“We demonstrate for the first time that a light-driven molecular motor can actually perform work by rotating a microscale object,” said Ben Feringa of the University of Groningen. “The object is at least 10 000 times the size of each motor but to rotate it a collective action of several motor molecules is needed, which is reminiscent of the combined work of many protein motor molecules in our muscles.”

Feringa and colleagues used a molecule with a central carbon-carbon double bond that functioned as an axle. The upper part of the molecule acted as a rotor while the lower part was the stator. The team added the molecules to a liquid crystal film and illuminated them with light with a wavelength of 365 nm.

The light caused a photochemical isomerization around the double bond and changed the helicity of the molecule from right- to left-handed. A thermal step then caused the molecule to revert to its right-handed state. Two sets of a photochemical step followed by a thermal step resulted in the molecule rotating through 360°.

“The change in shape of the propeller part of the motor during the rotary process causes a change in organization of the molecules of the liquid crystal material and a rotary change in the surface profile,” said Feringa. “An object placed on top of the surface follows the change in surface profile, which leads to rotary motion. A comparison is a small boat floating on rotating waves.”

The team used the motors to move glass rods with dimensions of 5 x 28 µm. The rod rotated at an average speed of 0.67 rpm during the photochemical steps and at 0.22 rpm during the thermal steps.

“The most important [result] is that we can induce motion and perform work,” said Feringa. “This will not be directly useful as such, but a stepping stone to functional motors that can act as tiny machines or perhaps power a nanocar.”

The researchers reported their work in Nature.