title: Development of Polar Order by Liquid-Crystal Self-Assembly of Weakly Bent Molecules.
abstract: Organic ferroelectrics are of growing importance for multifunctional materials. Here we provide an understanding of the distinct stages of the development of sterically induced polar order in liquid-crystalline (LC) soft matter. Three series of weakly bent molecules derived from 4-cyanoresorcinol as the bent core unit with laterally fluorinated azobenzene wings have been synthesized, and the effects of the position of fluorine substitution, alkyl-chain length, and temperature on the LC self-assembly and polar order were studied. In the LC phases a paraelectric-ferroelectric transition took place as the size of the polar domains gradually increased, thereby crossing a permittivity maximum, similar to inorganic solid-state ferroelectrics. An increase in polar coherence length simultaneously led to a transition from synpolar to antipolar domain correlation in the high-permittivity paraelectric range. Associated with the emergence of polar order was the development of a tilted organization of the molecules and a growing coherence of tilt. This led to a transition from non-tilted via tilt-randomized uniaxial to long-range-tilted biaxial smectic phases, and to surface-stabilized symmetry breaking with the formation of chiral conglomerates and field-induced tilt. Moreover, there is a remarkably strong effect of the position of fluorination; polar order is favored by peripheral core substitution and is suppressed by inside-directed fluorination.
authors.affiliation: Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, 06120, Halle (Saale), Germany.|Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt.
authors.forename: Mohamed
authors.identifier: http://orcid.org/0000-0003-4155-8644
authors.initials: M
authors.lastname: Alaasar
authors.affiliation: Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, 06120, Halle (Saale), Germany.
authors.forename: Marko
authors.initials: M
authors.lastname: Prehm
authors.affiliation: Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, 06120, Halle (Saale), Germany.
authors.forename: Silvio
authors.initials: S
authors.lastname: Poppe
authors.affiliation: Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt Mothes Str. 2, 06120, Halle (Saale), Germany.
authors.forename: Carsten
authors.identifier: http://orcid.org/0000-0002-4163-3272
authors.initials: C
authors.lastname: Tschierske
keywords: azo compounds; ferroelectricity; fluorine; liquid crystals; mirror-symmetry breaking