Molecular Switches and Unidirectional Molecular Motors: Light-Induced Switching and Motion
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The fascinating principles exploited by nature to control organization, switching, and motion are a major source of inspiration for the design and synthesis of artificial molecular systems with such functions. The highly efficient retinal cis–trans photoisomerization in the process of vision is a superior example of a molecular switch, and the intriguing biomolecular motors set an extremely high standard for the development of synthetic counterparts. The bottom-up construction of switches and motors, with the realization of machines and robotics of nanosize dimensions as the ultimate goal, offers a formidable challenge to scientists. Synthetic approaches toward artificial machinery have already resulted in several molecular systems whereby switching and/or motion is controlled by means of chemical, electrochemical, photochemical, or thermal input. For instance, molecular propellers, brakes, switches, turnstiles, ratchets, and shuttles have been constructed. Catenanes and rotaxanes have shown to be particularly promising systems in the development of molecular machines. Jimenez et al., for example, reported the contraction and stretching of a linear rotaxane dimer resembling a natural muscle at work, and Chia et al. demonstrated the threading and dethreading of rotaxanes assembled on a surface. Recently, Hugel et al. reported the first single molecule machine based on photoactive azo dyes.
In this article, we do not attempt to give a complete and detailed overview of all molecular switches and motors developed thus far and the reader is referred to various reviews. Molecular switches, based on a large variety of principles, have been reviewed extensively. Molecular motors found in nature as well as synthetic counterparts are reviewed elsewhere. We will focus on systems based on sterically overcrowded alkenes, in which light is used to induce switching or motion, to illustrate several of the key principles involved.