Light and chemically driven molecular machines showing a unidirectional four-state switching cycle.

The imitation of macroscopic movements at the molecular level is a key step in the development of nanomachines. The challenge is the synthesis of molecules that are able to transform external stimuli into a direction-controlled mechanical movement. The more complex such motion sequences are, the more difficult is the construction of the corresponding nanomachine. Here, we present a system that demonstrates a unidirectional, four-state switching cycle that bears similar characteristics to the arm movements of a human breaststroke swimmer. Like the latter, the molecules have a torso and two arms. The arms consist of bipyridine units and can be folded and stretched by addition and removal of copper ions. The unidirectional rotation of the arms is achieved by light-induced switching of an azo unit.