Cybotactic Nematic Phase Research Articles

Liquid-Crystalline Supermolecules Inducing Layer Fluctuations: From Hierarchical to Dissipative Structures

Liquid crystals, which have both liquid and solid properties, inevitably exhibit fluctuations. Some frustrated liquid-crystalline phases with a hierarchical structure, such as cybotactic nematic, modulated smectic, and bicontinuous cubic phases, are fascinating fluctuation-induced phases. In addition to these equilibrium phases, a pattern formation that is a nonequilibrium order through fluctuation is one of the most attractive research areas in soft matter. In this review, the studies on producing these fluctuation-induced orders in liquid crystals are described. Liquid-crystalline supermolecules in which several mesogens are connected via a flexible spacer have been designed. They have not only a characteristic shape but also an intra-molecular dynamic order. The supermolecules induce the fluctuations in layer structures at a molecular level, producing from the frustrated hierarchical to dynamic dissipative structures. In addition to reviewing molecular design for the hierarchical structures, the pattern propagation in a smectic phase is discussed based on the rotation of smectic blocks through Rayleigh–Bénard convection.

Synthesis of four-ring unsymmetrical bent-core mesogens and cybotactic cluster formation in their nematic phase

The synthesis and characterization of novel achiral unsymmetrical four-ring bent-core molecules resembling a hockey-stick shape are described. The present compounds bearing a methyl substituent in the central phenyl ring exhibit the skewed cybotactic nematic (NcybC), smectic C (SmC) and additional tilted smectic phase with tilted molecular arrangement. The skewed cybotactic nematic phase is formed by the bent-core molecules with long range orientational ordered clusters possessing local smectic C type order and within a cluster of tilted molecular assembly. The previously reported four-ring compounds without the methyl moiety did not exhibit nematic phase but show polarization modulated B1revtilt phase. The x-ray diffraction pattern clearly indicated the existence of SmC-type cybotactic clusters over a wide N temperature range; the layer reflection peak is weak at a higher temperature region and becomes more conspicuous with decreasing temperature. The lower temperature phase is distinctly the SmC phase, and in addition another tilted smectic phase emerges below SmC.

Polar response and Fréedericksz transition in cybotactic nematic fluids of unsymmetrical bent-core molecules

The appearance of polar order in nematic fluids is of broad interest rejuvenating affluent concepts of physics and chemistry. In bent-core nematics (BCNs), ferroelectric switching is a challenging quest of considerable interest after its primary observation in 1,2,4-oxadiazole-based BCNs. Herein, we observed induced polar switching in the cybotactic nematic phase (Ncyb) with a polarization value of ∼185 nC/cm2 in an unsymmetrical bent-core compound 8-F-OH. Temperature-dependent SAXS/WAXS analysis and dielectric measurements revealed nano-sized cybotactic clustering within the nematic phase. Electric-field-induced Fréedericksz transitions (FT) in the Ncyb phase at different frequencies and temperatures were investigated. Compound 8-F-OH exhibited two domains that formed and competed when the system was driven by an ac field. Thus, the build-up process of FT involved the reorientation of molecules and the transport of domains, where each had a distinctive characteristic time. We envisage that these detailed investigations can pave a bright way toward the development of advanced-level optical and electro-optical applications of BCN systems.

Electric-field-induced patterns in a hockey-stick nematic

The response to applied electric field of a newly synthesized hockey-stick nematic 8-F-OH has been studied. Unusual pattern forming scenarios were found, which show rich variety as a function of frequency, temperature and initial director alignment. The behavior has been compared with those, observed in calamitic and bent-core nematics. Experiments show that contrary to calamitic nematics, here the elastically strongly deformed layers due to Fréedericksz transition play a decisive role in the formation of regular patterns. This is attributed to the cybotactic nematic phase with polar smectic clusters, where flexoelectricity might also have influence. Static and dynamic behaviors of the patterns have been characterized and an application idea is formulated.

Molecular clustering behaviour in the cybotactic nematic phase of a spin-labelled liquid crystal

A newly synthesised liquid crystalline nitroxide radical with a terminal trifluoromethyl group exhibits a cybotactic nematic phase. EPR spectroscopy of the spin-labelled LC implies that molecules move into and out of the cybotactic clusters.

Alkylthio-based asymmetric liquid crystals: unravelling the substituent effects and intercalated cybotactic nematic and smectic phases

A homologous series of alkylthio- and alkoxy-based asymmetric calamitic liquid crystal (LC) molecules was developed. The effects of alkylthio groups on the phase transition properties and unexpected intercalated LC phase structures were studied.

Dielectric, electrooptic and viscoelastic properties in cybotactic nematic phase doped with ferroelectric nanoparticles

Hydrogen bonded liquid crystal, possessing a wide temperature range of cybotactic phase was used as a matrix to disperse ferroelectric LiNbO3 nanoparticles at the concentrations from 0.5 wt.% to 3.0 wt.%. The influence of nanoparticles on the physical properties has been studied with the help of polarizing optical microscopy (POM), impedance spectroscopy and electro-optic measurements. POM investigation revealed a nematic droplet dispersed in the isotropic phase due to the addition of ferroelectric nanoparticles. The suppression of the polarization has been observed and explained by the occurrence of antiparallel dipole configuration. The isotropic-nematic phase transition temperature was decreased due to the reduction in the parameter order. The decreasing in dielectric anisotropy (∆ε), response time (τ) and in threshold Freedericksz voltage (Vth) was associated to the decrease in splay constant elastic (K11) and in the rotational viscosity (γ). The curves of ∆ε, Vth, τ, K11 and γ when NPs concentration increases, showed a minimum at the same concentration (c = 1 wt.%) allowing the determination of the optimal NPs concentration.

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

Abstract The new compounds 4-decyloxy-2-fluorobenzoic acid and 4-dodecyloxy-2-fluorobenzoic acid display hydrogen-bonded liquid crystals (HBLC) of smectic and nematic phases. Their phase behavior established using polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) consistently occurs as the sequence upon cooling: Isotropic liquid – Nematic – SmC – crystalline solid. The dielectric study in the nematic phase shows a single relaxation process at low frequency characteristic of a cybotactic nematic phase. The optical properties of the nematic phase show a decrease of birefringence as alkyl chain length increases in the high-temperature part of the nematic domain, which reveals a decrease of order parameter for longer chains.

Cybotactic nematic phases with wide ranges in photoresponsive polycatenars

ABSTRACTWe report on the synthesis and phase behaviour of new light-responsive polycatenars. The new materials represent tetracatenar molecules with four terminal chains which are substituted at 2,3,4-position in one benzene ring at one terminus and at the other terminus only one chain is present. The liquid-crystalline behaviour of the prepared materials was characterised by differential scanning calorimetry (DSC), polarised optical microscope (POM) and X-ray diffraction (XRD). We investigated the effect of changing the type of one terminal chain, while keeping the remaining three chains fixed on the mesomorphism of these materials. All of the tetracatenars exhibit nematic and smectic C phases. By investigating the nematic phase with XRD it was found that it represents nematic phase with cybotactic clusters of the SmC type (NCybC). Moreover, these nematic phases exist over wide temperature ranges for all compounds. Additionally, the photosensitivity of these polycatenars was studied upon light irradiation.

Nematic—cybotactic nematic phase transition in a liquid crystal: a dielectric spectroscopic study

The cybotactic clustering in the nematic phase seems to be responsible for the unconventional behaviors such as biaxiality, achiral ferroelectricity, giant flexoelectricity, special rheological properties of bent-core nematics. Dielectric spectroscopy has been used to investigate the nematic—cybotactic nematic (N−Ncyb) phase transition and the effect of the external electric field on Ncyb phase of an U-shaped liquid crystalline compound 1,2-Bis{6-[4-(4-(4-hexyloxyphenyl)-2,3-difluorophenyl)phenyloxy]hexyloxy}benzene that exhibits SmC like skewed structured cybotactic clusters in the nematic phase over a relatively wider range of temperature (∼20 K). Dielectric measurements have been performed with thick planar cells. From the present study, it is observed that the N−Ncyb transition is acquainted with the sudden enhancement in dielectric parameters viz dielectric permittivity, dielectric loss, dielectric strength and relaxation time. External electric field induces more cybotactic clusters in cybotactic phase that results in enhancement of the dielectric parameters.

Liquid crystal thermosets. A new class of high-performance materials

ABSTRACT The evolution of the liquid crystalline order during the cure of a reactive thermotropic liquid crystal (LC) is studied using two phenylethynyl-terminated biphenol and naphthalenediol model compounds. The nematic order in the melt of both model compounds at elevated temperature is monitored with X-ray Diffraction (XRD) and C-13 Nuclear Magnetic Resonance (13C NMR). XRD shows a cybotactic nematic phase; the global nematic order decreases with curing and eventually the resulting cross-linked liquid crystal thermoset (LCT) exhibits an isotropic morphology. The NMR method tracks the phenylethynyl end-group and yields data about the cure kinetics of this class of reactive LC mesogens. The observations may be used to make inferences about the cure behaviour and morphology of phenylethynyl-terminated random copolyester macromonomers.

Polar smectic phases in quasi-bent-core liquid crystals with hockey-stick-shaped mesogen: synthesis and crossover mesomorphism

ABSTRACTNew hockey-stick-shaped liquid crystals (HSLCs) have been designed and synthesised. By extreme shortening of one ‘arm’ in the HSLCs, we have constructed the pronounced overall hockey-stick-shaped molecular conformation of these materials, and called the pseudo-rod-like (PRL) molecules with hockey-stick-shaped mesogen. The chemical structures were identified using spectroscopy and elemental analysis. The mesomorphic behaviour was investigated by polarising optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffractometry (XRD), and via electro-optical experiments. As a result, in spite of the PRL molecules, the various mesomorphic polymorphism, the interdigitated bi-layered structure and macroscopic polarity were found. Similar to up-to-date HSLCs, the quasi-bent-core LCs showed a transition from the synclinic SmC phase (SmCs) to anticlinic SmC (SmCa) phase, and exhibited SmA-type (NCybA) and SmC-type (NCybC) cybotactic nematic phases. At the same time, similar to practical rod-like LCs, the PRL molecules showed good surface alignment behaviour under planar boundary conditions.

Polar Order, Mirror Symmetry Breaking, and Photoswitching of Chirality and Polarity in Functional Bent-Core Mesogens.

In recent years, liquid crystals (LCs) responding to light or electrical fields have gained significant importance as multifunctional materials. Herein, two new series of photoswitchable bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected to an azobenzene based wing and a phenyl benzoate wing are reported. The self-assembly of these molecules was characterized by differential scanning calorimetry (DSC), polarizing light microscopy (POM), electro-optical, dielectric, second harmonic generation (SHG) studies, and XRD. Depending on the direction of the COO group in the phenyl benzoate wing, core-fluorination, temperature, and the terminal alkyl chain length, cybotactic nematic and lamellar (smectic) LC phases were observed. The coherence length of the ferroelectric fluctuations increases continuously with decreasing temperature and adopts antipolar correlation upon the condensation into superparaelectric states of the paraelectric smectic phases. Finally, long-range polar order develops at distinct phase transitions; first leading to polarization modulated and then to nonmodulated antiferroelectric smectic phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror symmetry breaking. Fine-tuning of the molecular structure leads to photoresponsive bent-core (BC)LCs exhibiting a fast and reversible photoinduced change of the mode of the switching between ferroelectric- and antiferroelectric-like as well as a light-induced switching between an achiral and a spontaneous mirror-symmetry-broken LC phase.

Cybotactic nematic phase of achiral unsymmetrical bent-core liquid crystals – Quelling of polar ordering and the influence of terminal substituent moiety

Nematic phase of bent-core liquid crystals (LCs) exhibiting cybotactic clusters (NCyb) have gained significant importance owing to its promising ability to demonstrate macroscopic biaxiality and the ferronematic phase. In this context, three achiral unsymmetrical four-ring bent-core LC compounds, bearing a long alkyloxy chain and differing only in the terminal substituent moiety (methyl, chloro, nitro), are designed and synthesized followed by their optical, dielectric, electro-optic and structural investigations. The presence of NCyb in the methyl and chloro substituted compounds was confirmed via dielectric spectroscopy and X-ray diffraction observations. The absence of ferroelectric behaviour in any of these compounds, even in the cybotactic nematic phase and in presence of polar substituent moieties (chloro and nitro), is attributed to the increased alkyloxy chain length and antiparallel molecular arrangement. The density functional theory (DFT) optimized molecular structure along with the experimental observations further substantiates these findings. The study establishes that cybotactic clusters and polar end moiety, although being a prerequisite for ferroelectric-like nature, do not necessarily result in a ferronematic phase.

Imaging of nanoscale molecular order in the cybotactic nematic phase

Imaging of nanoscale molecular order in the cybotactic nematic phase

PA46 棒状液晶が示すサイボタクチックネマチック相(ソフトマター,ポスター発表,2012年日本液晶学会討論会)

PA46 棒状液晶が示すサイボタクチックネマチック相(ソフトマター,ポスター発表,2012年日本液晶学会討論会)

Oxazole-based liquid crystals with low temperature nematic phases

ABSTRACTThis paper describes the synthesis and phase behavior of new oxazole containing liquid crystals. These compounds are related to oxadiazole derivatives which show a cybotactic nematic phase. The compounds described here possess a lateral bromo or chloro substituent and, in some cases, lateral methyl groups as well. Derivatives without lateral methyl groups exhibit enantiotropic behavior, while derivatives with lateral methyl groups show only monotropic behavior. However, several of the latter compounds supercool to room temperature in the nematic phase. In addition, the nematic phase of these derivatives retain fluidity to much lower temperatures than was observed for the oxadiazole analogs.

Temperature-dependent infrared study of phase transitions in bent-core liquid crystalline compound 12O(OH)2MeO10

Temperature-dependent Fourier-transform infrared (FTIR) investigation on a novel four-ring bent-core liquid crystal ((4′-n-decyloxyphenylazo)-phenyl-4-yl-3-[N-(4′-n-dodecyloxy-2-hydroxybenzylidene) amino]-2-methyl benzoate) has been done to study the dynamics of the molecules in mesophases at the molecular level. The spectroscopic analysis revealed that specific modes gave a clear evidence of Cr⇄SmC and SmC⇄NcybC phase transitions and successfully depicts the mechanism at these transitions. One of the important objectives to find the spectroscopic signature of the coexistence of SmC and cybotactic nematic NcybC phases below NcybC→SmC phase transition up to 3°C could not be achieved. FTIR spectroscopy confirmed the phase reversibility. In order to get the theoretical support density functional theory (DFT) has been adopted for conformational analysis that predicted the most stable conformer I. Conformer I might be expected closer to the real system and utilized for vibrational assignment along with potential energy distribution (PED). It has been demonstrated that the combination of FTIR spectroscopy and DFT paved a way to understand a candid picture of structural modification and dynamics of mesophases at the macromolecular level during the phase transitions.

3a03 サイボタクチックネマチック相を発現する棒状液晶(フォトニック・光デバイス/ソフトマター,口頭発表,2013年日本液晶学会討論会)

3a03 サイボタクチックネマチック相を発現する棒状液晶(フォトニック・光デバイス/ソフトマター,口頭発表,2013年日本液晶学会討論会)

Hydrogen bonding liquid crystalline benzoic acids with alkylthio groups: phase transition behavior and insights into the cybotactic nematic phase

A novel class of hydrogen bonding liquid crystalline benzoic acids with alkylthio groups was established and their phase transition behavior was investigated in detail.