Generation of Axially Polar Ferroelectricity in a Columnar Liquid Crystal Phase by Introducing Chirality

Abstract

Axially polar ferroelectric columnar liquid crystals (AP‐FCLCs) are materials in which the molecules are self‐organized into axially polar columns and exhibit switching and holding of their polar directions. If the polar directions along the columnar axis can be controlled by using a nanosized electrode, AP‐FCLCs have the potential to realize ultrahigh‐density memory devices. Though switching polarities in columnar liquid crystal phases have been studied by many scientists, it remains difficult to realize bistable switching. In this study, it is found that replacement of the normal alkyl groups (decyl groups) with chiral groups ((S)‐citronellyl groups) in an N,N’‐bis(3,4,5‐trialkoxyphenyl)urea realizes perfect ferroelectricity in the rectangular columnar liquid crystal phase, and it is confirmed that the phase is an AP‐FCLC phase. The differences in the properties and molecular packing structures between the achiral and chiral urea compounds are investigated. Furthermore, the mechanism for achieving ferroelectricity by introduction of the chiral alkyl chains is postulated.

Chiral Self-Sorting and the Realization of Ferroelectricity in the Columnar Liquid Crystal Phase of an Optically Inactive N,N′-Diphenylurea Derivative Possessing Six (±)-Citronellyl Groups

“…In 2020, our group also reported ferroelectricity in a rather thick layer (5 μm) of a chiral urea compound, N,N′-bis(3,4,5tri(S)-citronellyloxyphenyl)urea [(S)-1] 31 (Scheme 1), which achieved both a low E c (4.12 V μm −1 ) and highly stable polarization (the polarity quantitatively remained even after 8 h). From the comparison of (S)-1 with non-chiral ureas [e.g., Urea-10 (R = (CH 2 ) 9 CH 3 )], 29,31 we concluded that the helical structure generated by the chirality played an important role in generating its unique ferroelectricity. This result was because the one-handed, tightly wound helix kept the intermolecular distances between the substituent groups short, which effectively strengthened the attractive intermolecular interactions, such as alkyl−alkyl, phenyl−phenyl, and dipole−dipole interactions, in each column.…”

“…The AP-FCLC phase has the potential to realize writable, rewritable, and nonvolatile nano-sized memory devices if one-bit of information can be recorded by a nano-sized electrode. 2,4,31 However, it is still challenging to maintain the induced polarization after removal of the electric field, and we believe that polarization maintenance is most important to realize nonvolatile memory devices.…”

“…Although the XRD profile of rac-1 in the Col r phase did not show any peaks based on the helical structure due to the rather low symmetry of the helicity, our previous study clarified that (S)-1 molecules stack in 30°increments to form an (M)-helical column structure. 31 it is assumed that (R)-1 molecules stack to form the mirror image, (P)-helical column structure. Figure 7a shows the molecular model of (S)-1.…”

“…Recently, axially polar ferroelectric columnar liquid crystal (AP-FCLC) phases (Figure ), in which the polarity along the column axis is induced by applying an electric field and is maintained after removal of the electric field, have been increasingly studied. − In the columnar liquid crystal (LC) phase, the polar directions of the molecules (Figure a) are fixed to generate a polar column (Figure b) by intermolecular steric interactions − or intermolecular hydrogen bonding between the groups such as amide, ,− urea, − triazole, and vinylidene fluoride oligomer (VDFO) groups. The polar columns that are self-organized into a nonpolar state (Figure c) are polarized by applying an electric field, and the induced columnar polarities are switched (Figure d,e) by changing the directions of the applied electric field.…”

“…The polar columns that are self-organized into a nonpolar state (Figure c) are polarized by applying an electric field, and the induced columnar polarities are switched (Figure d,e) by changing the directions of the applied electric field. The AP-FCLC phase has the potential to realize writable, rewritable, and nonvolatile nano-sized memory devices if one-bit of information can be recorded by a nano-sized electrode. ,, However, it is still challenging to maintain the induced polarization after removal of the electric field, and we believe that polarization maintenance is most important to realize nonvolatile memory devices.…”