Molecular-level artificial machines based on photoinduced electron-transfer processes

The concept of macroscopic machine can be extended to the molecular level. A molecular-level machine can be defined as an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to perform mechanical-like movements (output) as a consequence of appropriate external stimulation (input). Molecular-level machines operate via nuclear rearrangements and, like macroscopic machines, are characterized by (i) the kind of energy input supplied to make them work, (ii) the manner in which their operation can be monitored, (iii) the possibility to repeat the operation at will, i.e., establishing a cyclic process, (iv) the time scale needed to complete a cycle of operations, and (v) the performed function. The extension of the concept of machine to the molecular level is of great interest not only for basic research, but also for the growth of nanoscience and the development of nanotechnology. In this chapter recent examples of molecular-level machines based on pseudorotaxanes, rotaxanes, and catenanes, and operating by means of photoinduced electron-transfer processes are presented.