Light-Driven Molecular Crystal Actuators: An Approach to Molecular Machinery

It is a dream for chemists to construct molecular systems, which can transform shape changes of molecules induced by external stimuli, such as chemicals, photons, or electrons (or holes), to macro-scale motion of materials and perform mechanical work. Although elaborate molecular systems, such as molecular shuttles, molecular muscles, molecular elevators, and molecular rotors, have been extensively studied, attempts to link the molecular-scale motion of these man-made devices to macroscopic mechanical work have failed. Any certain methodology how to rationally design the molecular systems to perform macroscopic mechanical work has not yet been developed. In this article we propose to use molecular crystals of diarylethene derivatives for transforming photostimulated shape changes of individual molecules to macro-scale motion of materials. The surface morphology as well as the bulk shape of the single crystals reversibly changes upon photoisomerization of the component diarylethene molecules in the crystals. The crystal shape changes are ascribed to the anisotropic deformation of the crystal lattice induced by the geometrical structure changes of diarylethene molecules upon photoisomerization. The photoinduced crystal deformation performs macroscopic mechanical works, such as launching a silica micro-particle, lifting up a heavy metal ball, and rotating a tiny gearwheel. These results suggest that fabrication of single crystalline assemblies is a promising approach to integrate stimuli-responsive molecules into macroscopic materials that perform significant mechanical work.