We report the development of chiral smectic C (SmC*) ferroelectric liquid crystals (FLCs) obtained by mixing of a nematic liquid crystal (NLC) as a host and chiral non-mesogenic dopants. The NLC is a binary eutectic mixture of phenyl- and biphenyl-pyrimidines, while chiral dopants belong to homologous series of secondary dilactates with terphenyldicarboxylate core. The induction in the mixtures not only the cholesteric phase, but also the wide temperature range ferroelectric SmC* phase with the helix pitch p0≈100 nm and the spontaneous polarization Ps about 70 nC/cm2 has been confirmed by dielectric, optical, and electro-optical measurements. The developed FLC mixtures are highly efficient for the deformed helix ferroelectric liquid crystal electro-optical mode because the mixtures combine mechanical stability (or mechanical shock resistance) of NLCs and kilohertz switching frequency (due to short helix pitch) of FLCs. These FLCs can be used to create various photonic devices, such as generators of vortex light fields and field-sequential color displays.

The origin of ferroelectricity in smectic liquid crystals is attributed to the chirality of their molecules. The chirality reduces the space symmetry and leads to the helical (helicoidal) spatial distribution of molecular axes in ferroelectric and antiferroelectric liquid crystals (FLCs and AFLCs), called a helical structure, or helical nanostructure, if its period is much less than any wavelength of visible light. Deformations of FLCs and AFLCs helical structures under the applied electric field E lead to the emergence of a number of specific electro-optical effects. This review analyzes the main results of experimental and theoretical studies of various electro-optical manifestations of FLCs and AFLCs helical nanostructures deformations, which are already used to develop displays and photonic devices of a new generation.(c) 2022 Elsevier B.V. All rights reserved.

ABSTRACT We propose an approach for development of ferroelectric liquid crystals (FLC) with low birefringence Δn. Two basic principles have been used to get lowering of Δn: selection of molecules with short chains of conjugation as components for achiral matrix and averaging of local refractive indices by FLC helical structure. FLC mixtures with low birefringence (0.07 < Δn < 0.10 at wavelength 589.3 nm of sodium line) were elaborated and investigated. They consist of an achiral matrix including both nematic and smectic liquid crystal components and of phenylpyrimidine derivatives as chiral dopants. The materials developed can be used for all basic electro-optical FLC modes such as surface stabilised FLC (SSFLC), deformed helix ferroelectrics (DHF) and electrically suppressed helix (ESH). The mixtures developed allow to reduce the FLC cells chromatic retardance variation due to the weaker birefringence dispersion as compared with the known FLC materials to date. Graphical Abstract

The dynamics of the director reorientation in new helix-free ferroelectric liquid crystals (FLC) is considered. These materials are specially designed helix-free FLCs with a rather low value of the spontaneous polarization (less than 50 nC/cm2) and high viscosity (from 0.3 to 1.0 Poise), which are characterized by a spatial periodic deformation of smectic layers in the absence of an electric field. FLC director reorientation is due to the motion of solitons – spatially localized waves of a stationary profile that arise in an alternating electric field upon transition to the Maxwellian mechanism of energy dissipation. A theoretical model is proposed for describing the spatial-periodic deformation of FLC and reorientation of its director. The frequency and field experimental dependences of FLC electro-optical response time are presented for the modulation of the light transmission with fastest response among all LC materials. The novel helix-free FLC are able to efficiently modulate the visible and near IR radiation at frequencies up to 7 kHz at the electric field strength of the order of 1-2 V/μm. The conditions for the continuous hysteresis-free electro-optical response were determined, and such a response was realized for the first time in the frequency range up to 6 kHz.

A two-dimensional model of Fredericks effect was used for the investigation of the static electric field influence on nematic liquid crystal director orientation in the side-electrode cell. The solutions of the equations describing the model were obtained by finite-difference methods. Fredericks transition threshold for the central part of the cell, as well as dependencies of the distribution of the director orientation patterns on the electric field and location, were obtained. The numerical results are found to agree qualitatively with the experiment. Further investigations are needed to elucidate completely the Fredericks effect.

It was reliably proved in recent years both theoretically and experimentally that the electrically controlled birefringence Delta n(E) of chiral smectic C* phase with subwavelength helix pitch is proportional to the square of the electric field E. The goal of the present work is to investigate the restrictions of the quadratic effect imposed by the thickness of the smectic C* layer between two solid substrates, and by the frequency of applied voltage. It is shown that these restrictions are mainly associated with the dielectric dispersion of the Goldstone mode under various boundary conditions.

Dielectric dispersion of a chiral smectic liquid crystal has been measured in the very broad frequency range from 0.1mHz to 50kHz. Two regions of dispersion have been recognized: the first one, from 0.1mHz to 0.1Hz was for the first time measured, the second one, from 100Hz to 50kHz, which is known as mainly due to Goldstone mode, has been confirmed.

A chiral ferroelectric smectic C* liquid crystal (FLC) with the helix pitch p(0) = 330 nm was developed to avoid any scattering of visible light when the helix is not unwound over a certain limit. Planar cells with different FLC layer thickness (16 and 44 mu m) have been assembled with helix axis parallel to the glass plates and aligned along the rubbing direction. The ellipticity of the light passing through the cells vs. the electric field was investigated, and a method for evaluating the electrically controlled birefringence via ellipticity measurements has been established. We have found that the FLC cell is an optical retardation layer driven by the electric field, the effective birefringence being proportional to the square electric field. The physical origin of the electrically controlled phase shift of the light passing through the FLC layer has been analysed.

The novel tandem two-diode organic amplifier/converter of light based on successively connected photosensitive and light emission cells with spatial disjunction of processes of photocurrent multiplication (PM) and electroluminescence (EL) has been proposed and realized. The terbium complex Tb(Sal)3(TPPO)2 (HSal – salicylic acid, TPPO – triphenylphosphine oxide) or aluminum tris-(8-hydroxyquinoline (Alq3)) have been used as active layers in light emission cell of device as well as the perylene pigment Me-PTC (N,N″-dimethylperylene-3,4,9,10-bis(decarboximide)) in the photosensitive one. The suggested method of amplification/conversion has some advantages: (1) it avoids the necessity to adjust HOMO- and LUMO levels of photosensitive and electroluminescent layers and thus, extends the range of suitable light emissive materials including ones with high emission characteristics, (2) it also eliminates reabsorption of light by photosensitive part of the system and so, extends spectral band of amplification/conversion. A modernized model of field-activated structural traps has been suggested. Kinetics of PM during light- and voltage switching with time pauses is explained using this model. The peculiarities of PM and its kinetics at different light intensities, temperatures and applied voltages are analyzed too. Optimal values of temperatures, voltages and light excitation intensities providing high PM have been found. The PM gain up to 105-fold has been achieved. The conversion of long-wave (λ=600nm) light into narrow emission bands of Tb(Sal)3(TPPO)2 (λmax 545nm) and wide-band (from 490nm) emission of Alq3 has been obtained. Up- and down-conversion of light along with enhancement of emission have been observed in both devices. A computer simulation of the tandem-diode amplifier/converter has been performed and operation conditions providing transition from the up-conversion mode to the enhancement of light mode have been studied. It has been demonstrated that the processes of PM and the bias redistribution between the units of device exert strong influence to one another and only their unification leads to the tandem-diode operation.

In the last years bent banana-shaped mesomorphic molecules found an increasingly great interest among theoreticians and experimentalists due to the nature of this type of liquid crystals to exhibit a spontaneous polarization in the absence of any chiral group in the molecules. Similar property was found in mesomorphic 1,3,4-oxadiazoles containing as a central unit the five-membered heterocycle with the Symmetric positions of heteroatoms and as the link units -the ester groups [1]. In the present work 1,2,4-oxadiazoles - asymmetric isomers of the liquid crystals mentioned above, keeping the same banana-shape, were synthesized and their properties were investigated.

Viscoelastic spectra of homeotropic nematic layers of MBBA oriented by self-assembled films with different degree of desorbtion: high (dilauroyl phosphatidyl choline, DLPC) and low (chromolan) have been studied by a phase-sensitive flexoelectric spectroscopy method. Orientants desorb from the surface and dissolve in the nematic, producing a surface gradient of concentration of the orientant. Extended theoretical treatment was implemented resulting in spectral shapes, which have been successfully compared to the experiment, yielding information about the surfactant gradient. These results permit us to reveal the existence and the structure of a desorbed layer of orientant in homeotropic oriented nematics. PACS number: 61.30.Hn, 61.30.Dk, 68.43-h

Homeotropic nematic layers of MBBA and BMAOB liquid crystals, oriented by self-assembled films (on glass plates) of dilauroyl phosphatidyl choline (DLPC) and chromolan have been studied by a phase-sensitive flexoelectric spectroscopy method [1-3]. The viscoelastic spectra of these layers, reflecting the surface dissipation of the orientational energy, have been analyzed taking into account a strong (DLPC) or weak (chromolan) surfactant desorption. Surfactant molecules desorb from the surface and dissolve in the nematic, producing a surface gradient of the concentration of the surfactant. A theoretical treatment of the first harmonic [4] has been implemented. The temperature dependence of the thickness of the desorbed subsurface layer of the surfactant was revealed. The surface viscosity was also obtained from the theoretical fits to the spectra. These results are able to give new insights on the physics of the nematic liquid crystal-solid surface interaction, where flexoelectricity and surfactant desorption play a fundamental role.

In the last decade, it has been experimentally found that in certain conditions a periodic domain pattern arises in ferroelectric liquid crystals (FLC) in bookshelf geometry. Such a periodic texture appears after switching-off an external electric field, even in strong anchoring conditions. It has a static character and is bidimensional, being dependent on directions normal to both the smectic planes and the cell plates. Here a new model explaining this phenomenon is proposed, valid in the case of FLC with strong anchoring. The model is based on the coupling between the spontaneous polarization field in the first semi-period of its modulation along the cell plates and the same field in the second semi-period. This coupling competes with the FLC medium elasticity and is trapped by the anchoring. According to our model, in the ferroelectric state the biperiodic texture is favored by increasing the values of spontaneous polarization. The critical value of the spontaneous polarization for which the undeformed state becomes unstable is found. It is shown to be proportional to the square root of the ratio between the FLC elastic constant and the cell thickness. Moreover, it is inversely proportional to the sinus of the pre-tilt angle with respect to the cell walls.

In the last decade it has been experimentally found a periodic domain pattern arising in smectic C* liquid crystals in surface stabilized bookshelf geometry. Such a periodic texture appears after switching-off an external electric field, even in strong anchoring conditions. It has a static character and can be bidimensional, being dependent on both directions normal to the smectic planes and normal to the cell plates. In the present work an explanation to this phenomenon is proposed. According to our model in the antiferroelectric phase the biperiodic texture is a threshold phenomenon, appearing for values of the spontaneous polarization greater than a critical value, whereas in the ferroelectric phase this type of bidimensional instability is hindered.

S. Torgova, L. Karamysheva, and A. Strigazzi 1 FSUE"SRC"NIOPIK" (Organic Intermediates & Dyes Institute), B. Sadovaya1/4, Moscow 103787, Russia 2 Dipartimento di Fisica, and Istituto Nazionale di Fisica della Materia (INFM), Politecnico di Torino, C. Duca degli Abruzzi 24, I-10129 Torino, Italy 3 Joint Laboratory of Orientationally Ordered Media (OOM-Lab), C. Duca degli Abruzzi 24, I-10129 Torino, Italy

The correlation between chemical structure and mesomorphic properties is one of the most important problems in liquid crystals science. 3,5-Disubstituted 1,2,4-oxadiazoles are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. The transition temperatures and dielectric properties of 1,2,4-oxadiazoles depend significantly both on the position of the substituents with respect to the heterocycle and on their donor or acceptor features.

The present paper mainly consists of a brief review about organic electroluminescent materials. Different types of organic light emitting substances belonging to conjugated polymers and low molecular mass compounds are described: the advantages of light emitting liquid crystals are shown.