Cybotactic Clusters Research Articles

The influence of bromine substitution and linking groups on the phase behaviour of light-responsive rod-like molecules

The effect of aromatic core bromination as well as different linking groups on the phase behaviour of light-responsive rod-like liquid crystalline materials is reported. For this purpose, two new series of laterally brominated rod-like molecules are designed and synthesized. The two series have three benzene rings connected using different linking groups either azo or ester units with different directions. Both series have pentyloxy chain at one end and an octyl chain at the other end in the first group or an octyloxy chain in the second group of materials. Additional lateral bromine substituent was inserted at ortho position with respect to the pentyloxy chain. The molecular self-assembly of the new materials was investigated using polarized optical microscope (POM), differential scanning calorimetry (DSC) and X-Ray diffraction (XRD). Depending on the type of the terminal chain (octyl or octyloxy) at one end and nature of the linking groups nematic phases as well as smectic C phases are observed, with variable temperature ranges. Interestingly, the nematic phases were found to exhibit cybotactic clusters with tilted SmC or orthogonal SmA structure (NCybC or NCybA) with wide temperature ranges. Finally, the isothermal photo switching because of cis–trans photoisomerization was investigated in solution as well as between different liquid crystal phases. This report could be of interest for optical information storage device applications.

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.

Nematic Phase of Polar Unsymmetrical Bent-Core Molecules.

In liquid crystalline systems, the presence of polar groups at lateral or terminal positions is fundamentally and technologically important. Bent-core nematics composed of polar molecules with short rigid cores usually exhibit highly disordered mesomorphism with some ordered clusters that favourably nucleate within. Herein, we have systematically designed and synthesized two new series of highly polar bent-core compounds comprised of two unsymmetrical wings, highly electronegative -CN and -NO2 groups at one end, and flexible alkyl chains at the other end. All the compounds showed a wide range of nematic phases composed of cybotactic clusters of smectic-type (Ncyb ). The birefringent microscopic textures of the nematic phase were accompanied by dark regions. Further, the cybotactic clustering in the nematic phase was characterized via temperature-dependent XRD studies and dielectric spectroscopy. Besides, the birefringence measurements demonstrated the ordering of the molecules in the cybotactic clusters upon lowering the temperature. DFT calculations illustrated the favourable antiparallel arrangement of these polar bent-core molecules as it minimizes the large net dipole moment of the system.

Nematic twist-bend phase of a bent liquid crystal dimer: field-induced deformations of the helical structure and macroscopic polarization

The twist-bend nematic (Ntb) phase is a recent addition to the family of nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntb phase under an external electric field is interesting and it was predicted in several recent theoretical studies. We investigated the field-induced polarization behaviour, dielectric, and electro-optic properties of a bent LC dimer CB7CB in the N and Ntb phases. A threshold-dependent polarization current response was obtained in both the phases under triangular and square-wave input electric fields, existing till frequencies as high as 150 Hz. The polarization switching times were found in ∼1 ms region, especially in the N phase. In the Ntb phase, electric field-induced deformation of the helical structure was observed, like ferroelectric LCs. Dielectric measurements revealed the presence of cybotactic clusters via collective relaxations. The dielectric anisotropy (Δϵ) is negative at the frequencies of polarization measurements. The net polarization resulted from field-induced reorientation of cybotactic clusters and additionally from the field-induced deformation of helical structures in the Ntb phase. We explored the possibility of ionic contributions to the net polarization by synthesizing TiO2 nanoparticles (NPs) dispersed CB7CB LC nanocomposite. Incorporation of the NPs resulted in reduction of the collective order, increase in the ionic impurity content and conductivity, but an extinction of the field-induced polarization response. Our results demonstrate that the net polarization has competing contributions from both ferroelectric-like and ionic origin (up to ∼10 Hz) in the LC phases, but it becomes dominantly ferroelectric-like at higher frequencies.

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.

Azobenzene-based polycatenars: Investigation on photo switching properties and optical storage devices

We report the photo switching behaviour of azobenzene-based rod-like polycatenar molecules having three hexyloxy chains at one terminus and a variable chain at the other terminus. The latter was varied with OC6H13, SC6H13, C7H13, OC10H21 and O(CH2)4C6F13 chains. All molecules are liquid crystalline exhibiting wide ranges of nematic phases with cybotactic clusters in addition to Smectic C phase. The effect of variation of terminal chains on the photo-switching properties of the above series of azo compounds upon light irradiation and their kinetic reactions are presented. Surprisingly, trans-cis conversion in solutions takes around 85 s, while relaxing back i.e. cis-trans conversion takes ~30–60 min depending on the type of the terminal chain. On the other hand, irrespective of the nature of the terminal chain all of them exhibit fast thermal back relaxation time. A possible reason for the observed phenomena is presented based on a caterpillar model. Moreover, the potential of one selected example in optical storage device is elucidated. Therefore, this work shows the structure-property relationship in such type of polycatenar molecules related to their photo switching properties.

Quantum-dots-dispersed bent-core nematic liquid crystal and cybotactic clusters: Experimental and theoretical insights.

We study a quantum-dots (QDs) dispersed bent-core nematic liquid crystalline system in planar geometry and present experimental measurements of the birefringence (Δn), order parameter (S), dielectric dispersion, absorption spectra, and optical textures with attention to variations with temperature. A bent-core liquid crystal (LC) 14-2M-CH_{3} is used as the host material and CdSe/ZnS core-shell type QDs are used as the dopant. The nematic (N) phase of the pristine (undoped) LC 14-2M-CH_{3} contains cybotactic clusters, which are retained by its QDs incorporated LC nanocomposite. Our experimental findings support: (i) reduced orientational order parameter of the QDs dispersed LC system compared to its pristine counterpart at fixed temperatures, (ii) reduced cybotactic cluster sizes due to the incorporation of QDs, and (iii) increased activation energies related to reduced cluster sizes. We complement the experiments with a novel Landau-de Gennes-type free energy for a doped bent-core LC system that qualitatively captures the doping-induced reduced order parameter and its dependence on the properties of the QDs and its variation with temperature.

Pristine and ferroelectric liquid crystal dispersed four ring bent core nematic: Impact of dispersion on electro-optic and dielectric properties

The electro-optic and dielectric behaviors of a four-ring bent core nematic (BCN) liquid crystal (5-2F-2Me-10) in its pristine and two different concentration proportions (0.5 wt% and 1.0 wt%) of ferroelectric liquid crystal (W206E) dispersed states, have been studied. Polarized optical microscopy (POM) confirms the presence of a nematic and SmA phases in pure and W206E doped states of BCN. The transition temperatures corresponding to different phases of doped samples decrease as compared to the pure BCN. An interesting ferroelectric like switching has been observed in pure 5-2F-2Me-10 and 5-2F-2Me-10 + 0.5 wt% W206E samples in their nematic phases. However, it is not significant in the case of 5-2F-2Me-10 + 1.0 wt% W206E. Dielectric spectroscopy reveals the presence of cybotactic clusters in pure and doped states of BCN, which is considered the main reason for the observed polar switching in these samples in their nematic phases. This study will help to understand the nature of interactions between the host BCN and doped FLC, and gives a direction to understand the effect of doping of FLC in BCN. Further, one major problem commonly associated with BCNs is high clearing temperature, as well as their different LC phases exist relatively at high temperatures, which restricts their potential applications in electro-optic devices. In this regard, the findings of this study are useful in reducing the isotropic temperature and the temperature corresponding to the different phases of a BCN.

Study of ferro- and anti-ferroelectric polar order in mesophases exhibited by bent-core mesogens

Ferroelectric nature in the NCybC phase and anti-ferroelectric nature in the SmC(I) and SmC(II) phases.

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.

Development of nematic and orthogonal smectic phases in short-core fluorinated hockey-stick shaped liquid crystal compounds

Here we report synthesis and investigations on novel short-core hockey-stick shaped molecules bearing a 4-n-alkyloxy-2-hydroxybenzylidene moiety at one end and polar fluoro-biphenyl moiety at the other end. The polar chloro and fluoro substituent at the lateral position of the core confers nematic and orthogonal smectic mesomorphism. The mesophase morphology is characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), electro-optical study and dielectric spectroscopy. The fluorinated compounds 1b (n = 16) and 1c (n = 18) have long terminal hydrocarbon tail and show orthogonal smectic phases with antiferroelectric switching behaviour. The chlorinated compound 2e (n = 8) with relatively short terminal chain length exhibits only a monotropic nematic phase on cooling. The nematic phase is comprised of cybotactic clusters as confirmed by XRD and exhibits electro-optic switching. The compound 2i (n = 12) manifests a short range enantiotropic nematic and additional orthogonal smectic phase. Dielectric spectroscopy reveals polar correlations in the nematic and smectic phases.

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.

Biaxial ordering in the supercooled nematic phase of bent-core mesogens: effects of molecular symmetry and outer wing lateral groups

ABSTRACT In previous research, we have found that the incorporation of three lateral substituents on bent-core oxadiazole-based liquid crystals has allowed for the presence of a nematic phase that supercooled to room temperature. X-ray diffraction (XRD) studies of these derivatives have confirmed that the nematic phase consists of cybotactic clusters. Moreover, the wide-angle XRD pattern showed a splitting indicative of local biaxial ordering within this phase. In this paper, we report the phase behaviour of two series of compounds that contain up to four lateral methyl groups. In one series, the arrangement of the lateral groups is symmetrical, and in the other case, the groups are not symmetrically distributed. All of the non-symmetric derivatives supercool to room temperature in the nematic phase, while symmetric derivatives tend to crystallise well above room temperature. XRD analysis of all of these compounds indicates the presence of cybotactic order. However, a splitting of the wide-angle XRD pattern is observed only in derivatives with lateral groups on the outer benzene rings, regardless of their supercooling behaviour, indicating the crucial influence of these substituents on the molecular packing.

One-dimensional theoretical analysis of coupling and confinement effects on the cybotactic clusters of bent-core nematic liquid crystals.

The bent-core liquid crystals (LCs) are highly regarded as the next-generation materials for electro-optic devices. The nematic (N) phase of these LCs possesses highly ordered smecticlike cybotactic clusters which are promising for electro-optic applications. We have studied a one-dimensional Landau-de Gennes model of spatially inhomogeneous order parameters for the N phase of bent-core LCs. We investigate the effects of spatial confinement and coupling (between these clusters and the surrounding LC molecules modeled by a coupling parameter γ) on the order parameters. The coupling is found to increase the cluster order parameter significantly, suggesting enhancement in cluster formation, and also predicts a transition to a phase with weak nematiclike ordering above the nematic supercooling temperature.

Formation and development of nanometer-sized cybotactic clusters in bent-core nematic liquid crystalline compounds.

Two homologue achiral bent-core liquid crystals (LCs), BCN66 and BCN84, in their nematic phases are studied by dielectric spectroscopy in the frequency range 10 Hz–10 MHz. In each of these compounds, two relaxation processes are identified and assigned to (i) collective dynamics of molecules in nanometer-sized cybotactic clusters and (ii) individual molecular relaxations, in the ascending order of frequency of the probe field. The temperature and the bias electric field dependence of the dielectric strength and relaxation frequency for these processes are shown to give rise to sharpness in cluster boundaries, increased size and volume fraction in the LC nematic phase. The effect of the bias field on the LC cell is similar to reducing its temperature; both variables increase the cluster size and volume fraction and give rise to sharp cluster boundaries. The findings confirm that dielectric spectroscopy is a powerful and an extremely useful technique to provide a deeper understanding of the mechanism of cybotactic cluster formation in the isotropic liquid and the nematic phase of LCs as a function of temperature and the bias field.

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.

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

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.

Nanodoping: a route for enhancing electro-optic performance of bent core nematic system

We report the effect of dispersion of barium titanate (BaTiO3) nanoparticles (BNPs) in a four ring bent core nematic (BCN) liquid crystal. Polarizing optical microscopy reveals the presence of a single nematic phase in pure and doped states. Polar switching has been observed in the bent core system and the value of spontaneous polarization (Ps) increases with increase in doping concentration of BNPs in BCN. Dielectric study shows a lower frequency mode, which can be ascribed to the formation of cybotactic clusters. These clusters are also responsible for the observed polar switching in pure, as well as, in doped BCNs. Another higher frequency mode, observed only in pure BCN, indicates the rotation of molecules about their long molecular axis. The conductivity of doped samples is also found to decrease as compared to the pure BCN. This reduction helps in the minimization of negative effects caused by free ions in liquid crystal based devices. This study demonstrates that the interaction between BNPs and BCN molecules improves the Ps, dielectric behaviour, viscosity and reduces the conductivity of pure BCN. Hence, nanodoping in a BCN is an effective method for the enhancement of electro-optic performances and will lead to the development of faster electro-optic devices.

Cybotactic clusters seen in the nematic phase of four-ring achiral bent-core liquid crystal by dielectric spectroscopy

ABSTRACTCybotactic nematic phase of the bent-core liquid crystals are considered as the new functional material for the next-generation liquid crystal display because of its unusual physical properties which has no calamitic counterpart. In this work, the cybotactic nature of the bent-core liquid crystal are confirmed by dielectric spectroscopy. The liquid crystal sample also shows a lower tendency towards crystallization paving the way for room temperature liquid crystal devices.

Influence of underlying local organisations in typical nematic phase of 6CHBT and induced nematic phase of bicomponent liquid crystalline systems 4DBT–12CB – a Fast Field Cycling NMR investigation

ABSTRACTVariable-temperature proton magnetic relaxation dispersion (PMRD) profiles are collected in the induced nematic phases of the binary liquid crystalline mixtures composed of smectic mesogens: 5-butyl-2-(4-isothiocyanatophenyl)-1,3-dioxane (4DBT) and 4′-dodecyl-4-cyanobiphenyl (12CB) and the low viscous nematogen 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to assess the influence of local organisations on the power spectrum of director fluctuation modes. Two distinct compositions of the binary mixture i.e. 50 mol% of 12CB (without a smectic phase) and 70 mol% of 12CB (with a smectic phase) are used. Fast Field Cycling nuclear magnetic resonance relaxometry is employed to measure proton spin-lattice relaxation rates (R1) as a function of Larmor frequency (10 kHz–30 MHz). PMRD data analysis with the choice of a suitable model indicated nematic clusters of moderate size (~200 Å) found in the broad nematic region of 50 mol% 12CB, whose size is almost invariant with temperature. On the other hand, cybotactic clusters, i.e. local smectic organisations of relatively larger size (~2000–3000 Å), are observed near the nematic–smectic transition (T > TAN) of 70 mol% 12CB. Interestingly, the shape geometry of such local organisations accessed from PMRD analysis is weakly anisotropic near TAN, while being isotropic near TNI for 70 mol% 12CB.