Photoactive pseudorotaxanes and rotaxanes as artificial molecular machines

A molecular machine is an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to perform mechanical-like movements as a consequence of appropriate external stimuli. The most convenient way to supply energy to an artificial molecular-level machine is through a photochemical reaction, that is, by using light. The two main types of photochemical reactions that can be employed to drive molecular machines are photoisomerization and photoinduced electron-transfer processes. By means of spectroscopic techniques, light can also be used to monitor the operation of the system. Two prototypes of photochemically driven molecular machines are described, namely, (i) a pseudorotaxane undergoing threading/dethreading motions, and (ii) a molecular shuttle based on a rotaxane. The extension of the concept of machine to the molecular-level is important not only for basic research, but also for the growth of nanoscience and the development of nanotechnology. © 2003 Elsevier Science B.V. All rights reserved.