Cyanobiphenyl Liquid Crystals Research Articles

The magic of cyanobiphenyls: celebrity molecules

The research achievements of George Gray extended over more than 50 years and embraced many areas of liquid crystal science. Above all he was lauded for the discovery of cyanobiphenyl liquid crystals which were a vital component of the launch of a new liquid crystal display industry in the 1970s. Important as this discovery was, Gray expressed concern that his work in other areas of liquid crystal science might be overshadowed by the impact of technological success derived from cyanobiphenyl liquid crystals. This article explores other research topics in liquid crystal science where the liquid crystallinity associated with the cyanobiphenyl moiety has been exploited to develop new and diverse liquid crystal systems. The particular properties of the cyanobiphenyl group are such as to promote liquid crystal formation in diverse structural environments, and so can be regarded as special. The importance of cyanobiphenyl liquid crystals to the development of liquid crystal displays is unchallengeable, but cyanobiphenyls have also made possible many other research advances in materials chemistry. The genius of George Gray was not only to realise the potential of cyanobiphenyls for display materials, but also to uncover their chemical magic for forming liquid crystals.

Flexo-Dielectro-Optical Spectroscopy as a Method of Studying Nanostructured Nematic Liquid Crystals

The method of flexo-dielectro-optical spectroscopy was initially developed to study the spectra of the linear and quadratic electro-optic response of continuous nematic liquid crystal layers as a function of the frequency of the applied electric field at low frequencies (0.1–10000 Hz) [1]. Subsequently, it was applied to the microconfined PDLC system [2].In the present study a nanoconfined nematic system of 4-n-heptyl cyanobiphenyl liquid crystal (7CB) containing hydrophilic silica nanoparticles (Aerosil 300) was investigated. The method was implemented by a lock-in detection of the 1st and 2nd harmonic of transmitted laser light through a nanonematic layer subjected to a.c. electric field, the results of which are compared with those for pure 7CB. The optical spectra recorded at various temperatures in the nematic phase of pure and nanostructured samples are presented along with data from static and dynamic measurements of the Freedericksz transition.The data so obtained contain information about liquid crystal disorder induced by the nanoparticles both in the bulk and in the nanoscale vicinity of each particle or network of paticles. They are interpreted in terms of two subsystems model, and dimensions of bulk and nanostructured domains evaluated.

Application of isotropic periodic sum method for 4-pentyl-4′-cyanobiphenyl liquid crystal

In future large-scale molecular dynamics (MD) simulations that will use parallel computing, the isotropic periodic sum (IPS) method is expected to effectively reduce the cost of interaction calculations while maintaining adequate accuracy. To assess the accuracy of this method in estimating low-charge-density polymer systems, we performed atomistic MD simulations of the bulk state of liquid crystal systems based on 4-pentyl-4′-cyanobiphenyl (5CB). In conditions of 270 K ≤ T ≤ 320 K and a normal pressure, the temperature dependence of the density, potential energy and order parameter was estimated using the IPS and Ewald sum method. The results of the IPS method and Ewald sum were consistent within the range of error. In conditions close to the phase transition point, however, the averaged values of potential energy and order parameter had a small difference. We concluded that the fundamental physical properties for the bulk state of 5CB systems are determined reasonably by using the IPS method, at least in conditions that are not close to the phase transition point.

Multicomponent nematic systems with doped gold nanoparticles

We study the dispersion of gold nanoparticles in binary systems of two homologues of cyanobiphenyl liquid crystals (nCB with n = 5, 6). Using differential scanning calorimetry method, we show that the studied systems have much lower crystal-nematic transition temperatures as compared to the pure nematics transition points, and nematic-isotropic liquid transition temperatures are within the range of pure nematics transition points. Using electrical and dielectric measurements, we reveal that the studied multicomponent mixtures have significantly less threshold voltages and switching off times for Fredrick’s transition (display parameters) as compared to the pure nematics’ values. We propose that this improvement of nematics’ display parameters is due to the effect of doped nanoparticles on nematics’ material parameters.

Preparation of Liquid Crystal Doped with Nanomaterials and Studying of Electrical and Dielectric Properties

The effects of multiwall carbon nanotube on dielectric ansitropy and molecular reorientation properties of 4-hepty1-4'-cyanobiphenyl liquid crystal have been investigated. The 4-hepty1-4'-cyanobiphenyl liquid crystal exhibits the negative resitance effect at higher voltages (3.5 V), whereas rnultiwall carbon nanotube doped 4-hepty1-4'-cyanobiphenyl liquid crysatl shifts it to the lower voltages (0.3 V). The 4- hepty1-4'-cyanobiphenyl and multiwall carbon nanotube doped 4-hepty1-4'cyanobiphenyl liquid crystals indicate a p-type dielectric anisotropy behaviour. The multiwall carbon nanotube dopant increases the dielectric anisotropy Delta epsilon of 4-hepty1-4'-cyanobiphenyl as the dopant-used facilitates the reorientation of the molecules in liquid crystal. It is has shown that the negative resitance effect and molecular reorientation properties of the 4-hepty1-4'-cyanobiphenyl are changed with multiwall carbon nanotube dopant.

A new approach to study interaction parameters in cyanobiphenyl liquid crystal binary systems

The phase transition of heptylcyanobiphenyl 7CB and pentylcyanobiphenyl 5CB liquid crystals was investigated using the differential scanning calorimetry DSC technique. Then, the phase transition of different compositions of 7CB/5CB binary mixture was studied to determine the eutectic point. The phase diagram of mentioned binary system in 7CB mole fraction of 0.45 at T=273.45K is in good agreement with that of predicted from Schroder–van Laar equation. The thermodynamic excess functions and interaction parameters were calculated to describe the phase transition physically using the non-random mixing for the first time. The P∗ randomicity parameter was used to describe the phase transitions of C–N and N–I in which a small amount of P∗ shows a non-random identity of C–N phase transition. Contrarily, the P∗ is greater in N–I phase transition showing a random mixing process.

In situ observation of the molecular ordering in the lubricating point contact area

The organization of lubricant molecules confined between two solid surfaces when the lubricant film thickness is at the nanoscale is unknown. In this work, an ordering process of nematic liquid crystal molecules is observed by in situ polarized Raman spectroscopy of the lubricated point contact area. Our experimental results indicate that 4-n-pentyl-4′-cyanobiphenyl liquid crystal molecules orient along the rotation direction when the linear speed exceeds 12.6 mm/s, and the degree of order increases with linear speed. The relationship between the observed orientation and physical properties of the lubricant film is investigated. Isotropic orientation is observed at the outlet area of the contact region. The orientation behavior of liquid crystal molecules in a confined area is observed and the relationship between lubrication conditions and molecular orientation is discussed.

Optical birefringence and its critical behavior in the vicinity of nematic–smectic A phase transition in a binary mixture

Optical birefringence (Δn) measurements as a function of temperature have been performed for binary mixtures of octyloxy cyanobiphenyl (8OCB) and octyl cyanobiphenyl (8CB) liquid crystals by means of a high resolution temperature scanning technique. The temperature dependence of the birefringence (Δn) was determined from the transmitted intensity data for wavelengths λ=532nm. Using a fitting procedure consistent with the mean field theory and the first order nature of nematic–isotropic (N–I) phase transition, the order parameter critical exponent β has been determined. The critical behavior of the nematic–smectic A (N–SmA) phase transition has been investigated from the high resolution birefringence data and the nature of this transition in the mixtures has been assessed. From the evolution of the critical exponent α, it has been possible to predict the limiting value of the McMillan ratio for the tricritical point (TCP) as well as the 3D-XY universality class.

Order and Conformation of Biphenyl in Cyanobiphenyl Liquid Crystals: A Combined Atomistic Molecular Dynamics and 1H NMR Study

The alignment of biphenyl (2P) in the liquid-crystal phases of 4-n-pentyl-4'-cyanobiphenyl (5CB) and 4-n-octyl-4'-cyanobiphenyl (8CB) is investigated by using a combination of predictive atomistic molecular dynamics (MD) simulations and (1)H liquid-crystal nuclear magnetic resonance (LXNMR) residual dipolar coupling measurements. A detailed comparison and validation of the MD results with LXNMR is provided, showing a good agreement between the simulated and experimental dipolar couplings at the same reduced temperature. MD is then used to examine the location of 2P in the smectic phase, which is unavailable to LXNMR, and 2P is found to be rather uniformly distributed. The combination of MD and NMR spectroscopy provides detailed information about the order, interconnection between orientation and conformation, local positional order, and interactions with the liquid-crystalline solvent.

Field-induced director dynamics in confined nematic liquid crystals imposed by a strong orthogonal electric field

The field-induced director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4'-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage of 200 V across the 194.7 μm LC film and the magnetic field of 7.05 T directed at the angle α=1.57(~89.99°) between two fields, there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α [<1.565(~88.81°)] dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destruction of the initially periodic structure to a monodomain state.

Field-induced non-uniform director reorientation for a low molar mass nematic imposed by a strong orthogonal electric field

The director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion. Analysis of the numerical results for the turn-on process provides an evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4′-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage in 200V across the 194.7μm LC film and the magnetic field of 7.05T directed at the angle α=1.57(~89.99°) between two fields there is the threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provide the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α (<1.565(~88.81°)) dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destroying of the initially periodic structure to a monodomain state.

Photovoltage enhancement from cyanobiphenyl liquid crystals and 4-tert-butylpyridine in Co(ii/iii) mediated dye-sensitized solar cells

Two cyanobiphenyl liquid crystals (LCs), 5CB (4-cyano-4'-pentylbiphenyl) and 8CB (4-cyano-4'-octylbiphenyl), are introduced as additives into Co(II/III) electrolytes for dye-sensitized solar cells (DSCs). An electrolyte containing a combination of these LCs and 4-tert-butylpyridine (TBP) exhibits higher photovoltage than one with only TBP, resulting in higher power conversion efficiency.

Cyanobiphenyl liquid crystal composites with gold nanoparticles

We study composites of mainly room temperature nematic cyanobiphenyl liquid crystals (nCB with n = 5, 6, 7, and 8) with gold nanoparticles (GNP). We report that doped GNP dramatically affects the material and, consequently, display parameters and mesophase stability of 5CB, 6CB, and 7CB homologues, while no homogeneous dispersion of GNP occurs in the 8CB homologue. We report several composites with values of switching voltages and times a few times lower than those for pure nematics, and with a broader operational thermal range, which makes them good candidates for application in liquid crystal (LC) display devices. We show that the amount of carbon atoms n in the alkyl “tail” of CB's constituent molecules and the concentration of doped GNP are critical factors in the behavior of the material parameters in the nanocomposites. We report certain correlations of the behavior of the material parameters with odd-to-even-to-odd changes in n. We propose that the doped nanoparticles aggregate with LC molecules affecting the local ordering, and consequently changing nCB's material parameters. The study of the morphology of the samples by SEM and AFM imaging techniques articulates our proposed ideas.

Optical Detection of Lithocholic Acid with Liquid Crystal Emulsions

The concentration level of bile acids is a clinical biomarker for the diagnosis of intestinal diseases because individuals suffering from intestinal diseases have a sharply increased concentration of bile acids at micromolar levels. Here, we report the detection of lithocholic acid (LCA) in aqueous solution by using surfactant-stabilized 4-n-pentyl-4'-cyanobiphenyl (5CB) liquid crystal droplets as an optical probe. We find that the surfactant adsorbed at the 5CB/water interface can be replaced by LCA, triggering a radial-to-bipolar configuration transition of the 5CB in the droplets. By simply observing the LCA-triggered transition with a polarizing optical microscope, micromolar levels of LCA in aqueous solution can be detected. The detection limit and selectivity of surfactant-stabilized 5CB droplets for LCA depend on the chain length and headgroup of the surfactants used for stabilizing 5CB droplets.

Liquid crystalline cyanobiphenyl homologues doped with gold nanoparticles

The following paper reviews our study of the dispersion of gold nanoparticles (GNPs) in different homologues of cyanobiphenyl (CB) liquid crystals (nCB with n = 5, 6, 7). We show that doped GNPs dramatically affect the material, and consequently the display parameters and mesophase stability of these mainly room temperature nematics. We report several homologues at certain GNP percentages with their values a few times lower than that for pure nematics, switching voltages and times, and with a broader operational thermal range. These materials are good candidates for application in liquid crystal (LC) display devices. We show that the amount of carbon atoms n in the alkyl ‘tail’ of the homologues' constituent molecules and the concentration of dispersed GNPs critically affect the behaviour of the material parameters in the nanocomposites. We report a certain correlation of the material parameters behaviour with an odd−even−odd change in the number of CB homologues. We propose that the influence of GNPs on the material parameters and the thermal stability of the nematics is caused by the formation of different kinds of aggregations between nanoparticles and LC molecules. These clusters affect the local ordering of the LC molecules, consequently changing the material parameters and mesophase stability.

Ordering of the 7CB liquid crystal induced by nanoscale confinement and boundary lubrication

In this study, the thermodynamic properties of nanoscale-thick 4-heptyl-4′-cyanobiphenyl (7CB) liquid crystals (LCs) confined in carbon nanotubes were investigated using differential scanning calorimetric methods. Both the isotropic-nematic phase transition temperature and the enthalpy increase decrease as the nanoscale confining size decreases, indicating the ordering effect of the 7CB molecules under nanoscale confinement. An average-order parameter <S> is calculated to quantitatively identify the ordering effect of LCs by utilising a simple expression of the Landau–de Gennes (LdG) free energy. The average-ordering parameter <S> of the 7CB LC can be 0.7 within a confinement thickness of ∼25 nm and above 0.5 within a film thickness of up to 100 nm. When lubricating the 7CB LC between a steel ball and chromium-coated glass disk at low speeds, a stable boundary layer with a thickness of 10 nm is formed that is independent of the entrainment speed. At higher speeds, the thickness of the 7CB lubrication film ranges from several tens of nanometres to 100 nm, resulting in a stable velocity-independent friction coefficient of approximately 0.03. The ordered microstructure of the 7CB molecules under confinement provides the underlay mechanism of its boundary lubrication property.

Rheological properties of a reentrant nematic liquid crystal

We report experimental studies on small angle light scattering (SALS), and rheodielectric and electrorheological properties of a binary mixture of octyloxy cyanobiphenyl and hexyloxy cyanobiphenyl liquid crystals. The mixture exhibits nematic (N) to smectic-A (SmA) phase transitions, and then again to a reentrant nematic (N(R)) phase transition. Rapid shear thinning in the quenched samples in the low shear rate region in the N and SmA phases observed from SALS experiments is attributed to the realignment of the director within the domains. The domains are elongated along the shear direction at higher shear rates. The temperature variation of the effective viscosity and static dielectric constant reveals the changes in the director orientation across N-SmA-N(R) phase transitions. At a steady shear rate the effective viscosity increases with the electric field in all the phases and saturates at much higher fields. It also exhibits two anomalous peaks across N-SmA-N(R) phase transitions beyond a particular field. The shear modulus of the SmA phase in an intermediate field is significantly larger than that measured at both low and high fields. This enhanced viscoelasticity of the SmA phase is argued to originate from the increased dislocation density.

Microstructure and electrical conductivity of hybrid liquid crystalline composites including 5CB, carbon nanotubes and clay platelets

The microstructure and electrical conductivity of 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal (LC) filled with multiwalled carbon nanotubes (NTs), organophilic montmorillonite (OM) clay platelets or their hybrid mixture OMNT were studied in the temperature range 290–350 K. The OMs were prepared by organic modification of montmorillonites obtained from different mineral deposits. In general, the samples 5CB+OMNT containing OM demonstrated more homogeneous distribution of filler particles than the samples 5CB+NT with the same amount of nanotubes. For the OM based on montmorillonite from Pyzhevsk deposit (Ukraine) we obtained that change of OMNT filler concentration from 1 to 4.5 wt% resulted in transition from the well developed nanoparticle network to the structure of isolated agglomerates. This was accompanied by an increase (1% of OMNT filler) and a decrease (4.5% of OMNT filler) of electrical conductivity in more than one order of magnitude as compared with the 5CB+NT composite with the same amount of nanotubes. The microstructure and electrical conductivity of the composites were also dependent on the geological origin of the clay minerals.

Probing the pore structure of a chiral periodic mesoporous organosilica using liquid crystals

Periodic mesoporous organosilicas (PMO) are prepared by the surfactant-templated condensation of bridged organosilsesquioxane monomers. By controlling the nature of the organic segment, the type of surfactant and the condensation conditions, one can control the physical and chemical properties of the resulting PMO and produce highly ordered porous structures with a periodicity on the nanometer scale. The development of chiral PMO materials has been of significant interest given their potential in heterogeneous asymmetric catalysis, chiral chromatography and non-linear optics. Characterization of the chirality of pore structures in these materials thus far has been achieved by indirect methods including polarimetry and solid-state circular dichroism. We report herein a general and convenient approach to probe directly the pore structure of chiral PMO materials based on their interactions with inexpensive liquid crystalline solvents, which result in the induction of measurable chiral properties in the nematic (N) and smectic A (SmA) phases of the liquid crystals. The templated co-condensation of a biphenylene organosilsesquioxane monomer and a chiral binaphthyl organosilsesquioxane monomer produced a new chiral PMO material that was investigated as dopant in two different liquid crystal hosts. Measurements of induced circular dichroism and helical pitch in the nematic phase of the cyanobiphenyl liquid crystal 5CB, and the measurement of an induced electroclinic effect in the SmA phase of the phenyl benzoate liquid crystal 9OO4 were carried out. The induced chiral properties measured in these experiments are consistent with chirality transfer taking place inside the pores, and suggest that the inner structure of the pores in the PMO material is indeed chiral.

Surface ordering and anchoring behaviour at liquid crystal surfaces laden with semifluorinated alkane molecules

We study the surface ordering and anchoring behaviour at liquid crystal/air interfaces laden with the semifluorinated alkane C18H37–C12F25. On alkanes, the semifluorinated alkane is known to form Gibbs films exhibiting a sharp transition from a dilute state at higher temperatures to a dense state at lower temperatures. We observe a similar transition in Gibbs films of this compound on the surface of cyanobiphenyl liquid crystals. The transition temperature can be tuned by controlling the C18H37–C12F25 concentration in the liquid crystal bulk phase. Our results show that the transition in the Gibbs film changes the anchoring of the liquid crystal from homeotropic at higher temperatures to planar at lower temperatures. When the transition temperature of the Gibbs film is tuned so that it takes place above the nematic–isotropic or smectic–isotropic transition of the liquid crystals, a pronounced effect on the nematic or smectic surface order is observed. Our study demonstrates for the first time that the ordering and anchoring behaviour of thermotropic liquid crystals can be tuned at a liquid crystal/air interface.