Chiral Smectic Phases Research Articles

The Influence of the Molecular Structure of Compounds on Their Properties and the Occurrence of Chiral Smectic Phases.

We have designed new chiral smectic mesogens with the -CH2O group near the chiral center. We synthesized two unique rod-like compounds. We determined the mesomorphic properties of these mesogens and confirmed the phase identification using dielectric spectroscopy. Depending on the length of the oligomethylene spacer (i.e., the number of methylene groups) in the achiral part of the molecules, the studied materials show different phase sequences. Moreover, the temperature ranges of the observed smectic phases are different. It can be seen that as the length of the alkyl chain increases, the liquid crystalline material shows more mesophases. Additionally, its clearing (isotropization) temperature increases. The studied compounds are compared with the structurally similar smectogens previously synthesized. The helical pitch measurements were performed using the selective reflection method. These materials can be useful and effective as chiral components and dopants in smectic mixtures targeted for optoelectronics and photonics.

Effect of the linking bridge type on the self-assembly behaviour of 2',3'-difluoroterphenyl derivatives.

The self-assembly behaviour and the crystallization kinetics of two liquid crystal compounds containing 2',3'-difluorosubstituted terphenyl as the mesogenic core have been described. Calorimetric studies show that the linking bridge type affects the polymorphism of smectic phases. The CH2O.3 compound with the -CH2O- linking bridge connected with a longer methylene spacer possesses the chiral smectic phase with antiferroelectric properties (SmCA* phase), while the COO.6 liquid crystal with the -COO- linking bridge connected with a shorter alkyl chain forms the chiral smectic phase with ferroelectric properties (SmC* phase). Both compounds crystallize upon slow cooling, while fast cooling causes the vitrification of the conformationally disordered crystal phase. Dielectric measurements reveal the complex relaxation dynamics in the identified thermodynamic states. DFT calculations allow us to estimate the nature of relaxation processes.

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.

Determination of tilt angle and its behavior in chiral smectic phases by exploring molecular conformations using complementary methods.

Density functional theory (DFT) calculations and x-ray diffraction techniques were employed to evaluate the value of the tilt angle in ferroelectric smectic C^{*} and antiferroelectric smectic C_{A}^{*} phases. Five homologues from the chiral series denoted as 3FmHPhF6(m=2,4,5,6,7), based on 4-(1-methylheptyloxycarbonyl) phenyl 4'-octyloxybiphenyl-4-carboxylate (MHPOBC), were studied. Two types of conformations for the nonchiral terminal chain (fully extended and gauche) and three types of deviation from the rodlike shape of the molecules (hockey stick, zigzag, and C shape) were computationally considered. The nonlinear shape of the molecules was accounted for by introducing a shape parameter δΘ. We observe that calculations of the tilt angle which consider the C-shaped structures, in both the fully extended or gauche conformations, lead to good agreement with the values of the tilt angle obtained from electro-optical measurements below the saturation temperature. The results allow us to conclude that such structures are adopted by molecules in the examined series of smectogens. Additionally, this study proves the presence of the standard orthogonal SmA^{*} phase for the homologues with m=6, 7, and the de Vries SmA^{*} phase for m=5.

Statement of Retraction: Induced Chiral Smectic Phase in Mixtures of Mesogenic and Non-Mesogenic Compounds

Statement of Retraction: Induced Chiral Smectic Phase in Mixtures of Mesogenic and Non-Mesogenic Compounds

Enantioselective Synthesis of 3-Aryl-3-hydroxypropanoic Esters as Subunits for Chiral Liquid Crystals.

Chiral liquid crystals (LCs) with their unique optical and mechanical properties are perspective functional soft materials for fundamental science and advanced technological applications. Herein, we introduce the chiral 3-aryl-3-hydroxypropanoic ester moiety as a versatile building block for the preparation of LC compounds. Three chiral subunits differing in the aromatic part were obtained through asymmetric transfer hydrogenation using Ru(II) complexes with ee from 98% to >99%. Chiral LC compounds of diverse topologies were further prepared without deterioration of the ee during the synthesis. The mesomorphic behavior of rod-shaped, bent-shaped flexible dimeric, and polycatenar LCs is consistent with their topology─chiral nematic and smectic phases were identified as well as the rarely observed twist grain boundary A and blue phases. The utilization of synthetic chiral building blocks offers the possibility of fine tuning the intermolecular interactions by subtle changes in the molecular structure as well as the preparation of the corresponding racemic forms. This paves the way for the study of self-organization and the structure-property relationship in chiral soft materials.

Effect of Alkyl Chain Length on the Phase Situation of Glass-Forming Liquid Crystals

The phase behaviour of the latest synthesised compound belonging to a family of (S)-4′-(1-methyloctyloxycarbonyl) biphenyl-4-yl 4-[‘m’-(2,2,3,3,4,4,4-heptafluorobutoxy) ‘m’alkoxy]-benzoates (where ‘m’ means 3, 5 or 7 methylene groups) is described by polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and Fourier-transform infrared absorption spectroscopy. It has been shown that as the length of the alkyl chain increases, a given liquid crystal possesses a greater number of mesophases and at a higher temperature it goes into the isotropic liquid phase. All examined compounds form a chiral smectic phase with antiferroelectric properties (SmCA* phase), in which the temperature range of occurrence increases with the length of the molecule. The number of methylene groups also affects the glass transition. The compound with the shortest alkyl chain (‘m’ = 3) is vitrified from the conformationally disordered crystal phase. For the compound with five -CH2- groups (‘m’ = 5), a glass transition from the monotropic high-order hexatic smectic SmXA* phase is observed. In the case of the liquid crystal with the longest carbon chain (‘m’ = 7), the vitrification from the less ordered SmCA* phase is visible. Differences in the crystallization kinetics, e.g., the nucleation-controlled mechanism for the compound with the shortest carbon chain vs. the complex phenomenon for its longer homologs, are discussed.

Investigation of Chiral Smectic Phases and Conformationally Disordered Crystal Phases of the Liquid Crystalline 3F5FPhH6 Compound Partially Fluorinated at the Terminal Chain and Rigid Core.

Complementary methods are applied to investigate the phase transitions and crystallization kinetics of the liquid crystalline compound denoted as 3F5FPhH6. Two crystal phases are confirmed, and one of them is the conformationally disordered (CONDIS) phase. Complexity of the melt crystallization process is revealed by the analysis with Friedman’s isoconversional method. The melt crystallization of 3F5FPhH6 shows different mechanisms depending on temperature, which is explained by the relation between the thermodynamic driving force and the thermal energy of translational degrees of freedom. The studied compound crystallizes even during fast cooling (30 K/min), unlike similar compounds with different fluorosubstitutions of the benzene ring, which form the smectic glass for moderate cooling rates. The tendency to vitrification of the smectic phase decreases apparently with the decreasing stability width of the SmCA* phase and the increasing relaxation time of the collective relaxation process in this phase, at least for homologues differing from 3F5FPhH6 only by the type of fluorosubstitution.

P-Terphenyl-containing symmetric tetraesters for nano-scale pitch ferroelectric liquid crystal materials

Design and studying of derivatives of fluorinated and non-fluorinated p-terphenyl-containing symmetric tetraesters as chiral components (CCs) of ferroelectric liquid crystal (FLC) materials working in deformed helix FLC (DHFLC) mode are described. The tetraesters combine a lactate and alcoholic residues in chiral tails. The new CCs induce short helix pitch (as short as 78 nm) and large spontaneous polarization (PS > 100 nC/cm2) at moderate concentrations (as low as 12 mol. %) in a non-chiral biphenylpyrimidine host. We have studied the effects of the (S)-lactate linker, terminal alkyl chains and fluorine atoms on helix pitch and other useful properties of the chiral tilted smectic (SmC*) phase induced by these CCs in the biphenylpyrimidine host. Depending on positions of the fluorine atoms, the fluorination of the p-terphenyl core shows little effect or even reduces the helical twisting power (HTP) of the CCs. At the same time, modifications in the chiral tails can essentially enhance the HTP and other CC performance characteristics. Thus, the combination of the (S)-lactate linker with a (S)-alkan-2-olic residue results in high HTP (up to 85 μm−1) even without fluorine atoms in the chiral tails. Substitution of the (S)-alkan-2-olic residue with a fluorinated analogue, viz. (R)-1,1,1-trifluoroalkan-2-ol derivative, provides further increase of the HTP in the induced SmC* phase up to the highest known to date value of 101 μm−1.

A chiral smectic phase induced by an alternating external field.

Using simple achiral building blocks modulated by an external field to achieve chiral liquid crystal phases remains a challenge. In this study, a chiral helix liquid crystal phase is obtained for a simple Gay-Berne ellipsoid model under an alternating external field by using molecular dynamics simulations. Our results show that the chiral helix liquid crystal phase can be observed in a wide range of external field strengths when the oscillation period is smaller than the rotational characteristic diffusion timescale of ellipsoids. In addition, we find that the pitch and tilt angle of the helix structure can also be adjusted by changing the strength and oscillation period of the applied alternating external field. This may provide a feasible route for the regulation of chiral liquid crystal phases by an alternating external field.

Synthesis and mesomorphic properties of new organosiloxane chiral calamitic liquid crystals

A new chiral calamitic compound consisting of a phenyl core that is connected through ester linkers to a benzoate carrying a chiral (S)-2-methylbutoxy group at one side and a biphenyl carboxylate with an undecenyloxy chain at other side has been synthesized. Additionally, the siloxane substituted derivatives and a dimer with these two identical chiral calamitics linked by a disiloxane spacer have been obtained via hydrosilylation reaction to study the effect of combining the chiral moiety and siloxane segments within the same structure on liquid crystalline properties. The chiral calamitic compound and derivatives with a siloxane end-group or a siloxane spacer have been characterized using classical spectroscopic methods (1H NMR, 13C NMR, 29Si NMR and MS) and elemental analysis (EA). The liquid crystalline properties of all new compounds were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray scattering and electro-optic methods. With the introduction of a siloxane end-group to the vinyl-terminated calamitic compound exhibiting enantiotropic chiral nematic phase (N*) and chiral tilted smectic phase (SmC*), the occurrence of stable smectic phase was strongly promoted. In a similar way, the dimer with two identical calamitic mesogens connected by a disiloxane spacer exhibited a wider smectic mesophase interval in addition to the presence of N* mesophase. The new mesogens exhibit a ferroelectric switching with PS of around 200 nC cm−2 in their chiral tilted smectic (SmC*) mesophase range. The transition temperatures of organosiloxanes are considerably lower than that of olefinic precursor.

Liquid crystalline dimers containing a cholesteryl group: Chiral smectic phases, and re-entrant TGBA* phase

A new series of liquid crystalline dimers containing a cholesteryl group were synthesized, exhibiting multiple mesophases including chiral smectic phases (SmX*, SmC* and SmA*), twist grain boundary phases (TGBA* and TGBC*), and chiral nematic phase (N*). Re-entrant phenomenon occurs, which is rarely reported for pure matters of dimeric liquid crystals. The chemical structure and liquid crystal properties of the dimers were characterized by FTIR, 1H NMR, DSC, POM and XRD. It was in evidence that the length and the parity of the spacer and the length of the terminal tail were the decisive factors of the mesophase behaviours.

Effect of end groups with aliphatic chain or ring structures on the thermal properties and optical textures of achiral and chiral liquid crystal compounds

Thermal properties and orientation in optical textures of liquid crystal (LC) are two crucial factors for LC application. Here, a series of achiral and chiral LC compounds (Cn) with different end groups were prepared. The chemical structures were investigated by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR). The thermal properties and optical textures were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). The results showed that all the compounds possessed good thermal properties and enantiotropic mesomorphism. For achiral compounds C1C4, droplets and schlieren textures of nematic phase (N) and broken fan-shape textures and dull grayish/cloudy textures of smectic phase (Sc) were seen. Oily streak and focal conic textures of chiral nematic phase (N*), fan-shape textures and dull grayish/cloudy textures of chiral smectic phase (Sc*) were observed for chiral compounds C5C7. For C1, C2, C3 and C5 with aliphatic chain in the end groups, it was found that lateral substitution in the end groups had a greater effect on the clearing point than that of the increase in the length of end groups. Moreover, due to the increasing free volume for the increase of branch chain on six-membered ring structure, the clearing point (Ti) of C4 was higher than those of C6 and C7.

Liquid crystalline dimers containing a cholesteryl group: chiral smectic phases and TGBA* phase

ABSTRACT A new series of liquid crystalline dimers containing a cholesteryl group were synthesised, exhibiting multiple mesophases including chiral smectic phases (SmC* and SmA*), twist grain boundary phases (TGBA*), and chiral nematic phase (N*). The chemical structure and liquid crystal properties of the dimers were characterised by FTIR, 1H-NMR, DSC, POM and XRD. It was in evidence that the lengths of the terminal tail and the flexible spacer were the decisive factors of the mesophase behaviours.

Self-assembling behaviour of chiral calamitic monoacrylates targeted for polymer stabilisation of polar smectic phases in chiral liquid crystals

The stabilisation and control of synclinic and anticlinic chiral liquid crystalline phases remain an actual and highlighted task. This work presents the design and mesomorphic properties of new chiral calamitic reactive mesogens, monoacrylates, based on biphenyl benzoate and phenyl biphenyl-4-carboxylate cores with reactive terminal groups placed far from the chiral chain. The compound's structures, compatible with the components of modern ferroelectric and antiferroelectric liquid crystalline mixtures, are confirmed by mass spectrometry (electron ionization) analysis and proton/carbon nuclear magnetic resonance. All the reactive mesogens possess the self-assembling, i.e. liquid crystalline behaviour with high tendency to create smectic phases in a broad temperature range, which was confirmed by a polarizing optical microscopy, differential scanning calorimetry and broad-band dielectric spectroscopy. Doping of advanced multicomponent mixtures possessing the chiral smectic phases by the designed monoacrylates with further cross-linking procedure, can be an effective and functional tool for stabilising ferroelectric and antiferroelectric phases in various states and hence, can allow the symmetrisation of the switching times in the modes employed surface stabilised geometry; this is a very highlighted task for optoelectronics. Moreover, an evident considerable tendency for thermal polymerisation of specific reactive mesogens can reduce the drawbacks of polymer stabilisation.

Temperature ranges of the chiral polar smectic phases

The paper presents the analysis of static and dynamic properties of achiral polar smectic doped with chiral dopant. In this system the two phases with equal symmetries exist, the and the phase, which continuously transform one to the other upon temperature changes. The analysis of dynamical properties allowed for the development of the criterion which determines the temperature ranges of the two phases. Besides, the analysis of collective modes revealed that they are either amplitude or phase. However, in many of them the character changes from polar to tilt or vice versa. The collective modes occur in pairs of opposite chiralities. The chiral character of modes explains also the gaps which occur between the amplitude modes.

High open-circuit voltage under the bulk photovoltaic effect for the chiral smectic crystal phase of a double chiral ferroelectric liquid crystal doped with a fullerene derivative

A bulk photovoltaic effect was studied in a chiral smectic crystal phase of a bisfluorophenylterthiophene derivative bearing two lactate moieties doped with a fullerene derivative.

Chiral Photoresponsive Liquid Crystalline Materials Derived from Cyanoazobenzene Central Core: Effect of UV Light Illumination on Mesomorphic Behavior

One of the most frequently utilized liquid crystalline (LC) materials is a rod-like (calamitic) compound 4-cyano-4′-pentylbiphenyl (5-CB). The main objective of this work is to enhance its functionality by introducing a photoresponsive diazenyl spacer in the aromatic core and replace the non-chiral pentyl chain with various chiral alkyl carboxylate units. The mesomorphic properties of the prepared materials have been studied using polarizing optical microscopy and differential scanning calorimetry. It has been found that materials with an extended aromatic system possess the liquid crystalline behavior. The studied LC materials have shown mesophases at lower temperatures than previously reported analogous substances. Furthermore, one of them exhibits a chiral orthogonal frustrated twist grain boundary smectic phase, which has not been previously observed for this structural type of materials. We also investigated photoresponse of the mesophases under illumination with UV-light (365 nm) using a polarizing optical microscope. A non-conventional photoresponse of the prepared materials in a crystalline phase is presented and discussed.

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

The chiral smectic phases of calamitic liquid crystals, SmC^{*} and SmC_{A}^{*}, 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, q_{T}=[F]/([A]+[F]). Sandhya etal. [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 q_{T}=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 q_{T}; 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.

Chiral Platinum‐Based Metallomesogens with Highly Efficient Circularly Polarized Electroluminescence in Solution‐Processed Organic Light‐Emitting Diodes

AbstractCircularly polarized luminescence (CPL) is of interest due to its wide potential application in semiconductors. To balance the emission efficiency and luminescence dissymmetry factor (gPL) of a CPL emitter, in this context, two chiral, phosphorescent and liquid‐crystalline cyclometalated platinum complexes, abbreviated R‐Pt and S‐Pt, are prepared. The complexes, which show an intense green emission at 504 nm both in solution and in the solid state, contain a simple, ortho‐metalated 2‐phenylpyridine unit functionalized with a chiral 2‐octanol chain, with liquid crystallinity being induced by modifying the β‐diketonato ligand with mesogenic groups. Interestingly, both the chiral smectic (SmA*) and nematic (N*) phases are found by a combination of polarized optical microscopy, differential scanning calorimetry, and small‐angle X‐ray scattering. By annealing, distinct CPL emission is achieved in the solid state with a gPL around 0.02. Employing the chiral platinum complexes as the dopant, solution‐processable organic light‐emitting diodes present an external quantum efficiency of 11.3% and strong, circularly polarized electroluminescence with an extremely high luminescence dissymmetry value (gEL) of 0.06 after annealing at 100 °C. This work opens an avenue for designing CPL‐active emitters with high emission efficiency and high dissymmetry factor.