Creation of optically active polymers using a helical liquid crystal model

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June 17, 2022

(News from Nanowerk) A scientist from the Faculty of Pure and Applied Sciences of the University of Tsukuba has developed a method of producing electrically conductive polymers that adopt a helical configuration. Using a liquid crystal as a model, he was able to produce optically active polymers capable of converting light into circular polarization (Molecular crystals and liquid crystals, “Optical activity of self-amplification by reaction field induction during polymerization in a liquid crystal”). This approach can help reduce the cost of smart displays.

A researcher at the University of Tsukuba is developing a method for creating optically active polymers using a helical liquid-crystal model, which could allow future computer and television screens to operate on the basis of a circularly polarized light. (Photo: University of Tsukuba)

Walking into an electronics store these days can be an overwhelming experience if you’re walking down the TV aisle. TV sizes have increased dramatically in recent years, while prices have come down. 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 switched rapidly using electrical voltage, which affects their coloration.

However, future progress may require new materials capable of taking advantage of other types of optical properties, such as circular polarization.

Now, a researcher from the University of Tsukuba has introduced a technique to create polymers locked in a helical configuration, using a sacrificial liquid crystal model.

“Polymers that have both optical activity and luminescent function can emit circularly polarized light,” says author Professor Hiromasa Goto.

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 imparts a “chirality” or laterality to the structure, making it oriented clockwise or counter-clockwise. An electric voltage was applied, which triggered the polymerization of the monomers. The liquid crystal pattern was then removed, leaving a frozen polymer in a helical shape. By breaking mirror symmetry, the polymer has the ability to convert linearly polarized light into circular polarization. The furan rings in the polymer not only contribute to electrical conductivity, they also help stabilize the helical structure.

“The pi-stacking interactions between the rings allow the polymer to aggregate into a highly ordered chiral system,” says Professor Goto.

The resulting polymer was tested using circular dichroism absorption spectroscopy and found to have strong optical activity at visible wavelengths. Future applications for this process could include cheaper and more energy-efficient electronic displays.

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