecular crystals will deform themselves as they grow under some conditions. Whether they respond to intrinsic stress or external forces or interactions among the fields of growing crystals remains an open question. Photoreactivity in single crystals has been a leading theme in organic solid-state chemistry; however, the focus of research has been traditionally on reaction stereo- and regio-specificity. However, as light-induced chemistry builds stress in crystals anisotropically, all types of motions can be actuated. The correlation between photochemistry and the responses of single crystals—jumping, twisting, fracturing, delaminating, rocking, and rolling—has become a well-defined field of research in its own right: photomechanics. The advancement of our understanding requires theoretical and high-performance computations. Computational crystallography not only supports interpretations of mechanical responses, but predicts the responses itself. This requires the engagement of classical force-field based molecular dynamics simulations, density functional theory-based approaches, and the use of machine learning to divine patterns to which algorithms can be better suited than people. The integration of mechanics with the transport of electrons and photons is considered for practical applications in flexible organic electronics and photonics. Dynamic crystals that respond rapidly and reversibly to heat and light can function as switches and actuators. Progress in identifying efficient shape-shifting crystals is also discussed. Finally, the importance of mechanical properties to milling and tableting of pharmaceuticals in an industry still dominated by active ingredients composed of small molecule crystals is reviewed. A dearth of data on the strength, hardness, Young's modulus, and fracture toughness of molecular crystals underscores the need for refinement of measurement techniques and conceptual tools. The need for benchmark data is emphasized throughout.

Photochromic reactivity of diarylethenes in the single-crystalline phase was found to be controlled by the distance between the reactive carbon atoms in the antiparallel conformation.