Photochromism and molecular mechanical devices

It is a dream for chemists to synthesize molecular systems, which can transform changes of molecular shape induced by external stimuli, such as chemicals, photons, and electrons (or holes), to macro-scale motion of the materials and perform mechanical work. To realize this dream we first prepared photoresponsive polyamides having azobenzene chromophores in the main chain. Although the polyamides changed conformation in solution upon photoinduced transcis isomerization, we failed to link the conformational change to macro-scale motion of polymer films and gels. During the Course of study on photochromism of diarylethene single crystals we found that 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 photoinduced shape changes of the molecules in the densely packed crystals give rise to mechanical motion of the crystals and launch a tiny silica-particle. This is the first molecular system, which directly transforms changes of molecular shape to macro-scale motion of the materials and performs mechanical work. This account describes the progress of the research.