Liquid Crystalline Materials Research Articles

Application of thermotropic ferroelectric liquid crystals in electrical vehicle

This intermediate phase between solid and liquid enables liquid crystal engineers for various interesting yet fascinating thermo-optical applications. One such application of exploited for electric vehicle safety is optical shutter realized using thermotropic liquid crystals. Optical shutters are essential components in various optical systems, in providing control over the transmission of visible light. These shutters are categorized into various types of mechanical shutters, electro-optical shutters, liquid crystal optical shutters (LCOS) etc. liquid crystal cells and polarizing elements to quickly change the transmittance of light, making them ultimate for applications such as imaging and precision illumination. In this work, we explored the experimental techniques, key specifications, features, and applications of an optical shutter developed using liquid crystalline material LCOS.

Investigation of thermodynamical, electro-optical and dielectric properties of nematic liquid crystals dispersed with low-concentration barium titanate nanoparticles

ABSTRACT A liquid crystals nanoparticles composite was prepared by dispersing barium titanate nanoparticles (BT-NPs) into a multi-component liquid crystalline material having wide range room temperature nematic phase. The thermodynamic, optical textures, electro-optical and dielectric properties of the composite sample were studied along with pristine materials. Isotropic to nematic phase transition temperature was enhanced for nano-composites as observed from thermodynamic, optical textures and dielectric investigations. Effect of BT-NPs dispersion on various display parameters of nematic liquid crystals, namely, threshold voltage, dielectric anisotropy, DC conductivity and splay elastic constant has been explored. A relaxation mode in kHz frequency range has been observed for pure and nano-composites samples, which varies with temperature and follows Arrhenius behaviour. DC conductivity enhances approximately fivefold for BT-NPs dispersed samples. The host liquid crystalline material has nematic ordering which supports alignment of BT-NPs parallel to the nematic director which consequently improves thermal stability, dielectric and electro-optical parameters of the nano-composite system.

Molecular Design of Sexiphenyl-Based Liquid Crystals: Towards Temperature-Stable, Nematic Phases with Enhanced Optical Properties.

This research introduces a novel liquid crystal molecular design approach based on the para-sexiphenyl (6P) structure. Six new liquid crystalline materials were synthesized, incorporating an alkyl terminal and lateral substitutions of the sexiphenyl core to achieve temperature-stable and broad nematic phases. The synthetic pathway involved cross-coupling, resulting in derivatives with strong nematogenic characteristics. Optical investigations demonstrated that the tested material had high birefringence values, making it promising for optical and electronic applications. These results open up new avenues of research and offer potential practical applications in electronics, photonics, optoelectronics and beyond.

Enhanced dielectric, electrical and electro-optical properties: Towards understanding the interaction in mesophases of 8OCB liquid crystal dispersed with CdSe/ZnS quantum dots

The current advancements in developing new liquid crystalline materials for the next generation applications focus mainly on improving the physical characteristics of the liquid crystal (LC) systems. Recent progress has shown that the functionalized nanoparticles embedded in LC matrices can significantly alter the characteristics of the LC material based on the mutual interaction between the host molecules and guest particles. In this aspect, the present study reports the impact of dispersing core-shell type CdSe/ZnS quantum dots (QDs) on the dielectric, electrical and electro-optical properties of the 8OCB LC doped at various concentrations. The doped samples exhibit an ion releasing behavior and this effect becomes more evident as the doping concentration increases up to 0.2 wt% in the LC system. It is explained as a result of the growing tactoid-like ellipsoidal form acquired by QDs owing to the enhanced mutual interaction among QD ligands and rod-like LC molecules. The temperature variation of the diffusion constant, conductivity, ionic mobility and mean relaxation time of ions significantly follows each other in all the studied samples. Moreover, thermal profiles of the dielectric anisotropy, threshold voltage and splay elastic constant show a decreasing pattern with an increment in doping concentration. A dual-relaxation regime is experimentally investigated, corresponding to the contribution of nematic director and dimers, providing two kinds of rotational viscosity in the pristine and QDs dispersed LC samples. The transmittance-voltage curve reveals the presence of residual value in the dispersed samples and is related to the volatile memory effect. A slight enhancement is observed in the photoluminescence intensity for the lower doped sample and reduces further with increasing doping concentration in the pristine LC system. All these findings suggest the considerable contribution of functionalized QDs to the studied properties in terms of interaction between the LC and dopant material. This study will further elucidate the selection of optimal QDs concentration based on the required properties for their potential applications in futuristic devices based on LCs.

Advances in 4-cyanobiphenyl tethered discotic oligomers and supermolecular liquid crystalline materials

ABSTRACT A novel family of rod-like liquid crystals designed by Prof. G.W. Gray having nematic phase has been extensively used to make stable informative liquid crystal displays. These 4’-alkyl- and 4’-alkoxy-4-cyanobiphenyl molecules were created using his extensive understanding of liquid crystal chemical structure and physical properties. Following the discovery of this family of nematogens for display applications, the development of liquid crystal molecule designing and synthesising has resulted in the emergence of a broad new family of functional liquid crystals. This feature article presents the review of reports on the liquid-crystalline behaviour of 4-cyanobiphenyl-based oligomeric systems, which are ‘in-between’ polymeric and low molar mass systems and exhibit distinct molecular structures.

Synthesis, chiral separation and physical properties of the cinnamic acid derivatives exhibiting short-pitch cholesteric and TGB phases

ABSTRACT Two new photosensitive chiral liquid crystalline materials derived from cinnamic acid were synthesised in (R) and (S) enantiomer forms. The materials were characterised by NMR spectroscopy, purified by preparative liquid chromatography, and the chemical purity of the final products was determined by high performance liquid chromatography (HPLC). High optical purity was reached for all enantiomers. The enantiomer elution order was confirmed by the analysis of non-racemic (R) and (S) mixed samples. The cinnamic acid derivatives are capable of undergoing photoisomerisation. Sample irradiation was performed to realise E-Z isomerisation and investigate its effect on the enantioseparation. The conditions of the enantioselective HPLC method were optimised to achieve a baseline separation for all studied materials both before and after irradiation by the UV light. The mesogenic properties of the studied enantiomers and their racemic mixtures were obtained from differential scanning calorimetry measurements, optical textures observed under the polarising light microscope and X-ray diffraction measurements. Cholesteric and TGB phases have been found for the enantiomers and, analogously, nematic and smectic phases have been confirmed for their racemates. The temperature dependence of the optical birefringence has been studied for the racemates in the temperature range of the mesophases.

Properties of Antiferroelectric Mixtures Differing in the Amount of Added Racemate

Novel three-component liquid crystalline mixtures composed of chiral and achiral (racemic) liquid crystalline materials were designed and studied by polarizing optical microscopy, differential scanning calorimetry, and UV–VIS spectroscopy. The compositions of liquid crystalline mixtures were developed based on the composition of a two-component (binary) mixture marked as W-1000 with the following phase sequence: Cr ↔ SmCA* ↔ SmC* ↔ SmA* ↔ Iso. This mixture has an antiferroelectric (SmCA*) phase over a wide temperature range and exhibits a helical pitch inversion in this phase. All newly obtained mixtures occur in a wide temperature range of the SmCA* phase, while the ferroelectric (SmC*) phase and the orthogonal (SmA*) phase occur in a narrow temperature range. The new mixtures also have a very long helical pitch in the antiferroelectric phase and a short helical pitch in the ferroelectric phase.

The Influence of the Molecular Structure of Compounds on Their Properties and the Occurrence of Chiral Smectic Phases.

We have designed new chiral smectic mesogens with the -CH2O group near the chiral center. We synthesized two unique rod-like compounds. We determined the mesomorphic properties of these mesogens and confirmed the phase identification using dielectric spectroscopy. Depending on the length of the oligomethylene spacer (i.e., the number of methylene groups) in the achiral part of the molecules, the studied materials show different phase sequences. Moreover, the temperature ranges of the observed smectic phases are different. It can be seen that as the length of the alkyl chain increases, the liquid crystalline material shows more mesophases. Additionally, its clearing (isotropization) temperature increases. The studied compounds are compared with the structurally similar smectogens previously synthesized. The helical pitch measurements were performed using the selective reflection method. These materials can be useful and effective as chiral components and dopants in smectic mixtures targeted for optoelectronics and photonics.

A comparative DFT analysis of chiral laterally semi-fluorinated liquid crystals

Liquid crystalline materials have the property to flow like a liquid while maintaining their molecular order. They are valuable material for technological integration. In this work, we have investigated a chiral semi-fluorinated liquid crystal material with the chemical structure C21H17F2NCS in two distinct laterally fluorine-substituted configurations. Employing density functional theory computations, we undertake a comprehensive comparative analysis of both configurations, assessing their charge dynamics, electrical characteristics, thermochemical properties, and nonlinear attributes. Intriguingly, the configuration featuring fluorine substitution on adjacent benzene rings exhibits remarkably superior properties in contrast to its counterpart, where fluorine is substituted on the same benzene ring. This research underscores the significance of structural nuances in elucidating the enhanced performance of liquid crystalline materials.

Uncommon building blocks in liquid crystals

ABSTRACT Just as nematic liquid crystals are widespread in display technology, a small number of building blocks find widespread use in the design and synthesis of liquid crystalline materials. This review explores the intricate relationship between the molecular structure of thermotropic liquid crystals (LCs) and their phase behaviour, emphasising the role of specific structural fragments in determining LC properties. We also detail into the impact of non-conventional building blocks on LC properties by comparing families of materials differentiated by only a single structural variation, allowing us to probe the extent to which deviations from traditional structures can maintain liquid crystallinity. This comprehensive overview not only underscores the importance of specific molecular fragments in LC design but also opens avenues for the innovative use of non-traditional building blocks in the development of new LC materials. Just as the ubiquity of 1,4-disubstituted benzene rings has its genesis (partly) in the extremely robust and predictable synthesis of such systems through cross-coupling chemistry; we consider that, the rapid progress in coupling sp3 fragments, coupled with the growing availability of suitable building blocks, makes the inclusion of said fragments ever more practical and attractive for use in liquid crystalline systems.

Gold nanoparticles doped organic liquid crystalline material for fast charge migration in nano-devices

ABSTRACT With the ever-growing need for low-cost, easily processable multifunctional materials and increasing miniaturisation in devices, we have landed in the era of nanodevices. Discotic liquid crystals are multifaceted materials that have shown their applicability as photonic devices, nanowires, optical switches, memory storage devices and many more. These materials show 1D conductivity which can be boosted upon doping with a suitable doping agent in appropriate concentrations. In this work, we present nanocomposite materials composed of a triphenylene discotic liquid crystal material, doped with thiol capped gold nanoparticles, and propose to apply it as a charge transportation layer in nanodevices for faster charge migration. Various techniques have been used to analyse the effect of doping on the different electrical, optical and lattice parameters of these nanocomposite systems. An increase in the value of the conductivity is obtained at higher temperatures, and it is successfully preserved till room temperature due to the presence of thiol capped gold nanoparticles in the columnar matrix. Thus, an increase in conductivity, lowering of the optical band gap, broadening of UV-Visible absorbance peak and decrease in core-to-core distance infers the increase in stability and optical and conduction properties of the host material after doping.

Chirality induction in confined chromonics: A case study

Chromonic liquid crystals are a sub-category of lyotropic liquid crystalline materials whose self-assembling mechanisms in complex supramolecular structures are still not fully understood. Chirality can be induced in chromonics using chiral aminoacids with alternating results. Understanding the key factors that lead to maximize this phenomenon is important for the design of novel materials able to self-assemble in chiral macroscopic structures with tailored optical properties, as selective reflection.In this work, we present a comparative study involving a not commercial chromonic, a synthesized metal containing chromonic compound [(Bpy-OH)2Ag][CH3COO]H2O, and two model chromonics that behave in different manner with respect to chiral induction. Spherical confinement provides a quick way to qualitatively evaluate the elasticity properties in new materials. Also, optical textures of the metal compound, after doping the material with a chiral moiety, show that its chiral phase is more similar to one of the two model chromonics. This observation points to a role played by the flexibility of the molecular structure that it’s reflected into the resistance of the columnar aggregates to undergo a twist. This last observation can be useful when selecting the most suitable building blocks in designing new materials.

Clustering triggered emissive liquid crystalline template for dual mode upconverted and downconverted circularly polarized luminescence.

Liquid crystalline materials have attracted significant attention in chiroptical research due to their ability to form long range ordered helical superstructures. Research focus has been on exploiting the unique properties of liquid crystalline materials to demonstrate highly dissymmetric circularly polarised luminescent (CPL) systems. In this study, we present a thermally driven, facile approach to fabricate CPL-active materials utilizing cholesteryl benzoate as the active substrate. Cholesteryl benzoate, a well-known thermotropic liquid crystal, has been found to manifest intriguing optical characteristics upon subjecting to repeated heating-cooling cycles. Despite the absence of conventional fluorescent moieties, the material exhibited luminescence through aggregation induced clustering triggered emission mechanism. Systematic investigations revealed excitation-dependent CPL for solid cholesteryl benzoate films when subjected to multiple thermal cycles. The excited state chiroptical investigation performed after multiple thermal cycles showed a luminescence anisotropy (glum) of 8 × 10-2, which is a high value for simple organic molecules. Moreover, upon co-assembly with lanthanide-based upconversion nanophosphors (UCNPs), the hybrid system demonstrated upconverted circularly polarised luminescence (UC-CPL). Benefiting from the ability to endow upconversion nanoparticles of various sizes, fabrication of UCNP-ChB hybrid nanocomposites exhibiting multicoloured upconversion CPL was demonstrated. These findings highlight the potential of liquid crystalline materials for diverse applications, including 3D optical displays and anticounterfeiting technologies.

Optical deformations of azobenzene polymers: orientation approach vs. other concepts.

It has been 30 years since the discovery of surface restructuring in thin azopolymer films by two independent research groups. A wide variety of topographical structures have been created by the application of two-/four-beam interference patterns, space light modulators and even helical beams. There are a number of comprehensive reviews which describe in detail the advances in superficial photopatterning of azopolymer films and macroscopic deformations of azonetworks. The theoretical approaches are only briefly touched on in these reviews and often are accompanied by the remark that the phenomenon is far from being understood. In this review, we would like to present the polymer theoretist's point of view on this intriguing problem. We begin by describing a multitude of theoretical approaches and commenting on the pluses and drawbacks of each. Importantly, we show that in most cases the presence of an azopolymer matrix is either ignored or limited to a specific class of azopolymers (liquid-crystalline or elastomeric). We then move to early orientation approaches based on the hypothesis that reorientation of azo-chromophores by modulated polarized light is the sole cause of superficial patterning. At the end of the review a modern orientation approach, as proposed by our own group, is presented. This approach has high predictive power because it can explain a large pool of experimental data for different classes of azopolymers including glassy and liquid-crystalline materials. This is made possible by taking into account both the light-induced orientation process and the change of anisotropic interactions between the chromophores upon their isomerization. Last but not least, this is the only approach that provides an estimate of the light-induced stress large enough to cause plastic deformations of glassy azopolymers. Recent finite element modeling results show remarkable similarity to real patterns and even time-dependent data are well explained. With this, we claim that the puzzle is finally understood and the orientation approach is ready for its implementation for major azopolymer classes.

MR-TADF liquid crystals: towards self assembling host-guest mixtures showing narrowband emission from the mesophase.

Creating (room temperature) liquid crystalline TADF materials that retain the photophysical properties of the monomolecular TADF emitters remains a formidable challenge. The strong intramolecular interactions required for formation of a liquid crystal usually adversely affect the photophysical properties and balancing them is not yet possible. In this work, we present a novel host-guest approach enabling unperturbed, narrowband emission from an MR-TADF emissive core from strongly aggregated columnar hexagonal (Colh) liquid crystals. By modifying the DOBNA scaffold with mesogenic groups bearing alkoxy chains of different lengths, we created a library of Colh liquid crystals featuring phase ranges >100 K and room temperature mesomorphism. Expectedly, these exhibit broad excimer emission from their neat films, so we exploited their high singlet (S1 ∼2.9 eV) and triplet (T1 ∼2.5 eV) energies by doping them with the MR-TADF guest BCzBN. Upon excitation of the host, efficient Förster Resonance Energy Transfer (FRET) resulted in almost exclusive emission from BCzBN. The ability of the liquid crystallinity of the host to not be adversely affected by the presence of BCzBN is demonstrated as is the localization of the guest molecules within the aliphatic chain network of the host, resulting in extremely narrowband emission (FWHM = 14-15 nm). With this work we demonstrate a strategy for the self-assembly of materials with previously mutually incompatible properties in emissive liquid crystalline systems: strong aggregation in Colh mesophases, and narrowband emission from a MR-TADF core.

A one-pot telescopic synthesis of benzo[b]carbazoles and exploration of their liquid crystalline properties.

We describe a diversity-oriented one-pot telescopic synthesis of various benzo[b]carbazoles with the naphthannulation of indoles as the key step, enabled by an intramolecular furan-olefin Diels-Alder reaction. This strategy is general and efficient across a wide range of substrates. We applied this method to synthesize and characterize the first benzo[b]carbazole-based liquid crystalline materials, where the unique molecular design led to the formation of a rare nematic phase at room temperature.

Dissipation in nonequilibrium thermodynamics and its connection to the Rayleighian functional

We examine quantitatively the role of dissipation in nonequilibrium thermodynamics and its connection to variational principles and the Rayleighian functional. The extremum of the Rayleighian is sometimes used to describe the inertialess (dissipation-dominated) dynamics of continuum systems, and it has been applied recently for the modeling of soft matter dynamics. We discuss how dissipation is considered within one of the modern complete descriptions of nonequilibrium thermodynamics, namely the single generator bracket formalism. Within this formalism, dissipation is introduced through the use of the dissipation bracket, describing irreversible dynamics, which is added to a Poisson bracket that describes the reversible dynamics of the system. A possible connection with the Rayleighian functional is then demonstrated that in all cases considered herein, the Rayleighian is equal to minus one half of the effective dissipation rate of the Lagrangian functional. The effective dissipation rate is obtained starting with an inertial (i.e., flux-based or velocity-based) system description, involving the Poisson bracket and the primitive part (i.e., without the entropy correction term) of the dissipative bracket. Several examples are discussed in detail, ranging from an algebraic model (damped oscillator) to continuum ones: modeling of fluid flow in porous particle media, viscous Newtonian compressible and incompressible fluid flows, and more interestingly, flow of a nematic liquid-crystalline material.

Influence of the polymer network on the stability of the heliconical structure in the ferro- and antiferroelectric liquid crystalline phases

The use of polymer networks to stabilise chiral liquid crystalline materials (ChLC), which canfind applications in electro-optical and photonic devices, is becoming increasingly common. Stabilisingthe heliconical structure of a ChLC using a polymer network involves forming a spatial polymer matrix ina liquid crystal medium. The polymer network acts as a kind of scaffold for the liquid crystal molecules,resulting in the helix being stiffened in a particular desired state. This method of helix stabilisation hasbeen mainly used in chiral nematic liquid crystals so far. Effective stabilising the helix with a polymerin chiral smectic liquid crystals could eliminate the main drawback of these materials, i.e., the changein their performance, in electro-optical effects based on the deformation of the helix, with a changein temperature. The main parameter characterising the heliconical structure is its pitch, whose valuechanges rapidly with temperature change, in most smectic liquid crystals, which in turn translates intothe above-mentioned drawbacks. Stabilisation with the polymer should translate into temperaturestability of the helical pitch and the rest of the material parameters. This paper presents the results ofthe temperature dependence of the helical pitch before and after polymerisation of a liquid crystalmaterial with an antiferroelectric phase. A tetrafunctional monomer structurally compatible to thecomponents of the antiferroelectric mixture and, for comparison, a commercially available monomerwith a different structure were used in the study. The results relevant that stabilisation of the heliconicalstructure is easier after the use of a monomer more structurally compatible to the components ofparent liquid crystal material, while maintaining homogeneity of the heliconical structure orderingand a wide temperature range of the antiferroelectric phase occurrence.Keywords: liquid crystals, antiferroelectric phase, helical pitch, polymerization

Aquatic Functional Liquid Crystals: Design, Functionalization, and Molecular Simulation.

Aquatic functional liquid crystals, which are ordered molecular assemblies that work in water environment, are described in this review. Aquatic functional liquid crystals are liquid-crystalline (LC) materials interacting water molecules or aquatic environment. They include aquatic lyotropic liquid crystals and LC based materials that have aquatic interfaces, for example, nanoporous water treatment membranes that are solids preserving LC order. They can remove ions and viruses with nano- and subnano-porous structures. Columnar, smectic, bicontinuous LC structures are used for fabrication of these 1D, 2D, 3D materials. Design and functionalization of aquatic LC sensors based on aqueous/LC interfaces are also described. The ordering transitions of liquid crystals induced by molecular recognition at the aqueous interfaces provide distinct optical responses. Molecular orientation and dynamic behavior of these aquatic functional LC materials are studied by molecular dynamics simulations. The molecular interactions of LC materials and water are key of these investigations. New insights into aquatic functional LC materials contribute to the fields of environment, healthcare, and biotechnology.

Design and rheological behaviour of lactic acid based cholesteric liquid crystalline materials with hydroquinone unit in the molecular core

In order to contribute to molecular structure – physical property relationship for chiral self-assembling materials, two chiral compounds derived from the lactic acid have been designed. Mesogenic core of materials consists of two benzoate units and one hydroquinone unit connected by the ester linkage groups and two flexible alkyl chains. Different lengths of the chiral alkyl chain were tested, while the length of the achiral alkyl was kept same for all materials. The mesomorphic properties were established by polarizing optical microscopy and differential scanning calorimetry. Both materials possess reasonably broad and stable cholesteric (N*) phase down to room temperature. The rheological characterisation was performed in the N* phase on the compound with shorter chiral alkyl chain using a rotational rheometer in a plate/plate geometry with and without external electric field applied perpendicularly to the flow direction. The results reveal a shear thinning behaviour, even without electric field applied. The N* phase exhibits a slight electrorheological effect, at low shear rates, that continuously decrease with the increase of the shear rate, due to the competition of the flow and electric fields. The mesomorphic behaviour of the designed materials was compared to structurally similar materials. Quite broad temperature range of the N* phase down to room temperature makes studied lactic acid derivatives as suitable chiral dopants for design of new multicomponent mixtures targeted for various applications in photonics and optoelectronics.