The bottom-up construction and operation of machines and motors of molecular size is a topic of great interest in nanoscience, and a fascinating challenge of nanotechnology. Researchers in this field are stimulated and inspired by the outstanding progress of molecular biology that has begun to reveal the secrets of the natural nanomachines which constitute the material base of life. Like their macroscopic counterparts, nanoscale machines need energy to operate. Most molecular motors of the biological world are fueled by chemical reactions, but research in the last fifteen years has demonstrated that light energy can be used to power nanomachines by exploiting photochemical processes in appropriately designed artificial systems. As a matter of fact, light excitation exhibits several advantages with regard to the operation of the machine, and can also be used to monitor its state through spectroscopic methods. In this review we will illustrate the design principles at the basis of photochemically driven molecular machines, and we will describe a few examples based on rotaxane-type structures investigated in our laboratories. <alternatives> [...] </alternatives>
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