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In this paper, we report on the design, synthesis and characterisation of two new ferroelectric liquid crystalline compounds based on using the cores of (i) 5-phenyl-pyrimidine benzene and (ii) 5-phenyl-pyrimidine benzoate terminated by achiral trisiloxane chain, and chiral (S) alkyl chain on the opposite ends of their respective mesogens. These compounds exhibit broad temperature ranges of SmA∗ and SmC∗ phases, whereas the latter exists at room temperature down to 11 °C. This is one of the desirable features of the compounds for perspective applications in devices. In both compounds, the differential scanning calorimetry shows a weak first order SmA∗-SmC∗ transition. This observation is supported by determining the critical exponent of the power law as ∼0.2 that describes the dependence of the tilt angle and the spontaneous polarization on temperature. The two compounds show excellent de Vries characteristics in terms of lower layer shrinkages in SmC∗, the maximum shrinkage in both cases is ∼1.5%. Furthermore (i) the birefringence decreases with a reduction in temperature instead of an increase in SmA∗, and (ii) a large field-induced birefringence is observed in SmA∗ close to the SmA∗-SmC∗ transition temperature, (iii) the maximum birefringence measured is much lower than that of a typical FLC material. The characteristics of these compounds when compared to a conventional/typical ferroelectric liquid crystalline mixture show that these materials surpass in performance in terms of having a (i) wider temperature range of both SmA∗ and SmC∗ phases and (ii) higher switching speed. The layer thickness in SmA∗ and SmC∗ is measured using the techniques of (a) small-angle X ray diffraction, and the (b) optical interferometry of free standing smectic films. A comparison of the results is used to determine an exceptionally useful result of the apparent tilt angle as a function of temperature in SmA∗ without the need to apply the electric field across the cell. This journal is © The Royal Society of Chemistry.

Physics

School of Advanced Sciences

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Journal of Materials Chemistry C

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

The chiral smectic phases of calamitic liquid crystals, SmC∗ and SmCA∗, are characterized by the synclinic ferroelectric F ordering and the anticlinic antiferroelectric A ordering in adjacent layers. Various states with mixed A and F orderings are degenerate at the frustrated phase-transition point. The degeneracy lifting is commonly caused by the long-range interlayer interactions (LRILIs), producing a series of biaxial subphases specified by a relative ratio of both orderings, qT=[F]/([A]+[F]). Sandhya et al. [Phys. Rev. E 87, 012502 (2013)PLEEE81539-375510.1103/PhysRevE.87.012502] established, however, the importance of thermal fluctuations in the degeneracy lifting in some binary mixtures of MC881 and MC452. They observed the most intriguing interplay of thermal fluctuations and LRILIs in the stabilization of an apparently single subphase. Since no other detailed experimental study of the subphase has so far been made, we carry out its dielectric investigations and clarify the following five points: (1) the subphase is surely a single phase from ≈80°C down to room temperature; (2) the imaginary part of complex permittivity ϵ″ shows the weak antiphase mode and hence it must be antiferroelectric qT=1/2; (3) ϵ″ becomes much stronger above ≈80°C, indicating the emergence of ferroelectric and/or ferrielectric states; (4) the dielectric amplitude gradually increases at least just above the 1/2 subphase, suggesting it be due to a continuous increase of qT; and (5) at low temperatures the antiphase relaxation mode shows irregularities that indicate the important role played by the cooperative motion of the layer undulation as well as of the director tilting. © 2020 American Physical Society.

Physical Review E

Dielectric study of a subphase stabilized in an exceptionally wide temperature range by a delicate balance of interlayer interactions and thermal fluctuations

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 P 2 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. © 2019 Author(s).

Fulltext

Journal of Chemical Physics

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

An external electric field applied across a planar-aligned cell in Smectic A* phase of de Vries smectic liquid crystal induces director redistribution over a cone, resulting in a substantial increase in the birefringence and the apparent optical tilt angle. Such an electro-optic response is modelled by Shen et al. [Y. Shen et al., Phys. Rev. E 88, 062504 (2013)], who modified their previous hollow cone with a diffuse cone model by introducing the molecular distribution function limited over a range of tilt angles, that lie in between θmin and θmax. The limits in these two tilt angles are assumed to be temperature independent though the tilt angle in between the two values can be temperature dependent. However, the high resolution measurements of birefringence and the layer thickness indicate the presence of temperature dependent diffuse cone angle in SmA* phase. In the proposed model, we replace θmin by θT, a temperature dependent fitting parameter and the change shows that a better fit of the experimental data to the model is obtained. We determine the temperature dependence of θmin and show that this angle increases as SmA* to SmC* phase transition temperature is approached. © 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.

Liquid Crystals

A modified Langevin-Debye model for investigating the electro-optic behaviour of de Vries smectic liquid crystals

We report experimental results of the orientational order parameter, the apparent tilt angle, and the field-induced tilt angle for three chiral smectic liquid crystalline materials investigated using infrared (IR) polarized spectroscopy. The common feature in these materials is use of the core 5-methyl-2- pyrimidine benzoate as the central part of the mesogen. This core is terminated by siloxane or carbosilane chains on one of the ends and by the chiral alkoxy chains on the opposite. These compounds exhibit low concomitant layer shrinkage at the smectic A∗ (SmA∗) to smectic C∗ (SmC∗) transition temperature and within the SmC∗ phase itself. The maximum layer shrinkage in SmC∗ is observed as ∼1.5%. We calculate the apparent orientational order parameter, Sapp in the laboratory reference frame from the observed IR absorbance for homeotropic aligned samples, and the true order parameter, S, is calculated using the measured tilt angle and is also interpolated from Iso-SmA∗ transition temperature closer to SmC∗ phase. The apparent tilt angle in the SmA∗ phase calculated from a comparison of order parameters S and Sapp is found to be significantly large. A low magnitude of Sapp found for homeotropic aligned samples in the SmA∗ phase indicates that the order parameter plays a vital role in determining the de Vries characteristics, especially of exhibiting larger apparent tilt angles. Furthermore there is a significant increase in the true order parameter at temperatures close to SmA∗ to SmC∗ transition temperature in all three compounds. The planar-aligned samples are used to study the dependence of induced tilt angle on the applied electric field. The generalized Langevin-Debye model given by Shen et al. reasonably fits the experimental data on the field-induced tilt angle. The results show that the dipole moment of the tilt correlated domain in SmA∗ diverges as temperature is lowered to the SmA∗-SmC∗ transition temperature. The generalized Langevin-Debye model is also found to be extremely effective in confirming some of the conclusions of the de Vries behavior. © 2019 American Physical Society.

Elucidation of the de Vries behavior in terms of the orientational order parameter, apparent tilt angle, and field-induced tilt angle for smectic liquid crystals by polarized infrared spectroscopy

An addition of chiral dopant to two achiral smectic liquid crystals from a homologous series, by varying weight percentages with known low values of layer shrinkage, leads to chiral smectic-C* phase with a finite value of the spontaneous polarisation. The electro-optical response arising from changes in the induced apparent tilt angle brought about by a weak electric field in the SmA* phase gives rise to power law dependency on the reduced temperature. The critical exponent γ of the power law depends on the dopant concentration but its value is found to be greater than the typical value of 1.32. This implies that the short-range correlation extends from two dimensions to three dimensions in these materials in the SmA phase. The layer thickness of smectic layers in the guest–host system remains unaltered up to the 15 wt % addition of the chiral dopant to two achiral smectics. The system thus retains the low layer shrinkage of the achiral smectic as evidenced by measurements of the layer thickness from X-ray scattering and thickness measurements from optical interferometry. Results on the optical birefringence and the apparent tilt angle lead us to the conclusion of having successfully obtained chiral smectic materials for devices with de Vries characteristics by chiral doping. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

The effect of chiral doping in achiral smectic liquid crystals on the de Vries characteristics: smectic layer thickness, electro-optics and birefringence

Infrared and dielectric spectroscopic techniques are used to investigate the characteristics of two chiral smectics, namely, 1,1,3,3,5,5,5-heptamethyltrisiloxane 1-[4′-(undecyl-1-oxy)-4-biphenyl(S,S)-2-chloro-3-methylpentanoate] (MSi3MR11) and tricarbosilane-hexyloxy-benzoic acid (S)-4′-(1-methyl-hexyloxy)-3′-nitro-biphenyl-4-yl ester (W599). The orientational features and the field dependencies of the apparent tilt angle and the dichroic ratio for homogeneous planar-aligned samples were calculated from the absorbance profiles obtained at different temperatures especially in the smectic-A∗ phase of these liquid crystals. The dichroic ratios of the C-C phenyl ring stretching vibrations were considered for the determination of the tilt angle at different temperatures and different voltages. The low values of the order parameter obtained with and without an electric field applied across the cell in the Sm-A∗ phase for both smectics are consistent with the de Vries concept. The generalized Langevin-Debye model introduced in the literature for explaining the electro-optical response has been applied to the results from infrared spectroscopy. The results show that the dipole moment of the tilt-correlated domain diverges as the transition temperature from Sm-A∗ to Sm-C∗ is approached. The Debye-Langevin model is found to be extremely effective in confirming some of the conclusions of the de Vries chiral smectics and gives additional results on the order parameter and the dichroic ratio as a function of the field across the cell. Dielectric spectroscopy finds large dipolar fluctuations in the Sm-A∗ phase for both compounds and again these confirm their de Vries behavior. © 2017 American Physical Society.

Chiral smectic- A and smectic- C phases with de Vries characteristics

Calamitic liquid crystals based on 5-phenyl-pyrimidine derivatives have been designed, synthesized, and characterized. The 5-phenyl pyrimidine core was functionalized with a chiral (R,R)-2,3-epoxyhexoxy chain on one side and either siloxane or perfluoro terminated chains on the opposite side. The one involving a perfluorinated chain shows SmA∗ phase over a wide temperature range of 82 °C, whereas the siloxane analog exhibits both SmA∗ and SmC∗ phases over a broad range of temperatures, and a weak first-order SmA∗-SmC∗ transition is observed. For the siloxane analog, the reduction factor for the layer shrinkage R (relative to its thickness at the SmA∗-SmC∗ transition temperature, TAC) is ∼0.373, and layer shrinkage is 1.7% at a temperature of 13 °C below the TAC. This compound is considered to have "de Vries smectic" characteristics with the de Vries coefficient CdeVries of ∼0.86 on the scale of zero (maximum-layer shrinkage) to 1 (zero-layer shrinkage). A three-parameter mean-field model is introduced for the orientational distribution function (ODF) to reproduce the electro-optic properties. This model explains the experimental results and leads to the ODF, which exhibits a crossover from the sugar-loaf to diffuse-cone ODF some 3 °C above TAC. © 2017 American Physical Society.

Design and investigation of de Vries liquid crystals based on 5-phenyl-pyrimidine and (R,R)-2,3-epoxyhexoxy backbone

Two approaches exist in the literature for describing the orientational distribution function (ODF) of the molecular directors in SmA∗ phase of liquid crystals, though several models are recently proposed in the literature for explaining the de Vries behaviour. These ODFs correspond to either the conventional unimodal arrangements of molecular directors arising from the mean field theory that leads to the broad or sugar-loaf like distribution or to the "diffuse-cone-shaped" type distribution proposed by de Vries. The hypothesis by de Vries provides for a realistic explanation as to how at a molecular level, a first-order SmA∗ to SmC∗ transition can occur where the uniform molecular director azimuthal distributions condense to values lying within a narrow range of angles; finally these condense to a single value while at the same time ensuring a little or no concomitant shrinkage in the layer spacing. The azimuthal distribution of the in-layer directors is probed using IR and polarized Raman spectroscopic techniques. The latter allows us to obtain the ODF and the various order parameters for the uniaxial and the biaxial phases. Based on the results of these measurements, we conclude that the "cone-shaped" (or volcano-shaped) de Vries type of distribution can most preferably describe SmA∗ where "a first-order phase transition from SmA∗ to SmC∗" and a low layer shrinkage can both be easily explained. © 2017 Author(s).

Observation of the de Vries behavior in SmA∗ phase of a liquid crystal using polarised Raman scattering and infrared spectroscopy

A heptamethyltrisiloxane liquid crystal (LC) exhibiting I-SmA∗-SmC∗ phases has been characterized by calorimetry, polarizing microscopy, x-ray diffraction, electro-optics, and dielectric spectroscopy. Observations of a large electroclinic effect, a large increase in the birefringence (Δn) with electric field, a low shrinkage in the layer thickness (∼1.75%) at 20 °C below the SmA∗-SmC∗ transition, and low values of the reduction factor (∼0.40) suggest that the SmA∗ phase in this material is of the de Vries type. The reduction factor is a measure of the layer shrinkage in the SmC∗ phase and it should be zero for an ideal de Vries. Moreover, a decrease in the magnitude of Δn with decreasing temperature indicates the presence of the temperature-dependent tilt angle in the SmA∗ phase. The electro-optic behavior is explained by the generalized Langevin-Debye model as given by Shen et al. [Y. Shen, Phys. Rev. E 88, 062504 (2013)10.1103/PhysRevE.88.062504]. The soft-mode dielectric relaxation strength shows a critical behavior when the system goes from the SmA∗ to the SmC∗ phase. © 2017 American Physical Society.

Phase behavior and characterization of heptamethyltrisiloxane-based de Vries smectic liquid crystal by electro-optics, x rays, and dielectric spectroscopy