Molecular motors are common in nature — for example the proteins kinesin and myosin, which perform transport functions in cells — and are very efficient at converting chemical to kinetic energy. Light-driven molecular motors have proved to be the most effective of the artificial systems, with potential applications as functional components in nanoscale devices. One particularly promising set of such molecules comprise a 'rotor' and 'stator' connected through a carbon–carbon double bond 'axle'.

Although unidirectional rotation has been achieved in these systems previously, the maximum rate was on the order of 100 Hz. Now, Ben Feringa and colleagues from Groningen and Dublin Universities have optimized1 the structure of the molecular motors such that they can reach megahertz frequencies in unidirectional rotation. The motors work by using light to drive two cis–trans isomerization steps around the C=C bond, each of which are followed by thermal inversion of the helical structure to give the rotational motion. The photochemical isomerization takes only picoseconds, so the thermal helical inversion is the rate-limiting step.

In previous systems, each end of the C=C bond is attached to a six-membered ring, but now, Feringa and co-workers have replaced one of them with a five-membered cyclopentane ring that is less sterically demanding. As a consequence, the energy barrier for the helical inversion step is reduced and the rate of rotation increases, with speeds of up to 3 MHz obtained.