Molecular Shape and Polar Order in Columnar Liquid Crystals

Abstract

Conspectus Bottom-up materials design by the conceiving of new molecular building blocks is powerful and chemists are uniquely qualified to innovate. Liquid crystals (LCs) and related soft crystals, collectively called mesophases, naturally create materials with dynamic properties. The thermotropic LC state has a liquid-like intermolecular disorder, but the cooperative nature of these materials facilitates a long-range directional order (alignment) that couples strongly to applied electric/magnetic fields and interfaces. Thermotropic LCs are held together by mesogen cores, which are often unsaturated with anisotropic polarizability, and are appended with flexible (often n-alkane) side chains. Thermal excitation of the side chains produces large amplitude motions that drive a melting transition, and the anisotropic attractions between mesogenic cores produce a directional organization that produces the LC order. LCs are liquids as defined by thermodynamics and may not contain three-dimensional (3D) organization. However, in many cases there are 3D ordered phases below the melting temperatures, which are soft (deformable) plastic materials. Unconventional mesogens offer opportunities to create responsive molecular assemblies with optical, electronic, or magnetic activity. I will detail in this account my efforts to control these dynamic states with the goal of creating polar organizations in columnar LCs. The use of molecular shape, dative bonding, and dynamic correlations between molecules in fluid/plastic phases will be highlighted and how applied electric fields can polarize select materials.

Room-Temperature Ferroelectric Columnar Liquid Crystals Driven by a Low Electric Field

“…The realization of room-temperature (RT) liquid crystals (LCs) with electric field (E-field) responsiveness has greatly contributed to fundamental and applied studies in the fields of chemistry, − physics, − and materials science. − Since the discovery of RT-nematic LCs with E-field responsiveness in the 1960s, extensive research has been conducted on LCs, which have been utilized for various applications, including LC displays (LCDs) . Ferroelectric LCDs utilizing smectic LC molecules, which have a memory effect in addition to E-field responsiveness at RT, have also been developed as high-speed LCDs. − Ferroelectric columnar liquid crystals (FCLCs) consisting of disk- or tapered-shaped molecules − have also been reported, which generate polarization in the direction perpendicular (lateral polarity) , or parallel (axial polarity) ,− to the column axis. Compared to lateral polar FCLCs, axial polar FCLCs have been noted for their unique potential applications in ultrahigh-density memory devices based on column-by-column polarization switching.…”

Positions of Chiral Alkoxy Groups Responsible for Ferroelectricity in a Columnar Liquid Crystal Phase of Diphenylureas with Six Alkoxy Groups

“…Based on these structural features, AP-FCLCs could be utilized for ultrahigh-density memory devices if polarization switching and polarization maintenance in each column are achieved. Many examples of AP-FCLCs − have been reported in which molecules with polar groups, such as amide, − ,,,,,,,,, urea, ,,, and triazole groups, self-organize into columnar molecular aggregates via intermolecular hydrogen bonds and π–π interactions, and the columnar polarization direction is controlled by applying an E-field. However, only a few studies have evaluated the polarization maintenance time of these AP-FCLC compounds, and most of the reports describe only P – E hysteresis loops obtained at high frequencies.…”

Fixation of Polar Assembly of Columnar Liquid Crystals with a Phosphine Oxide Group

“…The fixation of well-aligned polar structures is particularly promising for the development of piezo-responsive devices. − However, achieving polar ordering in liquid crystals is inherently challenging because molecules usually align to cancel out the dipoles in dynamic LC assemblies . While extensive research has been conducted on polar calamitic , and discotic − liquid crystals, the polar structures of these molecules are not retained after heating to their isotropization temperatures and revert to nonpolar states. Polarity-fixed LC polymers were prepared by photopolymerization of polar liquid crystals under electric fields (E-fields). − However, the development of polarity-fixed polymers based on E-field responsive polar columnar liquid crystals is still in its early stages.…”