Let's do the twist! Molecular crystals of a diarylethene derivative exhibit photoreversible twisting upon irradiation with UV and visible light. The left‐handed and right‐handed helix formation is ascribed to the contraction of the crystal in a diagonal direction, depending on which face is irradiated with UV light.

Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twisting of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) crystals depends on illumination direction. The ribbon-like crystal of 1a could be successfully prepared by a sublimation method. The ribbon crystal exhibited reversible photomechanical crystal twisting upon alternating irradiation with ultraviolet (UV) and visible light. Moreover, changing the UV illumination direction with respect to the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. Control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.

The photoinduced crystal bending of a photochromic diaryl­ethene derivative, 1,2-bis­(2-methyl-5-(4-(1-naphthoyl­oxymethyl)­phenyl)-3-thienyl)­perfluoro­cyclo­pentene (1a), has been particularly investigated. The rodlike crystal of 1a shows reversible photoinduced bending upon alternating irradiation with ultraviolet (UV) and visible light. The photoinduced crystal bending can be repeated over 80 cycles. The rodlike crystal of 1a shows different bending behavior depending on the faces irradiated with UV light. This is ascribed to the molecular orientation viewed from the faces. Furthermore, we found that the bending speed depends on the crystal thickness, and the curvature change against the crystal thickness is well-fitted to Timoshenko’s bimetal model. These findings provide a new useful strategy to design for the photomechanical actuators.

Gold-coated diarylethene crystals exhibited photoreversible bending upon alternating irradiation with ultraviolet (UV) and visible light. The bending behavior can be well explained by the extended bimetal model, which we propose here. Moreover, we have demonstrated that it can be used as an actual electrical circuit photoswitch.

Molecular crystals have shown remarkable adaptability in response to a range of external stimuli. Here, we survey this emerging field and provide a critical overview of the experimental, computational and instrumental tools being used to design and apply such materials.

When photochromic molecules are organized in a crystal, the small-scale forces generated by molecular photoisomerization events can combine together to generate work on micro- or macroscopic length scales. In this work, photomechanical nanocrystals themselves are organized on macroscopic length scales using a porous inorganic template. The organic diarylethene component provides the reversible photoresponse, whereas the porous alumina component provides structural support and directionality. This hybrid organic–inorganic photomechanical material acts as a bending actuator. Using ultraviolet and visible photons as power inputs, as little as 0.1 mg of reacted material generates enough force to tilt a 1.28 g mirror and steer a laser beam. The motion can be cycled multiple times in air and under water. Actuator figures-of-merit such as energy-to-work conversion efficiency and stiffness are probably limited by the high elastic modulus of the inorganic template, providing an obvious pathway for optimization.

We herein report a unique mechanical behavior of a molecular crystal induced by combination of a photochromic reaction and a reversible single-crystal-to-single-crystal (SCSC) phase transition. A crystal of a diarylethene having octyl group at both sides (1a) was found to undergo a reversible thermodynamic SCSC phase transition accompanying a change in crystal length, which was clarified by DSC measurement, X-ray crystallographic analysis, and direct microscopic observation of the crystal length. Furthermore, upon irradiation with ultraviolet light, the diarylethene crystal exhibited an unusual photomechanical behavior. The mechanism of the behavior was proposed based on photoisomerization of the diarylethene from the open-ring isomer to the closed-ring isomer and a reversible thermodynamic SCSC phase transition, which was well supported by thermal bending behavior of a photoirradiated crystal.

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.