We have studied several compounds of non-chiral mesogens composed of symmetrically and non-symmetrically bent-shaped
molecular core with thermally stable ester linkages and laterally substituted by methoxy group. Recently,
mesomorphic properties have been described and the SmCP (B2) phase has been identified in a wide temperature range
below the isotropic phase. Additionally, symmetrical bent-shaped mesogens exhibit the B7 phase on cooling from the B2
phase. Herein we concentrated on nonlinear optical properties in the observed mesophases. Second harmonic generation
(SHG) supports the idea that the ground state of the B2 phase in studied bent-shaped mesogens has antiferroelectric
character and demonstrates the switching from antiferroelectric to ferroelectric state under applied electric field.
The field of ferroelectric liquid crystals (FLCs) is one of the most fascinating aspects of the Science of Materials for their
interesting electro-optic applications. Among different chemical and physical properties those related to the molecular
dynamics are very stimulating due to the relationship between molecular motions and macroscopic response to external
fields, such as electric and magnetic ones. In this work, the molecular dynamics of a ferroelectric smectogen, namely
the (S)-2-methylbutyl-[4'-(4"-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')-benzoyl)-propionyl)]-propionate (ZLL 7/*)
has been investigated by means of 2H NMR and dielectric relaxation techniques. The first method allows us to get
information on the molecular motions in the fast motion regime, in particular the diffusion reorientational motions of the
whole molecule, as well as of the internal motions affecting the phenyl and biphenyl fragments.
The second technique, which covers the slow motion regime, has been used to detect collective motions and fluctuation
modes. This study allows us to have a complete idea of molecular motions in the different smectic phases formed by the
ZLL 7/* mesogens, in particular the paraelectric smectic A (SmA), the ferroelectric smectic C* (SmC*), the
antiferroelectric smectic C*A (SmC*A) and the re-entrant ferroelectric smectic C* (SmC*r) phases. This last phase has
been investigated for the first time in this work, from the point of view of the dynamic behaviour. All results will be
discussed in the framework of the molecular dynamics of ferroelectric liquid crystals, reported in the literature so far.
New series of chiral liquid crystalline materials with the same rigid core with keto group and a various number of (S)-lactate groups in the chiral part of the moelcule have been studid by DSC, X-ray, electro-optics and dielectric spectroscopy. All compounds contain branched terminal alkyl chain derived from (S)-2-methylbutanol. The influence of the lactate number on mesogenic behavior of those compounds has been studied. All compounds show the paraelectric SmA phase and the ferroelectric SmC* phase in a rather wide temperature range. The antiferroelectric SmC*A phase was detected for the compound possessing two lactate groups. A polar field-induced smectic phase have been observed by electro-optic studies.
Two series of ferroelectric liquid crystalline materials, non-substituted and containing a methoxy lateral group at the aromatic ring of the molecular core have been investigated by dielectric relaxation spectroscopy in the range from 1 Hz to 1 MHz. All compounds exhibit chiral SmC* phase. A low temperature ferroelectric phase, consisting of the ordinary SmC* phase and a crystalline phase, was detected in three of the investigated compounds. The frequency dispersion reveals two modes (denoted as mode 1 and mode 2) detected in both ferroelectric phases. The analysis of the frequency dispersion of the complex permittivity provides relaxation frequency and dielectric strength of these modes. In the SmC* phase, high frequency mode 1 is the Goldstone mode. Low frequency mode 2 is attributed to a non-homogenous distribution of ionic space charge, which is localized near the surfaces. The values of Ps in the low temperature ferroelectric phase are lower than in the ordinary SmC* phase for the methoxy substituted compounds but shows no anomaly for the non-substituted material.
Two binary mixtures have been studied of homologues from a series derived from 4-alkoxybiphenyl-4'-carboxylic acid with three chiral centers exhibiting ferroelectric (FE), antiferroelectric (AF) and tilted hexatic phases. One homologue exhibits a re-entrant SmC* (SmC*re) phase, which is miscible with the SmC* phase. The phase diagrams of the studied mixtures exhibit a close pocket of the SmC*A phase. The SmC*re phase is induced even in the mixtures composed from homologues without this phase. Dielectric spectroscopy reveals the Goldstone mode in both ferroelectric SmC* and the SmC*re phases, this mode being stronger when increasing concentration of the antiferroelectric homologue: In the SmC*A phase a relaxation mode is detected with the relaxation frequency three orders higher, which exhibits critical slowing down when approaching the SmC*re phase. This mode, which can be attributed to the anti-phase mode is responsible for the appearance of the SmC*re phase. In mixtures exhibiting the SmC*A phase a helix twist inversion occurs in the vicinity of the low temperature border of the AF phase.
Dielectric dispersion has been studied in the frequency range of 10Hz divided by 1MHz in liquid crystals with 2- alkoxypropriate chiral group. Two of them exhibit the spontaneous helix twist inversion in the SmC* phase. The dispersion reveals two modes, dielectric strengths as well as relaxation frequencies of which exhibit a strong dependence on the sample thickness. Mode 1 with the relaxation in the kHz range is the Goldstone mode, which is combined with a thickness mode, but it becomes the pure thickness mode in the temperature range near the helix twist inversion temperature, where the helicoidal structure is unwound. This situation is reflected in the temperature dependence of characteristics parameters of mode 1. The lower frequency mode 2 is attributed to a mode connected with a non-homogeneous distribution of ionic space charge, which is localized near the sample surface.
New series of ferroelectric liquid crystals derived form alkoxycinnamic acid containing chiral alkyllactate group was synthesized. All compounds have a wide temperature range SmC* phase at temperatures above 100 degrees C and high spontaneous polarization. Some of homologues exhibit spontaneous helix twist unwinding in the SmC* phase accompanied with the helix twist inversion.
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