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日本語

The N_TB is a newly identified fluidic liquid crystal (LC) phase, which possesses a helical nanostructure with a pitch ranging from single to double digit nanometers, and which is attracting considerable interest in the LC science community. Recently, various approaches to applying the N_TB materials to wavelength-tunable LC laser and photo-alignment technologies have been investigated. In terms of practicality, it is desirable for LC materials to form LC phases over a broad temperature range of 100°C or higher which encompasses room temperature. Unfortunately however, molecules that can maintain the N_TB phase over a wide temperature range, including at room temperature, remain exceptionally rare. This has impeded deep evaluations of various properties and the development of new applications.

Assistant Professor Yuki Arakawa and his team from Toyohashi University of Technology have been taking an interest in developing novel sulfur-containing LC materials, especially for high-birefringence materials and twist-bend nematic LCs, based on thioether (R–S–R) linkages that contain sulfur. Sulfur, which is found naturally occuring in hot springs, is one of the few surplus resources in Japan. Sulfur or thioether bonds have high polarizability and are expected to be useful functional moieties for improving certain physical properties, such as refractive index and birefringence, compared with other bonds based on conventional atoms, such as methylene (carbon) and ether (oxygen).

Previously, Assistant Professor Yuki Arakawa and his team had successfully developed thioether-based bent molecules that exhibited the N_TB phase. In this study, we attempted to devise new LC dimers by introducing oppositely directed ester bonds (i.e., –COO– and –OCO–) to the thioether-based bent dimeric molecules and elucidating the influence of the ester bond direction on the N_TB phase behaviors. The team succeeded in developing new molecules that exhibit N_TB phases over a wide temperature range, including room temperature.

Furthermore, the team observed a phenomenon, in which the helical pitches (6–9 nm) of the molecules with COO ester were approximately half (11–24 nm) of those with OCO ester. This is because the COO-ester dimers have more bent molecular geometries than the OCO-ester dimers, resulting in a more enhanced molecular precession in the helical structure for the former than for the latter. Finely tuning the molecular design (i.e., the ester bond direction) enables the manipulation of helical nanostructures, which is particularly important for optical applications.

According to Assistant Professor Arakawa, “LC molecules that exhibit the helical N_TB phase over a broad temperature range, including room temperature, remain rare. No studies have clearly revealed the structure-property relationship between molecular design and the resultant helical structure, i.e. how the helical nano-structures can be controlled by molecular design. We believe that our studies provide insights into this question.”

This work was partly supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI grant numbers 17K14493 and 20K15351, the Naito Research Grant, and research grants from the Toukai Foundation for Technology and Toyohashi University of Technology.

Technical Terms

• 1) A structure type of LC molecules in which two rigid structures are connected with a flexible alkyl chain spacer.
• 2) A helical liquid crystal phase in which bent molecules heliconically assemble to make a helical nano-structure.