Unidirectional rotation in a mechanically interlocked molecular rotor

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Abstract
Molecular motor proteins are ubiquitous in nature1 and have inspired attempts to create artificial machines2 that mimic their ability to produce controlled motion on the molecular level. A recent example of an artificial molecular rotor is a molecule undergoing a unidirectional 120° intramolecular rotation around a single bond3,4; another is a molecule capable of repetitive unimolecular rotation driven by multiple and successive isomerization of its central double bond5,6,7,8. Here we show that sequential and unidirectional rotation can also be induced in mechanically interlocked assemblies comprised of one or two small rings moving around one larger ring. The small rings in these [2]- and [3]catenanes9 move in discrete steps between different binding sites located on the larger ring, with the movement driven by light, heat or chemical stimuli that change the relative affinity of the small rings for the different binding sites10,11,12. We find that the small ring in the [2]catenane moves with high positional integrity but without control over its direction of motion, while the two rings in the [3]catenane mutually block each other's movement to ensure an overall stimuli-induced unidirectional motion around the larger ring.