Angew. Chem. Int. Ed. http://doi.org/fz5gq7 (2012)

A key design feature of both macroscopic and nanoscopic motors is energy efficiency. On the nanoscale, molecular motors can convert light, chemical or electrical energy into directed motion through the excitation of electronic states in the molecule. However, if the motors could operate only on the potential energy surface of the electronic ground state they would be more energy efficient. Jin Zhang, Anastassia Alexandrova and colleagues from the University of California Los Angeles and Utah State University have now shown that boron clusters composed of 13 atoms could be used as such a device.

The planar B13+ clusters are composed of an inner triangular ring of atoms surrounded by an outer ring of ten atoms. Previously it was shown that the outer ring of atoms can rotate with respect to the inner ring due to delocalized bonding, but such thermally driven motion is bidirectional. With the aid of theoretical calculations, Zhang, Alexandrova and colleagues demonstrate that unidirectional rotation can be achieved by using a circularly polarized infrared laser as an energy source. In particular, the researchers examine the energy profile of two elementary rotational moves of the outer ring — one clockwise, the other counterclockwise — and find that without an electric field both directions are equally possible. However, when a constant electric field is applied, the counterclockwise rotation is strongly favoured: the transition-state barrier for counterclockwise rotation almost completely disappears, whereas the barrier for clockwise rotation increases significantly.

With circularly polarized infrared radiation near 3 THz, the rotating frequency of the outer ring would be approximately 250 GHz and the US team suggest that the electric field strength used in their simulations could be achieved experimentally with ultraintense lasers.