Spinning Both Ways

Using a flash of light, a bit of base, and principles of molecular chirality, researchers led by Ben L. Feringa of the University of Groningen, in the Netherlands, have designed a rotating molecular motor that can be instructed to spin forward or in reverse (Nature Chem., DOI: 10.1038/nchem.872).

"There are relatively few examples of artificial molecular machines in the literature where unidirectional motion has been demonstrated, let alone it being shown that it is possible to reverse the direction of motion," comments Fraser Stoddart, a chemist at Northwestern University. This "tour de force" advance could be used in "the construction of adaptive materials, wherein the function of some kind of integrated system is controlled by the chemist," Stoddart says.

The motor consists of two components: a stationary aromatic base and a rotor component. The two pieces are connected by an alkene bond that acts as the motor's axle. Incident light causes a photochemical isomerization of the rotor, which begins to spin as it thermally relaxes to a more stable state. Continuous exposure to light causes repeated isomerization, which keeps the system rotating at about one revolution per second at elevated temperatures (around 80 °C), Feringa says.

When the pH of the system is increased, base-catalyzed epimerization alters the molecule such that incident light pushes the rotor in the opposite direction. The distinct feature of this reversible motor "is that by a simple stereochemical trick—namely base-mediated inversion at a single stereocenter—the directionality can be reversed without compromising the motor function," Feringa says.

Next up, the team is adjusting the structure to make the rotation faster. Ideally, the researchers would like it to complete one revolution every millisecond. Feringa says they are also planning to work on applications—for example, by introducing a variety of functional groups that control specific properties such as catalytic ability, both in a dynamic way and in response to light.