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HOME > No.25, May 2021 > Modulating helical nanostructures in a liquid crystal phase by molecular design

Modulating helical nanostructures in a liquid crystal phase by molecular design

The ester bond direction in liquid crystal dimers impacts the helical pitch in the twist-bend nematic phase By Yuki Arakawa
Yuki Arakawa

A research team led by Assistant Professor Yuki Arakawa, from the Department of Applied Chemistry and Life Sciences at Toyohashi University of Technology, has successfully developed sulfur-containing liquid crystal (LC) dimer molecules1) with oppositely directed ester bonds, which exhibit a helical liquid crystal phase, viz. twist-bend nematic (NTB) phase2), over a wide temperature range, including at room temperature. Collaboration with a team at the Advanced Light Source research facility at Lawrence Berkeley National Laboratory, USA, revealed that the ester bond direction in the molecular structures largely impacts the pitch lengths of helical nanostructures in the NTB phase. It is expected that this molecular design can be used to tune the resultant physical properties of LC materials that would contribute to new LC technologies, such as LC laser, photo-alignment, and display technologies.
This research paper was selected in “Materials Advances HOT Article Collection, 2020”.

The NTB 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 NTB 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 NTB 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 NTB 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 NTB phase behaviors. The team succeeded in developing new molecules that exhibit NTB phases over a wide temperature range, including room temperature.

Molecular structures of the synthesized LC dimers and images of the resultant helical nanostructures with different helical pitches.
Molecular structures of the synthesized LC dimers and images of the resultant helical nanostructures with different helical pitches.

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 NTB 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.


Yuki Arakawa, Kenta Komatsu, Jun Feng, Chenhui Zhu, Hideto Tsuji. "Distinct twist-bend nematic phase behaviors associated with the ester-linkage direction of thioether-linked liquid crystal dimers." Materials Advances, 2020.


By 荒川 優樹

豊橋技術科学大学 応用化学・生命工学系の研究チームは、ナノスケールのらせん構造を有するツイストベンドネマチック(NTB)相が室温まで冷却可能な二量体液晶分子を開発しました。米国のAdvanced Light Source (Lawrence Berkeley National Laboratory)との共同研究により、それら二量体分子構造中のエステル結合の向きが、NTB相のらせん構造の周期長に大きな影響を与えることを明らかにしました。このようなNTB液晶材料は、波長可変液晶レーザー、光配向材料などへの応用が期待されます。
本研究論文は、「Materials Advances HOT Article Collection, 2020」に選出されました。



本研究では、チオエーテル系の二量体液晶分子をベースとし、向きの異なるエステル結合(COOおよびOCO)をそれぞれ導入した新しい類縁体を開発し、それら類縁体が室温を含む広い温度範囲でNTB相を示すことを明らかにしました。共鳴X線散乱を用いたらせんピッチの評価により、COOエステル二量体とOCOエステル二量体のらせんピッチはそれぞれ6~9 nmおよび11~24 nmであることがわかり、エステル結合の向きの違いによりらせん構造の周期長が約2倍異なることが明らかとなりました。このように、Å単位の分子設計で、ナノスケールの高次構造の制御が可能になります。


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Researcher Profile

Yuki Arakawa
Name Yuki Arakawa
Affiliation Department of Applied Chemistry and Life Science
Title Assistant Professor
Fields of Research Liquid crystals, Polymer chemistry, Organic sulfur chemistry, High birefringence materials, Optical
Degree Ph. D. (Engineering), Tokyo Institute of Technology