Rod-shaped Liquid Crystals Research Articles

Fluorinated liquid crystals and their mixtures giving polar phases with enhanced low-temperature stability

ABSTRACT The fluid ferroelectrics, called ferroelectric nematics (NF), have recently become available by incorporating strong polarity into rod-shaped liquid crystal molecules. Its unprecedented electro-optic properties have created significant excitement in soft matter research. The further progression from the NF phase to the antiferroelectric smectic Z (SmZA) phase, and ultimately to the ferroelectric Smectic A (SmAF) phase, represents a remarkable journey in emerging polar liquid crystal states. Nevertheless, the limitation of NF liquid crystal materials remains one of the prominent obstacles to physical property optimization and optoelectronic device development. In this work, we synthesized a series of fluorinated liquid crystal molecules with large dipole moments and systematically investigated their phase behavior. We designed them with a similar fluorinated aromatic skeleton and varied the structures of the terminal group and bridging bond. We found that the dipole moment density and shape anisotropy significantly affect the phase behavior. Notably, diverse polar liquid crystal phases, including NF, SmZA, and SmAF were observed. Through a multi-component mixing strategy, we successfully achieved a much-expanded temperature window and improved low-temperature stability not only in the NF phase but also in the SmZA and SmAF phases.

Mesomorphic property and gelation ability of rod-shaped compounds having a coumarin skeleton

ABSTRACTRod-shaped liquid crystal materials with a lactone group at the terminal position of the molecule were designed, synthesised and their mesomorphic properties in bulk state and gelation ability in several organic solvents were examined. The effects of perfluoroalkyl chain and chemical structure on liquid crystal property and gelation ability were investigated. These compounds showed smectic A phase in bulk state and gelation ability in polar organic solvents. It was found that the presence of perfluoroalkyl chain and the direction of orientation of the esters had a great effect on these properties.

General phase-structure relationship in polar rod-shaped liquid crystals: Importance of shape anisotropy and dipolar strength

Ferroelectricity is a property with spontaneous polarization, which is of paramount importance in modern electrooptic applications. Yet, its observations are mostly limited to solids or chiral smectic C liquid crystals with nearly no fluidity. The fluidic ferroelectrics, called ferroelectric nematics, have recently become available by incorporating strong polarity into the apolar liquid crystalline orientational field. Here, we present a rational principle for designing the ferroelectric materials and controlling their phase behaviors. We reveal that, while the apolar nematic order with the second-rank tensor originates from the shape anisotropy in the apolar nematic state, the polarization field serves as an additional mechanism for stabilizing polar nematic orders. Based on a comprehensive set of examinations and machine-learningdriven analyses for about 150 chemically distinct polar rod-shaped molecules, we uncover that the interplay between the shape anisotropy and polarization field gives rise to a diversification of the polar liquid crystal behaviors. The strategy offers much broader design flexibility and may facilitate the development of future polar functional liquid crystals.

Luminescent Liquid Crystals Based on Carbonized Polymer Dots and Their Polarized Luminescence Application.

Traditional luminescent liquid crystals (LLCs) suffer from fluorescence quenching caused by aggregation, which greatly limits their further application. In this work, a kind of novel LLCs (named carbonized polymer dot liquid crystals (CPD-LCs)) are designed and successfully synthesized through grafting the rod-shaped liquid crystal (LC) molecules of 4'-cyano-4-(4″-bromohexyloxy) biphenyl on the surface of CPDs. The peripheral LC molecules not only increase the distance between different CPDs to prevent them from aggregating and reduce intermolecular energy resonance transfer but also make this LLC have an ordered arrangement. Thus, the obtained CPD-LCs show good LC property and excellent high luminous efficiency with an absolute photoluminescence quantum yield of 14.52% in the aggregated state. Furthermore, this kind of CPD-LC is used to fabricate linearly polarized devices. The resultant linearly polarized dichroic ratio (N) and polarization ratio (ρ) are 2.59 and 0.44, respectively. Clearly, this type of CPD-LC shows promising applications for optical devices.

Influence of terminally attached branched alkyloxy chains on the mesophase behaviour of calamitic liquid crystals

ABSTRACT Three sets of new rod-shaped liquid crystals with one terminal fixed branched alkyl chain, other terminal with varying normal alkyl chain, and central naphthalene with terminal branched chain derived symmetric molecules were synthesized and characterized. Chemical structures were confirmed by standard analytical methods. Liquid crystal properties were evaluated with the help of differential scanning calorimetry, polarized optical microscopy and X-ray studies. All the compounds favoured to exhibit highly birefringent smectic and nematic mesophase textures. Further, symmetric naphthalene derivatives preferably exhibited nematic mesophase with elevated transition temperatures.

Liquid/Single Crystal Structure Analysis: Synthesis and Characterization of a Trimethylsilyl Derived Rod Shaped Mesogen

4-[4'-Cyanophenoxy-carbonyl-phenyl-4-(trimethylsilyl)ethynyl]benzoate a rod shaped liquid crystal (SmA) is synthesized and characterized. The single-crystals were grown in triclinic crystal system in the space group of Pı - with unit cell parameters a = 5.9577(2) Å, b = 8.0398(3) Å, c = 25.8842(9) Å, α = 86.096(2)o, β = 89.912(2)o, γ = 2.919(2)o, Z = 2, and V = 1182.16(7). The crystal structure is stabilized by C-H···O intra-molecular interactions. Further, the structure also involves C-H···π interactions and weak π-π stacking interactions [centroid-centroid separation = 3.806 (3) Å].

DSC Study of Bent-Core and Rod-Shaped Liquid Crystal Mixtures

Phase transitions of different binary mixtures of a bent-core (10DClPBBC) and a rod-shaped (6OO8) liquid crystal were studied using differential scanning calorimetry. For the binary mixture with 50:50 weight ratio of bent-core and rod-shaped molecules, the nematic to smectic transition occurred below the temperature of 40°C and crystallization was shifted to sub-ambient temperature. It was found that crystallization was the phase transition with the lowest apparent activation energy.

Synthesis and Phase Behaviors of New Heterocyclic Liquid Crystals: Nematic and Smectic Properties

A series of new rod-shaped liquid crystal, 6-methoxy-2-(4-alkyloxybenzylidenamino) benzothiazoles possessingeven numbers of carbon atoms at the alkyloxy chain (CnH2n+1O-, n = 10, 12, 14, 16, 18) are prepared and characterized. The phase behaviour of these new compounds was studied by differential scanning calorimetry and polarising optical microscopy. All members exhibited mesophase. Influence of alkyl chain length on the mesomorphic properties was studied. Enantiotropic nematic phase was identified in all the members except for n = 18. Lower member (n = 10) exhibited monotropic (metastable) smectic A phase. The enantiotropic smectic A phase was observed from n-dodecyloxy derivatives onward to the last homologous synthesized.

Enhanced Photorefractive and Third-Order Nonlinear Optical Properties of 5CB-Based Polymer-Dispersed Liquid Crystals by Graphene Doping

The enhanced photorefractive and third-order nonlinear optical properties of graphene/graphene oxide (GO)-doped polymer-dispersed liquid crystals (PDLC) were studied by two-beam coupling and z-scan methods. A higher interior electric field will be produced according to the photorefractive performance because of the high electrical conductivity of graphene. The doping of graphene also improved the uniformity of alignments of 4-cyano-4′-pentylbiphenyl (5CB) in PDLC because the rod-shaped liquid crystals tended to align along the surface of sheet structures and the alternate positions of the hexagons on the graphene surface matched well with the alkyl chains of 5CB molecules. Higher third-order nonlinear refraction and absorption coefficient can be attributed to the geometric relationship between graphene and 5CB. The GO-doped samples exhibited higher photorefractive efficiency, third-order nonlinear refraction, and absorption coefficient than undoped PDLC samples, but lower values than the graphene-doped sa...

Synthesis and photoswitching properties of azobenzene liquid crystals with a pentafluorobenzene terminal

The fluorine-substituted benzoate ester rod-shaped liquid crystals containing an azobenzene side chain linked with terminal double bonds were synthesized and characterized. The mesophase and photoswitching properties were determined by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and UV–visible spectrometry. The rod-shaped compound 4a having an odd number of carbon atoms in the alkyl chains exhibits nematic phase and SmA type phase whereas compound 4b having an even number of carbon atoms showed only nematic phase. Both fluorine-containing compounds 4c and 4d showed only SmA type phases. The photoswitching properties of these compounds showed a rate of trans to cis isomerization ranging 19–20 s, whereas reverse process took around 230 min in solution. These materials may be ideal in the field of optically rewritable applications where both on and off rates should be crucial.

Synthesis and mesomorphic behaviours of a homologous series of 2-(4-alkyloxybenzylidenamino)benzothiazoles

The synthesis and characterisation of a series of new rod-shaped liquid crystal (LC), 2-(4-alkyloxybenzylidenamino)benzothiazoles possessing even number of carbon atoms at the alkyloxy chain (CnH2n + 1O-, n = 2, 4, 6, 8, 10, 12, 14, 16, 18) are reported. The molecular structures of title compounds were elucidated using FTIR and nuclear magnetic resonance spectroscopic techniques. The phase behaviour of these new compounds was characterised and studied by differential scanning calorimetry and polarising optical microscopy. Lower members (n = 2, 4 and 6) did not exhibit mesophase. Octyloxy derivative was monotropic LC. Enantiotropic smectic A phase was observed from n-decyloxy derivatives onward to the last homologous synthesised.

Synthesis and characterization of liquid crystalline azobenzene chromophores with fluorobenzene terminal

Two series of fluorine-substituted benzoate ester type rod-shaped liquid crystals incorporating the azobenzene as side arm linked with terminal double bonds as polymerizable functional groups were synthesized and characterized by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC) and UV–visible spectroscopy investigations. Thus, rod-shaped monomers, namely 4a and 4b having odd and even number of carbon in the terminal group exhibited nematic phase and SmA type phase was found at lower temperature. Compound 5 showed nematic phase whereas compound 6 showed SmA phase. These rod-shaped molecules exhibit strong photoisomerization behaviour in solution. The photoswitching properties of the compounds showed trans to cis isomerization in about 10s, whereas the reverse process takes place about 120min in solutions. Proposed materials may have potential to use it in optical storage devices.

Alkoxyphenyl-Substituted Symmetric Liquid Crystalline Diamantane Derivatives

Abstract Eleven novel thermotropic rod-shaped liquid crystals composed of alkoxyphenyl (methoxy to decyloxy) or hexanoyloxyphenyl groups on both ends of the diamantane core have been synthesized and their liquid crystalline properties investigated. All of them behaved as liquid crystals at considerably high temperatures. When the alkoxy side chains are propyloxy or shorter, the clearing points are over 300 °C. The clearing point of a liquid crystalline compound with two hexanoyloxy groups also exceeds 300 °C. When the alkoxy groups on both ends of the molecule are octyloxy or longer, they showed only smectic phases as the mesophase. When they are shorter than octyloxy, the compounds showed nematic phases just below their clearing points. A nematic phase was also observed for the hexanoyloxy compound as well as the smectic A and B phases. However, as the bent-shaped adamantane-derived hexyloxyphenyl molecule did not show any liquid crystalline properties, the linear structures of the diamantane-derived molecules were crucial in generating liquid crystalline phases.

Photo-Instability in a Binary Mixture of Banana-Shaped and Rod-Shaped Liquid Crystals

By triangular wave method, we studied the effects of ultraviolet irradiation on the polarization of a binary liquid crystal mixture. The mixture was comprised of 94 wt% of banana-shaped liquid crystal 4-chloro-1,3-phenylene bis [4-(4-n-tetradecyl) phenylimino methyl] benzoate and 6 wt% of an azo-containing rod-shaped liquid crystal hexanoic acid 4-(4-ethoxy-phenylazo)-phenyl ester. Under the 365 nm irradiation for 40 minutes, the polarization of the mixture was observed to decrease 10% although the component of the banana-shaped liquid crystal was photostable. Our results demonstrated that photolysis took place in the component of azo-containing rod-shaped liquid crystal.

Polarity-sensitive switching induced by the electric field in a self-assembled nematic system from two different achiral species of molecules

A polarity-sensitive switching behavior was observed in the nematic phase of a system of unit blocks self-assembled from an achiral rod-shaped liquid crystal (LC) and an achiral tripod-shaped dopant. This unit block has a shape of a tripod with extended legs and showed a spontaneous polarization under the applied electric field. Although the polarity-sensitive switching in the achiral LCs is intrinsically impossible, our system of the tripod-shaped unit blocks made with the linear-shaped achiral 8CB bound to the stretched branches of the tripod-shaped achiral dopant by the steric interaction or by the hydrogen bonding showed the polarity-sensitive switching. At zero applied field state, the unit blocks were aligned helically to compensate the macroscopic polarization value. As the field was applied, the helical structure was unwound and the optic axis of the sample rotated clockwise or counterclockwise depending on the sign of the applied field. The threshold field to show the polarity-sensitive switching was lower in the sterically assembled sample (0.1V∕μm) than in the hydrogen-bonded one (1.3V∕μm) and the field response time was much faster in the former case. In addition, the response time of the sterically assembled sample was linearly proportional to the strength of the electric field in the strong field regime, but the one of the hydrogen-bonded sample was saturated in the strong field regime.

Development of a new traceless aniline linker for combinatorial solid-phase parallel synthesis of rod-shaped liquid crystals with an azomethine linkage

A new traceless linker was developed to synthesize a library of 28 compounds possessing an azomethine linkage using combinatorial solid-phase parallel synthesis. The loading of the substrates on a solid support and cleavage from the solid support were performed by an imine synthesis and by imine-exchanged process in the mild conditions, respectively. Products with a thioester group exhibited liquid crystalline properties with the higher transition temperatures than the others.

A Chameleon Chiral Polar Liquid Crystal: Rod-Shaped When Nematic, Bent-Shaped When Smectic

The first antiferroelectric liquid crystal (AFLC) exhibiting a (chiral) nematic phase, a combination which has long been the goal of synthetic chemists working with polar liquid crystals but which at the same time represents a fundamental contradiction in terms of translational order, was recently reported by Nishiyama and co-workers. We have investigated this chiral twin dimer by optic, electrooptic, and dielectric methods and conclude that it is not an ordinary AFLC material, but one where the peculiar properties of bent-core smectics are combined with those of ordinary rod-shaped liquid crystals. The compound exhibits a new type of nematic−smectic phase transition, connected with a change of molecule conformation from rod- to bent-shaped. This also has an important impact on the chiral interactions in the system. Toward the high-temperature end of the smectic phase, the energy balance between bent conformation smectic and straight conformation nematic can be shifted by an electric field such that the t...

Achiral non-branched rod-shaped liquid crystals showing anticlinic smectic phase

Both peripheral ends of some straight-shaped mesogenic compounds, possessing a biphenyl or azobenzene moiety as the central rigid core, were systematically modified, and the effect on the liquid-crystalline properties investigated. Introduction of phenyl rings into the middle of the flexible chains was found to produce a well-defined smectic layered structure. Further introduction of another phenyl ring at each terminal end of the peripheral chain had a significant effect on the determination of the molecular tilt direction in the smectic phase, i.e., synclinic or anticlinic. Thus, novel molecular architecture for stabilizing the anticlinic smectic phase was introduced and the origin of the anticlinic ordering discussed.

Enhanced thermal response of the SAd1layer thickness in highly fluorinated thermotropic liquid crystals

Abstract X-ray studies of a homologous series of rod-shaped liquid crystal molecules with one tail perfluorinated and the other protonated, reveal large decreases in the smectic A layer spacing with increasing temperature. These materials form unique dimer phases in which the smectic layer spacing is dependent on the length of the perfluorinated tail and independent of the length of the protonated tail. The chain statistics of the perfluorinated tail significantly influence the thermal expansion coefficient since the length of the fluorinated tail defines the smectic layer spacing. Thermal expansion coefficients for the layer spacing observed here are negative and nearly an order of magnitude greater than for typical protonated rod-shaped thermotropic liquid crystals in the SA phase.