Nematic Compounds Research Articles

Dielectric anisotropy in self-assembling MXene-based lyotropic nematic compounds

The formation of a lyotropic liquid crystal phase at the level of MXene-based aqueous dispersions due to the self-assembling capacity of these two-dimensional (2D) materials has been previously successfully reported. However, the influence of electrical external factors has been less studied in spite of the fact that understanding the electrical behaviour of 2D-based liquid crystal compounds is vital for fostering the development of new innovative applications. Consequently, within this paper, we report an in-depth evaluation of the dielectric properties of liquid crystalline phases in Ti3C2 MXene aqueous dispersions. The dependence on the low voltage of planar ε⊥′ to homeotropic ε∥′ orientational transition is emphasized and discussed, while the influence of the electric field frequency at the level of dielectric anisotropy is assessed in respect to various concentrations of MXene flakes within the same 2D-based compounds. Nonetheless, dielectric spectroscopy highlights the biphasic nature (containing both nematic and isotropic phases) of the studied aqueous Ti3C2 MXene inks visible up to 20% MXene. Above this concentration, the existence of a pure nematic phase in the dispersion of highMXene concentration (>20 wt%) was detected based on the evaluation of dielectric permittivity against various voltage values.

A ferroelectric nematic liquid crystal vitrified at room temperature

ABSTRACT Most of the current highly polar rod-shaped molecules that form ferroelectric nematic (NF) phase do so only at elevated temperatures and multicomponent mixtures are generally needed to obtain a broad and room temperature range NF phase. In this work, we describe the synthesis, phase characterisation and measurement of various physical properties of a new ferroelectric nematic compound 4-[(4-nitrophenoxy)carbonyl]phenyl 2-isopropoxy-4-methoxybenzoate (RT11165). The molecular structure of RT11165 with a 2-isopropoxy group differs only by a substitution of the 2-methoxy group found in the prototype ferroelectric nematic material 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). This small structure change produces a rather dramatic change in phase behaviour leading to an NF phase from 63°C down to room temperature. Below about 45°C the rotational viscosity of RT11165 increases critically and the temperature dependence indicates a glass transition at ~19°C. The transparent and polar glassy state of RT11165, which should be also piezoelectric, is a good candidate for energy storage, piezoecatalysis, data storage and other applications.

Size-dependent optical and dielectric anisotropy of CdS nanowire doped high-birefringent nematic liquid crystalline compound

Highly birefringent liquid crystals have grabbed ample attention because of their potential applications as an active medium for devices in visible, infrared and other spectral regions. Here, we have attempted to study the effect of the size and concentration of CdS nanowires on a nematic liquid crystal. Anisotropy in liquid crystals is one of those distinguishing traits that gain their superiority over other materials. We hereby have analyzed how optical and dielectric anisotropy respond to variations in the size and concentration of the dopant. The study is of significant scientific value because there is very little literature available on how dopant size can alter liquid crystalline properties and also introduces a method to tune the properties of pure liquid crystals (LCs) by varying the size and concentration of the dopant. The study explains how the size of the nanowires changes the analyzed parameters and up to what extent. The obtained results yield a significant increase in both optical and dielectric anisotropy by doping CdS nanowires making them an excellent choice for dopant selection. CdS-doped nematic LC mixtures have proved to be highly birefringent and possess a high positive dielectric anisotropy as well. Such a compound can be of great application in device fabrications such as laser beam steerers, electronic lenses, attenuators and light shutters, etc.

Influence of the molecular structure of compounds differing with terminal polar group on the induction of the smectic A phase. Part III

ABSTRACT The aim of this work is to determine the influence of the terminal polar group of nematic compounds, namely -OCF3, -NCS, -CN, -F, on the induction of the SmA phase in their mixtures with compounds having terminal non-polar groups. The former group of compounds has positive dielectric anisotropy, and the latter negative. The phase transition temperatures of binary mixtures were measured using polarising thermomicroscopy, and phase diagrams for investigated systems were constructed. The strength of the SmA phase induction increases upon the increase of the polarity of compounds. The strongest induction is observed in systems with compounds having a terminal trifluoromethoxy group. Although compounds with the cyano group have the highest polarity, their ability for the SmA phase induction is smaller due to their dimerisation.

Highly enhanced fluorescence parameters in a dye-doped liquid crystalline compound

ABSTRACT Fluorescent liquid crystalline compounds have been the focal point of research due to their rich applications in organic optoelectronics. Fluorescent dyes have also found great applications in the development of random lasers and non-linear optics. Other than that, materials with high fluorescence intensity and quantum yield have been very desirable for various electro-optical applications over the years. We have attempted to engineer a highly fluorescent nematic liquid crystalline compound (4′-pentyl-4-biphenylcarbonitrile/5CB) with high quantum yield by doping anthracene dye. We have obtained the desired increase in fluorescence intensity due to anthracene being fluorescent as well and a further significantly high quantum yield for higher concentrations of anthracene in the nematic compound. The reduction in non-emissive radiations and the increase in the concentration of fluorophores in the excited state can be held responsible for this increased quantum yield. Lifetime of the fluorophores has been found to increase with the increase in the dye concentration. The obtained results make the anthracene-doped 5CB compound highly desirable for various device applications.

Synthesis and mesomorphic properties of novel multi-arm side-chain liquid crystal polymers with different terminal substituents

ABSTRACT In this study, two novel aromatic ester multi-arm liquid crystal monomers M 1 and M 2 with different terminal substituents centred on 3,4,5-trihydroxybenzoic acid, and a rod-shaped chiral liquid crystal monomer N were designed and synthesised. The synthesised liquid crystal monomers M and N were grafted and copolymerised with polymethylhydrosiloxane (PMHS). The structural characterisation of the synthesised liquid crystal compounds was carried out by FT-IR and 1H-NMR. The mesomorphic properties of the synthesised liquid crystal compounds were characterised by POM and DSC, and the results showed that the multi-arm liquid crystal monomers M 1 and M 2 are typical enantiotropic nematic liquid crystal compounds and N is typical thermotropic enantiotropic cholesteric liquid crystal. Polymers P 1 and P 2 are both cholesteric liquid crystal polymers. The glass transition temperatures and liquid crystal isotropic transition temperatures of polymers P 1 and P 2 series both tend to increase with the increase of multi-arm liquid crystal monomer M.

Collective Relaxation Processes in Nonchiral Nematics

Nematic–nematic transitions in a highly polar nematic compound are studied, in thick cells in which the molecules are aligned parallel to the substrates but perpendicular to the applied electric field, using dielectric spectroscopy in the frequency range 1 Hz to 10 MHz over a wide temperature range. The studied compound displays three nematic phases under cooling from the isotropic phase: ubiquitous nematic N; high polarizability NX; and ferroelectric nematic NF. Two collective processes were observed. The dielectric strength and relaxation frequency of one of the processes P2 showed a dependence on the thickness of the cell. The process P1 is the amplitude mode, while the process P2 is the phason mode.

Optical activity and linear electrooptic response in the paraelectric subphase of an achiral ferroelectric nematic compound

ABSTRACTPolarizing optical microscopy and electro optics are used to study a compound with constituent non-chiral molecules, DIO, known to exhibit three nematic phases, namely N, Nx and ferronematic NF. N is found to be a conventional nematic but shows two additional unusual features: the presence of optically active domains and a linear electro optic response to the applied signal. The optical activity is explained by a segregation of the stable helical conformers of the opposite sense; these preferring to form domains with an optical rotation of ±4°/μm. This is the first example of helical segregation observed in non-chiral molecules in the high temperature nematic phase. Unlike the ordinary nematic liquid crystalline material, this compound shows a linear electro optical response to the applied electric field, thereby confirming the polar nature of this phase. NF is the ferroelectric nematic as reported previously. The strong polar azimuthal surface interaction energy in the NF phase stabilises a homogeneous structure in planar-aligned cells that are rubbed parallel and gives a twisted structure in cells rubbed antiparallel.

Studies of temperature dependent Raman spectroscopy of two nematic liquid crystalline compounds of homologous series

Raman spectroscopy is an important tool to understand the structural and molecular behaviour of the liquid crystals when they undergo through different temperatures. It also helps to understand the different phase changes of the liquid crystal material as temperature changes. In this work, the structural properties of two nematic liquid crystals having relatively high clearing temperature namely 4 butylcyclohexyl-3, 5-difluoro-4- isothiocyanato biphenyl and 4-pentylcyclohexyl-3, 5-difluoro-4-isothiocyanato biphenyl are studied. The study is done using temperature dependent Raman spectroscopy. From the studies of the two compounds it has been found that the experimental values are agree well with the various functional groups and different bond assignments recorded in literature. This agreement validates the presence of different functional groups and different stretching bonds in the two studied liquid crystal compounds. Deformations of some of the peak positions of the two liquid crystal compounds have been observed with the change in phase at different temperature. Also to understand the behaviour of the Raman peak near the clearing temperatures of the liquid crystal compounds the linewidth of the different peak values at different temperature have also been studied. From the line width study, the various phase transition temperatures of the two liquid crystalline compounds can be confirmed.

Dianhydride moieties involvement on the interactions of some polyimides with nematic compounds

This article has the aim to perform a comparative analysis between a semi-aromatic polyimide (PI) and a fully aliphatic one, with focus on the dianhydride moiety effect on certain physical properties of interest in display applications. Molecular modeling proved that the distinct PI conformational features have implications on the polymer interaction with the nematic molecules. Optical studies indicate different levels of transparency as a function of the aromatic extent of the sample structure. For achievement of the nematic orientation on the PI layers, their surface is subjected to mechanical erosion. The characteristics of the created topography reveal the involvement of the type of dianhydride used in the PI synthesis. Wettability data show a relevant enhancement in the nematic adhesion work values after the surface rubbing of both PIs.

Two mechanisms for the formation of the ferronematic phase studied by dielectric spectroscopy

A non-chiral ferroelectric nematic compound with a 1,3-dioxane unit in the mesogenic core called 2,3′,4′,5′-tetrafluoro-[1,1′-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) was studied by dielectric spectroscopy in the frequency range 0.1 Hz–10 MHz over a wide range of temperatures. The compound exhibits three nematic phases on cooling from the isotropic phase, i.e., the ordinary paraelectric nematic N; the intermediate nematic NX and the ferroelectric NF phase. The least frequency process is due to the dynamics of ions. The middle frequency relaxation process P1 is like as observed in other ferronematic compounds and this mode is a continuation of the molecular flip-flop motion in the isotropic phase to the collective dynamics of dipoles which are strongly coupled with the splay fluctuations in nematic phases. In addition to this process, DIO shows an additional collective relaxation process P2 at higher frequencies both in the N and the NX phases. This mode originates from the polar/chiral domains of the opposite chirality, these arise from the spontaneous symmetry breaking of achiral mesogens in the N phase. Both collective processes, P1 and P2, show soft mode-like characteristic behavior on cooling from the N to the NX-NF phase transition temperature and are shown to contribute independently to the formation of the ferronematic NF phase.

Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic, and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry.

High-resolution calorimetry has played a significant role in providing detailed information on phase transitions in liquid crystals. In particular, adiabatic scanning calorimetry (ASC), capable of providing simultaneous information on the temperature dependence of the specific enthalpy h(T) and on the specific heat capacity c_{p}(T), has proven to be an important tool to determine the order of transitions and render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound 2,3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) and on mixtures with 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). Both compounds exhibit a low-temperature ferroelectric nematic phase (N_{F}) and a high-temperature paraelectric nematic phase (N). However, in DIO these two phases are separated by an intermediate phase (N_{x}). From the detailed data of h(T) and c_{p}(T), we found that the intermediate phase was present in all the mixtures over the complete composition range, albeit with strongly decreasing temperature width for that phase with decreasing mole fraction of DIO (x_{DIO}). The x_{DIO} dependence on the transition temperatures for both transitions could be well described by a quadratic function. Both these transitions were weakly first order. The true latent heat of the N_{x}-N transition of DIO was as low as L=0.0075±0.0005J/g and L=0.23±0.03J/g for the N_{F}-N_{x} transition, which is about twice the previously reported value of 0.115 J/g for the N_{F}-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing x_{DIO}. At all the N_{x}-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the N_{F} to the higher-temperature phase exhibits substantial pretransitional behavior, in particular, in the high-temperature phase. Power-law analysis of c_{p}(T) resulted in an effective critical exponent α=0.88±0.1 for DIO and this value decreased in the mixtures with decreasing x_{DIO} toward α=0.50±0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior provided the total transition enthalpy change was used in the analysis.

Fast photo- and electro-optical switching of the polymer- stabilised cholesteric liquid crystal composite prepared by the template method

ABSTRACTMultifunctional cholesteric liquid crystalline (LC) polymer-stabilised composite with photo- and electro-controllable photonic band gap was prepared and its fast photo- and electro-optical switching was demonstrated. The composite was prepared by a template method implying the polymerisation of cholesteric mixture consisting of several components, such as nematic liquid crystal compounds, chiral dopant, polymerisable mono- and diacrylates as well as photoinitiator. UV-induced polymerisation and subsequent removing of the low-molar-mass components enables to obtain porous cholesteric polymer matrix, which was used for filling with photochromic nematic mixture. Application of electric field or UV-irradiation shifts the photonic band gap to the short-wavelength spectral range. In the case of an electric field, this shift is associated with alignment of LC molecules along the electric field and deformation of the cholesteric helical structure. UV light action induces E-Z isomerisation of the azobenzene-containing molecules of photochromic mixture and decrease in their molecular anisometry leading to decrease in birefringence. Both electro- and photo-induced changes are completely reversible and switching the electric field off and/or irradiation with visible light completely recovers initial shape of selective reflection band. The switching is very fast and takes several seconds for the light action and less than one second for the electric field.

Optical and thermodynamic studies on binary mixtures composed of two-isothiocyanato liquid crystals

Phase diagram and optical properties of the nematic mixtures containing two-isothiocyanatobenzen liquid crystal (7CHBT and 9CHBT) were studied by refractometry and DSC measurements. The thermodynamic and optical properties of the eutectic mixture (E79) composed of 7CHBT and 9CHBT were identified and studied. The phase diagram of the eutectic system was compared with the Schroder–van Laar calculation. The excess thermodynamic properties of the eutectic mixture were calculated and interpreted in terms of intermolecular interactions. The modified four-parameter model was used to fit the temperature dependence of the nematogen parameters on the refractive index. In addition, the temperature gradient of the ordinary refractive index (dno/dT) of the nematogens was analyzed in a convenient framework. The order parameters and polarizabilities of the nematic compounds, which are important parameters, were calculated through different methods based on birefringence measurements. Finally, the eutectic mixture and one of the constituent component (7CHBT) display crossover temperatures (TCO), which is very important liquid crystal parameter for specific applications such as optical devices and nonlinear optics.

Induction of the Orthogonal Smectic A Phase in Mixtures of Nematic Tolanes with Positive and Negative Dielectric Anisotropy

Induction of the orthogonal smectic A phase occurs in mixtures of nematic compounds with positive and negative dielectric anisotropy. The aim of this work is to determine influence of a molecular structure of compounds on the induction strength of this phase. Cyclohexyl tolanes with the terminal group OCF3, differing with a lateral substitution of a rigid core with fluorine atoms, were mixed with tolanes with two fluorine atoms in a lateral position and different lengths of a terminal alkoxy chain. Based on phase diagrams, obtained with the use of a polarising optical microscope, the influence of a structure of both types of compounds on the induction of the SmA phase is presented. Both, an increase in a number of fluorine atoms in a rigid core and a decrease in an alkoxy chain length cause a decrease in the strength of the SmA phase induction in the investigated systems.Keywords: chemistry, liquid crystals, smectic A phase induction, miscibility studies

Photo-induced change in alignment of mesogens using reduction of viologens as a trigger

Liquid crystals (LCs), whose refractive index can be changed by light irradiation, can be used as photoresponsive optical materials. In this study, an LC viologen was proposed as a novel photoresponsive compound that induced a change in the alignment of LC molecules (mesogens). An extended viologen with an asymmetric core, that exhibits a low-order LC phase at room temperature, was mixed with the well-known nematic LC compound (5CB) to prepare homogeneously aligned films. The change in the alignment of the mesogens was successfully induced by triggering a one-electron reduction reaction of the viologen moieties by light irradiation.

Induction of the smectic A phase in liquid crystalline mixtures formulated using non-chiral compounds with positive and negative dielectric anisotropy

ABSTRACT The induction of the SmA phase in mixtures of non-chiral nematic compounds with the positive and the negative dielectric anisotropy is presented which is obtained by different substitutions of fluorine atoms. One component is a three-ring tolane with polar group – OCF3 in position 4 and two fluorine atoms in positions 3 and 5 in the terminal phenyl ring. The other component is two ring tolane with two or four fluorine atoms in the lateral position. Based on phase diagrams, the influence of the alkyl chain length of both types of compounds on the induction phenomenon is shown. Results of the X-ray measurements confirm that the induced phase is a monolayer SmA phase. The increase of the alkyl chain length causes the increase in the strength of the induction in mixtures, i.e. the induced SmA phase is observed for a wider range of the temperature and concentration of mixture components.

Determination of refractive indices for liquid crystals using a double transmission technique.

We report a new, to the best of our knowledge, approach to determine the refractive indices (RIs) of liquid crystal (LC) systems by considering a double transmission phenomenon of a light beam within the LC medium and the principle of the wedged-cell refractometer. This modified system delivers better spatial resolution of refracted beams in terms of well-resolved spots, compared to the single transmission of a beam in the conventional thin-prism method. The deviation angle obtained in this method is found to be comparatively greater than that of the thin-prism method. The higher values of deviation angle are described using a theoretical model and further experimentally demonstrated for the 5CB nematic LC compound. The variation of the principal RIs (no, ne), and hence the birefringence (Δn) with temperature as well as electric field is investigated for both transmission methods. A relative comparison is carried out between the simulated and standard values obtained from literature. The present technique provides a higher spatial beam resolution to determine the RIs and birefringence of LC systems, which can be very advantageous for different modern photonic applications based on beam shaping and steering.

Greenly synthesized porous carbon nanoparticle (bio‐waste‐based)‐doped nematic liquid crystal composite with optimized electric and electro‐optical properties for devices

AbstractThe present investigation on the impact of porous carbon nanoparticles (PCNPs) on a room temperature nematic liquid crystalline compound (TP‐188‐01) reports extensive electro‐optical analysis of composite followed by dielectric and luminescence study as well. The pure nematic compound has been doped with 0.1%, 0.25%, and 0.5% of PCNPs (spherical), which have been synthesized by green synthesis technique without use of any activating agents. Low‐frequency dielectric analysis (50 Hz to 1 kHz) has been done to confirm ion generation with increase in PCNS concentration. The dielectric data also indicate preferential orientation of molecules from homogeneous to homeotropic alignment, which can be used in performance optimization of electro‐optical devices. The analysis of dielectric data also illustrates the application of PCNS‐doped systems in circuits operating at high voltages. Diffusion coefficient, mobility, and DC conductivity have been further examined to support the above observations. A significant decrease (of about 47%) in the response time of porous carbon nanoparticle liquid crystal (NP‐LC) composites has also been observed. The photoluminescence spectrum of NP‐LC composites as a function of carbon nanosphere (CNS) concentration has shown Quenching (following the Stern–Volmer quenching mechanism) in the luminescence intensity with increasing CNS concentration. The highly improved response time and quenching in the luminescence intensity further make the PCNS‐doped LC composites as ideal materials to be used in devices, encouraging LC‐aided green nanotechnology.

Dielectric relaxation of some fluorinated bi-cyclohexyl phenyl nematic compounds

Dielectric properties of six bi-cyclohexyl phenyl compounds (3ccp-f, 3ccp-ff, 3ccp-fff, 5ccp-f, 5ccp-ff, 5ccp-fff) have been investigated. The compounds exhibit broad range of temperatures in nematic phase. Only one strong absorption process is observed in the absorption spectra; for clarity spectra only at some selected temperatures are presented. In each case relaxation frequencies are found to increase with temperature. The relaxation behavior is a result of Cole-Cole type process, the complex dielectric permittivity was fitted with the modified Cole-Cole equation to get the actual values of relaxation frequency (fc ) and symmetric distribution parameter (α).