Chiral Nematic Research Articles

The effect of molecular structure on mesophase behaviour of non-symmetric liquid crystal dimers containing mandelic acid and fluorine group

ABSTRACT A series of non-symmetric dimers with the same chiral core, mandelic acid (MA), have been synthesised by varying the terminal group and the rigidity of the molecules, termed QBBMA-3F3BT, QBMA-3F3BT, QBMA-3F2BT, 3F3BMA-BBCN, 3F2BMA-BBCN and 3F1BMA-BBCN. Chemical structures and liquid crystal (LC) properties of the dimers were characterised by Fourier transform infrared spectroscopy, 1H NMR, polarised optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric analysis (TG). The six dimers displayed different optical activity (in THF). QBBMA-3F3BT and QBMA-3F3BT displayed enantiotropic chiral smectic A (SmA*) phase. QBMA-3F2BT displayed N phase. The Tm of QBMA-3F3BT is the highest among QBBMA-3F3BT, QBMA-3F3BT and QBMA-3F2BT, which indicated that molecular conformation played an important role on Tm besides molecular weight and molecular rigidity. 3F3BMA-BBCN and 3F2BMA-BBCN displayed chiral nematic (N*), which indicated that the mesogenic arm BBCN is beneficial to the formation of N* phase. The LC range of 3F2BMA-BBCN is wider, which may be due to that the similar length of the two LC arms is advantageous to the interaction between C≡N group and other molecules so as to stabilise the LC phase.

Fluorinated chiral nematic liquid crystal dimers based on (S)-1-phenylethane-1,2-diol

ABSTRACTA series of symmetric liquid crystal (LC) dimers with the same chiral core (S)-1-phenylethane-1,2-diol ((S)-PE) have been synthesised, termed TBDA-(S)-PE, 3F3B-(S)-PE, 3F2B-(S)-PE, 1F3B-(S)-PE, 1F2B-(S)-PE, respectively. Chemical structures and LC properties of the five symmetric LC dimers were characterised by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectrometer (1H NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and polarised optical microscopy (POM). TBDA-(S)-PE displayed enantiotropic chiral smectic A (SmA*) phase while 3F3B-(S)-PE, 3F2B-(S)-PE, 1F2B-(S)-PE all displayed enantiotropic chiral nematic (N*) phase and 1F3B-(S)-PE displayed monotropic N* phase. The results indicated that the removal of the flexible spacers between the rigid mesogenic arm and the chiral core facilitated the formation of the N* phase. When the rigid mesogenic units are connected to the chiral core directly, the structure of the terminal fluorine group and the rigidity of the mesogenic unit played certain influence on the thermal properties of the LC dimers, but did not change the type of mesomorphic phase. Compared to 3F3B-(S)-PE and 1F3B-(S)-PE, 3F2B-(S)-PE and 1F2B-(S)-PE displayed wider LC ranges, respectively, suggesting molecular regularity had greater influence on LC-isotropic (I) transition temperature.

Synthesis and properties of side-chain liquid crystalline polymers grafted with chiral dimers containing cholesteryl groups

ABSTRACTTwo series of novel side-chain liquid crystal (LC) polysiloxanes grafted with chiral liquid crystalline dimers containing cholesteryl mesogens were synthesised. The chemical structure and LC properties of comonomers and polymers were characterised by FTIR, 1H-NMR, DSC, TGA, POM and XRD. M1 and M2 were chiral nematic (N*) dimers, and M3 was an achiral LC monomer displaying nematic mesophase in a narrow mesomorphic temperature range, while the copolymers exhibited N* mesophase whose mesomorphic temperature ranges were much wider than those of the comonomers. Moreover, the glass transition temperatures and isotropization temperatures of the polymers all decreased with decreasing the dimer components. Reflection spectra showed that Pa series tend to attain wide-band selective reflection at long wavelengths, while Pb series were more potential at short wavelengths with narrow bandwidths. Decreasing the dimer components led the wavelength of the selective reflection to blue shift, which was an abnormal phenomenon in chiral mixture system.

Light-Emitting Chiral Nematic Dimers with Anomalous Odd-Even Effect.

In this report, based on the results derived from the extensive study into the thermal and photophysical properties, an anomalous mesomorphic behavior of photoluminescent, chiral nematic (N*) liquid crystalline dimers, belonging to two different series has been revealed. They comprise cholesterol and fluorescent three-ring Schiff base or salicylaldimine core interlinked via an ω-oxyalkanoyloxy spacer of varying length and parity. The effect of molecular structure on the liquid crystal (LC) behavior and photophysical properties of both the series has been probed by varying the length of the terminal n-alkoxy tails for a fixed (odd or even) parity of the spacer. The detailed investigations using complementary techniques not only evidenced the existence of the N* phase in all the dimers synthesized but also the occurrence of an intriguing odd-even effect; blue phases (BPs) exist in all the dimers comprising even-membered spacer, which surprisingly remains totally absent in their odd-membered counterparts. While the results reported hitherto are exactly opposite to the aforesaid findings, this atypical behavior has been interpreted in terms of the over-all shape of the dimers rendered by the orientation of terminal tails. Photophysical studies carried out clearly revealed the intrinsic light emitting feature of the dimers not only in their dilute solutions but also in their three condensed states viz., solid, N* phase, and isotropic liquid state; the emission intensities of the N* phase varies with the change in temperature, as expected. CD spectra of the N* phase recorded as a function of temperature show bisignate CD band characteristically, signifying large chiral correlations in the molecular self-assembly, while the origin of bands from positive to negative region suggests a right-handed twist of the N* helix.

Clusters of B7 Fibers Reveal Origin of Blue Phase Stability in a Binary Mixture of Chiral Rod-like and Achiral Bent-Core Molecules.

Liquid crystal (LC) blue phases (BPs) have gained relevance because of their potential applicability as tunable photonic band gap materials. However, their narrow temperature range often restricts technical usage. Doping with an LC made of achiral bent-core (BC) molecules is one of the strategies employed to increase BP stability. It is now shown that mixing a BCLC exhibiting the polarization-modulated lamellar B7 phase, with a calamitic chiral LC made of rod-like (R) molecules, enhances the BP range considerably. The special feature in this system is the spontaneous expulsion of clusters of B7 fibers in the chiral nematic (N*) phase occurring below the BPs. This gives a clear indication that islands rich in BC molecules lie interspersed between the R molecules. Based on several experimental studies, it is shown that the BP stability may be attributed to an interplay of conformational and intrinsic chirality of the BC and R molecules across the interface of these islands. This study provides new insights from a molecular point of view and provides a novel technique for designing stable-induced BPs. The additional novelty is the occurrence of a phase transition within the fibers. Further, the electro-responsive fibers may also have a potential to form new materials.

Understanding the influence of carbon nanotubes on the flow behavior of liquid crystalline hydroxypropylcellulose: A Rheo-NMR study

Hydroxypropylcellulose (HPC) is an important cellulose derivative that has been widely studied due to its water-solubility, biocompatibility and biodegradability, but even more significant due to its ability to form liquid crystalline phases. HPC is able to form, under certain conditions, chiral nematic (cholesteric) structures in water solutions. Previous work confirmed that films prepared from liquid crystalline HPC/water solutions (LC-HPC) gave rise to anisotropic networks, with similar mechanical and optical characteristics of Liquid Crystalline Elastomers (LCE), capable to respond to humidity. It was also demonstrated that the incorporation of carbon nanotubes (CNTs) significantly improved the actuator responsiveness. In the work presented herein, we investigate how the incorporation of carbon nanotubes affects the flow behavior of LC-HPC solutions, and thus the structure-properties relationship, through a detailed Rheo-NMR study. As observed from the results, when shearing the samples, the degree of order reached (maximum quadrupolar peak splitting) by LC-HPC solutions increases with CNT content. Regarding the subsequent relaxation process, only the incorporation of 0.01 wt% of CNTs (lowest content) contributes to a faster recovery of cholesteric structure.

Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

ABSTRACT New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (TSmC*-SmA*/TSmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan’s molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.

Appearance of induced chiral nematic phase in solutions of 4-n-alkyloxy-4′-cyanobyphenyles with symmetric camphorsubstituted hemiporphyrazines

The synthesis of chiral symmetric camphorsubstituted hemiporphyrazine ((+)HPA) and its racemic mixture ((±)HPA) was carried out by crossover condensation of corresponding camphoradicyanopyrazines with 1,3-phenylenediamine under microwave radiation.By the method of polarizing microscopy, a chiral nematic phase formation of the 4-(n)alkoxy-4′-cyanobiphenyls (n = 3,4,5,6,7,8) nematic mixtures (CB6) was observed under the influence of the symmetric camphorsubstituted hemiporphyrazine, the clearing temperatures and helix pitch were measured, and the twisting power value β was calculated.The degree of destabilizing effect of the racemic mixture (±)HPA on the mesophase was shown to be higher than that of the chiral dopant. Under study the electric permittivity of the CB6 + (±)HPA and CB6 + (+)HPA binary systems the dielectric anisotropy of the achiral composition was found to weakly depend on the dopant concentration, whereas in the case of the spiral mesophase, there is a significant decrease in anisotropy with an increase in the (+)HPA content.At high concentrations of the chiral dopant, jumps in the temperature dependences of the electric permittivity components in the mesophase were detected. Such behavior has been interpreted in the framework of the notions of competition between the dopant and the magnetic field, which attempts to untwist the spiral mesophase.A non-monotonic change in the kinematic viscosity was found on the temperature dependences in the chiral nematic region. In this case, the twisting of the mesophase with a chiral dopant is counteracted by the orientation of the molecules by the flow of a liquid crystal. The equally weak effect of both the chiral dopant (+)HPA and its racemic mixture on the birefringence of LCs is shown, which is caused by the coincidence of the local orientational order in the nematic phase and in the quasinematic layers of the chiral nematic.

The influence of flexible spacer and the chirality of the core on the formation of chiral nematic phase of symmetric dimers containing trifluoromethyl terminal

ABSTRACTSeven symmetric liquid crystal (LC) dimers containing different chiral cores and LC arms have been synthesised, termed EPDA-(R,S)PD, TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD, TFBA-(R)PD, TFBDA-IB and TFBA-IB, respectively. TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD and TFBDA-IB displayed chiral smectic A (SmA*) phase, while EPDA-(R,S)PD, TFBA-(R)PD and TFBA-IB exhibited chiral nematic (N*) phase. The effects of flexible spacer, structural type of LC arms and the chirality of the cores on the thermal properties of the dimers and the formation of N* phase have been studied. The results indicated that the chiral core was prone to induce the N* phase in LC dimer that contained nematic arms although the chirality of the core is very weak, while for the smectic LC arms containing CF3 terminal, the removal of the flexible spacer between chiral core and rigid LC units was conducive to the formation of N* phase. For example, TFBDA-(R)PD and TFBDA-IB displayed SmA* phase, while TFBA-(R)PD and TFBA-IB exhibited N* phase. However TFBA-(R,S)PD did not display N* phase, which reminded us that the chirality of the core and the conformation of the dimer also played an influence in the formation of the LC phase.

2-Chloroalkoxy-Substituted Pentafluorinated Bistolanes as Novel Light-Emitting Liquid Crystals

In this study, we designed and synthesized novel pentafluorinated bistolane derivatives with 2-chloropentyl or 2-chlorohexyl flexible units as novel light-emitting liquid crystals (LELCs). By measuring the phase-transition behaviors, all derivatives were found to display liquid-crystalline (LC) phases during both heating and cooling processes. Among the novel bistolanes, the S- and R-configured derivatives exhibited a chiral nematic (N*) phase with a typical Grandjean optical texture. Interestingly, the chiral derivatives also exhibited a blue phase with a typical platelet texture in a narrow temperature range (2–4 °C). Photophysical measurements revealed that the 2-chloroalkoxy-substituted pentafluorinated bistolanes exhibited intense photoluminescence (PL) both in solution and in crystalline phases. The PL characteristics, especially the maximum PL wavelength, were found to switch sensitively during the heating and cooling cycles depending on the molecular aggregates through the crystal (Cr) ⇄ N* phase transition. The 2-chloroalkoxy flexible units induced dynamic changes in the LC and PL properties, providing valuable insight into the potential of various LELCs as PL sensing materials.

The impedance and dielectric properties of a chiral biphenylcarboxylic acid liquid crystal

We have been synthesized a chiral rod-like liquid crystal 4′-(S)-3,7-Dimethyloctyloxy-4-biphenylcarboxylic acid (DMOBC) exhibiting enantiotropic chiral nematic (N*) as a high temperature phase as well as a smectic mesophase (SmI*) in the wide temperature range. The dielectric and impedance properties of 4′-(S)-3,7-Dimethyloctyloxy-4-biphenylcarboxylic acid (DMOBC) liquid crystal have been analyzed over the frequency range from 1 Hz to 10 MHz at the room temperature.

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.

Unwinding a spiral of cellulose nanocrystals for stimuli-responsive stretchable optics

Cellulose nanocrystals (CNCs) derived from biomass spontaneously organize into a helical arrangement, termed a chiral nematic structure. This structure mimics the organization of chitin found in the exoskeletons of arthropods, where it contributes to their remarkable mechanical strength. Here, we demonstrate a photonic sensory mechanism based on the reversible unwinding of chiral nematic CNCs embedded in an elastomer, leading the materials to display stimuli-responsive stretchable optics. Vivid interference colors appear as the film is stretched and disappear when the elastomer returns to its original shape. This reversible optical effect is caused by a mechanically-induced transition of the CNCs between a chiral nematic and pseudo-nematic arrangement.

Photonic titanium dioxide film obtained from hard template with chiral nematic structure for environmental application

In the present work, mesoporous TiO2 with a photonic structure was elaborated using cellulose nanocrystals (CNCs) as a biotemplate by two-step hard template methods. This strategy enables to replicate the chiral nematic (CN) structure of the photonic films (biotemplate) in TiO2 films. A series of iridescent CNCs films with different weight ratios of silica/CNCs composite photonic films were prepared via evaporation induced self-assembly (EISA) method. The films showed iridescent color and tuneable Bragg reflection wavelengths by solely changing the ratio between the silica and the CNCs biotemplate. Polarized optical microscopy (POM) performed on hydride SiO2/CNCs films showed a birefringence and typical fingerprint of chiral nematic structure. This birefringence was also observed for TiO2 films obtained using SiO2 films as a hard template, which suggested the transfer of the chiral nematic structure in TiO2 materials. Afterwards, their optical, morphological and electronic properties were studied by scanning electron microscope (SEM), POM, energy-dispersive X-ray spectroscope (EDX) and time resolved microwave conductivity (TRMC). The photocatalytic activities were evaluated by following the phenol degradation using high performance liquid chromatography (HPLC). The results showed that the structuration of the TiO2 film using a chiral nematic SiO2 film as hard template enhances the photocatalytic performance compared to non-structured mesoporous TiO2.

Effect of flexoelectricity on the Frèedericksz transition in chiral nematics with negative dielectric anisotropy

It is shown theoretically that due to the flexoelectric effect the electric field driven Frèedericksz transition can take place in chiral nematic (cholesteric) liquid crystal with negative dielectric anisotropy. It is especially noteworthy, since such a transition in a plane parallel cell is impossible in the absence of flexoelectricity. The spatial inhomogeneity of the electric field is is taken into account in the free energy alongside with the traditionally considered terms, describing the orientation elasticity and the anchoring at the boundaries. The phase diagrams are plotted and the orientation structures above the threshold are analyzed basing on the free energy minimization. A comparison between the phase diagrams for liquid crystals with positive and negative anisotropy of dielectric permittivity is made. It was found, that for sufficiently high flexoelectric coefficients the orientational structure can be described by simple functions. In this case, as a result of the Frèedericksz transition, the orientation structure of the liquid crystal changes from planar helicoidal to a hybrid one. This gives rise to new opportunities for the design of switching devices.

Room temperature helical fluids in single-component systems

Technologically significant chiral nematic (N*) phase is a helicoidal superstructure, that is generally formed by the spontaneous self-assembly of entire chiral mesogens (mostly calamitics) or by the LC mixtures where a chiral dopant is being added to the host nematic (N) phase. However, the stabilization of the N* phase at room temperature solely by chiral mesogens is highly challenging where the molecular designing is rather crucial, and thus scarcely reported. Working in this direction, we have rationally designed, synthesized and characterized new chiral materials namely, non-symmetric dimers derived from cholesterol. Notably, as expected, some of these dimers exhibit room temperature N* phase over a wide thermal range. The exclusive occurrence of N* phase has been evidenced unequivocally with the help of polarized optical microscopy (POM) and differential scanning calorimeter (DSC). The chiroptical property of the phase has been studied by recording circular dichroism (CD) spectra as a function of temperature where the intense bisignate bands (change of sign within the band) were observed. Notably, one of the dimers forms stable gel in an aprotic solvent such as hexadecane. Thus, our study reveals a new molecular architecture to stabilize the technologically important N* phase at ambient temperature.

Imine based chiral liquid crystals: Effect of varying the terminal substituent and orientation of ester linking unit

New liquid crystalline compounds composed of three-benzene-ring molecular core comprising both the imine and ester linkage units, (S)-3,7-dimethyloctyloxy chiral unit at one of terminals and n-dodecyl/dodecyloxy group as other side chain have been synthesized and characterized to study the effect of varying terminal chains, the orientation of polar ester linking group and the presence of molecular chirality on mesophase properties of imine based calamitic compounds. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and electro-optic methods. As a result of the strong effect of chirality of (S)-3,7-dimethyloctyloxy chain, all Schiff bases (imines) exhibit enantiotropic chiral smectic C (SmC*), chiral nematic (N*) as well as SmX phase just below the SmC* phase and blue phase (BP) appearing monotropically. The SmC* phase shows ferroelectric switching behaviour with PS values in the range 70–130 nC cm−2. The alteration of the n-alkyl tail with an alkoxy chain gives rise to a significant increase up to 30 °C in mesomorphic transitions. The change in the orientation of polar ester linking group has no effect on the type and sequence of observed LC phase. As compared with (S)-4-(3,7-dimethyloctyloxy)phenol based imine derivatives (7a, 7b), the chiral nematic (N*) mesophase temperature range of (S)-4-(3,7-dimethyloctyloxy)benzoate based compounds (8a, 8b) significantly broadened. Additionally, a decrease on the phase transition temperatures occurred with regardless of introducing n-dodecyl/dodecyloxy group as side chains.

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.

Liquid crystal phase behavior of a DNA dodecamer and the chromonic dye Sunset Yellow

Sunset Yellow (SSY) is an aromatic heterocycle nano-sheet functionalized by hydrophilic sulphanate groups. SSY forms chromonic stacks and liquid crystal (LC) phases in aqueous solution. The DNA oligomer 5\ensuremath{'}-GCGCTTAAGCGC-3\ensuremath{'} is a self-complementary strand which forms duplexed B-form helices in aqueous solution, which, similarly, aggregate into chromonic stacks and form LC phases. Binary aqueous solutions of these two molecules were investigated using polarized optical microscopy, x-ray diffraction, and spectroscopy. At lower solute concentrations and/or higher temperatures these solutions form uniformly mixed single phases, including isotropic, chiral nematic, and hexagonal columnar LCs. At higher solute concentrations and/or lower temperatures, the uniform columnar solution separates into two columnar phases, one containing SSY with trace DNA and the other containing both SSY and DNA aggregates. The study of these solutions indicates that the mixed and unmixed phases are composed of single component SSY or DNA chromonic stacks, with the DNA stacks containing a small fraction of intercalated SSY, evidenced by structurally induced circular dichroism in the SSY absorption band. In the columnar monophase, the hexagonal lattice sites are occupied randomly by either DNA or SSY columns, with the column spacing varying continuously with the SSY-DNA mass ratio. The results demonstrate significant selectivity in the chromonic stacking of both molecules. The binding mode of these chromonic LCs may have applications to adaptive optics and nucleic acid chemistry.

Homogeneous and heterogeneous nucleation of skyrmions in thin layers of cubic helimagnets

Formation of isolated chiral skyrmions by homogeneous and heterogeneous nucleation has been studied in thin layers of cubic helimagnets via elongation of torons and chiral bobbers, correspondingly. Both torons and bobbers are localized in three dimensions, contain singularities, and according to the theoretical analysis within the standard phenomenological models can exist as metastable states in saturated and modulated phases of noncentrosymmetric ferromagnets. Their elongation into the defect-free skyrmion filament is facilitated by small anisotropic contributions making skyrmion cores negative with respect to the surrounding parental state. We show that isolated magnetic torons pose the same problem of compatibility with a surrounding phase as the torons in confinement-frustrated chiral nematics [I. Smalyukh et al., Nature Mater 9, 139-145 (2010)]. We underline the distinct features of magnetic and liquid-crystals torons and calculate phase diagrams indicating their stability regions.