A ferroelectric dimeric liquid crystal with enormous spontaneous polarization and low-temperature dielectric constant

Within the Tokyo Tech LG Material & Life Solution collaborative research hub, a joint research team developed a ferroelectric dimer liquid crystal with a spontaneous polarization that exceeds (8 μCcm^-2) and a dielectric constant greater than 8,000 at low temperatures. The results are published in The Journal of Physical Chemistry B.

Ferroelectric liquid crystals are a unique type of liquid crystal that have high spontaneous polarization and dielectric constant. Among them, dimer molecules have a simple molecular structure and can form a ferroelectric phase at low temperature, so they are expected to be a material with many applications.

The joint researchers developed a dimeric molecule called di-₅ (₃FM-C₄T), which has a fluorine-substituted mesogenic core linked to the side wings by a pentamethylene spacer.

The researchers confirmed that this dimeric molecule exhibits liquid crystallinity at low temperatures (55°C to 211°C) and is composed of three polar phases: nematic, smectic and isotropic, with enormous spontaneous polarization (8 μCcm^-2) and dielectric constant (8,000).

The researchers successfully developed a dimeric liquid crystal exhibiting ferroelectricity at low temperatures. The use of the dimeric molecules developed in this research will enable the creation of technologies such as capacitors for smaller, lower-power electronic devices, piezoelectric elements and electrostatic actuators that can be driven at low voltage, as well as holographic screens displaying three-dimensional videos.

This development is expected to lead to new applications in areas such as automobiles, industrial robots and medical equipment.

These research results were obtained by the Tokyo Tech LG Material & Life Solution collaborative research hub, consisting of Shigemasa Nakasugi (joint researcher with industry and other organizations, including the private sector), Adj. Professor Hiroki Ishizaki, Adj. Assoc. Professor Sung Min Kang of LG Japan Lab, Professor Masato Sone, Adj. Professor Junji Watanabe and Assoc. Professor Tso-Fu Mark Chang of the Laboratory for Future Interdisciplinary Research in Science and Technology, and Professor Takaaki Manaka of the School of Engineering, which is a joint research organization of LG Japan Lab and Tokyo Institute of Technology .

The results were published in the Journal of Physical Chemistry B.

Ferroelectric liquid crystals are expected to have innovative applications in electronic devices because they exhibit higher spontaneous polarization and dielectric constant than conventional liquid crystals. Moreover, due to their high-speed switching properties and memory effect, they have recently attracted attention as a favorable material for realizing holographic displays requiring fine pixel structures.

Ferroelectricity requires reduction of molecular symmetry and chiral smectic-C phases with chiral molecules, nematic phases with specific functional groups and bent molecules with bent structure have been developed so far.

In particular, molecules with a bent shape have the property that the bent structure of the molecule decreases the intramolecular symmetry and that ferroelectricity can be expressed with a simple molecular structure that does not require the introduction of specific functional groups.

Additionally, some molecules with a curved shape are called dimeric molecules. While most bent-shaped molecules have a mesogen bonded to the 1,3 positions of the aromatic ring, dimeric molecules contain a flexible alkylene group (odd carbon number) as the mesogen bond.

This flexible alkylene group allows the dimer molecule to form the ferroelectric phases at lower temperatures than conventional curved-shaped molecules, which is superior in terms of application development.

In this study, the research team focused on dimer molecules to develop new materials with huge spontaneous polarization and dielectric constant.

Researchers developed a new dimeric molecule with a large dipole moment to achieve huge spontaneous polarization and dielectric constant. Specifically, they synthesized a dimeric molecule, di-5(3FM-C4T), which has a fluorine-substituted mesogenic core linked by pentamethylene spacers as side wings.

Thanks to efficient substitution of fluorine, the mesogenic core of di-₅ (₃FM-C₄T) was found to have a very large dipole moment of 11.2 D by density functional theory. Di-₅ (₃FM-C₄T) was structurally analyzed to reveal nematic ferroelectric (NF), smectic ferroelectric-A (SmAPF), and isotropic polar (IsoP) phases.

The NF phase consists of U-shaped molecules and exhibits a huge spontaneous polarization of around 8 μCcm^-2, reflecting the large dipole moment of the mesogenic core. On the other hand, the SmAPF phase consists of molecules with a bent shape and exhibits a high spontaneous polarization of approximately 4 μCcm.^-2.

The spontaneous polarization of the SmAPF phase is half that of the NF phase, which is due to the halved dipole moment in the bent molecule with a bent angle of 120° in a comparison with U-shaped molecules. IsoP phase on the high temperature side, which is still under structural analysis, still exhibits a polar structure and may exhibit polar aggregation of molecules in small domains.

These polar phases exhibit a dielectric constant of over 8,000, reflecting large dipole moments.

By applying the newly developed bent-shaped dimer molecules with huge spontaneous polarization and dielectric constant as a medium, it is possible to realize a variety of high-performance electronic devices. For example, the application to capacitors will allow the miniaturization and low power consumption of electronic devices.

Additionally, application to piezoelectric elements and electrostatic actuators will enable low-voltage drive, helping to improve control technology and energy-efficient industrial processes.

In application to 3D video display elements, the technology shows promise as an enabling technology for holographic displays because it is less likely to cause crosstalk between pixels in a fine pixel structure and enables optical switching at great speed. Thus, new applications are expected in areas such as automobiles, industrial robots, medical equipment and video display devices.

In this research, the three polar phases of the developed bent dimer molecules are viscous liquids, and research on immobilization techniques such as elastomerization and gelation is essential for practical applications.

With the development of immobilization techniques, the application areas of ferroelectric materials are expected to expand and develop into new application areas.