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Photomechanical molecular crystals that expand under illumination could potentially be used as photon‐powered actuators. In this study, we find that the use of high‐quality single crystals of 9‐methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c‐axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well‐defined linear expansions during 100 % photoconversion, suggesting that this class of solid‐state phase change material could be used for actuation. [ABSTRACT FROM AUTHOR]

The ability to exhibit life‐like oscillatory motion fueled by light represents a new capability for stimuli‐responsive materials. Although this capability has been demonstrated in soft materials like polymers, it has never been observed in molecular crystals, which are not generally regarded as dynamic objects. In this work, it is shown that molecular crystalline microwires composed of (Z)‐2‐(3‐(anthracen‐9‐yl)allylidene)malononitrile ((Z)‐DVAM) can be continuously actuated when exposed to a combination of ultraviolet and visible light. The photo‐induced motion mimics the oscillatory behavior of biological flagella and enables propagation of microwires across a surface and through liquids, with translational speeds up to 7 μm s−1. This is the first example of molecular crystals that show complex oscillatory behavior under continuous irradiation. A model that relates the rotation of the transition dipole moment between reversible E→Z photoisomerization to the microscopic torque can qualitatively reproduce how the rotational frequency depends on light intensity and polarization. [ABSTRACT FROM AUTHOR]

Keywords: kinetics; organic crystals; phase transitions; photochemistry; photomechanical actuators EN kinetics organic crystals phase transitions photochemistry photomechanical actuators 1 1 1 01/06/22 20220110 NES 220110 B Photomechanical crystals b composed of 9-methylanthracene molecules were investigated by Daichi Kitagawa, Christopher J. Bardeen, Seiya Kobatake, and co-workers in their Research Article (e202114089). Inside Back Cover: Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9-Methylanthracene Single Crystals (Angew. Kinetics, organic crystals, phase transitions, photochemistry, photomechanical actuators. [Extracted from the article]

Understanding photoreaction dynamics in crystals is important for predicting the dynamic property changes accompanying these photoreactions. In this work, we investigate the photoreaction dynamics of p‐phenylenediacrylic acid dimethyl ester (p‐PDAMe) in single crystals that show reaction front propagation, in which the photoreaction proceeds heterogeneously from the edge to the center of the crystal. Moreover, we find that p‐PDAMe single crystals exhibit a distinctive crystal shape change from a parallelogram to a distorted shape resembling a fluttering flag, then to a rectangle as the photoreaction proceeds. Density functional theory calculations predict the crystal structure after the photoreaction, providing a reasonable explanation of the distinctive crystal shape change that results from the spatially heterogeneous photoreaction. These results prove that the spatially heterogeneous photoreaction dynamics have the ability to induce novel crystal shape changes beyond what would be expected based on the equilibrium reactant and product crystal shapes. [ABSTRACT FROM AUTHOR]