The chirality of a molecule—the property of being non-superimposable on its own mirror image—can have significant effects on its optical properties. The ability to switch the chirality of a molecule from one handedness to another therefore opens up a myriad of opportunities for potential applications. Kenji Tsuchihara and colleagues1 from the National Institute of Advanced Industrial Science and Technology in Tsukuba are studying switching the chirality of molecules in thin solid films. They have achieved the rapid and reversible conversion of polymer molecules from one optical isomer to another using a solvent or heat.

Fig. 1: Structure of the optically active chiral polymer.

The researchers investigated an optically active poly(phenylacetylene) polymer (Fig. 1) having two enantiomers. The side chain of the polymer contains a hydroxyl group, which interacts with the polymer solvent resulting in the polymer chain adopting a helical structure. The helices of the two enantiomers twist in different directions.

Tsuchihara and colleagues cast thin films of their polymer onto a substrate, and show that it adopts the same helical structure in the solid state as in solution – with a preference for one twist direction.

When the film was exposed to chloroform or toluene vapor, however, the helical twist direction inverted. The speed of the inversion depended on how good the solvent was at solvating the molecules— for chloroform, the inversion was complete in one minute, whereas for toluene, a poor solvent, it took three hours. Notably, in both cases, the inversions were stable for more than three months, and contact with the vapor of a polar solvent such as methanol reversed the inversion.

Heating the films also caused inversion, and the helix remained inverted even after cooling. Only when the films were exposed to polar solvents, did the twists reverse.

Tsuchihara says that the change of chirality in the solid state is an important advance, “reversible chirality inversion of poly(acetylene)s had been usually conducted only in solution. Our simple method is easily used in films to obtain the desired chirality.”

For applications, the use of the films for “enantioselective permeation membranes” is a possibility. “Our film may have controlled enantioselectivity by selecting external conditions,” says Tsuchihara.