smectic-A Phase Research Articles

Thermal and structural analysis of binary mixtures of pyrimidine liquid crystals using Modulated Differential Calorimetry and Synchrotron X-ray Diffraction measurements.

We present a systematic experimental dataset on the temperature dependence of specific heat capacity in a binary mixture of the second and seventh homologous series of 5-alkyloxy-2-(4-nonyloxy-phenyl) pyrimidine(PhP) liquid crystal compound. These binary mixtures exhibit nematic, smectic-A, and smectic-C phases within a concentration range of xPhP1 = 0 to 0.45. The liquid crystalline phases are structurally characterized using synchrotron X-ray diffraction. We determine the apparent molecular length in the nematic phase, smectic layer spacing, average distance between the long axes of molecules, correlation length, and orientational order parameters (<P2> and <P4>) as functions of temperature. The tilt angle in the SmC phase is inferred from the layer spacing data. To examine the critical behavior near the nematic to smectic A(NA) and smectic A to the smectic C (AC) phase transitions, we evaluate the critical exponents: α from specific heat capacity, β from the fitting of the temperature-dependent tilt angle, and ν‖, ν⊥from the temperature-dependent longitudinal (ξ‖) and transverse (ξ⊥) correlation lengths. Modulated Differential Scanning Calorimetry (MDSC) measurements indicate the absence of phase shift, latent heat and imaginary specific heat capacity, suggesting that the AC transitions are second-order for all binary mixtures. The results obtained from heat capacity reveal that both the AC and NA transitions exhibit non-universal behaviours with effective exponents lying between the tricritical and 3D-XY values and follow nearly identical curve with decreasing width of the Sm-A and N phases. The Josephson hyperscaling relation is verified for both the NA and AC transitions in different mixtures. Moreover, knowing the heat capacity critical exponent α and the order parameter critical exponent β, the susceptibility critical exponent γ for the AC transition can be estimated from Rushbrooke equality α+2β+γ=2, with γ values ranging from 1.015 to 1.313, indicating the system's crossover character and apparently validating the Rushbrooke equality.

13C NMR studies and orientational order of rod-like mesogens with strongly polar terminal cyano group

ABSTRACT 13C NMR studies of two rod-like mesogens with core units consisting of two and three phenyl rings with strongly polar terminal cyano groups are reported. The two-ring mesogen displayed an enantiotropic smectic A (SmA) mesophase while for the three-ring mesogen, enantiotropic nematic and SmA mesophases were observed. The solution 13C NMR studies of both the mesogens are carried out besides static 13C NMR measurements in the liquid crystalline phase. For the two-ring mesogen, the chemical shift anisotropy (CSA) tensors of the core unit are determined by the 2D-separation of undistorted powder patterns by effortless recoupling (SUPER) experiment. Furthermore, 2D-separated local field (SLF) measurements are realised for both the mesogens in the liquid crystalline phase to determine the 13C-1H dipolar couplings to find the orientational order parameters of the phenyl rings. The main order parameter (Szz) of the two-ring mesogen varies from 0.63 to 0.53 across the SmA phase and for the three-ring mesogen, the Szz values are found to be ~0.73 in the SmA phase and ~0.69 in the nematic phase. The CSA values determined from the SUPER experiment are expected to be a significant utility for molecules with polar terminal nitro/cyano groups as ferroelectric nematic phases are realised in them.

Topological Defect Evolutions Guided by Varying the Initial Azimuthal Orientation

Topological defects are a key concern in numerous branches of physics. It is meaningful to exploit the topological defect evolutions during the phase transitions of condensed matter. Here, via varying the initial azimuthal orientation of the square alignment lattice in a hybrid cell, the topological defect evolution of liquid crystal during the nematic (N)–smectic A (SmA) phase transition is investigated. The director fields surrounding ±1 point defects are manipulated by predesigning the initial azimuthal orientation. When further cooled to the SmA phase, spiral toric focal conic domain (TFCD) arrays are formed as a result of twisted deformation suppression and unique symmetry breaking after the phase transition. The variation in the azimuthal orientation causes the TFCDs to degenerate from infinite rotational symmetry to quadruple rotational symmetry, thus releasing new textures for the SmA phase. Landau–de Gennes numerical modeling is adopted to reproduce the director distributions in the N phase and reveal the evolution of the topological defects. This work enriches the knowledge on the self-organization of soft matter, enhances the capability for the manipulations of topological defects, and may inspire new intriguing applications.

Effects of quantum dots on the nematic to smectic-A phase transition

ABSTRACT Experiments on the composites of liquid crystals and quantum dots showed the significant role of quantum dots on the properties of the nematic–smectic-A phase transition. The effects of quantum dots on the nematic-smectic-A phase transition have been discussed using Flory- Huggins theory of isotropic mixing and Landau-de Gennes theory. The impact of quantum dots on the smectic-A order parameters, nematic–smectic-A transition temperature, splay and bend elastic constants has been discussed. The smectic-A order parameters, transition temperature and elastic constants decrease with the increase of volume fraction of quantum dots.

Prolonged memory effect in smectic A phase of a thermotropic liquid crystal material

The long-duration memory effect in thermotropic liquid crystal (LC) materials has long been a topic of substantial interest and numerous pioneering works. Although the idea of exploring the memory effect had been started from the nematic (N) phase of LC itself, but due to its very short duration, it was soon shifted to the other LC phases such as ferroelectric LC (FLC) or their derivatives and composites. Therefore, we demonstrate here the prolonged memory effect in a smectic A (SmA) phase of a pristine thermotropic LC material namely 4-octyl-4′-cyanobiphenyl (8CB). The memory effect has also been investigated in the N phase to estimate its duration as compared to the SmA phase. To examine the memory effect, firstly, we measured the dielectric permittivity using the dielectric spectroscopic technique with and without 40 V DC biasing. Furthermore, these results have been confirmed by means of optical textures recorded through a polarizing optical microscope. Moreover, we have also observed the removal and retrieval of two stable states by sequent application of heat and DC biasing to examine the cyclability of the LC compound under testing. We anticipate that our results will definitely bridge the gap that occurred in the advancement of emerging futuristic LC-based memory devices.

Difluorinated tolane-based photoluminescent liquid crystals with imidazolium salt-terminated flexible chains: Thermophysical and photophysical properties

Photoluminescent liquid crystal (PLLC) molecules have attracted significant interest because they exhibit both photoluminescent (PL) and liquid crystal (LC) properties within a single-composition molecular system. In the quest to develop PLLC molecules, researchers have explored ionic compounds incorporating ionic moieties in flexible chains or mesogenic structures. Notable examples include disk-shaped columnar PLLCs and smectic (Sm) PLLCs, which exhibit distinct layers of ionic moieties, counterions, and luminescent mesogens. Ionic LC molecules exhibiting the Sm phase at low temperatures and whose phase transition temperatures can be modulated by tuning the ionic moiety show promise as PLLCs. However, reports on Sm PLLC molecules remain scarce. Based on previous findings that difluorinated tolanes can emit fluorescence in the crystalline (Cr) state, we explored their potential as luminescent mesogens for the development of efficient Sm PLLC compounds. In this study, we designed a rod-shaped compound featuring a difluorinated tolane skeleton as a luminescent mesogen, a decyleneoxy chain for stabilizing the LC phase, and an imidazolium ion as an ionic moiety at the chain end to induce the smectic A (SmA) phase. Upon investigating their thermophysical properties, all compounds demonstrated thermotropic LC attributes and contained a SmA phase. Notably, the melting temperature decreased with increasing anion volume. Photophysical property evaluation revealed that the compounds displayed blue photoluminescence in both dilute solutions and Cr states, with no counteranion-dependent variations in photoluminescence observed in the solution. However, the photoluminescence behavior in the Cr-state specimens varies with different counteranions, demonstrating an increased photoluminescence wavelength with increasing anion volume. Furthermore, the Cr ⇄ LC phase transition altered the photoluminescence behavior, highlighting the potential of our molecular design strategy for developing temperature-responsive PL materials. These findings provide valuable insights for designing new PLLC molecules that can serve as effective fluorescent sensors across a range of temperatures, including room temperature.

First Order Alignment Transition in an Interfaced Active Nematic Fluid.

We investigate experimentally the dynamic phase transition of a two-dimensional active nematic layer interfaced with a passive liquid crystal. Under a temperature ramp that leads to the transition of the passive liquid into a highly anisotropic lamellar smectic-A phase, and in the presence of a magnetic field, the coupled active nematic reorganizes its flow and orientational patterns from the turbulent into a quasilaminar regime aligned perpendicularly to the field. Remarkably, while the phase transition of the passive fluid is known to be continuous, or second order, our observations reveal intermittent dynamics of the order parameter and the coexistence of aligned and turbulent regions in the active nematic, a signature of discontinuous, or first order, phase transitions, similar to what is known to occur in relation to flocking in dry active matter. Our results suggest that alignment transitions in active systems are intrinsically discontinuous, regardless of the symmetry and momentum-damping mechanisms.

Asymmetric naphthalene centred bis-ester Schiff base: Self-assembling behaviour and DFT approach

New asymmetric 2,6-disubstituted naphthalene embedded Schiff base-bis esters NEKE-(2–18) synthesized and investigated for their mesomorphic behaviour. One end of naphthalene has been substituted by decyloxy long chain whereas the other end connected with imine-bis ester aromatic core with terminal long chains. New molecules have been found to exhibit polymorphic behaviour with good thermal stability and enhanced mesophase stability upon POM (polarising optical microscopy) and DSC (differential scaling calorimetry) investigations. Early members (C2 and C3) showed nematogenic behaviour along with SmA (smectic-A) mesophase, whereas molecules with C4 and C5 showed only SmA phase. Compounds with C6 and C7 showed monotropic SmC (smectic-C) upon cooling whereas molecules from C8-to-C18 exhibited enantiotropic SmC along with SmA mesophase. The presence of mesophase has been confirmed by VT-XRD (variable temperature X-ray-crystallography) analysis. For comparison and better understanding, the mesomorphic data of compounds from literature has been compared. The DFT (density functional theory) calculations were performed on both new compounds and structurally analogous molecules found in literature. A detailed discussion was carried out regarding the correlation between the mesomorphic behaviour observed in new compounds and literature, and the computed structural (length, aspect ratio) and electronic (dipole moment, polarizability) variables.

Effects of fluorine atoms introduced into flexible chains or mesogenic structures on their liquid-crystalline and photoluminescence characteristics

Introducing semifluoroalkoxy-type flexible chains into π-conjugated mesogens induces the formation of smectic A (SmA) liquid-crystalline (LC) phases, which are higher-order LC phases. Although tolanes are frequently used as π-conjugated mesogens, tolane and its derivatives are generally non-emissive in the solution and crystalline (Cr) phases. Tetrafluorinated tolanes, wherein four F atoms are introduced into the minor molecular axes, exhibited significantly increased photoluminescence (PL) efficiencies. However, even when a semifluoroalkoxy-type flexible chain was introduced into the tetrafluorinated tolane-based mesogen, no LC-phase formation was observed due to the increased Cr-phase stability. Therefore, we developed new semifluoroalkoxy-substituted difluorinated tolanes with two F atoms within the short molecular axes of the mesogens. The length of the fluoroalkyl segment of the semifluoroalkoxy chain and the positions of the F atoms introduced into the short molecular axis of the mesogen affected the LC and PL characteristics. Remarkably, introducing flexible chains with long fluoroalkyl segments decreased the PL intensity in the Cr phase, whereas after the phase transition to the SmA phase, the PL intensity was increased compared to that of the Cr phase in the case of difluorinated tolane with a 3,5-difluorobenzonitrile unit. With appropriate molecular design, strong PL may be realized, even in the LC phase, and new tolanes should be applied as useful photoluminescent temperature sensors in the future.

Splay-induced order in systems of hard tapers.

The main objective of this work is to clarify the role that taper-shaped elongated molecules, i.e., molecules with one end wider than the other, can play in stabilizing orientational order. The focus is exclusively on entropy-driven self-organization induced by purely excluded volume interactions. Drawing an analogy to RM734 (4-[(4-nitrophenoxy)carbonyl]phenyl-2,4-dimethoxybenzoate), which is known to stabilize ferroelectric nematic (N_{F}) and nematic splay (N_{S}) phases, and assuming that molecular biaxiality is of secondary importance, we consider monodisperse systems composed of hard molecules. Each molecule is modeled using six collinear tangent spheres with linearly decreasing diameters. Through hard-particle, constant-pressure Monte Carlo simulations, we study the emergent phases as functions of the ratio between the smallest and largest diameters of the spheres (denoted as d) and the packing fraction (η). To analyze global and local molecular orderings, we examine molecular configurations in terms of nematic, smectic, and hexatic order parameters. Additionally, we investigate the radial pair distribution function, polarization correlation function, and the histogram of angles between molecular axes. The last characteristic is utilized to quantify local splay. The findings reveal that splay-induced deformations drive unusual long-range orientational order at relatively high packing fractions (η>0.5), corresponding to crystalline phases. When η<0.5, only short-range order is affected, and in addition to the isotropic liquid, only the standard nematic and smectic-A liquid crystalline phases are stabilized. However, for η>0.5, apart from the ordinary nonpolar hexagonal crystal, three additional frustrated crystalline polar blue phases with long-range splay modulation are observed: antiferroelectric splay crystal (Cr_{S}P_{A}), antiferroelectric double-splay crystal (Cr_{DS}P_{A}), and ferroelectric double-splay crystal (Cr_{DS}P_{F}). Finally, we employ Onsager-Parsons-Lee local density functional theory to investigate whether any sterically induced (anti)ferroelectric nematic or smectic-A type of ordering is possible for our system, at least in a metastable regime.

Liquid Crystalline and Gel Properties of Luminescent Cyclometalated Palladium Complexes with Benzoylthiourea Ligands.

The design and development of new luminescent metallogels formed by cyclometalated palladium(II) complexes in protic solvents were investigated by a combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM), and rheology. Cyclometalated palladium(II) complexes based on imine ligand and ancillary benzoylthiourea (BTU) ligand showed red emission in solid and gel states. The formation of a lyotropic liquid crystal phase was observed for the complex bearing shorter alkyl groups on the BTU ligand. This complex also behaved as a thermotropic liquid crystal that displays a monotropic smectic A phase (SmA). Dynamic rheology measurements (frequency sweep in the 5-90 °C range) of the 1-decanol solution of palladium(II) complexes highlighted their supramolecular self-association ability to generate 3D networks and form gels as a final result.

SYNTHESIS AND MESOGENIC PROPERTIES OF NEW AROMATIC SCHIFF BASE’S ESTERS

A homologous series of new Schiff base’s esters, 4-bromobenzylidene-4’-alkanoyloxyanilines were syn-thesized. The structure of the synthesized compounds was elucidated using IR and NMR spectroscopic techniques along with mass spectrometric method. Nine homologous members were prepared whereby the length of alkanoyloxy chain, Cn-1H2n-1COO is varied from n = 2, 4, 6, 8, 10, 12, 14, 16 and 18. Their thermotropic properties were analyzed using differential scanning calorimetry and polarized optical mi-croscopy techniques. Whilst short chain members (number of carbons, n = 2, 4) are non-mesogens, member with n = 6 exhibited monotropic smectic A (SmA) phase. As the n increased to 8, this com-pound becomes the only member that exhibited SmA and SmB as enantiotropic phase. Varying from n = 10 to n = 16, these members displayed enantiotropic SmA and monotropic SmB phases. Longest chain member, n-octadecanoyloxy derivatives exhibited only single mesophase (monotropic SmA). The ester linkage and polar terminal (bromo) group in the present series are essential for the smectic polymorphism in Schiff bases

The effect of surfaces on the structural properties of liquid crystals

The effect of the surface microrelief on the structural properties of both the nematic and smectic A (SmA) phases, and the effect of the emerging smectic structure induced by the surface at temperatures close to the second-order nematic-SmA (NA) phase transition temperature TNA, on the Freedericksz threshold voltage for homogeneously aligned polar liquid crystals, is theoretically analyzed.It is shown that the orientation distortion of the director field caused by the wave-like microrelief on solid surfaces exponentially decays in the bulk of both phases. Since in the case of the SmA phase, the characteristic depth of deformation is comparable to the length of the molecule, then when the period of undulating relief passes several micrometers, the depth of penetration of the deformation is several tens of micrometers. This is two orders of magnitude greater than the penetration length of the distortion in the case of the nematic phase.The effect of the surface-induced SmA phase on the threshold voltage, for homogeneously aligned of polar liquid crystals is discussed from the energy point of view. It is shown that the Freedericksz critical voltage, for 4-n-octyl-4′-cyanobiphenyl, in a very thin cell near the second-order nematic-SmA (NA) phase transition temperature, exhibits an anomalous increase with decreasing of temperature T towards TNA.

Three Distinct Polar Phases, Isotropic, Nematic, and Smectic-A Phases, Formed from a Fluoro-Substituted Dimeric Molecule with Large Dipole Moment.

A dimeric molecule, di-5(3FM-C4T), with fluoro-substituted mesogenic cores composed of three-aromatic rings and linked by a pentamethylene spacer is prepared. Di-5(3FM-C4T) forms the ferroelectric nematic (NF), ferroelectric smectic-A (SmAPF), and polar isotropic (IsoP) phases. The NF phase is composed of molecules in U-shaped conformation that behave like polar rod-like molecules. The reversal spontaneous polarization (Ps) is approximately 8 μC cm-2, which is extremely large and reflects the huge dipole moment (11.2 D) of the one-side mesogenic core. On the other hand, the SmAPF phase is formed by bent-shaped molecules. The NF-SmAPF phase transition thus follows the conformational change of molecules. The reversal Ps of the SmAPF phase is around 4 μC cm-2, which is half of that in the NF phase, and this is an expected value from the bent shape of the molecules. It is interesting that the highest temperature IsoP phase still exhibits the polar structure and possibly retains some polar aggregation of molecules in small domains. The three polar phases exhibit the dielectric mode due to the collective polarization fluctuation at around 100 Hz, giving the high dielectric constant over 8000.

Critical behaviour near the nematic-smectic-C and nematic-smectic-A phase transitions of a binary system: a comparative study from calorimetric, optical and dielectric measurements

ABSTRACT Detailed heat capacity, birefringence and dielectric anisotropy measurements have been carried out near the nematic – smectic-C (NC) and the nematic – smectic-A (NA) phase transitions of a mesogenic binary system. Important feature is the existence of two tricritical points one for the NC transition and other for the NA transition. The dependence of critical exponent (α) is well explained in terms of temperature ratios (TNC/TNI and TNA/TNI) for the NC and NA phase transitions. Both transitions exhibit non-universal behaviour and showing effective critical exponents in between tricritical and 3D-XY values. However, the nematic range necessary for the NC tricritical point is quite large (>20°C). The enthalpy, phase shift and imaginary heat capacity are also analogous to the nature of the critical exponent (α). Moreover, within a very small concentration range, the smectic-A – smectic-C (AC) transition line appears. Although the smectic-A width above the AC transition is quite small~2K, heat capacity and birefringence data clearly confirm second-order nature. Additionally, there exists a region where the NC, NA and AC line segments converge, i.e. a NAC multicritical point. At this point, the latent heat for the NC, NA and AC transition lines disappears, indicating an obvious second-order nature of the NAC multicritical point.

Landau-de Gennes theory of the nematic to smectic—A phase transition under confinement conditions

We study the nature of the nematic to smectic-A phase transition under confinement conditions in the context of Landau-de Gennes theory. The effect of quenched disorder on the nematic to smectic-A transition temperature, order parameter, enthalpy, and thermal hysteresis are discussed followed by phase diagrams. The tricritical behavior of the nematic to smectic-A phase under confinement conditions is predicted.

Preparation of High-quality Vapor-deposited Crystalline Organic Thin Films through Crystallization from Their Liquid Crystal Phase

High-quality crystalline organic thin films with large crystalline domain size were fabricated by vacuum deposition, followed by a post-annealing and cooling crystallization from their respective smectic A (SmA) liquid crystal precursor states. Crystallinity, surface smoothness and air exposure stability of ionic liquid crystalline thin films were much improved by this process, and the electric double layer action was for the first time observed at the SmA phase. Moreover, a seeded temperature gradient cooling process from the SmA phase was proposed and its capability was demonstrated in the fabrication of benzothienobenzothiophene derivative thin films, achieving their directional lateral crystal growth with high precision controllability.

Conformational degrees of freedom and stability of splay-bend ordering in the limit of a very strong planar anchoring.

We study the self-organization in a monolayer (a two-dimensional system) of flexible planar trimer particles. The molecules are made up of two mesogenic units linked by a spacer, all of which are modeled as hard needles of the same length. Each molecule can dynamically adopt two conformational states: an achiral bent-shaped (cis-) and a chiral zigzag (trans-) one. Using constant pressure Monte Carlo simulations and Onsager-type density functional theory (DFT), we show that the system consisting of these molecules exhibits a rich spectrum of liquid crystalline phases. The most interesting observation is the identification of stable smectic splay-bend (S_{SB}) and chiral smectic-A (S_{A}^{*}) phases. The S_{SB} phase is also stable in the limit, where only cis- conformers are allowed. The second phase that occupies a considerable portion of the phase diagram is S_{A}^{*} with chiral layers, where the chirality of the neighboring layers is of opposite sign. The study of the average fractions of the trans- and cis- conformers in various phases shows that while in the isotropic phase all fractions are equally populated, the S_{A}^{*} phase is dominated by chiral conformers (zigzag), but the achiral conformers win in the smectic splay-bend phase. To clarify the possibility of stabilization of the nematic splay-bend (N_{SB}) phase for trimers, the free energy of the N_{SB} and S_{SB} phases is calculated within DFT for the cis- conformers, for densities where simulations show stable S_{SB}. It turns out that the N_{SB} phase is unstable away from the phase transition to the nematic phase, and its free energy is always higher than that of S_{SB}, down to the transition to the nematic phase, although the difference in free energies becomes extremely small when approaching the transition.

General weak segregation theory with an application to monodisperse semi-flexible diblock copolymers.

A general theory has been developed for a polydisperse semi-flexible multi-block copolymer melt. Using the Bawendi-Freed approach to model semi-flexible chains, an expression for the Landau free energy is derived in the weak segregation regime, which includes density and orientation order-parameters. The orientation order-parameter is described in the smectic phase and in more complicated structures, such as the hexagonal phase. The Landau free energy contains contributions of two kinds of interactions. The first kind is the Flory-Huggins interaction, which describes the incompatibility of chemically different blocks and may induce microphase separation. The second kind is the Maier-Saupe interaction, which may induce nematic ordering. In the framework of the weak segregation limit, the Landau theory allows us to predict phase structures in the melt as a function of the composition, persistence length, and the strength of the Flory-Huggins and Maier-Saupe interaction. The general theory is applied to a simple system of monodisperse semi-flexible diblock copolymers. In several phase diagrams, a number of possible phase structures are predicted, such as the bcc, hexagonal, smectic-A, smectic-C, and nematic phase. The influence of the Maier-Saupe interaction on the microphase structure is thoroughly discussed.

Statement of Retraction: Induced Reentrant Smectic-A Phase in Binary Mixtures of Liquid Crystals

Statement of Retraction: Induced Reentrant Smectic-A Phase in Binary Mixtures of Liquid Crystals