Nematic phases of bent-core mesogens

Abstract

Bent-core mesogens derived from 4-cyanoresorcinol with terminal alkyl chains have been synthesized and investigated by polarizing microscopy, XRD and electro-optical methods. Short chain compounds have exclusively nematic phases which can be cooled to ambient temperature. These nematic phases are similar to ordinary nematic phases with only nearest neighbour correlation (N) whereas long chain compounds form SmC-type cybotactic clusters and these cybotactic nematic phases (N cybC ) can be regarded as strongly fragmented SmC phases. The chain length dependent as well as temperature dependent structural transition from N to N cybC is continuous and associated with a change of the position and intensity of the small angle scattering in the XRD patterns. Moreover, a temperature dependent stepwise transition from cybotactic nematic phases to different types of non-polar and tilted smectic phases (SmC (I) and SmC (II) ) is observed with a mesophase composed of elongated, but not yet fused cybotactic clusters (CybC) as an intermediate state of this transition. This improves the understanding of the nature and special properties of the nematic phases formed by bent-core molecules as well as their transition to smectic phases and it paves the way to new materials with spontaneous or field-induced biaxial nematic phases at ambient temperatures.

Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series

“…The slow crystallization of C4 as observed is also being reported here. The rate of cooling is much lower compared to the DSC results presented in [7].…”

“…A variety of orthogonal smectic phases have recently been discovered in several bent-core systems [5] and several types of the nematic phase (differing in detail from each other) are also observed in some compounds synthesized by several groups [6]. Towards the end of the last and the beginning of the current decade, * jvij@tcd.ie the transition temperatures from the Isotropic-Nematic (I-N) phase have consistently been lowered, especially by the works of Tschierske et al [7]. Though smectic phases of bent-core and calamitic systems offer huge potential for a range of applications, the commercially successful liquid crystal (LC) applications, so far, are mostly based on nematics.…”

“…When cooled from the isotropic phase to lower temperatures, the materials exhibit I-N transition; C4 and C7 show a monotropic nematic phase while C6 exhibits an enantiotropic phase. X-ray scattering studies have shown that the smectic phase is not present in either of C4 or C6 even at low temperatures but smectic-C-like clusters are present in both C6 and C7 but these are much smaller or measurably absent in C4 [7]. The dielectric and electro-optic studies of C4 revealed that it exhibits ε > 0 for lower frequencies and higher temperatures, whereas ε < 0 for higher frequencies and lower temperatures [22].…”

“…In Ref. [7], the cooling rate was faster, 10 K/min, and the boundary conditions for DSC were different and hence crystallization was not observed there.…”

“…The parallel and perpendicular components of the dielectric permittivity, ε and ε ⊥ were obtained from homeotropic and planar cells, respectively, using Novocontrol Broadband High Resolution dielectric spectrometer, Alpha-A Frequency Analyzer, interfaced with a PC. The reasons for [7]. Cr, crystalline solid; Iso, isotropic liquid; N, nematic phase similar to the ordinary nematic phases; NcybC, nematic phase composed of SmC type cybotactic clusters.…”