Artificial molecular-level machines

The concept of "machine" can be extended to the molecular level by designing supramolecular species capable of performing mechanical-like movements as a consequence of an appropriate energy supply. Molecular-level machines operate via electronic and nuclear rearrangements, for example, through some kind of chemical reaction. Like macroscopic machines, they are characterized by: (i) the kind of energy input supplied to make them work, (ii) the kind of movement performed by their components, (iii) the way in which their operation can be controlled and monitored, (iv) the possibility to repeat the operation at will and establish a cyclic process, (v) the time scale needed to complete a cycle of operation, and (vi) the function performed. A crucial issue is that concerning energy supply. Artificial machines powered by chemical energy ("fuels") produce waste products whose accumulation compromises the operation of the machine unless they are removed from the system. Photochemical and electrochemical energy inputs, however, can be used to make a machine work without formation of waste products. Examples of chemically, electrochemically, and photochemically powered machines investigated in our laboratory are reviewed, and future directions for the construction of novel machines are illustrated. The two most interesting kinds of applications of molecular-level machines are related to the mechanical aspect, which can be exploited, for example, for molecular-level transportation purposes, and the logic aspect, which can be exploited for information processing at the molecular level and, in the long run, for the construction of molecular level (chemical) computers.