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

Abstract

An axially polar-ferroelectric columnar liquid crystal (AP-FCLC) phase that exhibits both switching and maintenance of the macro-polarity in the column axis direction has been achieved in an N,N′-bis(3,4,5-trialkoxyphenyl)urea compound (rac-1) prepared from (±)-citronellyl bromide. Although it had been thought that chirality is necessary to achieve the AP-FCLC phase from our previous study, the optically inactive compound which is a mixture of 21 stereoisomers, generated an AP-FCLC phase. We confirmed its ferroelectricity and investigated the mechanism for realizing the AP-FCLC phase using optoelectronic experiments, Xray diffraction, and circular dichroism spectroscopy. As a result, it was suggested that chiral self-sorting occurs in the columnar liquid crystal phase, in which molecules with a similar stereochemistry form a one-handed helical column, and columns with the same helicity gather together to form a chiral domain. Accordingly, we conclude that the optically inactive compound rac-1 also indicates ferroelectricity similar to that of an optically pure urea compound because of chiral self-sorting.

Axially Polar-Ferroelectric Columnar Liquid Crystalline System That Maintains Polarization upon Switching to the Crystalline Phase: Implications for Maintaining Long-Term Polarization Information

“…They were attributed to the Cr and Col r phases based on the presence or absence of a short-range order in the wide-angle region. In addition, the comparison of the electron density maps of the Cr and Col r phases (Figure S3) revealed that they have similar molecular packing structures. Then, by POM observation on heating (Figures , S4, and S5), two similar fan-shaped textures were observed before and after the Cr–Col r transition at 64 °C.…”

“…To investigate the ferroelectricity of 1 , several SHG experiments were conducted. SHG measurement is an established method to confirm the macroscopic polarities of materials. − ,,, Compound 1 was heated to its Iso state, introduced in a sample cell consisting of two glass plates coated with indium tin oxide and polyimide films (cell gap: 5 μm) (Figure S9) by capillarity, and cooled to room temperature. The obtained sample in the cell was used as the original sample in Figure a.…”

“…In recent years, the rapid increase in the amount of information requires an entirely new recording system with higher density, lighter weight, and lower environmental impact. As a material for realizing the system, axially polar-ferroelectric columnar liquid crystals (AP-FCLCs) are attracting attention. − AP-FCLCs consist of parallel polar columns formed by the self-assembly of polar molecules, in which the polar directions of the columns can be switched by applying an external electric field (E-field) and maintained even after the removal of the E-field (Figure a). AP-FCLCs are expected to have a lower environmental impact and better processability than the inorganic ferroelectrics currently used. − In AP-FCLC phases, if all polar columns (a few nanometers in diameter) can be aligned perpendicular to a substrate and then each of their polar directions can be switched by using a nanoelectrode, as shown in Figure b, an ultrahigh-density memory device can be realized.…”

“…Recently, many unique AP-FCLC compounds showing switching behaviors through the application of an alternating current (AC) E-field have been reported. − However, few studies have revealed polarization maintenance by investigating the polar state over time after removing the E-field. To the best of our knowledge, only two types of AP-FCLC compounds have been reported that maintain almost 100% polarization after voltage removal.…”

“…To the best of our knowledge, only two types of AP-FCLC compounds have been reported that maintain almost 100% polarization after voltage removal. Those are benzene-1,3,5-tricarboxamide (BTA) compounds ,, and N , N ′-diphenylurea (DPU) compounds. − BTA compounds exhibiting polarization maintenance require large switching thresholds (= coercive E-fields ( E c s)) of 100–200 V μm –1 . On the other hand, our DPU derivatives possessing chiral and racemic alkyl chains also achieved perfect polarization maintenance, although their E c s are 4.0 and 3.6 V μm –1 , respectively.…”