A scientist from the Faculty of Pure and Applied Sciences at the University of Tsukuba developed a method for producing elec­trically conductive polymers that assume a helical confi­guration. By using a liquid crystal as a template, he was able to produce optically active polymers that can convert light into a circular polari­zation. This approach may help lower the cost of smart displays.

The sizes of TVs have signi­ficantly expanded in recent years, while the prices have dropped. This is mainly due to the adoption of organic light emitting devices (OLEDs), which are carbon-based polymers that can glow at tunable optical wavelengths. These conjugated polymers, which have alternating single and double bonds, are both elec­trically conductive, and have colors that can be controlled by chemical doping with other molecules. Their oxidation state can also be rapidly switched using an electric voltage, which affects their colora­tion. However, future advance­ment may require new materials that can take advantage of other kinds of optical properties, such as circular polari­zation.

Now, the researcher has introduced a technique for creating polymers locked into a helical confi­guration, using a sacri­ficial liquid crystal template. “Polymers that both have optical activity and luminescent function can emit circularly polarized light,” Hiromasa Goto says. For this process, the liquid crystal molecules were originally in a straight confi­guration. The addition of monomer molecules caused the liquid crystals to twist into a helical configuration. This imprints a chirality to the structure, making it oriented either clockwise or counter­clockwise. An electric voltage was applied, which triggered polymeri­zation of the monomers.

The liquid crystal template was then removed, leaving a polymer frozen in a helical shape. By breaking the mirror symmetry, the polymer has the ability to convert linearly polarized light into a circular polari­zation. The furan rings in the polymer not only contribute to the electrical conductivity, they also help stabilize the helical structure. “The pi-stacking inter­actions between the rings allows the polymer to aggregate into a highly ordered chiral system,” Goto says. The resulting polymer was tested using circular dichroism absorption spectro­scopy and was found to have strong optical activity at visible wavelengths. Future appli­cations of this process may include cheaper and more energy efficient elec­tronic displays. (Source: U. Tsukuba)

Reference: H. Goto: Reaction field induction self-amplification optical activity during polymerization in liquid crystal, Mol. Crys. Liq. Crys., online 24 May 2022; DOI: 10.1080/15421406.2022.2073421

Link: Dept. of Material Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan