Chiral Smectic Liquid Crystals Research Articles

Linear and Nonlinear Electro-optic Response of MHPOCBC

This study investigated the linear and second-order electro-optic responses in chiral smectic liquid crystal 4-(1-methylheptyloxycarbonyl) phenyl 4-octylcarbonyloxybiphenyl-4-carboxylate (MHPOCBC). The result revealed a single Debye-type relaxation in the linear electro-optic frequency dispersion, with a relaxation frequency extending up to one hundred kHz. Conversely, the second-order electro-optic response exhibited intricate temperature-dependence, accurately depicted by a phenomenological Landau theory around the SmA–SmCa* phase transition point. Notably, in the SmCa* phase, critical slowing down of the amplitude mode occurred near the transition to the SmA phase, while at lower temperatures of the SmCa* phase, a distinct low-frequency relaxation mode emerged. Furthermore, the relaxation frequency of the antiferroelectric Goldstone mode in the SmCA* phase remained constant across the entire temperature range. These findings should significantly contribute to the understanding of dynamic behaviors in chiral smectic liquid crystals, shedding light on their complex phase transitions and electro-optic properties.

Solid-Liquid Crystal Biphasic Ferroelectrics with Tunable Biferroelectricity.

Ferroelectricity has been separately found in numerous solid and liquid crystal materials since its first discovery in 1920. However, a single material with biferroelectricity existing in both solid and liquid crystal phases is very rare, and the regulation of biferroelectricity has never been studied. Here, solid-liquid crystal biphasic ferroelectrics, cholestanyl 4-X-benzoate (4X-CB, X = Cl, Br, and I), which exhibits biferroelectricity in both the solid and liquid crystal phases, is presented. It is noted that the ferroelectric liquid crystal phase of 4X-CB is a cholesteric one, distinct from the ordinary chiral smectic ferroelectric liquid crystal phase. Moreover, 4X-CB shows solid-solid and solid-liquid crystal phase transitions, of which the transition temperatures gradually increase from Cl to Br to I substitution. The spontaneous polarization (Ps ) of 4X-CB in both solid and liquid crystal phases can also be regulated by different halogen substitutions, where the 4Br-CB has the optimal Ps because of the larger molecular dipole moment. To the authors' knowledge, 4X-CB is the first ferroelectric with tunable biferroelectricity, which offers a feasible case for the performance optimization of solid-liquid crystal biphasic ferroelectrics.

Broad temperature range ferrielectric liquid crystal as a highly sensitive quadratic electro-optical material.

A chiral smectic liquid crystal in which a ferrielectric phase with a helix pitch p0 less than 125 nm exists over a temperature range of at least from -3°C to +36°C has been developed. Such a wide temperature range (including room temperatures) of the ferrielectric phase with a subwavelength helix pitch has been achieved for the first time, to the best of our knowledge, which creates opportunities for the practical use of ferrielectric liquid crystals. A quadratic electro-optical effect caused by deformations of the helix in an electric field is observed in the ferrielectric phase, and the Kerr coefficient, which reaches almost 200 nm/V2, is significantly higher than the same coefficient for the blue phase and for the smectic C* phase, which means a higher sensitivity of the developed ferrielectric liquid crystal to the electric field. The electro-optical response time does not exceed 300μs at room temperatures for this ferrielectric liquid crystal.

Helical Network Polymers Embodying High Dissymmetry Factors in Circularly Polarized Luminescence: Photocrosslinking Polymerization of Acrylate Derivatives in Chiral Smectic Liquid Crystals

Radical photocrosslinking polymerizations of functional acrylate derivatives are achieved in a chiral smectic C liquid crystal (SC*-LC) used as an asymmetric medium to synthesize helical network polymers (HNPs) with thermally and chemically stable chiroptical properties that are represented by circularly polarized luminescence (CPL). Two types of acrylate derivatives that bear an LC moiety and a fluorene moiety as luminophores are synthesized and used as functional monomers for photocrosslinking polymerizations. A trisubstituted acrylate derivative is used as a crosslinker to enhance the amount of photocrosslinking to produce a spatially linked network structure of polymers. The synthesized HNPs show a striated fan-shaped texture that is characteristic of the SC*-LC, and they exhibit notable Cotton effects owing to their circular dichroism in the absorption and luminescence processes. The CPL of the (R)- and (S)-HNP films shows considerably high dissymmetry factors (|glum|) of 2.4 × 10–2 and 2.5 × 10–2, respectively. The CPL further increases in intensity by more than 4 times after heating the HNP film at 150 °C to provide |glum| values of 1.0 × 10–1 and 1.1 × 10–1 for (R)- and (S)-HNP films, respectively. This increase in CPL intensity is ascribed to the thermally stimulated progress of the crosslinking between the monomers that still remain unreacted during photopolymerization at room temperature, thus leading to the completion of the helical network in the HNP film. The higher structural order of the HNP films prepared with the SC*-LC than those prepared with a chiral nematic LC (N*-LC) is confirmed through X-ray diffraction measurements. The present work is the first successful asymmetric polymerization using the SC*-LC as a chiral medium.

Second-order NLO response in chiral ferroelectric liquid crystals: Molecular and bulk consideration

Nonlinear optical (NLO) properties of three chiral smectic liquid crystals were investigated by Hyper-Rayleigh Scattering (HRS) and the Second-Harmonic Generation (SHG) methods. The HRS measurements of two compounds (MHPOBC and D16), with the biphenyl unit in their center, in solution demonstrate the quadratic NLO activity. The third compound (D17), having three isolated phenyl rings in the core, does not exhibit the NLO activity. The NLO properties in smectic phases of all investigated compounds have been also studied by wide-field second-harmonic and two-photon fluorescence microscopy confirming noncentrosymmetry for compounds with biphenyl π coupling (MHPOBC and D16).

Vibrational Dynamics of a Chiral Smectic Liquid Crystal Undergoing Vitrification and Cold Crystallization

Vibrational dynamics in the glass transition and the cold crystallization process of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-benzoate (3F7HPhH7) was studied by Fourier transform infrared spectroscopy (FTIR) during cooling/heating experimental runs. The measured spectra processing was supported by quantum chemical density functional theory (DFT) calculations (frequency assignments). The perturbation-correlation moving window two-dimensional analysis (PCMW2D) was performed to examine how the height of individual absorption bands change under with temperature. Two-dimensional correlation analysis (2D-COS) was used to detect freezing-in or activation of the stochastic movements during the vitrification and the cold crystallization processes. Upon cooling, the vitrification process involves freezing-in of the stochastic movements of ester groups. Upon heating, as the cold crystallization process begins, the first to respond are the vibrations of the C–O–C and C=O groups in the rigid core.

Design and electro-optic investigations of de Vries chiral smectic liquid crystals for exhibiting broad temperature ranges of SmA* and SmC* phases and fast electro-optic switching

Two new ferroelectric liquid crystalline compounds using 5-phenyl-pyrimidine core terminated by achiral trisiloxane chain, and chiral (S) alkyl chain on the opposite ends of their respective mesogens are designed with better characteristics of switching speed and broader temperature range.

Study of the orientational ordering in the field-induced intermedate phase from the SmC *Fi2 phase in chiral smectic liquid crystals

Under an electric field, chiral smectic liquid crystals transit usually to the unwound SmC* phase where the helical structure is completely unrolled. Sometimes the sample transits initially towards an intermediate polar state before the total destruction of the helix. The discovery of this intermediate phase opens a promising field of application especially in the field of color displays. This gives importance to the study and the characterization of this new phase. Based on the extension of the Hammaneh-Taylor model (H&T model), a theoretical study of these field-induced phase transitions was carried out. Two hypotheses of the dynamics that give rise appearance of the intermediate phase have been discussed. Numerical analysis confirms the known experimental results; the intermediate phase has a three-layer periodicity structure.

Molecular orientational distribution function of a chiral de Vries smectic liquid crystal from birefringence measurements.

An alternative method for determining the orientational distribution function and the order parameter from the electric field-induced birefringence measurements of a chiral liquid crystal compound in its Smectic A* is being introduced. A chiral mesogen based on a 5-phenyl-pyrimidine benzoate core terminated by a trisiloxane group on one side and the chiral alkyloxy chain on its opposite side is designed and synthesized to exhibit the "de Vries" smectic characteristics. The compound exhibits first order Smectic A*-Smectic C* phase transition, evidenced by the results of differential scanning calorimetry. The material is being investigated by electro-optical experiment in its smectic phases. We present a model that incorporates the generalised Langevin-Debye model which includes the Maier-Saupe effective mean-field potential term in order to explain the change in birefringence with the electric field. A good agreement between the experimental results and the predictions from the model leads to the determination of the molecular orientational distribution function in Smectic A phase. Furthermore, the temperature dependency of the Saupe orientational order parameter ⟨P2⟩ is obtained using the parameters of the model. Based on the experimental and theoretical results, we show that de Vries Smectic A* phase exhibits a broad volcano-like tilt angle distribution with the two maxima occurring at finite tilt angles closer to the Smectic A*-Smectic C* transition temperature, and a sugarloaf-like distribution occurs in the tilt for temperatures close to the Isotropic-Smectic A* phase transition.

Tuning the electro-optic and viscoelastic properties of ferroelectric liquid crystalline materials

This work deals with determination of the viscoelasticity coefficient in chiral smectic liquid crystals possessing the helical structure and is the continuation of the work, in which the elasticity coefficient was presented [Phase Transitions 89 (2016) 368]. The measurements have been performed using optical detection in the small deformation limit. The viscosity coefficient was measured on two commercially available pure chiral materials, namely, 4-(n-hexyloxy phenyl)-1-(2-fuethyl butyl) biphenyl-4-carboxylate and 4-(2-methylbutyl) phenyl-4-n-octylbiphenyl-4-carboxylate, and on resulting binary mixture composed from those materials in 50:50 weight ratio. All three liquid crystalline materials exhibit a tilted ferroelectric phase over a reasonably broad temperature range. Each of the two pure liquid crystalline materials have its own disadvantages. However, by design of the binary mixture in definite weight concentration, we tried to improve the behavior and to tune the properties; specifically, the effect of the viscoelastic properties on the mixture composition has been established.

Solitons in discrete linear chains of chiral smectic liquid crystals with competing interactions

Chiral liquid crystals exhibit in-plane spontaneous polarizations, however, in their smectic (Sm) phase the primary-order parameter is the tilt vector associated with molecular rotations around the long axis parallel to the molecular directors. These molecular rotations can lead to several distinct phases among which an incommensurate order with a domain-wall texture is referred to as soliton. In this study, the formation of domain walls in smectic chiral (SmC) liquid crystals is analyzed, with emphasis on the competition between an Ising-like symmetric intermolecular interaction, an anti-symmetric intermolecular interaction due to molecular chirality and an applied in-plane electric field. It is found that the anti-symmetric intermolecular interaction increases the width of kink structures in the domain wall at moderate intensity of the y-component of the applied in-plane electric field. Increasing the x-component of the electric field creates unstable condition for soliton formation irrespective of magnitudes of the symmetric and chiral intermolecular interactions. The stability condition for single-kink structures in the discrete molecular chain is discussed, by estimating the Peierls stress experienced by the single-kink soliton. Results suggest that chirality lowers the Peierls–Nabarro barrier, hence increasing the lifetime of single-kink structures in the discrete medium.

Electro-optical properties and () phase diagram of fluorinated chiral smectic liquid crystals

Fluorinated smectic liquid crystals each with a biphenyl benzoate rigid core are investigated. Molecular structures of the studied compounds have difference only in fluorine position and the length of the carbon chain. Dielectric relaxation study and electro-optical measurements are carried out with the classical SSFLC geometry. The field-induced phase transitions are studied and the (E,T) phase diagram is established.

On DNA Motions under Action of Enzymes of Different Types. I

Different types of enzymes in rotational macromolecular motors of living cells are presented based on the data in the literature. Particular attention has been given to the restriction–modification enzymes and helicases, in which conformational changes in sites are physical reasons of linear motions and rotation of DNA molecules (caused by polar forces). It is shown that the structure and functioning of enzymes, similar to those in chiral smectic C-type liquid crystals, can be described within the formalism of generalized forces and currents. These forces include the membrane electrochemical gradient, the velocity of rotor mechanical motion, and the measure of change in free energy in the “fuel” adenosine triphosphate molecule.

Resonant x-ray scattering observation of transitional subphases during the electric-field-induced phase transition in a mixture of Se-containing chiral smectic liquid crystals.

Using resonant x-ray scattering techniques, transitional subphases during the electric-field-induced phase transition of a mixture of Se-containing chiral liquid crystals, 80% AS657 and 20% AS620, in a planar-aligned cell geometry were investigated, where the prototypical phase sequence SmC_{A}^{*}-SmC_{γ}^{*}-AF-SmC^{*} was observed; the transitional subphases were formed during the transition from the three-layer periodicity phase to the ferroelectric phase. In the lower-temperature range where the three-layer SmCγ^{*} phase appeared under the low electric field, nine- and six-layer subphases and a "streak" pattern appeared in sequence after the transition from the SmCγ^{*} phase with increasing applied electric field; the ferroelectric phase was realized. In the higher-temperature range where the four-layer AF phase appeared under a low electric field, the AF phase changed to a three-layer phase at the medium electric field. The twelve-, nine-, and six-layer subphases subsequently appeared in sequence, and finally the ferroelectric phase was generated with increasing electric field. The molecular arrangements of the field-induced subphases, especially the newly found nine-layer periodicity phase, was analyzed. The successive field-induced phase transition of the present results was compared with that of our previous results for pure Se-containing and Br-containing liquid crystals, and the relation to the three-layer ferrielectric phase was discussed.

Calculation of gyrotropy coefficients in media with low-pitch helical structures.

Chiral smectic liquid crystals are known for their huge optical activity due to the precession of the anisotropic dielectric tensor around the helicoidal axis. For an oblique direction of the propagating wave, the helix acts as a grating which splits an incident beam in different directions as long as the pitch is not too small with respect to the light wavelength. When the pitch of the helix is smaller than the wavelength, the effect of the helix is a renormalization of the gyrotropic coefficients (g_{⊥} and g_{∥}) of the resulting uniaxial medium. We report here on a method to compute these coefficients in that limit. Resolution of the Maxwell equations, using a perturbative approach, gives expressions for g_{⊥} and g_{∥} as a power development of the ratio (p/λ). The various terms of these developments coincide with the approximate expressions of these coefficients known in the literature.

Calculation of electric field–temperature (E, T) phase diagram of a ferroelectric liquid crystal near the SmA– transition

In this work we perform a theoretical calculation in order to reconstitute the (E–T) phase diagram of a chiral smectic liquid crystal in the vicinity of the SmA– transition. This reconstruction is carried out on the basis of a thermodynamic calculation of the slope of the curve joining the domain and the unwound SmC*. An empiric correction of the mean field term of Landau De-Gennes development is necessary to accomplish this reconstruction. Thereafter, an experimental validation is performed to verify our calculations.

Room-temperature magnetoelectric effect in a chiral smectic liquid crystal

A direct magnetoelectric effect, an induction of electric polarization by a magnetic field, was observed at room temperature in a chiral smectic liquid crystal without magnetic metals or radicals. Our detailed measurements of the magnetoelectric effect and the magnetodielectric effect suggest a unique origin of the magnetoelectric coupling in the liquid crystal, that is, a magnetic-field manipulation of the molecular orientation state, which is closely coupled with a local electric dipole moment due to the chirality-induced symmetry breaking. This unconventional strategy based on the softness of liquid crystals provides one of the promising directions towards the achievement of room-temperature metal-free magnetoelectrics.

PB51 電圧印加時のカイラルスメクチック液晶の螺旋構造と光学特性の解析(液晶フォトニクス・光デバイス,ポスター発表,2015年日本液晶学会討論会)

PB51 電圧印加時のカイラルスメクチック液晶の螺旋構造と光学特性の解析(液晶フォトニクス・光デバイス,ポスター発表,2015年日本液晶学会討論会)

Transitional subphases near the electric-field-induced phase transition to the ferroelectric phase in Se-containing chiral smectic liquid crystals observed by resonant x-ray scattering.

Resonant x-ray scattering experiments revealed transitional subphases near the electric-field-induced phase transition of a Se-containing chiral liquid crystal in a planar aligned cell geometry. In the lower-temperature range (Sm-C_{A}^{*} and three-layer periodicity Sm-C_{γ}^{*} phases), the six-layer periodicity subphase appeared with increasing electric field during the field-induced transition from Sm-C_{γ}^{*} to Sm-C^{*}. In the higher-temperature range [four-layer periodicity antiferroelectric (AF) phase], the peak positions of the three-layer satellites shifted to those of the four-layer satellites and then the satellites corresponding to the five- through seven-layer periodicity appeared in sequence. Near the AF to Sm-C_{α}^{*} phase transition temperature, the layer periodicity increased with applied field. The molecular configurations of the subphases near the field-induced transition are discussed based on the Ising, distorted clock, and perfect clock models.

Effect of defects and surface anchoring on the phase behavior of chiral smectic liquid crystals

ABSTRACTThe influence of defects and surface effects on the stability of phenomena accompanying phase transitions, mainly coexistence phenomenon, has been studied. Experimental measurements were carried out by varying the cell thickness and using simple experimental on chiral smectic liquid crystal product. The analysis of the results with different cell thickness leads us to interpret the anomaly shown by our product related to impact of defects and surface effects. Moreover, the phase transition is affected by the trace of defects for thick cell and by the surface effect for thin cell. These effects have the same role as an electric local field which can induce phase transition.