Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound

Abstract

Here, we report the observation of ferroelectric nematic and ferrielectric smectic mesophases in an achiral bent-core azo compound consisting of nonsymmetrical molecules with a lateral fluoro substitution on one of the wings. These mesophases are enantiotropic in nature with fairly low transition temperatures and wide mesophase ranges. The liquid crystalline properties of this compound are investigated using polarizing optical microscope, differential scanning calorimeter, X-ray diffraction, and electro-optical studies. As revealed by X-ray diffraction measurements, the nematic mesophase is composed of skewed cybotactic clusters and, in the smectic mesophase, the molecules are tilted with respect to the layer normal. The polar order in these mesophases was confirmed by the electro-optical switching and dielectric spectroscopy measurements. The dielectric study in the nematic mesophase shows a single relaxation process at low frequency ( f < 1 kHz) measured in the range 10 Hz to 5 MHz, which is attributed to the collective motion of the molecules within cybotactic clusters. The formation of local polar order in these clusters leads to a ferroelectric-like polar switching in the nematic mesophase. Of particular interest is the fact that the smectic phase exhibits a field induced ferrielectric state, which can be exploited for designing of the potential optical devices due to multistate switching.

Dielectric and electro-optic properties of cybotactic nematic phase in hydrogen-bonded liquid crystals

“…Such reversal of the temperature dependence has not been observed for 8OBAF [30]. It has already been detected in some achiral bent-core liquid crystals and has been explained by an increasing size of ferroelectric clusters [33]. In this later case, a polarization has also been detected in a narrow temperature range in the isotopic phase indicating the persistence of clusters in the isotropic phase.…”

“…5. The natures of these phases were assigned following reference [33]. The dielectric anisotropy, dispersion, relaxation phenomena and loss tangent characteristics of the dielectric behavior of liquid crystals have been determined by fitting the Cole-Davidson distribution of relaxation times (Eq 1) to the real ε' and imaginary ε" parts of the complex dielectric permittivity [30,33]:…”

“…Fluoro-substitution of chiral LC [41], bent-core LC [33,42] and HBLC [43] bring about specific behavior which deserves investigation. Indeed, enhancements of viscoelastic, dielectric and optical properties have been reported in several instances.…”

Huge dielectric constants of the ferroelectric smectic-A phase in bent-shaped dimeric molecules

“…The high Δε has been reported in the ferroelectric phases of the bent-shaped molecules. 14,[20][21][22] W586 showed the maximum value of the Δε (~300) as observed in a 3 μm-thick cell. 14 Furthermore, the bent-shaped azo molecule consisting of nonsymmetrical molecules with a lateral fluoro substitution on one of the wings achieved the maximum value of the Δε (~400) in a 9 μm-thick cell.…”

Nanostructure of Unconventional Liquid Crystals Investigated by Synchrotron Radiation

“…The first experimental evidence of such a ferroelectric-like response to switching electric fields in a low molar mass nematic was reported in 2009 for BCM 2 of Scheme 1 [ 11 ] and later confirmed for other low molar mass bent-core nematics [ 6 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]. The study was carried out combining XRD with repolarization current measurements, electro-optical characterization and computer simulations.…”