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 numbe r 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) th e 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 oper ation at will, i.e., establishing a cyclic process, (iv) the time scale nee ded 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 inte rest not only for basic research, but also for the growth of nanoscience an d the development of nanotechnology. In this chapter recent examples of mol ecular-level machines based on pseudorotaxanes, rotaxanes, and catenanes, a nd operating by means of photoinduced electron-transfer processes are prese nted.