Effects of molecular chirality on self-assembly and switching in liquid crystals at the cross-over between rod-like and bent shapes

Abstract

A bent-core compound derived from a 4-cyanoresorcinol core unit with two terephthalate based rod-like wings and carrying chiral 3,7-dimethyloctyloxy side chains has been synthesized in racemic and enantiomerically pure form and characterized by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical investigations to study the influence of molecular chirality on the superstructural chirality and polar order in lamellar liquid crystalline phases. Herein we demonstrate that the coupling of molecular chirality with superstructural layer chirality in SmCsPF domain phases (forming energetically distinct diastereomeric pairs) can fix the tilt direction and thus stabilize synpolar order, leading to bistable ferroelectric switching in the SmC* phases of the (S)-enantiomer, whereas tristable modes determine the switching of the racemate. Moreover, the mechanism of electric field induced molecular reorganization changes from a rotation around the molecular long axis in the racemate to a rotation on the tilt-cone for the (S)-enantiomer. At high temperature the enantiomer behaves like a rod-like molecule with a chirality induced ferroelectric SmC* phase and an electroclinic effect in the SmA'* phase. At reduced temperature sterically induced polarization, due to the bent molecular shape, becomes dominating, leading to much higher polarization values, thus providing access to high polarization ferroelectric materials with weakly bent compounds having only "weakly chiral" stereogenic units. Moreover, the field induced alignment of the SmCsPF(()*()) domains gives rise to a special kind of electroclinic effect appearing even in the absence of molecular chirality. Comparison with related compounds indicates that the strongest effects of chirality appear for weakly bent molecules with a relatively short coherence length of polar order, whereas for smectic phases with long range polar order the effects of the interlayer interfaces can override the chirality effects.

Mirror symmetry breaking in liquids and liquid crystals

“…The combined effects of strong longitudinal and transversal helix coupling in the highly tilted and polar smectic phases of chirality synchronised bent-core molecules might contribute to DC phase formation by layer deformation into a disordered sponge-like structure (see Section 3). Likewise, the introduction of permanent chirality into bent-core mesogens [207,208] and bent mesogenic dimers [209] leads to a series of chirality induced and chirality frustrated LC phases. In any case, local or long range lamellar organisation provides significant restrictions to helical self-assembly in LC phases.…”

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

“…Uniaxiality of a tilted smectic phase requires that either the tilt is randomized as in the de Vries SmA phases, or that a short pitch helical superstructure with a helix axis parallel to the layer normal is formed . A helical structure was first proposed for the polar and uniaxial smectic phase of compound 1/14 .…”

Development of Polar Order by Liquid‐Crystal Self‐Assembly of Weakly Bent Molecules

“…Under an applied field the domains grew and adopted au niform tilt correlation, which led to af ield-induced biaxial ands ynpolar SmC s P S state (Figure 6c,d)t hat relaxed to an antipolarS mC s P A state at E = 0( Figure6e, f). This field-induced formation of at ilted organizationc ould be considered as an on-classical type of electroclinic effect [45,22] or,a lternatively, as at ransition to af ield-induced and surface-stabilized state. [16b, 46] The tilt directionw as retained during relaxation and also after field reversal, which indicated an antiferroelectric Figure 4.…”