Liquid Crystal Behavior Research Articles

Acousto- and acousto-optic effects in liquid crystals: An overview

An overview of the following three aspects of the acoustical and related optical behavior of liquid crystals, namely i) ultrasonic studies, ii) acousto-optical effects, and iii) surface wave induced optical effects have been discussed. The temperature dependent ultrasound velocity has been used for the evaluation of the adiabatic compressibility (kS) and the acoustic impedance (Z). A correlation between thermodynamic functions and thermo-elastic properties facilitated the evaluation of the specific heat at constant volume (CV), the ratio of specific heats (g) and the isothermal compressibility (kT) across liquid crystal-isotropic phase transition. Acousto-optical effect in liquid crystals has been described in terms of molecular reorientation related to flows and turbulences due to acoustic vibration and radiation pressure in the medium. One important aspect of the acousto-optical effects is the surface wave induced optical effects. The change in transparency of a normal layer of liquid crystal under crossed polarizers are discussed which is interpreted as a consequence of the rotation of the optic axis. In addition, the acoustic emission occurring from homeotropically-aligned liquid crystals irradiated by surface acoustic waves are also discussed.

Efficient helical columnar emitters of chiral homoleptic Pt(ii) metallomesogens for circularly polarized electroluminescence.

Chiral organometallic Pt(ii) complexes have been demonstrated to be excellent circularly polarized luminescence (CPL) materials due to their rich phosphorescence and strong self-assembly characteristics. However, it remains a formidable task to simultaneously achieve high luminance (L) and electroluminescence dissymmetry factor (g EL) values for circularly polarized electroluminescence (CP-EL) devices of Pt(ii) complex-based emitters. In this study, we carry out a straightforward and efficient protocol to construct highly CPL-active helical columnar () emitters by using chiral homoleptic triazolatoplatinum(ii) metallomesogens (R/S-HPt). The peripheral flexible groups can not only improve solubility but also favor the induction of chirality and liquid crystal behavior. The resultant complexes R/S-HPt can self-assemble into the mesophase over a broad temperature range (6-358 °C) and exhibit excellent phosphorescence (Φ: up to 86%), resulting in intense CPL signals after thermal annealing (λ em = 615 nm and |g em| = 0.051). Using emitting layers (EML) based on R/S-HPt in solution-processed CP-EL devices, L max and |g EL| of CP-EL can reach up to 11 379 cd m-2 and 0.014, respectively. With comprehensive consideration of L max and g EL, this investigation shows the excellent performances among Pt(ii) complex-based CP-EL devices.

Phase behavior of polymer dispersed liquid crystals, comparison between mean-field theory, and coarse-grained molecular dynamics simulations

We report a simulation methodology to quantitatively predict the thermodynamic behaviour (phase diagrams) of polymer mixtures, that exhibit phases with broken orientational symmetry.

Impact of the position of nitrogen atom on the liquid crystal, optical and multi-stimuli response behaviors of rod-like mesogens containing multiple functional building blocks

Two novel rod-like mesogens containing one cyanostyrene, one pyridine and one pyrene as π-conjugate rigid core were designed and synthesized by Suzuki coupling and Knoevenagel reactions. The effect of the position of nitrogen-atom on the thermodynamic behavior, liquid crystalline self-assembly, optical behavior, mechanochromism behavior and acidochromism behavior was investigated. The rod-like mesogen in which the nitrogen atom is adjacent to the pyrene displays a phase transition from smectic A to smectic C by increasing temperature, whereas the other rod-like mesogen can only self-assemble into smectic C phase. This difference in mesophase transition behavior might be attributed to the position of nitrogen-atom which induces the change in molecular configuration. Both mesogens exhibit positive solvatochromism and high fluorescence in both solution state and aggregated state which might be attributed to the intramolecular charge transfer and twisted molecular configurations, respectively. Reversible mechanochromism behavior due to changes in molecular stacking and acidochromism behavior due to protonation and deprotonation were also achieved. These multi-stimuli responsive behaviors of the designed rod-like mesogens demonstrate potential applications in the field of multifunctional materials.

Synthesis and performance of novel fluoro acrylate polymer containing rigid cyclohexane group as a mesogenic core

Fluorine-containing liquid crystal polymers have excellent hydrophobic stability, making them attractive for use in the field of stable hydrophobic coatings. To improve the hydrophobic stability of fluoropolymers, a novel fluoro acrylate monomer (FAM) containing a rigid cyclohexane structure was simply synthesized using isophorone diisocyanate (IPDI), hydroxyethyl methacrylate (HEMA), and perfluorohexylethanol (S104) as raw materials and its homopolymer (PFAM) was prepared by radical solution polymerization. The structure, liquid crystal behavior, thermal properties, hydrophobic properties, and surface properties of FAM or PFAM and a reference substance, dodecafluoroheptyl methacrylate (DFMA) and its homopolymer (PDFMA), were studied using various techniques including FT-IR, NMR, GPC, XRD, POM, DSC, TGA, XPS, AFM, and static/dynamic contact angle measurements. The results indicate that FAM and PFAM are thermally liquid crystalline. PFAM's thermal stability is higher than that of PDFMA. The presence of a rigid cyclohexane structure facilitates the dense distribution of fluorocarbon chain tails of the homopolymer and also prevents surface reconstruction when the homopolymer comes into contact with polar liquids, resulting in improved hydrophobicity and hydrophobic stability. Thus, this study provides a straightforward method for the synthesis of fluoro acrylates with surface stability, highlighting their potential application in hydrophobic coatings.

The effect of fluorination on the liquid crystal and optical behaviors of amphiphilic cyanostilbene-based mesogens

Three series of amphiphilic mesogens containing a cyanostilbene core with zero, one or two lateral fluorine atoms, which attach one polar glycerol group at one end and three lipophilic alkyl chains at the other end were synthesized by Suzuki coupling and Knoevenagel reactions. The thermodynamics, liquid crystalline and photophysical properties of these amphiphilic mesogens were evaluated by polarized optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction, density functional theory calculation, absorption and emission spectra. The non-fluoro and mono-fluoro amphiphilic mesogens display a mesophase transition from hexagonal columnar phase to three-dimensional micellar cubic phase with Im3¯m lattice by the increase of temperature or the elongation of terminal lipophilic alkyl chains. The bi-fluoro amphiphilic mesogens can only self-organize into hexagonal columnar mesophase. Similar spectral properties and weak solvatochromism in these amphiphilic mesogens are observed attributed to the weak intramolecular charge transfer and weak influence of fluorine atom on the electronic properties in both ground and excited states. In addition, aggregation-induced emission behavior could be achieved in these amphiphilic mesogens due to the twisted molecular configuration. Therefore, complex self-assembly structures and interesting photophysical properties may be altered by varying the degree of fluorination of amphiphilic mesogens, which could promote the application of liquid crystalline materials.

PATH-60. BRAIN TUMORS: FROM ONCOSTREAMS TO LIQUID CRYSTALS

Abstract High grade gliomas (HGG) are the most common, destructive, and varied of all malignant brain tumors. HGG are heterogeneous at the histological, cellular, and molecular level, which makes them hard to study and treat. One particular tumor pathological differentiation is mesenchymal differentiation, containing oblong, motile cells. We recently noted distinct fascicles of elongated, aligned, mesenchymal-like cells in mouse and human gliomas, which we denote as oncostreams. Time-lapse confocal microscopy in ex vivo slices, and in in vivo two photon imaging, indicated that cells in oncostreams are motile. Oncostream motility was classified based on cellular orientation. The molecular characteristics of oncostreams were determined by laser capture microdissection, RNA-sequencing, and bioinformatics. 43 genes were differentially expressed; COL1A1 was overexpressed in oncostreams. Inhibition of COL1A1 in mouse gliomas, using the Sleeping Beauty transposon model, reduced oncostreams, tumor aggressiveness, proliferation, tumor vasculature, and collective glioma invasion. More recently, we elucidated that glioma cell growth in vitro, and potentially in vivo displays domains of nematic orientation (oncostreams) and topological defects, suggesting that gliomas are organized as liquid crystals. Topological defects are singularities of local orientation, and characteristic of liquid crystals. Further, studies of gliomas in vivo, suggest the presence of ±1/2 topological defects. Thus, nematic orientation and topological defects are present in brain tumors in vitro and in vivo. This provides support for our hypothesis that brain tumors are organized as active liquid crystals. As topological defects have been exploited to alter liquid crystal behavior, we hypothesize that manipulating brain tumor liquid crystalline behavior will be of therapeutic importance. Comba et al. (2022) Spatiotemporal analysis of glioma heterogeneity reveals COL1A1 as an actionable target to disrupt tumor progression. Nature Communications: Wood et al. (2023) Scale-free correlations and potential criticality in weakly ordered populations of brain cancer cells. Science Advances (in press).

Effect of CdSe/ZnS quantum dot dispersion on phase transitional behavior of 8OCB liquid crystal.

We report the effect on the phase transitional behavior of 8OCB liquid crystal (LC) doped with functionalized CdSe/ZnS quantum dots (QDs) in different concentrations. The temperature-dependent data of high-resolution optical birefringence and dielectric anisotropy are utilized to characterize the critical anomaly for both the isotropic-to-nematic (I-N) and nematic-to-smectic-A (N-SmA) phase transitions. The obtained results reveal that the order parameter exponent (β) for the pure LC is found to be β=0.249 and does not vary upon the inclusion of QDs in the pure matrix. It describes the weakly first-order characteristic of the I-N phase transition for all the studied samples, which falls within the limit of the tricritical hypothesis. Conversely, depending on the range of the N phase, we observed a nonuniversal nature of the specific heat capacity critical exponent (α^{'}) linked with the N-SmA phase transition for all the studied samples. A relative comparison was made amongst the α^{'} values extracted from both the anisotropy data, and further, a theoretical relationship is established with these exponent values. The coupling strength among the N and SmA order parameters is determined using the optical birefringence data and discussed from the perspective of mutual interaction between the LC-QDs ligands. The results signify that a strong ligand-ligand interaction between neighboring QDs effectively reduces the N range and slightly influences the N-SmA phase transition.

Helical structures of achiral liquid crystals under cylindrical confinement.

Confined liquid crystals (LCs) exhibit complex and intriguing structures, which are fascinating fundamental problems in soft matter. The helical structure of cylindrical cavities is of great importance in LC studies, particularly for their application in optical devices. In this study, we employ molecular dynamics simulations to explore the behavior of achiral smectic-B LCs confined in narrow cylindrical cavities, where geometric frustration plays an important role. By increasing the cylinder size, LCs exhibit a transition from multi-helical to layered structures. Notably, we observe two stable structures, namely the helical structure and the layered structure, at moderate cylinder size. We also investigate the effects of the arrangement of cylindrical wall particles (hexagonal or square array) and anchoring strength on the LC structure. Our findings reveal that both the hexagonal array and strong anchoring strength promote the formation of helical structures. Our study provides novel insights into the confinement physics of LCs and highlights the potential for achieving helical structures in achiral LCs, which will expand the future applications of LCs.

Hydrogen bond ferroelectric liquid crystal for tripartite temperature sensor: Experimental and computational (DFT) studies

The mesogenic behavior and electronic properties of hydrogen bond ferroelectric liquid crystal (HBFLC) complex are studied using experimental and theoretical techniques. HBFLC complex is synthesized from mesogenic 4-hexyloxy benzoic acid (6O BA) as the proton donor and nonmesogenic chiral DL-tartaric acid (DLTA) as the proton acceptor through intermolecular hydrogen bonding. Molecular geometry of DLTA+6O BA HBFLC complex is optimized by density functional theory (DFT) calculation. An interesting observation is that polarizing optical microscope (POM) study confirms that the HBFLC complex exhibits thermochromic behavior in chiral nematic (N[Formula: see text] along with induced nontitled smectic G* phases. Another noteworthy observation is the manifestation of tripartite temperature stages. Fourier-transform infrared spectroscopy (FTIR) and natural bond orbital (NBO) analysis elucidate the intermolecular hydrogen bond between mesogenic (6O BA) and nonmesogenic (DLTA) compounds. Further, frontier molecular orbital (FMO) study and electrostatic potential (ESP) analysis explore the reactivity sites, intermolecular charge transfer and electronic behavior of HBFLC complex.

Electronic, optical and spectroscopic properties of N-dialkyl-imidazolium hexafluorophosphate (CNMIM.PF6) ionic liquid crystal molecules investigated by computational methods.

In this present work, we calculate the electronic, spectroscopic and nonlinear optical properties (NLO) of N-dialkyl-imidazolium hexafluorophosphate (CNMIM.PF6, where N = 10, 12, 14, 16, 18, 20) ionic liquid crystal molecules under the effect of alkyl chain length variation in cation moiety [CNMIM]+ with fixed anion [PF6]-. CONTEXT: The majority of research on ionic liquid crystal to date has been focused on experiments, while theoretical studies on the optical properties of ionic liquid crystal have been extremely rare. Nonlinear phenomena in optical devices have attracted many researchers. Therefore, results of NLO properties may favor facile synthesis and fabrication of novel-type of materials as well as optoelectronic devices. Spectroscopic studies elucidate further insight into ionic liquid crystal behavior. The results demonstrate that variations in alkyl chain length have an impact on the conformers' electrical, spectroscopic, and NLO properties as well as their stability. The stability of ionic liquid crystal molecules increases with increasein the alkyl chain length and the energy band gap range is 6.64-6.29 eV. Understanding ionic liquid crystal's physical behavior requires an understanding of their dipole moments and NLO features, which are covered in this article. The results of NLO characteristics for all ionic liquid crystal molecules show that their first-order hyperpolarizabilities are higher than the reference molecule (urea). METHODS: The electronic (molecular energy band gap, electrostatic potential map, as well as HOMO-LUMO orbitals) and spectroscopic (IR-RAMAN, UV) properties were evaluated with the help of theoretical model at B3LYP/6-31G(d) while the NLO study has been performed using B3LYP and M06-2X with different basis sets 6-31G(d) and 6-311++G(d,p), as implemented in Gaussian09 software.

Non-spherical assemblies of chitin nanocrystals by drop impact assembly

Preparing complex non-spherical assemblies of elongated nanoparticles and exploring their topological conformations is a challenge due to liquid crystals' mobility and elastic distortion. Here, we fabricated a variety of non-spherical liquid crystal assemblies of chitin nanocrystals (ChNCs) in a coagulation bath containing sodium triphosphate (STP) by drop impact assembly method, and the forming mechanism and internal topology were systematically investigated. The collection height, ChNCs concentration, and STP concentration have significant influence on the shape and size of the assembled structures. Long-range ordered structures and long-lived topological textures of the ChNCs liquid crystal can be obtained since a molecular interaction of hydrogen bonding and electrostatic attractions between ChNCs and STP occur during the impact assembly. Rheological and kinetic analysis suggested the shear thinning behavior of the ChNCs liquid crystals and the rapid gelation phenomenon of ChNCs induced by STP. Morphology results showed that the rod-like ChNCs in the non-spherical assemblies were orderly and closely arranged with periodic repetition and layered structure. The non-spherical assemblies of ChNCs liquid crystals can be used as carriers of carbon nanotubes, magnetic Fe3O4 nanoparticles, synthesized polymers, and anticancer drugs for functional composite applications. The drop impact assembly method of ChNCs liquid crystal structure is highly controllable on the composition, morphology, and function, which shows promising applications in energy, environmental-friendly, and bioactive materials.

Spectroscopy, electrochemistry and liquid crystal properties of monosubstituted ferrocene based azobenzene derivatives with 1,2,3-triazole heterocycle

Monosubstituted ferrocene based azobenzene liquid crystals were synthesized, in which ferrocene was flexibly attached to the mesomorphic core by click reaction. The counterpart compounds without ferrocene unit were also prepared. Two series of compounds displayed liquid crystal behaviours during heating and cooling, and had wide mesophase range. In the process of cooling, the compounds underwent a transformation from nematic phase to smectic C phase. When ferrocene was introduced into the molecule, the melting point of most compounds decreased by 30–40 °C, and the mesophase range was still as wide as 71–97 °C. These compounds emitted fluorescence at 400–550 nm and had strong absorption near 280 nm and 350 nm. Under the irradiation of 365 nm ultraviolet light, these compounds transformed rapidly from E to Z isomers, while the reverse reaction occurred slowly in the dark, especially the molecules containing ferrocene. Cyclic voltammetry studies showed that ferrocene based azobenzene derivatives exhibited one electron transfer processes with low formal redox potentials, indicating that these compounds are easily oxidized.

Chirped pulses for Nematicons in liquid crystals with cubic-septic law nonlinearity

This manuscript concentrates the behaviour of nematic liquid crystals (NLC) incorporating cubic-septic law property. The resulting solutions of the model play a significant role in the energy transport of soliton molecules in liquid crystals. In this study, the Jacobi elliptic (JE) function technique, which is one of the efficient integration procedure, is used to investigate chirped elliptic and solitary wave solitons like bright, dark, kink, singular, hyperbolic with some constraint conditions while the hyperbolic type traveling wave solutions of the equation defining the NLC incorporating cubic-septic law property is also represented as dark and singular solitons. The linear section of the obtained pulse chirp has two intensity-dependent chirping components that change the chirp. The results of this work could help us understand how chirped solitary waves (CSW) propagate in a weakly nonlocal cubic-septic law medium. This study improves the understanding of nonlinear light-matter interactions in liquid crystals by providing new insights into the role of chirped pulses in forming and modulating spatial optical solitons. Furthermore, we provide successful results in 3D, 2D, and contour forms. It is emphasized that the constrained technique is more effective and powerful than other ways, and the conclusions drawn in this work can contribute in understanding of the soliton molecules found in liquid crystals.

Study on synthesis, properties, and aggregation behavior of sodium alkyl glycoside sulfate

In this work, sodium dodecyl and tetradecyl glycoside sulfate (APGS-Na) was synthesized by sulfating dodecyl and tetradecyl glycoside (APG1214) with chloro-sulfonic acid (CSA). The product was then characterized using FT-IR, 1H NMR, MS, and elemental analysis. Response surface methodology (RSM) was used to explore the sulfation modification conditions in order to achieve the maximum APG1214 conversion rate (84.79% with the molar ratio of CSA to APG1214: 2.17:1, reaction temperature: 16 °C, and reaction period: 2 h). Further studies were performed to establish the relationship between the pertinent structure and physicochemical characteristics, such as Krafft point, equilibrium surface tension, dynamic surface tension, aggregation behavior of micelles and lyotropic liquid crystals (LLCs), as well as application characteristics like foam property, emulsification, detergency, and irritability. The results demonstrated that the Krafft point of APGS-Na was below 0 °C, and the surface tension (γcmc ) was 39.4 mN·m−1 when the critical micellar concentration (CMC) was 2.40 g·L−1. At low concentration, the adsorption process was controlled by a diffusion step, different to a mixed kinetic-diffusion adsorption mechanism at high concentration, can form large size micelles and attractive LLCs in water. Moreover, the surfactant is nonirritant, with excellent foaming and decontamination capability. Accordingly, APGS-Na is expected to be widely used in diverse fields as a green anionic glycoside surfactant.

Effect of quantum dots on the phase behavior and order of 8CB liquid crystal

We experimentally investigate the impact of surface functionalized hydrophobic core-shell CdSe/ZnS quantum dots on the phase behavior and order of the host liquid crystal compound 4-cyano-4'-octylbiphenyl (8CB) by means of polarized optical microscopy, polarized micro-Raman spectroscopy and X-ray diffraction. The orientational order of the nanocomposite system is probed by means of birefringence measurements as a function of temperature and mass fraction of the quantum dots. The dispersion quality of the dopant in the samples, partial-aggregation phenomena and/or nanoparticle organization give rise to non-monotonic variation of the probed thermodynamic quantities. A critical dopant mass fraction above which the nematic phase becomes paranematic is evidenced. A phenomenological model accounts for phase separation effects and a qualitative interpretation of the experimental results. The impact of nanoparticles on the macroscopic nematic order is discussed in the frame of a random anisotropy Lebwohl-Lasher model.

Prediction of the liquid-crystal phase behavior of hard right triangles from fourth-virial density-functional theories.

We have used an extended scaled-particle theory that incorporates four-body correlations through the fourth-order virial coefficient to analyze the orientational properties of a fluid of hard right isosceles triangles. This fluid has been analyzed by computer simulation studies, with clear indications of strong octatic correlations present in the liquid-crystal phase, although the more symmetric order tetratic phase would seem to be the most plausible candidate. Standard theories based on the second virial coefficient are unable to reproduce this behavior. Our extended theory predicts that octatic correlations, associated to a symmetry under global rotations of the oriented fluid by 45^{∘}, are highly enhanced, but not enough to give rise to a thermodynamically stable phase with strict octatic symmetry. We discuss different scenarios to improve the theoretical understanding of the elusive octatic phase in this intriguing fluid.

Structural transition in a ferroelectric nano-dispersed cholesteric liquid crystal

ABSTRACT We present a theoretical study of the orientational and electrical behaviors of a cholesteric liquid crystal dispersed with ferroelectric nanoparticles. We assume a soft planar coupling between the liquid crystal molecules and the nanoparticles. We consider two spiral structures in the ferroelectric nano-dispersed system under an external electric field. This assumption is due to the fact that the director and the polarization vector would have different pitches of spiral structure. We study the behavior of the average polarization and the pitch of the helical structure as a function of the field strength. The impact of ferroelectric nanoparticles on cholesteric-nematic phase transition is investigated by calculating the critical electric field. The influence of field strength and material parameters on the phase transition is also discussed. The calculations are based on a developed continuum theory and a modified form of free energy for the helical supramolecular structure. The influence of nanoparticle volume fraction on the helix unwinding of both spiral structures is studied. It is found that an electric field with a sufficiently high strength causes an increase in the pitch of the helical structure of polarization. We obtain a critical volume fraction of nanoparticles, after which the pitch of the polarization helical structure differs from the director pitch.

Surface acoustic wave investigation of magnetic nanoparticle size and concentration effect on liquid crystal behavior.

The effect of spherical magnetic nanoparticles with different size (5, 10, 15, and 20 nm) and volume concentration (10-3, 5 × 10-4, and 10-4) on liquid crystal 4-cyano-4'-hexylbiphenyl (6CB) behavior was investigated using surface acoustic wave (SAW). The attenuation response of SAW propagating along with the substrate/liquid crystal interface was used to study the structural changes induced by an applied magnetic field. The obtained results showed the shift of the threshold magnetic field with an increase in the volume concentration of nanoparticles toward lower fields and also the decrease in the isotropic-nematic phase transition temperature depending on the nanoparticle size and the nanoparticle volume fraction. Results confirmed again that the bulk viscosity coefficients should dominate the SAW attenuation and that the SAW investigation in the presented configuration is applicable to monitoring of the role of magnetic dopants in structural changes under external fields. Some theoretical background of the presented SAW investigation is introduced as well. Obtained results are discussed within the context of previous ones.

Phase behavior of nematic-nanoparticle mixtures

We study the effects of nanoparticles (NPs) on thermotropic nematic liquid crystals (LCs) in relatively dilute NP–LC mixtures. We are interested in the fundamental generic mechanisms that quantitatively and qualitatively affect the phase behavior of LCs. A simple molecular field analysis shows that a phase transition will likely occur upon entry into the ordered phase. Moreover, the interaction between nematogenic NPs and LCs could force a sergeant–soldier-like behavior, in which only the phase behavior of one component is affected despite the symmetric appearance of the coupling term. When NPs are anisotropic, their influence on LC phase behavior can be qualitatively different depending on the anchoring, even in the absence of the disorder. We illustrate numerically that a random-field-type disorder might impose either short-range, quasi-long-range, or even long-range order, which might survive.