Orthogonal Smectic Research Articles

Coumarin Schiff base derivatives with terminal cyano group: Synthesis, Mesomorphic properties and DFT investigation

Coumarin Schiff base derivatives with terminal cyano group were synthesized and analysed for their mesomorphic behaviour. The homologous series CN6–16 (n = 6,8,10,12,14,14,16) with 4-cyano benzaldehyde, lower member (n = 6,8) exhibited enantiotropic nematic mesophase, while smectic A mesophase started appearing with n = 8 during cooling process. Enantiotropic smectic A mesophase was observed with higher chain length n = 10,12,14,16. Similar trend of mesomorphic properties was exhibited by Schiff base derivatives BPCN8–14. Introduction of cyano biphenyl at aldehyde end of the molecule has led to formation of stable smectic A mesophase as compared to that by compounds CN8–14. Introduction of ester linkage in Schiff based derivatives, compounds ECN10–12 and RECN10–12 showed smectic A mesophase with high clearing temperature. VT-XRD analysis of two compounds has confirmed orthogonal smectic A mesophase. Additionally, DFT calculations were carried out for all the newly synthesized compounds and compared with similar literature molecules for various structural and electronic parameters followed by their correlation with the mesophase type and stability.

Ferroelectric and antiferroelectric chiral multilactate liquid crystalline materials with negative dielectric anisotropy

In this work, the mesomorphic, electro-optic and dielectric properties have been discussed in the light of molecular structure-property correlations of two chiral multilactate liquid crystalline materials possessing the orthogonal paraelectric Smectic A* phase, tilted ferroelectric Smectic C* phase and the tilted antiferroelectric Smectic CA* phase, over a substantially broad temperature range. Interestingly, a reasonably rare re-entrant Smectic C* phase (SmCre*) has also been identified in one of the investigated materials. These materials differ in their linkage groups (keto or ether) and an additional chiral unit in the terminal chain. The phase transition temperatures and transition enthalpies were determined from Polarizing Optical Microscopy and Differential Scanning Calorimetry (DSC) measurements. The compounds exhibit negative dielectric anisotropy (Δε) throughout the mesomorphic range (maximum −18 and −4 in SmA* for both the compounds) and moderately high values of spontaneous polarization (∼153 nC/cm2 in SmCre* phase). The temperature dependence of the response time (τ), bulk viscosity (η) and the activation energies (Ea) throughout the mesomorphic phase have been determined from the spontaneous polarization measurements. To emphasize the structure-property correlations in more detail, dielectric spectroscopy measurement has also been performed to measure the dielectric strength, dielectric loss, frequency dependent permittivities, relaxation time and relaxation frequencies. The clear evidence of the relatively rare SmCre* phase has also been confirmed from the temperature and frequency dependence of the dielectric permittivity. These results shed important light on the emergence of these materials as a smart alternative for their application in multicomponent mixtures targeted for advanced electro-optic and photonic devices.

Revisiting a family of liquid crystalline methacrylic phenyl benzoate monomers

The synthesis and phase characterization of a homologous series of 4′-alkyloxyphenyl 4-(6-(methacryloyloxy)hexyloxy))benzoate monomers have been carried out. The characterization comprises polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and temperature dependant small and wide-angle x-ray scattering (SAXS –WAXS). The X-ray data and POM images reveal both the nematic and monolayered orthogonal smectic A phase in M6En series. The monomers with a terminal chain length from five to twelve carbon atoms exhibit a wide temperature range of a stable smectic A phase with a narrow intervening nematic phase at higher temperatures. On cooling down from the isotropic phase the temperature range of the nematic and smectic A phase considerably increases. The specific features of the phase diagram and mesogenic properties of M6En series are discussed in terms of the peculiar molecular structure of these monomers.

The correlations between the molecular core structure and mesomorphic behaviour for chiral liquid crystals with several (S)-lactate groups

To contribute to better understanding of the structure–property relationship for chiral liquid crystalline materials a series of new liquid crystalline (LC) materials possessing several (S)-lactate groups as chiral centres have been designed and investigated. All the materials possess the orthogonal paraelectric smectic A* phase stable over a broad temperature range. For the specific lengths of the chiral and non-chiral alkyl chains the ferroelectric tilted smectic C* phase have been detected at lower temperatures. The mesomorphic behaviour and structural properties of new multilactate materials have been studied by polarizing optical microscopy, differential scanning calorimetry, small angle X-ray diffraction and dielectric spectroscopy. The obtained results are discussed and compared to that of the structurally similar lactic acid derivatives. The main objective of the present work is to establish the molecular structure – mesomorphic property relationship for this specific class of multilactate-based LC materials, which can be further successfully utilized as chiral dopants for the design of smart multicomponent LC mixtures targeted for application in optoelectronics and photonics.

Sign reversal of the spontaneous and induced polarisation in a mixture of achiral liquid crystal host and chiral azo dopant

ABSTRACT Achiral liquid crystal, possessing orthogonal smectic A and tilted smectic C phases in its phase sequence, was doped with a chiral photochromic azo dopant. It was found that the spontaneous and induced polarisation in the tilted smectic C* phase and in the orthogonal smectic A phase, respectively, change their sign, as well as their magnitude, under illumination with UV light. The origin of this sign reversal effect is considered to be the different sign of the molecular net dipole moment component µy of trans- and cis-isomers of the photochromic azo dopant, respectively. This light-induced sign reversal effect seems to have large potential for applications in the light-light controlled photonic liquid crystal devices, based on this effect.

Influence of the molecular structure of calamitic compounds with trifluoromethoxy terminal group on the induction of the smectic a phase. Part I

ABSTRACT For the liquid crystalline mixtures of compounds with positive and negative dielectric anisotropies, the induction of the liquid crystal smectic A phase can be observed. This effect can be very favourable for definite specific purposes. The main aim of this work is to determine the influence of the molecular structure of compounds with a polar trifluoromethoxy (-OCF3) group on the induction strength of the orthogonal smectic A (SmA) phase. In order to reach this target, the phase diagrams of the mixtures were established using the polarised optical microscope observations. The presence of a triple bond, phenyl and cyclohexyl rings as well as fluorine atoms in the core and the length of terminal alkyl chains of the tested compounds were considered. The length of the rigid core has the greatest effect on the induction strength of the SmA phase, while the length of the alkyl chain (3–5 carbon atoms) has relatively minor influence. Explaining the correlation between the molecular structure and the induction strength, the dipole moments and the polarisability of the molecules should be taken into account. For shorter molecules, changes in the molecular structure have more significant influence on the temperature and concentration range of the orthogonal SmA phase.

Induction of the Orthogonal Smectic A Phase in Mixtures of Nematic Tolanes with Positive and Negative Dielectric Anisotropy

Induction of the orthogonal smectic A phase occurs in mixtures of nematic compounds with positive and negative dielectric anisotropy. The aim of this work is to determine influence of a molecular structure of compounds on the induction strength of this phase. Cyclohexyl tolanes with the terminal group OCF3, differing with a lateral substitution of a rigid core with fluorine atoms, were mixed with tolanes with two fluorine atoms in a lateral position and different lengths of a terminal alkoxy chain. Based on phase diagrams, obtained with the use of a polarising optical microscope, the influence of a structure of both types of compounds on the induction of the SmA phase is presented. Both, an increase in a number of fluorine atoms in a rigid core and a decrease in an alkoxy chain length cause a decrease in the strength of the SmA phase induction in the investigated systems.Keywords: chemistry, liquid crystals, smectic A phase induction, miscibility studies

Investigation of the tilt angle and spontaneous polarisation of antiferroelectric liquid crystals with a chiral centre based on (S)-(+)-3-octanol

The electro-optical properties (tilt angle of the director and spontaneous polarisation) of eight smectic liquid crystalline compounds with a chiral centre based on (S)-(+)-3-octanol and different substitution of the benzene ring and different length of the oligomethylene spacer (−(CH2)nO−) were studied. The compounds possess the self-assembling behaviour with an antiferroelectric ordering in smectic Ca* phase at a broad temperature range, some of materials have also a ferroelectric smectic C* phase and/or an orthogonal smectic A* phase. Due to high chemical stability and very high values of the optical tilt angle (40–44°) at saturation, the tested compounds can be used as chiral components for orthoconic antiferroelectric liquid crystalline mixtures.

Self-Assembling Behavior of Smart Nanocomposite System: Ferroelectric Liquid Crystal Confined by Stretched Porous Polyethylene Film.

The control and prediction of soft systems exhibiting self-organization behavior can be realized by different means but still remains a highlighted task. Novel advanced nanocomposite system has been designed by filling of a stretched porous polyethylene (PE) film with pore dimensions of hundreds of nanometers by chiral ferroelectric liquid crystalline (LC) compound possessing polar self-assembling behavior. Lactic acid derivative exhibiting the paraelectric orthogonal smectic A* and the ferroelectric tilted smectic C* phases over a broad temperature range is used as a self-assembling compound. The morphology of nanocomposite film has been checked by Atomic Force Microscopy (AFM). The designed nanocomposite has been studied by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), small and wide-angle X-ray scattering and broadband dielectric spectroscopy. The effect of a porous PE confinement on self-assembling, structural, and dielectric behavior of the chiral LC compound has been established and discussed. While the mesomorphic and structural properties of the nanocomposite are found not to be much influenced in comparison to that of a pure LC compound, the polar properties have been toughly suppressed by the specific confinement. Nevertheless, the electro-optic switching was clearly observed under applied electric field of low frequency (210 V, 19 Hz). The dielectric spectroscopy and X-ray results reveal that the helical structure of the ferroelectric liquid crystal inside the PE matrix is completely unwound, and the molecules are aligned along stretching direction. Obtained results demonstrate possibilities of using stretched porous polyolefins as promising matrices for the design of new nanocomposites.

Self-assembling behaviour of new functional photosensitive cinnamoyl-based reactive mesogens

ABSTRACT One of the most exciting classes of organic materials that is able to self-assemble at nanoscopic scale are those exhibiting the liquid crystalline (LC) behaviour which can be distantly tuned by an external stimulus. Under UV/Vis light irradiation, the reversible photoisomerisation process can take place if the compounds possess the photosensitive group, e.g. N=N or cinnamoyl group incorporated in their molecular structure. This can drastically affect the LC behaviour. However, the compounds possessing the cinnamoyl fragment with C=C double bond are capable of both photoisomerisation and photocycloaddition processes. Several cinnamoyl-based reactive mesogens with polymerisable vinyl group have been designed; their mesomorphic and structural behaviour have been studied. Depending on the molecular structure, the reactive mesogens exhibit nematic, orthogonal smectic A and tilted smectic C phases; the kinetics of the phase transition temperatures has been checked. The structure of the mesophases is confirmed by small-angle X-ray scattering. The main objective of the present work is to contribute to better understanding of molecular structure – mesomorphic property relationship for new functional cinnamoyl-based photosensitive reactive mesogens which can be further used for design of smart self-assembling macromolecular materials.

Effect of molecular structure on dielectric and electro-optic properties of chiral liquid crystals based on lactic acid derivatives

Self-assembling behaviour of a few lactic acid derivatives with several ester linkage groups in the molecular core has been studied. The molecular structure–physical property correlations has been discussed in the light of the temperature dependences of the static permittivity, dielectric anisotropy, spontaneous polarization, response time and torsional bulk viscosity for eight chiral ferroelectric liquid crystalline compounds. Depending on the molecular structure these compounds exhibit the chiral nematic (N*) phase, the paraelectric orthogonal smectic A* (SmA*) phase and the ferroelectric tilted smectic C*(SmC*) phase being thermally stable over relatively broad temperature range. The temperature behaviour of the spontaneous polarization clearly confirms the second order nature of the SmA*-SmC* phase transition and the first order nature of the N*-SmC* phase transition. The effect of temperature and molecular configuration on the polarization and on the dispersion anchoring energy coefficients has been established. The obtained results are discussed in order to contribute to the structure – property relationship for the specific class of soft organic materials which can be used for design of smart multifunctional liquid crystalline mixtures aimed for optoelectronic and photonic applications.

The effect of the length of terminal n-alkyl carboxylate chain on self-assembling and photosensitive properties of chiral lactic acid derivatives

A new series of photosensitive azo materials possessing a chiral alkyl lactate moiety and terminal n-alkyl carboxylate unit close to the azo group has been synthesized and studied. The length of the n-alkyl carboxylate chain has been systematically varied in order to establish the effect of the molecular structure on the self-assembling behaviour. Two series of materials possessing hexyl and dodecyl alkyl chains in the chiral part of the molecule have been studied. It has been shown that the length of both the alkyl chains strongly influences the mesomorphic behaviour, however, each chiral/achiral chain has different utility to tune the mesomorphic properties. With exception of the compound with the longest chains, all studied compounds exhibited the chiral tilted ferroelectric smectic C* phase. Based on the combination of terminal alkyl chains, chiral nematic, orthogonal smectic A*, and twist grain boundary smectic A* phases have been detected on cooling beyond the SmC* phase. The presence of the photosensitive functional N=N group in the molecular core allowed further tuning of the material properties by UV light illumination. The E-to-Z photoisomerization of the azo group and subsequent thermal back-isomerisation have been studied in solution by nuclear magnetic resonance and most importantly in the mesophases on bulk samples. We report on UV-induced isothermal switching from chiral smectic and nematic mesophases into the isotropic phase, respectively, and differences in the textures of mesophase upon restoration of the ordered liquid.

Mesomorphic and structural properties of liquid crystalline side-chain polymethacrylates: from smectic C* to columnar phases

ABSTRACTThe chiral methacrylate monomers with photosensitive azobenzene group possessing the orthogonal smectic A* and tilted smectic C* (Sm-C*) phases have been synthesised and characterised. The monomers have been used as functional side chains for the design of corresponding polymethacrylates. X-ray diffraction has been applied to elucidate the structure and phase behaviour of liquid-crystalline side-chain polymethacrylates with azobenzene-containing central core, chiral fragments and aliphatic spacers and tails of different length. X-ray patterns of polymethacrylates oriented fibres impose the tilted Sm-C* order as a basic structure of these materials. This is complemented by a regular pattern of small-angle diffuse spots, which implies complex positional order on the local scale and serves as a precursor for the formation of a columnar phase. The increase of the total length of the aliphatic tail and spacer of the side-chain fragments leads to formation of the tilted columnar phase (Coltilt*) with two-dimensional monoclinic lattice. For the polymer containing 10 methylene units in both, spacer and aliphatic tail, the Coltilt* precedes the formation of the Sm-C* phase. The observed structural changes are explained as due to coupling between the smectic ordering of the mesogenic side groups and the polymer backbone conformation.

Exploration of Induced Blue Phases and Orthogonal Smectic Batonnets in Hydrogen-Bonded Ferroelectric Nanoliquid Crystalline Complexes for Optoelectronic Devices

A novel blue phase hydrogen-bonded ferroelectric liquid crystal (HBFLC) series has been synthesized from cholesteryl stearate (CHS) and p-n-alkyloxybenzoic acid (nOBA, where n = 2 to12). Blue phase (BP) liquid crystalline complex is a high potent material for next-generation optoelectronic devices. The structural, optical, and thermal properties of present HBFLC complexes have been characterized by X-ray diffraction techniques (XRD), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), polarizing optical microscope (POM), and differential scanning calorimetry (DSC) techniques. The band gap energy of the present HBFLC complex (4.6 eV) is estimated by UV-Visible spectrometer. X-ray diffraction and scanning electron microscope (SEM) studies which confirm the monoclinic nature and morphology of the present complex. Phase diagram of the present CHS + nOBA is constructed from DSC data and the same is discussed. The lower homologous of CHS + nOBA HBFLC (n = 2 to 6) complex shows BPsm1, BPsm2, BPsm3 while higher homologous (n = 7 to 12) exhibiting orthogonal smectic A* (SmA*) character with blue phases. A noteworthy observation is that the identification of extended thermal span of smectic blue phases and induced SmA* phase in the present HBFLCs. Another important observation is that the widest BP range is reported in the present hydrogen-bonded ferroelectric nanoliquid crystalline (HBFNLC) complex which is more suitable for photonic devices. In addition, LC parameters such as, phase width, thermal stability factor, phase transition temperature with enthalpy value, and the origination of orthogonal phases in CHS + nOBA HBFLC complex is also reported.

Mesomorphic, electro-optic and structural properties of binary liquid crystalline mixtures with ferroelectric and antiferroelectric liquid crystalline behaviour

ABSTRACT Two new binary liquid crystalline mixtures composed from the chiral liquid crystalline materials with partially fluorinated non-chiral chain and without lateral substitution at the molecular core have been designed and investigated by polarisation optical microscopy, differential scanning calorimetry, broad-band dielectric spectroscopy, and X-ray techniques. Both mixtures possess the paraelectric orthogonal smectic A*, the tilted ferroelectric smectic C* and the antiferroelectric smectic CA* phases over a reasonably broad temperature range. The gained results reveal a considerable improvement of (i) the broadness of the mesophases temperature range of the ferroelectric and antiferroelectric phases (up to 130 K breadth) and (ii) electro-optical properties, namely the spontaneous polarisation (up to 200 nC/cm2) and tilt angle (higher than 40o), with respect to that of the original pure compounds.

Experimental and computational (DFT) studies on induced orthogonal smectic A∗ phase in hydrogen-bonded ferroelectric liquid crystals

Hydrogen-bonded ferroelectric liquid crystals (HBFLC) are designed and synthesized from nonmesogenic chiral proton donor compound of (R)-([Formula: see text])-Methylsuccinic acid (MSA) and mesogenic proton acceptor compound of 4-undecyloxybenzoic acid (11OBA) in a different mole ratio. Intermolecular hydrogen bonds (H-bond) between the nonmesogenic and mesogenic compounds have been confirmed through experimental Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) computational studies. The steric hindrance and inductive effects of the present complex and its influence on the structure are discussed. A rich phase polymorphism in the liquid crystalline complex has been studied using polarizing optical microscope (POM) and differential scanning calorimetry (DSC). The chiral phases observed in the present complex are due to the presence of lone pair (n) to anti-bonding ([Formula: see text]) transition symmetry which is validated by DFT studies. A noteworthy observation of induced smectic A[Formula: see text](Sm A[Formula: see text]) by quenching of traditional phase (nematic) has been identified and the reason for the same has been discussed by DFT studies. The unusual phase order of Sm A[Formula: see text], smectic C[Formula: see text](Sm C[Formula: see text]) and smectic G[Formula: see text][Formula: see text](Sm G[Formula: see text]) mesogenic phases are observed. The other liquid crystalline parameters are evaluated by experimental and theoretical calculations and the same has been compared. Increased tilt angle in liquid crystal (LC) molecules has been theoretically analyzed by natural bond orbital (NBO) studies. Stability of the HBFLC phases and its origination mechanism have been discussed with the help of highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) energies.

On the orientational distribution functions in de Vries-type smectic liquid crystals

ABSTRACTIn de Vries smectic liquid crystals, the transition from the orthogonal smectic A (SmA) to the tilted smectic C (SmC) phase occurs with essentially no decrease in smectic layer thickness. This unusual behaviour is commonly explained assuming a ‘hollow-cone’ or ‘volcano-like’ orientational distribution function (ODF) of rod-like molecules in SmA and the transition to SmC being a pure disorder–order transition in the molecular tilt directions. However, even after 20 years of extensive investigations, the experimental confirmation of this ad hoc model is still inconclusive. While optical and electro-optic studies of many de Vries smectics can be readily explained with the hollow-cone model, X-ray diffraction (XRD) studies as well as many polarised Raman spectroscopy (PRS) studies support a broad Maier–Saupe distribution of a sugarloaf-like shape. We review and summarise X-ray, electro-optic and PRS results on the ODFs in the SmA phases of materials claimed to be of the de Vries type and discuss how seemingly contradicting findings can be true at the same time. In molecules where the core is far from collinear with the long molecular axis, the cores may exhibit a volcano ODF while the molecular axes exhibit a sugarloaf ODF.

Design and self-assembling behaviour of comb-like stereoregular cyclolinear methylsiloxane copolymers with chiral lactate groups

ABSTRACTSeveral new comb-like stereoregular cyclolinear methylsiloxane copolymers exhibiting the self-assembling behaviour have been designed and studied. For the resulting macromolecular materials, the chiral mesogenic unit, namely (S)-(–)-4-[1-ethoxycarbonyl)ethoxycarbonyl]phenyl-4′-[11-tetramethyldisiloxyl)-undecenyloxy)-biphenyl-4′-carboxylate, has been used as a side group to assure the mesomorphic behaviour. The mesomorphic and structural properties were studied by the polarising optical microscopy, differential scanning calorimetry and small-/wide-angle X-ray diffraction techniques. The designed copolymers form the orthogonal smectic A* and the tilted smectic C* phases over relatively broad temperature range, down to room temperatures. The obtained results are discussed in order to contribute to better understanding of the molecular architecture–nano-organisation relationship for a specific type of macromolecular system based on the lactic acid derivatives used as a flexible side groups.

Effect of lateral fluorine substitution far from the chiral center on mesomorphic behaviour of highly titled antiferroelectric (S) and (R) enantiomers

Several new liquid crystalline chiral ester enantiomers in the (S) and (R) configuration were designed and characterised. In order to tune their mesomorphic properties, the molecular core was laterally substituted by fluorine atoms in different positions on the benzene ring placed in the molecular core far from the chiral centre. All the materials possess the self-assembling behaviour, specifically the paraelectric orthogonal smectic A*, the ferroelectric tilted smectic C* and antiferroelectric tilted smectic CA* phases over a broad temperature range down to room temperature. Properties of the materials were characterised by the polarising optical microscopy, differential scanning calorimetry, electro-optics and dielectric spectroscopy. Due to high chemical stability, reasonably broad temperature ranges of polar smectic mesophases (up to 110°C) and very high values of the optical tilt angle (40–45°) at saturation, these materials can be used as chiral dopants for the multicomponent orthoconic mixture design aimed for development of advanced electro-optic and photonic devices.

Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups

AbstractThe tilted smectic C phase is a rather uncommon phase in ionic liquid crystals (ILCs), whereas the orthogonal smectic A phase is the most common phase in ILCs. We now present 2 new groups of mesogens with an azobenzene core that exhibit smectic C as well as smectic A phases. Their phase sequences and tilt angles were studied by polarizing microscopy, and their temperature‐dependent layer spacings and orientational order parameters were investigated by X‐ray diffraction. We present 1 new amidinium azobenzene mesogen that forms enantiotropic smectic C and A phases and another amidinium as well as 2 new guanidinium azobenzene mesogens that exhibit monotropic smectic C and enantiotropic smectic A phases. With this study, we show that azobenzene is indeed an SmC‐promoting group in ILCs. Comparing these results with our earlier results on azobenzenes with an N‐methylimidazolium head group (N Kapernaum et al, ChemPhysChem 2016, 17, 4116‐4123), we show that the aromaticity of the imidazolium head group plays an important role in the formation of smectic C phases.