Mechanically Responsive Organic Crystals by Light

Mechanically responsive molecular crystals that exhibit macroscopic motions such as bending, twisting, and locomotion by light and heat have been studied intensively over the past decade. Photoisomerization has been applied to induce various motions, especially the bending of typical photochromic crystals, e. g., diarylethene and azobenzene. Phase transition is another mechanism underlying crystal actuation. Moreover, photothermal effect is a promising mechanism that has the potential to actuate any crystals that absorb light, including those for which actuation cannot be achieved by photoisomerization or phase transitions. Molecular crystals have an advantage over polymers and gels in terms of having a higher elastic modulus and stronger output force. However, previous studies of mechanical crystals have been limited mostly to basic research. There is a need to address the practical application of such mechanically responsive crystals in sensors, switches, actuators, and soft robots.