‘Fussy’ molecules prefer one direction over the other, chiral copolymer study finds

Despite their identical composition, molecules that are mirror images can interact differently with light and electrical current depending on their “handedness,” which is called chirality. In a study published in Chemical Communications, a research team from Osaka University has produced spin-coated chiral copolymer films that display strong spin polarization, which enables the films to act as “spin filters” that behave differently toward currents with opposite polarization directions.

Chiral molecules are like left and right hands; although they have the same components, their overall structure is different, which can lead to distinct properties. Chiral molecules can interact differently with light and electrical current.

In the case of chirality-induced spin selectivity (CISS), the specific interaction of chiral molecules with an electrical current can provide spin-polarized current. This form of current is desirable in spintronics, which are cutting-edge electronic devices that use electron spin as well as charge.

Researchers from Osaka University have developed a way to synthesize polymers with chiral backbones. They developed a chiral indacenodithiophene (IDT) derivative that was used as a monomer unit in copolymers. The IDT units endowed the copolymers with chirality. The CISS of spin-coated films of the IDT copolymers was then evaluated.

“We used an atomic force microscope to measure the CISS of the IDT copolymer thin films,” says lead author Shuang Li. “The metal cantilever of the microscope was maintained in up or down polarization with a neodymium magnet, causing the copolymer films to be exposed to a polarized current.”

The films displayed different behavior—that is, CISS—depending on the polarization direction induced by the neodymium magnet. Spin polarization is used to indicate the strength of CISS and can be calculated from the currents measured under different polarization. The spin polarization of the films reached almost 70%, putting them among the best achieved to date for a chiral polymer.

“Such strong CISS will allow the IDT copolymer films to act as spin filters,” explains Fumitaka Ishiwari, senior author. “Our copolymer films are easily prepared by spin coating, which makes them attractive for practical applications in spintronics.”

Previous polymer films showing strong spin polarization have required complex preparation processes, so the simple formation of the IDT copolymer films is a clear advantage. The developed films are promising for use in the emerging field of spintronics research. It is anticipated that IDT copolymers will be useful in advanced clean energy technologies that use spin-polarized currents.