Jean-Pierre Sauvage and his team at Strasburg University, France, have developed a copper-based molecular shuttle whose motion between stopping stations, triggered via an electrochemical stimulus, is solely governed by steric factors. 

The driving force for the motions of molecular machines developed in Sauvage's group is the preference of copper complexes for certain geometries, such as four-coordinate for copper(I) and five-coordinate for copper(II). However, they also observed that the steric hindrance of the coordinating sites had a critical influence on the gliding kinetics and used this effect to build a molecular shuttle with two bidentate ligands on its thread. The two ligands only differ by their steric environment, one being less hindering than the other, while their electronic characteristics are kept very similar. 'We could not have predicted that a motion would occur only because of the difference of steric hindrance between the two coordination sites,' says Sauvage. 'The shuttling motion obtained in the present work is also fast (milliseconds to seconds), which is important for future applications of related molecular machines,' he adds. 

Coordination and sterics are now two parameters in hand to design more elaborate dynamic molecules. However, Sauvage notes that molecular machines in solution may not be very useful for future applications. The challenge is to organise these molecules on surfaces and show macroscopic effects arising from their nanoscopic movements and another goal will be the transport of matter over large distances, he says. These molecular machines could be developed so as to carry molecules in a way reminiscent of biological systems, such as the kinesin motor protein 'walking' on a microtubule and driving a 'cargo', dispatching the products of biosynthesis in the cell. 

Marie Cote