Abstract
A molecular-level machine is 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 stimuli (input). Like macroscopic machines, molecular-level machines 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. The most convenient way to supply energy to an artificial molecular-level
machine is through a photochemical reaction.