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Photomechanical materials exhibit mechanical motion in response to light as an external stimulus. They have attracted much attention because they can convert light energy directly to mechanical energy, and their motions can be controlled without any physical contact. This review paper introduces the photomechanical motions of photoresponsive molecular crystals, especially bending and twisting behaviors, from the viewpoint of symmetry breaking. The bending (right–left symmetry breaking) and twisting (chiral symmetry breaking) of photomechanical crystals are based on both intrinsic and extrinsic factors like molecular orientation in the crystal and illumination conditions. The ability to design and control this symmetry breaking will be vital for generating new science and new technological applications for organic crystalline materials.
Kitagawa et al. (2020) studied this question.