Liquid Crystalline Molecules Research Articles

Cholesteric-isotropic phase transitions of banana-shaped liquid crystalline molecules

A mean field theory is introduced to describe cholesteric (Ch) phases of banana-shaped liquid crystalline molecules. From a geometric point of view, we derive the tensor order parameter of the banana-shaped molecules as a function of a bend angle () and four orientational order parameters. Using the tensor order parameter, we derive the free energy of the cholesteric phase for the banana-shaped molecules and obtain the approximate formula of a cholesteric-isotropic phase transition (CIT) temperature as a function of the bend angle. We find a cholesteric C phase for < 70.5 or > 109.5 where the the two arm vectors of the banana-shaped molecule tend to parallel to the director and a cholesteric C phase for 70.5 < < 109.5 where the two arm vectors tend to perpendicular to the director. These Ch phases are also distinguished by using the primary axis of the orthogonal molecular axis.

Optical studies of n-octyloxy-cyanobiphenyl (8ocb) with dispersed ZnO nanoparticles for display device application

Liquid crystalline (LC) materials have the ability to transfer their long range orientation order in the dispersed materials such as nanoparticles and various colloids. ZnO nanoparticles have wide variety of applications and influence the birefringence properties of liquid crystalline molecules. The paper describes the systematic studies on liquid crystalline n-octyloxy-cyanobiphenyl (8ocb) compound with dispersed ZnO nanoparticles in different concentrations. The presence of ZnO nanoparticles in 8ocb are determined by optical, scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies. The refractive indices and birefringence measurements are carried out at different wavelengths (460, 500, 570 and 635 nm) using wedge shaped cell with modified spectrometer. The birefringence (δn) increases from 14 to 35% with increasing weight concentration of ZnO nanoparticles in the nematic thermal range. This is due to strong Vander Waal’s interaction between LC molecules and ZnO nanoparticles which increases the alignment of the LC molecules. Further the order parameter S obtained from Kuczynski internal field model, Vuks model and Haller extrapolation methods are in good agreement. It is found that the order parameter S increases with increase in dispersion of concentration of ZnO nanoparticles from 11.2 to 23.9%. The prepared materials find potential applications in many optical devices.

Uniaxial and biaxial nematic phases of banana-shaped molecules and the effects of an external field

ABSTRACTA mean field theory is introduced to describe uniaxial and biaxial nematic phases of banana-shaped liquid crystalline molecules. From a geometric point of view, we derive the tensor order parameter of the banana-shaped molecules as a function of a bend angle and four orientational order parameters. Using the nematic free energy of the Maier-Saupe type, we calculate the phase diagram on the temperaturebend angle () plane and find three uniaxial nematic () and three biaxial nematic () phases, where is the primary axis of the orthogonal molecular axis. We also derive the approximate formulas of a nematic-isotropic phase transition () and a biaxial-uniaxial phase transition () temperature as a function of the bend angle. We also discuss the effects of the external field on the temperature and find the negative dependence of the temperature on the external field.

Voltage controlled scattering from porous silicon Mie-particles in liquid crystals

Creating novel liquid crystalline (LC) photonic metamaterial structures is a highly promising approach for the development of tunable photonic devices particularly when nanoporous structures are used in which the LC can infiltrate and modify their properties. In the present work, a tunable scattering device made of partially oxidized porous silicon microparticles mixed with LC is proposed and demonstrated. Being porous in nature, the microparticles trap the LC molecules and enhance the scattering in the voltage OFF state. Due to the random orientational order inside the particles, the averaged refractive index (RI) inside these nanopores is close to that of the isotropic LC RI in the OFF state. Averaging between this LC and the oxidized porous Si brings the average RI of the microparticle down and maximizes the index mismatch with the surrounding LC regions. When the porosity is large enough, a high RI mismatch between the particles and its surrounding in the OFF state is obtained. In the ON state, the molecules orient along the electric field direction, decreasing the RI mismatch and increasing the transparency of the system. The scattering decreases gradually and reversibly with the applied voltage while at zero voltage state it decreases with temperature. At the high voltage state the scattering increases with temperature with some enhancement at the clearing point due to the LC order parameter fluctuations. A slight decrease of the clearing point is observed as a result of weakening the intramolecular interaction and lowering the order parameter. Using fluorescent confocal microscopy the changes in the LC and microparticles domains are monitored showing some reversible dragging of the microparticles towards one of the substrates as the voltage increases. Being a tunable light scattering device, it has a great potential for smart window and light shutter applications.

Formation of oriented luminescent organic thin films on modified polymer substrate

Nanostructured organic thin films with polarized luminescence are obtained by means of the slow vapor deposition of fuorescent liquid crystalline dipolar molecules on the surface of the rubbed polyamide layer. Based on the polarized photoluminescence measurements, the polarization degree of thin film based on blue-emitting organic liquid crystalline molecules 4-(n-pentyl)-3-chloro-4′′′-trifluoromethoxy-[1,1′, 4′, 1′′, 4′′, 1′′′-quaterphenyl] (M-BLUE) was found to be 29%, and for red-emitting 4-(dicyanomethylene)-2-(4-amylcyclohexyl)-6[4-(dimethylaminostyryl)]-4H-pyran molecule (M-RED) it is as high as 78%. It can be connected with the higher dipole momentum of M-RED molecule (15) than M-BLUE (3.45), and can be ascribed to its higher interaction with polyamide matrix.

Morphology formation in binary mixtures upon gradual destabilisation.

Spontaneous liquid-liquid phase separation is commonly understood in terms of phenomenological mean-field theories. These theories correctly predict the structural features of the fluid at sufficiently long time scales and wavelengths. However, these conditions are not met in various examples in biology and materials science where the mixture is slowly destabilised, and phase separation is strongly affected by critical thermal fluctuations. We propose a mechanism of pretransitional structuring of a mixture that approaches the miscibility gap and predict scaling relations that describe how the characteristic feature size of the emerging morphology decreases with an increasing quench rate. These predictions quantitatively agree with our kinetic Monte Carlo and molecular dynamics simulations of a phase-separating binary mixture, as well as with previously reported experimental observations. We discuss how these predictions are affected by non-conserved order parameters (e.g., due to chemical reactions or alignment of liquid-crystalline molecules), hydrodynamics and active transport.

Columnar liquid crystalline assembly of a U‐shaped molecular scaffold stabilized by covalent or noncovalent incorporation of aromatic molecules

ABSTRACTWe report on our serendipitous finding of the anomalous behavior of a novel liquid crystalline (LC) molecule UH/H, containing a U‐shaped handle. While UH/H itself shows a bicontinuous cubic (Cubbi) phase, UH/H in the presence of pyrene molecules as an external guest forms a hexagonal columnar (Colh) phase without phase separation. Interestingly, even when the pyrene molecules were covalently attached to UH/H (UPy/Py), the molecule exhibited a Colh phase, with a similar columnar geometry to that of UH/H mixed with pyrene molecules (1–4 equiv.). This result means that no matter how the pyrene moieties are incorporated in the LC system, the resultant material exhibited a similar molecular assembly with a columnar geometry. This finding is interesting because UH/H and pyrene molecules do not seem to have specific interactions or shape complementarity to form a columnar assembly, as in the case of UPy/Py. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 342–351

Influence of Pre-Tilt Angles of Liquid Crystalline Molecules Placed Onto Orienting Substrates on Characteristics of Liquid Crystalline Modulators Based on π-Cells

Influence of Pre-Tilt Angles of Liquid Crystalline Molecules Placed Onto Orienting Substrates on Characteristics of Liquid Crystalline Modulators Based on π-Cells

Effect of fluorine substitution on the core geometry and refractive indices in biphenyl-cyclohexane liquid crystals: A DFT study

Incorporating fluorine into liquid crystalline molecules has been found to bring about desirable changes to their physicochemical properties to improve the behavior of the parent compound. This paper attempts to determine the effect of fluorine incorporation on refractive indices in biphenyl-cyclohexane (BCH) liquid crystals. To understand the mechanisms of the effect, quantum chemistry calculations were carried out. For the investigated liquid crystals, refractive indices are estimated through the calculated molecular polarizabilities at different optical wavelengths. Through a systematic relationship between the fluorine substituents and the optical constants, it has been demonstrated that the refractive indices of the investigated liquid crystals do not change largely in the studied wavelength range. However, the fluorine substituents significantly affect refractive indices. The effects of fluorine substitution on the core geometry are also examined.

Columnar self-assembly of bowl-shaped luminscent oxadiazole calix[4]arene derivatives

In the present investigation, we wish to report a new class of blue light emitting liquid crystalline bowl-shaped molecules based on oxadiazole calix[4]arene has been designed and synthesized. These supramolecular compounds were investigated by polarizing optical microscope (POM), differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), high temperature X-ray diffraction and photophysical studies. We have prepared total six newly supramolecular derivatives having butyl alkyl bridge with oxadiazole ring and variable trisubstituted side alkoxy chain on the lower rim of calix[4]arene. The present synthesized bowl-shaped molecules are promising to stabilize the hexagonal columnar phase over a broad thermal range. Thermal behavior as well as photophysical behaviors of these new bowl-shaped molecules are extremely dependent on the number and types of peripheral tail group present in the lower rim of calix[4]arene inbuilt with schiff-base linking unit. Additionally, these synthesized oxadiazole calix[4]arene based supramolecular compounds showed blue luminescence in solution as well as thin films under long wavelength UV light. These properties of present material suggest that these may have promising applications in organic electronics.

Perylene-Based Liquid Crystals as Materials for Organic Electronics Applications.

Columnar phases formed by the stacking of disclike molecules with an intimate π-π overlap forms a 1D pathway for the anisotropic charge migration along the columns. Columnar phases have great potential in organic electronic devices to be utilized as active semiconducting layers in comparison to organic single crystals or amorphous polymers in terms of processability, ease of handling, and high charge carrier mobility. Intelligent molecular engineering of perylene and its derivatives provided access to tune the physical properties and self-assembly behavior. The columnar phase formed by perylene derivatives has great potential in the fabrication of organic electronic devices. There are several positions on the perylene molecule, which can be functionalized to tune its self-assembly, as well as optoelectronic properties. Thus, many liquid-crystalline molecules stabilizing the columnar phase, which are based on perylene tetraesters, perylene diester imides, and perylene bisimides, have been synthesized over the years. Their longitudinal and laterally extended derivatives, bay-substituted derivatives exhibiting a columnar phase, are reported. In addition, several liquid-crystalline oligomers and polymers based on perylene derivatives were also reported. All such modifications provide an option to tune the energy levels of frontier molecular orbitals with respect to the work function of the electrodes in devices and also the processability of such materials. In this feature article, we attempt to provide an overview of the molecular design developed to tune the applicable properties and self-assembly of perylene derivatives as well as recent developments related to their application in the fabrication of organic solar cells, organic light-emitting diodes, and organic field-effect transistors.

N-fused ring strategy toward orange/yellow light-emitting liquid crystalline molecules

A new synthetic procedure for fluorinated N-fused ring, 2F-fluorazone (1,2-difluoro-9H-pyrrolo[1,2]-indol-9-one), has been developed. It connected with acetylene to obtain the first example of fluorinated N-fused rings luminescent liquid crystals (LLCs). CH3O-fluorazone-2F and CH3O-fluorazone display yellow and orange fluorescence respectively, and their properties can be tuned by the substitution pattern.

Exploring “Dormant” Opto-Mechanical Properties of the Isotropic Phase of Liquid Crystals and Revealing Hidden Elasticity of (Ordinary) Liquids

There is little literature on the flow properties of the isotropic phase of liquid crystalline fluids. However, this phase is an ideal tool to bridge the physics of liquid crystals with those of (ordinary) fluids. Optical and mechanical studies are presented, demonstrating that away from any phase transition, the isotropic phase of liquid crystalline molecules (LCs) and liquid crystalline polymers (LCPs) can work as an optical oscillator in response to low-frequency mechanical excitation, establishing the elastic origin of the flow birefringence and “visualizing” the very existence of the elastic nature of the liquid state. Additionally, mimicking the excellent anchoring ability of liquid crystals, an alternative rheological protocol optimizing the fluid/substrate interfaces is presented to access the low-frequency shear elasticity in various one-component liquids and salt-free aqueous solutions.

Helical inversions and phase separations in binary mixtures of cholesteric liquid crystalline molecules

ABSTRACTA mean field theory is introduced to describe a helical inversion in nematic-cholesteric mixtures and binary cholesteric mixtures. Taking into account a chiral coupling between unlike molecules, the helical pitch is derived as a function of orientational order parameters and concentration. We find the conditions of helical inversions for the mixtures and derive the concentration of the helical inversions, depending on the chiral interaction. The numerical results are in agreement with the experimental results. We also examine phase separations on the temperature-concentration plane, including the helical inversions. A large variety of phase separations, such as the two-phase coexistence between a right-handed rich and a left-handed rich phase, etc., are predicted.

Preparation and cure kinetics of epoxy with nanodiamond modified with liquid crystalline epoxy

A novel epoxy composite with liquid crystalline epoxy resin modified nanodiamond (ND-LCER) was prepared by grafting diglycidyl ether of 4,4′-bisphenol (DGEBP) monomers onto the surface of acid treated nanodiamond (ND-COOH). Untreated nanodiamond (UND), ND-COOH and ND-LCER were composited with epoxy/4,4′-diamino diphenyl sulfone (DDS) in various weight ratios, respectively. The cure behavior of the prepared composite systems was investigated with dynamic differential scanning calorimetry (DSC) technology at different heating rates. The NDs exhibited acceleration effects on the curing reaction with increase of content amounts, which might attribute to the special structure of the nanodiamond (ND) particles itself and the functional groups of the ND. The modified ND particles showed the lowest activation energy value due to the diminution to the diffusion control process caused by liquid crystalline epoxy resins molecules. The two-parameter (m, n) autocatalytic model of Sestak-Berggren based on the Malek method is in-line with the epoxy/ND composites.

Low temperature processed high-performance thick film ternary polymer solar cell with enhanced stability

Recently, efficient thick-film polymer solar cells (PSCs) were reported to be promising candidates for large-area module device fabrication because of their potential compatibility with roll to roll processing technique. Nevertheless, most of thick-film PSCs with high power conversion efficiencies (PCEs) were fabricated under high temperature (over 100 °C, namely “Hot-Processing”), which restricts their potential commercialization application. Herein, a new pathway was developed to make high performance thick-film PSCs under a mild condition without hot-processing. It was found that the ternary strategy can efficiently tune the processing conditions of thick film PSCs due to the decrease of polymer aggregation intensity. Consequently, high-efficiency thick-film ternary PSCs (10.59%) without hot-processing are designed and fabricated by introducing the nematic liquid-crystalline small molecule (benzodithiophene terthiophene rhodanine, BTR) into fluorinated benzothiadiazole-based polymer (PffBT4T-2OD):fullerene host blend. The morphology studies reveal that the incorporation of BTR has a similar effect as hot-processing on the blend films, resulting in a much improved nano-scale phase separation. It was also demonstrated that the addition of BTR is beneficial for improving light harvesting, charge separation, transport and extraction of the resulting devices. Moreover, this ternary strategy shows a well general applicability for other systems that need hot-processing. Such as for naphthobisthiadiazole-based polymer PNTT, the optimized ternary device without hot-processing obtained an improved performance with a PCE of 11.44%, which is one of the highest-efficiency thick-film PSCs reported to date. Therefore, our results provide a facile approach to fabricate the high performance PSCs with thick-active layer at a lower temperature instead of hot-processing and may meet the needs of future roll-to-roll production of PSCs.

Effect of non-resonant polarized laser irradiation on the formation of nanostructured organic thin films

Nanostructured organic thin film with polarized luminescence is obtained by means of the vapor deposition of fluorescent liquid crystalline dipolar molecules in the presence of polarized laser irradiation outside the absorption band. It was shown that irradiation of molecules by non-resonant polarized laser beam during deposition strongly decreases the formation of aggregated molecules that is confirmed by atomic force microscopy, fluorescent microscopy and luminescent spectroscopy.

New fabrication approach to develop a high birefringence photo-crosslinked film based on a sulfur-containing liquid crystalline molecule with large temperature dependence of birefringence

ABSTRACTWe present a new fabrication approach to achieve a high birefringence film by means of photopolymerization based on an alkylthio-containing rod-like liquid crystalline molecule exhibiting large temperature dependence of birefringence. We designed a new reactive mesogen having alkylthio linkages (BPM–S). It was found that BPM–S had a larger increment of birefringence with decreasing temperature, relative to commercially available alkoxy analog LC242. This result could be thought to be due to enhanced intermolecular attractive interaction for an alkylthio mesogen implied by the proximity of laterally neighboring molecules and cybotactic nematic tendency based on wide-angle X-ray measurement. The uniaxially-aligned photo-polymerized film for BPM–S showed higher birefringence than that for LC 242.

Peripheral chain length-dependent complex phase behaviour in C3-symmetric hexa-alkylated liquid crystals

ABSTRACTWe report the thermal and self-assembly properties of C3-symmetric liquid crystalline (LC) molecules consisting of a conformationally tunable triazole-based mesogen and six-fold alkyl chains. Unlike the LC compound (1) based on non-crystallisable octyl chains, 2 and 3, which have crystallisable dodecyl and tetradecyl chains, respectively, exhibit a cold crystallisation which only takes place under slow heating conditions (2°C/min). In contrast with the vertically interdigitated lamellar crystalline phase of 1, a laterally interdigitated bilayered lamellar structure driven by the crystallisation of the dodecyl or tetradecyl chains is observed in the cold crystallisation temperature range. In addition to their crystalline morphology, 2 and 3 show LC morphological behaviour distinct from that of 1, 2 and 3 exhibit a hexagonal columnar LC phase consisting of T-shaped conformers rather than the lamellar LC phase of 1. The morphological transformation from the lamellar (1) to the columnar phase (2 and 3) can be rationalised by the alleviation of the conformational energy of the longer alkyl chains. Consequently, the simple variation of alkyl chain length in the C3-symmetric LC system results in contrasting thermal and assembly properties in the crystalline and LC phases.

Bowl liquid crystal based on cyclotriveratrylene derivatives with multiple triphenylene units: The influences of the numbers of triphenylene units on mesomorphic behaviors

Series of novel CTV derivatives with one, two and three triphenylene units were designed and synthesized via click reaction in 72–84% yields. The influences of the numbers of triphenylene units on mesomorphic behaviors were investigated. The results suggested that one triphenylene unit could not counteract the rigidity of CTV skeleton, resulting in no mesophase for CTV derivative 6 with one triphenylene unit. The strong induction of more triphenylene units leaded to the mesophase for CTV derivative 7 with two triphenylene units and 8 with three triphenylene units. The more triphenylene units decreased the phase transition temperatures. The XRD analysis suggested that compound 8 possessed higher ordered hexagonal columnar mesophase than compound 7, implying that the more triphenylene units were favorable for improving the structural symmetry and produced the ordered columnar stacking. Compound 8 was the first example of high ordered hexagonal columnar liquid crystalline molecule based on CTV skeleton.