Transition-metal-containing rotaxanes can behave as linear motors at the mo
lecular level. The molecules are set into motion either by an electrochemic
al reaction or using a chemical signal. In a first example, a simple rotaxa
ne is described that consists of a ring threaded by a two-coordination-site
axle. The ring contains a bidentate ligand, coordinated to a copper center
. The axle incorporates both a bidentate and a terdentate ligand. By oxidiz
ing or reducing the copper center to Cu(II) or Cu(I) respectively, the ring
glides from a given position on the axle to another position and vice vers
a. By generalizing the concept to a rotaxane dimer, whose synthesis involve
s a quantitative double-threading reaction triggered by copper(I) complexat
ion, a molecular assembly reminiscent of a muscle is constructed. By exchan
ging the two metal centers of the complex (copper(I) / zinc(III)), a large-
amplitude movement is generated, which corresponds to a contraction/stretch
ing process. The copper(I)-containing rotaxane dimer is in a stretched situ
ation (overall length similar to8 nm), whereas the zinc(II) complexed compo
und is contracted (length similar to6.5 nm). The stretching/contraction pro
cess is reversible and it is hoped that, in the future, other types of sign
als can be used (electrochemical or light pulse) to trigger the motion.