Symmetric Liquid Crystal Research Articles

Liquid crystalline behaviour of symmetrical thermotropic Schiff base compounds

A series of new symmetrical liquid crystal compounds with Schiff base as a linking unit attached to different terminal alkyl chains were synthesized and characterized. The intermediates of 4-(decyloxy)benzaldehyde, 4-(dodecyloxy)benzaldehyde, and 4-(tetradecyloxy)benzaldehyde were synthesized through alkylation reaction between 4-hydroxybenzaldehyde with three series of bromoalkanes. Next, it further reacted with 1,4-phenylenediamine through condensation reaction to produce symmetrical Schiff base linkage with different terminal group chain which indicated as N-(substitutedbenxylidene)benzene-1,4-diamine. The compounds were characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and CHN elemental analysis. The Schiff base linkages are detectable in the FTIR spectra within the range of 1601–1604 cm⁻¹ and in the NMR spectra at δ 8.47–8.48 ppm. The mesophase transition phase of this compounds were determined using polarized optical microscope (POM) and further confirmed using differential scanning calorimetry (DSC). The Schiff base compounds were found to be mesogenic with smectic C and nematic transitions phases. These compounds attached to decyl, dodecyl, and tetradecyl terminal chains were found to exhibit smectic C phase at 142.47, 135.60, and 131.90 °C and nematic phase at 199.62, 197.11, and 188.76 °C, respectively. The incorporation of a Schiff base linking unit with a symmetrical alkyl chain induces the mesogenic effect in the molecules. A long alkyl chain allows the compounds to adopt a lamellar packing, facilitating the formation of a smectic phase. Incorporating the Schiff base into the molecules maintains an appropriate permanent dipole moment, which enables the homologs to arrange in a parallel orientational order, thereby adopting a nematogenic character.

Synthesis, mesophase behavior, and computational studies of dimers composed of three-ring rod-shaped ester-imine linkage monomeric unit

New symmetrical liquid crystal dimers composed of three-ring ester imine rod-shaped monomers with flexible alkyl chains of different parity were synthesized. The thermal stability of the newly synthesized compounds was determined using thermogravimetric analysis. The phase transition and mesogenic behavior of the compounds were explored using polarising optical microscopy and differential scanning calorimetry. Dimers with odd parity spacers exhibited the phase sequence Iso-N-Nx-SmX-Cr. The Nx phase transition was observed by polarising optical microscopy. This transition was distinct from that of the uniaxial nematic phase. The Nx phase was absent in the dimer with even parity spacers. A density functional theory study was carried out to investigate the stable configuration and influence of the solvent on the structural parameters, chemical reactivity parameters, and excited state properties of the molecules.

Exploring the nematic to twist-bend nematic phase transition: calorimetric and optical measurements

ABSTRACT We investigate the nematic (N) to twist-bend nematic (NTB) phase transition in binary mixtures of a symmetric liquid crystal dimer (CB7CB) and a calamitic bi-phenyl mesogen (7OCB). As the molar concentration of 7OCB increases, both the Iso–N and N–NTB transition temperatures decrease gradually, consequently, the width of the nematic phase above the NTB phase can suitably be varied from ~12 K to 46 K. Notably, as the nematic range expands, there is a remarkable reduction in transitional entropy, eventually approaching negligible or zero values. Additionally, the heat capacity peak and the phase shift disappear when transitional entropy reaches zero. This behaviour diverges from typical N–SmA and N–SmC phase transitions, where transitional entropy decreases near the tricritical point but the heat capacity peak persists. Our findings indicate a finite positive δ(∆n) across all concentrations at the N–NTB phase transition, suggesting a gradually weakening first-order nature. This contrasts with theoretical predictions of a second-order N–NTB phase transition, as our experimental data consistently supports a first-order nature. The self-consistent mean-field Landau Model successfully validates our experimental data, establishing the universal first-order nature of the N–NTB phase transition, with experimental entropy values in good agreement with theoretical prediction.

Effects of polar end group, flexible spacer length and molecular conformation on properties of the liquid crystal dimers

ABSTRACT Two series of symmetric liquid crystal dimers, namely 1F and 3F, have been synthesised by varying the length of the flexible odd spacer. The melting temperatures and clear points of the 3F series are higher than those of the 1F series. The 1F series only exhibits nematic phase, while the 3F series displays both smectic A and nematic phases. As the length of the flexible spacer increases, the dominance of the smectic phase becomes more pronounced. This phenomenon may be attributed to the combined effect by the polar end group CF3 and the long flexible spacer on the molecular conformation.

The influence of the imine bond direction on the phase behaviour of symmetric and non-symmetric liquid crystal dimers

The synthesis and characterisation of two series of liquid crystal dimers are reported: the symmetric 1,5-bis(4′-oxyanilinebenzylidene-4-alkane)pentanes (m-O5O-m), and the non-symmetric 1-(4-cyanobiphenyl-4′-yloxy)-5-(4′-oxyanilinebenzylidene-4-alkane)pentanes (CBO5O-m). In the acronyms for each series m refers to the terminal chain length. The m-O5O-m series showed monotropic nematic phases for m = 1-7. Smectic behaviour emerged at m = 7 and nematic behaviour extinguished at m = 8. For the CBO5O-m series, an enantiotropic nematic phase is seen for m = 1-10. In addition, m = 1 shows a twist-bend nematic phase, and m = 9 and 10 an interdigitated smectic A phase. These series are compared to the corresponding dimers in which the imine link in the benzylideneaniline fragments is reversed, the m.O5O.m and CBO5O.m series. For the symmetric dimers, reversing the imine link has a small effect on the nematic-isotropic transition temperatures, TNI, but a much larger effect on the smectic-isotropic transition temperatures, TSmI. In both cases, the m.OnO.m series shows the higher values. The values of TNI are also slightly higher for the CBO5O.m series than the CBO5O-m series. Surprisingly, the values of TSmI are higher for the CBO5O-m than for the CBO5O.m series. These differences are discussed in terms of the changes in shape and electronic distributions arising from reversing the imine link.

Dielectric Study of Liquid Crystal Dimers: Probing the Orientational Order and Molecular Interactions in Nematic and Twist-Bend Nematic Phases.

Dielectric spectroscopy in frequencies that range from 10 Hz to 1 GHz has been used to study the molecular orientational dynamics of the two types of dimers that form the twist-bend nematic phase over a wide range of temperatures for both nematic and twist-bend nematic phases. The symmetrical and asymmetrical liquid crystal dimers with the cyanobiphenyl mesogenic groups were investigated. The results were analyzed within the framework of the molecular theory of dielectric permittivity for nematogens. The two molecular processes can be assigned to the reorientation of the monomeric unit: the high frequency one to the precessional rotation of the longitudinal components of the cyanobiphenyl groups (CN) and the second (low frequency) to the end-over-end rotation of the CN dipole around the short molecular axis. The precession mode, which is determined by the local order and is almost unaffected by the phase transition from the nematic to the twist-bend phase, while the end-over-end rotation clearly slowed down at the transition, as it is affected by the growth of the intermolecular interactions. The latter corresponds well to the fact revealed by IR spectroscopy about the longitudinal correlation of the molecular dipoles.

Fabrication of azimuthally/radially symmetric liquid crystal plates using two-step photoalignments.

Fabrication of azimuthally/radially symmetric liquid crystal plates (A/RSLCPs) using two-step photoalignments with the two dichroic dyes methyl red (MR) and brilliant yellow (BY) is demonstrated. BY molecules coated onto a substrate and MR molecules doped in LCs in a cell can azimuthally and radially align LCs by illuminating radially and azimuthally symmetric polarized light with specific wavelengths, respectively. In contrast to the previous fabrication methods, the fabrication method proposed here can prevent the contamination/damage of photoalignment films on substrates. A method to improve the proposed fabrication to avoid generating undesirable patterns is also elucidated.

Multiresponsive Cylindrically Symmetric Cholesteric Liquid Crystal Elastomer Fibers Templated by Tubular Confinement.

Cylindrically symmetric cholesteric liquid crystal elastomer (CLCE) fibers templated by tubular confinement are reported, displaying mechanochromic, thermochromic, and thermomechanical responses. The synthesis inside a sacrificial tube secures radial orientation of the cholesteric helix, and the ground state retroreflection wavelength is easily tuned throughout the visible spectrum or into the near-infrared by varying the concentration of a chiral dopant. The fibers display continuous, repeatable, and quantitatively predictable mechanochromic response, reaching a blue shift of more than -220 nm for 180% elongation. The cylindrical symmetry renders the response identical in all directions perpendicular to the fiber axis, making them exceptionally useful for monitoring complex strains, as demonstrated in revealing local strain during tying of different knots. The CLCE reflection color can be revealed with high contrast against any background by taking advantage of the circularly polarized reflection. Upon heating, the fibers respond-fully reversibly-with red shift and radial expansion/axial contraction. However, there is no transition to an isotropic state, confirming a largely forgotten theoretical prediction by de Gennes. These fibers and the easy way of making them may open new windows for large-scale application in advanced wearable technology andbeyond.

Low aberration and fast response liquid crystal lens with optically compensated bend structures

A hole-patterned electrode (HPE) liquid crystal (LC) lens with positive (0 D – 0.7 D) and negative focal lengths (0 D - −1.1 D) is demonstrated. The lens uses an optically compensated bend (OCB) cell filled with dual-frequency LC materials and alignment films with an intermediate pretilt angle. Better reduction in tilt and coma aberration can be achieved using this simple one-layer structure with its symmetric LC orientation in an OCB cell than can be obtained with a complicated two-layer HPE LC lens. The use of dual-frequency switching together with the reduction of the backflow effect in the OCB cell also improves the response by about 7 times compared with the conventional HPE LC lens.

Star-shaped Blue Emitting Truxene-based Discotic Liquid Crystalline Molecules: Synthesis, Self-assembly, and Electroluminescence Studies

Two new C3 symmetrical star-shaped truxene-based discotic liquid crystals have been designed to study their optoelectronic properties. Both the truxene derivatives exhibited columnar hexagonal self-assembly, confirmed by a thorough analysis using POM, DSC, and XRD. A multilayered OLED device was structured using these mesogens as blue emitters for electroluminescent studies. The best-performing configuration was perceived for devices based on emitter 1b at a dopant concentration of 1 wt% in a well-aligned CBP host, giving a maximum EQE of 2.9%.

The effects of alkylthio chains on the properties of symmetric liquid crystal dimers

The properties of four series of dimers are discussed to investigate the effect of thioether links on their mesogenic behaviour.

Comparative dielectric and thermally stimulated-depolarization-current studies of the liquid crystal dimers 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane and heptane and a binary mixture between them, close to the glass transition.

We have performed dielectric spectroscopy and thermally stimulated-depolarization-current experiments to study the molecular dynamics of the twist-bend nematic phase close to the glass transition of two members of the 1″,7'-bis(4-cyanobiphenyl-4'-yl)alkane homologous series (CBnCB): the liquid crystal (LC) dimers CB9CB and CB7CB, as well as a binary mixture of both. By doping CB9CB with a small quantity of CB7CB, the crystallization is inhibited when cooling the sample down, while the bulk properties of CB9CB are retained and we can investigate the supercooled behavior close to the glass transition. The study reveals that the inter- and intramolecular interactions of the mixture are similar to those of pure CB9CB and confirms that there is a single glass transition in symmetric LC dimers.

A Simulation of Liquid Crystal Grating with Field‐Induced Twist Deformation for Beam‐Steering Applications

Beam‐steering device using liquid crystal (LC) grating with field‐induced twist deformation, which has symmetric LC director distribution, is demonstrated. The proposed LC cell can convert the polarizations without multidomain LC alignments, and the polarization of the output light can also be changed with different input polarization. Therefore, the polarization of the input light is controlled, and the polarization of the output light is analyzed by additional layers that each consists of a switchable quarter‐wave plate (QWP) and a linear polarizer. The transmitted light of the proposed LC cell can be diffracted in three different directions by changing the bias voltage. The calculated efficiency and angle of steering are 46% and 8°, respectively. Potential applications of such a grating for diffraction optics, optical interconnect, and beam‐steering devices are foreseeable.

Thermal behavior, decomposition mechanism by TG/MS/FTIR technique and theoretical study of some symmetric and asymmetric bent-core liquid crystals based on 2,7-dihydroxynaphthalene

Thermal behavior, decomposition mechanism by TG/MS/FTIR technique and theoretical study of some symmetric and asymmetric bent-core liquid crystals based on 2,7-dihydroxynaphthalene

Detection of polarization state of a polarized light using azimuthally symmetric dye-doped liquid crystals

Detection of polarization state of a polarized light using azimuthally symmetric dye-doped liquid crystals (AS-DdLCs) is demonstrated. The first method is that the image of a polarized light with unknown polarization state after passing through the AS-DdLCs is captured by a charge-coupled device and transformed to a gray-level image, which is then analyzed by the designed program to identify the polarization state of the polarized light. The second method of comparing images using a database to identify the polarization state of an unknown polarized light is proposed. The techniques to increase detection accuracy of the two methods, and the comparison of the two methods are also discussed.

Phase transitions in a high magnetic field of an odd, symmetric liquid crystal dimer having two nematic phases, N_{U} and N_{TB}, studied by NMR spectroscopy.

Both ^{1}H and ^{13}C NMR spectra have been obtained in a static magnetic field of 23.5 T on a bent-shaped dimer molecule, 1^{''},7^{''}-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB), which shows the sequence of liquid crystal phases twist-bend nematic, N_{TB}, and uniaxial nematic, N_{U}, before entering the isotropic phase. The ^{1}H spectra are used to locate the temperature at which the sample melts to form a twist-bend nematic, T_{CrN_{TB}}, and then T_{N_{U}I} when the isotropic phase is entered, both in a magnetic field of 23.5 T, and to compare these with those measured at the Earth's field. The differences between these transition temperatures are found to be zero within the error in their measurement, in stark contrast to previous measurements by Salili et al. [Phys. Rev. Lett. 116, 217801 (2016)10.1103/PhysRevLett.116.217801]. In the isotropic phase in the presence of the field the sample exists in a paranematic phase in which the molecules of CB9CB are partially ordered. The ^{1}H and ^{13}C NMR spectra in the paranematic phase are used to measure the critical temperature T* below which this phase is unstable. The spectra are also used to study the structure, molecular orientational order, and distribution of molecular conformations in the paranematic phase.

Investigation of the spectroscopic features along with the media polarity effect in some symmetrical disc-shaped liquid crystals

Disc–shaped liquid crystals with interesting structural features are an important research field in the science and technology. Function of these strongly depends on their molecular structure and the media that surrounds them. This paper highlights spectral features of three symmetrical disc–shaped liquid crystals with similar structures and dissimilar substituent groups in various surrounding media to recognize their interactional and structural similarities and differences. Determining the type of molecular interactions in these liquid crystals and their effect is of great importance in accurate determination of host–guest interactions for diverse applications in polymer–dispersed liquid crystal displays, elastomers and optical industries, and so on. For this purpose, spectral variations were analyzed through KAT (Kamlet–Abboud–Taft) model and the dipole moments were measured through UV–visible spectroscopic technique, too. The dipole moment results reveal that the electron cloud resides in the central part of these liquid crystals. The experimental data were validated via DFT (Density Functional Theory) calculations. Both experimental and theoretical data assist to better identify the spectroscopic features of these liquid crystals and give useful information about their performance in various media that can be valuable for designing applicable guest–host or mixture liquid crystal systems.

Birefringence and photoluminescence properties of diphenylacetylene-based liquid crystal dimers

We herein report phase transitions, mesomorphism, birefringence behavior and photoluminescence properties of symmetric liquid crystal (LC) dimers based on diphenylacetylene or tolane.

Fluorinated chiral nematic liquid crystal dimers based on (S)-1-phenylethane-1,2-diol

ABSTRACTA series of symmetric liquid crystal (LC) dimers with the same chiral core (S)-1-phenylethane-1,2-diol ((S)-PE) have been synthesised, termed TBDA-(S)-PE, 3F3B-(S)-PE, 3F2B-(S)-PE, 1F3B-(S)-PE, 1F2B-(S)-PE, respectively. Chemical structures and LC properties of the five symmetric LC dimers were characterised by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectrometer (1H NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and polarised optical microscopy (POM). TBDA-(S)-PE displayed enantiotropic chiral smectic A (SmA*) phase while 3F3B-(S)-PE, 3F2B-(S)-PE, 1F2B-(S)-PE all displayed enantiotropic chiral nematic (N*) phase and 1F3B-(S)-PE displayed monotropic N* phase. The results indicated that the removal of the flexible spacers between the rigid mesogenic arm and the chiral core facilitated the formation of the N* phase. When the rigid mesogenic units are connected to the chiral core directly, the structure of the terminal fluorine group and the rigidity of the mesogenic unit played certain influence on the thermal properties of the LC dimers, but did not change the type of mesomorphic phase. Compared to 3F3B-(S)-PE and 1F3B-(S)-PE, 3F2B-(S)-PE and 1F2B-(S)-PE displayed wider LC ranges, respectively, suggesting molecular regularity had greater influence on LC-isotropic (I) transition temperature.

Symmetrical and asymmetrical liquid crystal dimers: synthesis, characterisation and mesomorphic behaviour

ABSTRACTA series of new asymmetrical liquid crystal dimers, RnO–S2O–ORn (n = 2–10), linked by an ethylene spacer having carbothiol – COS – and carboxyl – COO – linkages to the core centre of the molecule and bearing different lengths of terminal alkoxy chains were synthesised in order to study the effects of the length of the terminal alkoxy chains on mesomorphic properties. As well as, five symmetrical and asymmetrical dimers linked by butylene spacers bearing different linkages to the core and various terminal chains were also synthesized in order to study the effect of the nature of the spacer and terminal groups. The structures of the synthesised dimers were confirmed by physico-chemical techniques, i.e. FTIR, NMR and mass spectra. Differential scanning calorimetry and polarising optical microscopy verified the liquid crystal behaviour transition temperatures. The isotropic transition temperatures of the dimers RnO–S2O–ORn (n = 2–10) decreased with increasing length of alkoxy chain. Structural effects on the mesomorphic and physical properties were investigated in terms of alteration of carboxylate and thioester groups linking the spacer. The mesomorphic investigation reveals that all the dimers formed an enantiotropic Nematic phase except for dimer HO2SH which is not a liquid crystal.