Ferroelectric liquid crystals induced by dopants with axially chiral 2,2′‐spirobiindan‐1,1′‐dione cores: diether vs. diester side‐chains

A series of axially chiral 5,5′‐ and 6,6′‐dialkanoyloxy‐2,2′‐spirobiindan‐1,1′‐dione dopants, (R)‐2 and (R)‐4a–4c were synthesized in optically pure form and their ferroelectric polarization powers, δ_p, measured in four liquid crystal hosts with isotropic (I)–nematic (N)–smectic A (SmA)–smectic C (SmC) phase sequences. The results show that the sign of polarization P _S induced by (R)‐2 and (R)‐4a–4c follows the same trend as that previously reported for the 5,5′‐ and 6,6′‐diheptyloxy‐2,2′‐spirobiindan‐1,1′‐dione dopants, (R)‐1 and (R)‐3. The polarization induced by (R)‐2 in the host DFT is below detection limits, and the sign of P _S was found to invert as a function of temperature at mole fractions as low as 0.01. On the other hand, the polarization power of the 6,6′‐diheptanoyloxy dopant (R)‐4b in the host NCB76 is ?1449 nC cm^?2, the fourth highest value reported so far, and more than three times the δ_p value of the 5,5′‐diheptanoyloxy analogue (R)‐2 in that host (+474 nC cm^?2). Results of ²H NMR experiments suggest that (R)‐4b exerts stronger local perturbations in NCB76 than (R)‐2, and that these perturbations may be chiral in nature.     

Electrically induced and thermally erased properties of side‐chain liquid crystalline polymer/liquid crystal/chiral dopant composites

The properties of synthesized side‐chain liquid crystalline polymer (SCLCP)/liquid crystal (LC)/chiral dopant composites having a chiral nematic (N*) phase at room temperature were investigated by polarized optical microscopy (POM) and a UV/VIS/NIR spectrophotometer. The composite exhibited a planar texture after it was filled into cells under homogeneous boundary conditions and it was transparent. When an electric field was applied to the composite, a focal conic texture was formed and the composite became light scattering. After the electric field was turned off, the light‐scattering state remained stable for some time, i.e. the light‐scattering state exhibited a memory effect. The focal conic texture changed into the planar texture when the composite was heated and the composite became transparent again. Therefore, the composite had electrically induced and thermally erased properties. The SCLCP had some influence on the memory effect and on the thermo‐electro‐optical properties of the composite.     

Synthesis and characterization of side‐chain liquid‐crystalline poly(ethyleneimine)s with cyanobiphenyl groups

A series of polyethyleneimine‐based side‐chain liquid‐crystalline polymers substituted with different ratios of cyanobiphenyl as pendent mesogenic groups has been synthesized in which the spacer length varies between two and six methylene units. The structures of the synthesized polymers are confirmed by infrared and ¹H nuclear magnetic resonance spectroscopy. The thermal properties of these polymers have been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. The results indicate that the thermal behaviour of the polymers is strongly dependent on the degree of substitution. Polymers containing more than 69% of mesogenic groups exhibit nematic‐type thermotropic liquid‐crystalline behaviour with schlieren textures. Below this limit, the polymers are amorphous. Polymers with a higher degree of substitution present the crystalline states. The phase transition temperatures increase and the mesomorphic temperature ranges widen with increasing degree of substitution. The clearing temperatures decrease as the spacer length increases. An odd–even effect in the clearing temperatures is observed and the odd members display the higher values.     

Field dependence of the optical activity of a chiral liquid crystal near the isotropic–smectic A transition

Abstract

Chiral liquid crystals exhibit molecular optical activity in the isotropic phase. We have studied the evolution of the optical activity as a function of an applied electric field on a 76·2 μm film of the chiral liquid crystal W7, which exhibits an isotropic–smectic A transition at approximately 40°C. We measured the optical activity by recording the rotation of the plane of polarization of an incident linearly polarized ray of light, provided by a He–Ne laser. The applied biasing electric field is parallel to the direction of the incident beam. We find that at 41·0°C, the plane of polarization shifts from ?1·1° for an applied voltage of 30 V to a maximum of ?4·0° at 70 V. The absolute value of the signal decreases beyond this voltage. These shifts are in the direction of the smectic A phase and are in general larger than those observed as a function of temperature. Close to the isotropic–smectic A phase transition, molecules inside the liquid coalesce to form dynamic coherent groups, which have smectic nature. These groups are randomly oriented with respect to each other in the absence of an electric field. The application of an electric field causes the molecules within these groups to align along the direction of the field and to contribute coherently to the optical activity of the system. The way the molecules align with the field depends on the relative values of the polarizability α, which contributes to the alignment of the long axis of the molecule, and the dipole moment p, which contributes to the alignment of the short axis of the molecule. Our preliminary results and calculations suggest that for small fields, the electric field couples with the dipole moment p, whereas for fields in excess of 70 V, the field couples with the polarizability of the long axis of the molecule, causing a rotational reorientation of the molecules in the isotropic phase. The value of the field at which this reorientation occurs may be controlled by temperature.     

Electric field‐induced birefringence,optical rotatory power and conoscopic measurements of a chiral antiferroelectric smectic liquid crystal

Using a photoelastic modulator‐based novel set‐up, the electric field‐induced in‐plane birefringence and the optical rotatory power (ORP) were measured of an antiferroelectric liquid crystalline compound (12OF1M7) in its various phases using 30 µm homeotropic cells. Some specific signatures of the in‐plane birefringence and of the ORP for the various phases are being established. A relatively small threshold field is needed for the unwinding process of the antiferroelectric phase with a unit cell of four layers [SmC_A*(1/2)] compared with that for two layers [SmC_A*(0)]. On application of the electric field on the high temperature side of the SmC_A*(1/2) phase (80.1–81.5°C), a field‐induced phase transition is shown to occur directly to the SmC* phase, whereas on the lower temperature side (79.4–80.1°C) the transition takes place to SmC* via the SmC_A*(1/3) phase. The in‐plane birefringence exhibits a critical power law dependence for the SmC*–SmA transition. The ORP changes sign within the temperature range of the phase with a unit cell of three layers, reflecting a change in the handedness during this phase. Using tilted conoscopy, the results for the biaxiality and the apparent tilt angle for a smectic liquid crystal with a tilt angle greater than 18° in the ferroelectric phase are reported. The biaxiality implies the difference in the refractive indices between the two minor axes of the refractive index ellipsoid. The optical transmittance at visible and IR wavelengths for free‐standing films reveal characteristic reflection bands for these phases. The modulated structures of the reflected bands appear just above the SmC_A* phase and below SmC_A*(1/3); these are possibly due to an easy deformation of the phase by the surfaces.     

Ferroelectric liquid crystals derived from (S)‐2‐(6‐methoxy‐2‐naphthyl)propionic acid with non‐fluorinated or semi‐fluorinated alkanes at a chiral terminal chain

Two series of ferroelectric liquid crystals derived from (S)‐2‐(6‐methoxy‐2‐naphthyl)propionic acid, with non‐fluorinated or semi‐perfluorinated alkanes positioned at a chiral terminal chain, have been synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and electro‐optical measurements. The non‐fluorinated compounds, 1‐hexyl (S)‐2‐{6‐[4‐(4‐alkanoyloxyphenyl)benzoyloxy]‐2‐napthyl}propionates exhibit rich mesomorphism—the BP_II, N*, TGB_A*, SmA* and SmC* phases. The fluorinated compounds display only the SmA* and SmC* phases, suggesting that the fluorination promotes the formation of smectic phases. In addition, the SmA* and SmC* phases of the fluorinated compounds have enhanced thermal stability as compared with the corresponding phases of the non‐fluorinated compounds. The spontaneous polarization (P _s values) for the non‐fluorinated compounds are higher than those of the fluorinated compounds at any reduced temperature below the SmA*–SmC* transition. The electro‐optical responses measured for these compounds in the ferroelectric phase displayed thresholdless, V‐shaped switching.     

Infrared spectroscopic study of a phenyl benzoate side group—methacrylate main chain polymeric liquid crystal

Abstract

The temperature dependence of the infrared dichroism of a side chain polymeric liquid crystal was investigated for two homogeneously oriented samples, one aligned with a PVA coating and the other aligned by a magnetic field. From the apparent order parameters, it was found that the rotation of the mesogenic side group was restricted. Not only the rigid part of the side group, but also the alkyl chains were found to be partly ordered. Realignment of the side groups was observed in the case of the magnetically aligned sample at the smectic-nematic transition temperature. Unlike the other absorption bands, the C?O bond vibration band shifted to higher wavenumbers with increasing temperature. This behaviour was explained by the influence of the neighbouring mesogenic groups on the conjugation existing between the π-electron cloud of the benzene rings and the π-electrons of the C?O bond.     

Dimeric liquid crystals with 5‐(4‐alkoxybenzoyloxy)tropone or 4‐(4‐alkoxybenzoyloxy)phenyl cores: evaluation of the tilt angles of the cores,spacers, and side chains

Two types of symmetric dimers with 5‐(4‐alkoxybenzoyloxy)tropone cores or with 4‐(4‐alkoxybenzoyloxy)phenyl cores were synthesized to evaluate the effect of the core structure and the length of the spacer on the mesomorphic properties. The former had smectic C phases whereas the latter had smectic C and F phases. Both types of dimer showed a remarkable odd–even effect on varying the spacer on the mesomorphic properties. Comparison of the thermal stability between them demonstrated that benzenoid twins are more stable than troponoid ones. The layer spacings of the smectic C phases were measured to determine the tilt angles of the core part, the spacer, and the side chains on changing the length of the spacer and the side chains. Troponoid dimers had a larger tilt angle of the core part of the smectic C phase than benzenoid twins, which lowered the thermal stability of the troponoid. Entropy changes of the smectic C phase to the isotropic liquid showed a contrast between troponoids and benzenoids. The former had smaller values and odd–even effects than the latter, which indicated that the former troponoids had a limited number of conformers in mesophases.     

Cell gap‐dependent transmission characteristics of a fringe‐electric field‐driven homogeneously aligned liquid crystal cell,for a liquid crystal with negative dielectric anisotropy

Transmittance characteristics were studied as a function of cell gap for a homogeneously aligned liquid crystal (LC) cell driven by a fringe‐electric field—named fringe‐field switching (FFS) mode. The light efficiency of a conventional LC cell using in‐plane switching and twisted nematic modes, where the LC director is determined by competition between elastic energy and electrical energy, does not depend on cell gap as long as the cell retardation value remains the same; i.e. only dielectric torque contributes to the deformation of the LC director. However, the transmittance of the FFS mode is dependent on the cell gap such that it decreases as the cell gap decreases, although the cell retardation value remains the same. This unusual behaviour (unlike that of conventional LC cells) arises because in the device the elastic and dielectric torques have the role of determining the LC director, such that the driving voltage giving rise to maximum transmittance becomes strongly dependent on the electrode position when the cell gap is as small as 2?µm. In addition, the LCs at the centre of the pixel and common electrodes are not sufficiently twisted because of a competition between the two elastic forces, which tries to twist the LCs in plane and hold them in their initial state by surface anchoring.     

X-ray diffraction analysis of the layer structure in a ferroelectric liquid crystal with a chiral nematic–chiral smectic C phase transition

A half-V-shaped switching ferroelectric liquid crystal (FLC) is a promising candidate for fast response displays. In the half-V FLC display, a liquid crystal with a chiral nematic–chiral smectic C phase transition is used, and the smectic layer is formed by cooling from N* to SmC* with an applied d.c. field. We studied the layer structure by means of X-ray measurements for two axes (ω and χ). By using a point-focused X-ray tube and optimizing the slit width, we succeeded in the two-axis measurement with a commercial X-ray system. The ω–χ profile of the half-V FLC showed two broad peaks in an arc-shaped high-intensity area. Our interpretation of this result is that the major part of the layer consists of a tilted-bookshelf structure and that the minor part consists of a near-bookshelf structure. Since optical microscopy observations on the half-V FLC cells showed a stripe-shaped texture, we consider that the coexistence of the tilted-bookshelf and the near-bookshelf structures forms the stripe-shaped patterns. The radius of the arc-shaped high-intensity area was nearly equal to the molecular tilt angle. This result can explain why the half-V FLC showed a desirable black appearance in spite of the stripe-shaped texture.     

Dynamics of the acousto‐optic effect in a nematic liquid crystal

In a nematic liquid crystal cell, the application of an ultrasonic wave induces a rotation of the director, leading to a change in the optical transmission through the cell. In this study, we investigate the dynamic response of the optical intensity after the ultrasonic wave is switched on or off. Our experiments show that the optical intensity follows a double‐exponential function of time, indicating that the system has two relaxation modes with widely different time scales. The fast mode has an amplitude and time scale qualitatively consistent with the dynamics of the Fréedericksz transition, but the slow mode shows novel behaviour associated with the acousto‐optic effect.     

Ferroelectric compounds with chiral (S)-1-methylheptyloxycarbonyl terminal chain – their miscibility and a helical pitch

Compounds with the same chiral terminal chain but different structure of rigid core and non-chiral terminal chain were added to basic highly tilted ferroelectric compound in the aim of establishing whether the miscibility of phases, especially ferroelectric phase, exists in these compounds and what are the helical pitch and tilt angle temperature dependence in bicomponent systems. In certain systems, we obtained a mixture with very short helical pitch at broad temperature range of ferroelectric phase, which open opportunities for the formulation of multicomponent mixtures for fast switching ferroelectric liquid crystals modes.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.     

Synthesis and mesomorphic properties of new chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups

New chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups (Pn‐O‐PIMB5*‐4O, n = 7, 8, 9 and 10) were synthesized and their mesomorphic properties and phase structures investigated by means of electro‐optic measurements, polarizing optical microscopy, differential scanning calorimetry, and second harmonic generation measurements. Most of these chiral bent‐core mesogens (n = 7–9) showed the antiferroelectric B2 phase, whereas P10‐O‐PIMB5*‐4O exhibited the B7 phase. Comparing with the previously reported homologue Pn‐O‐PIMB(n‐2)*, we conclude that the terminal chain structure, particularly the position of chiral centres, plays an important role in the emergence of particular phase structures.     

A series of novel side chain liquid crystalline polysiloxanes containing cyano‐ and cholesterol‐terminated substituents: Where will the structure‐dependence of terminal behavior of the side chain reappear?

A series of polysiloxane side chain liquid crystal polymers with strong polarity cyano substitution‐terminated achiral side chains and cholesterol‐terminated chiral side chains was successfully synthesized via thiol‐ene click chemistry. ¹H‐NMR, FT‐IR, and thermogravimetric analysis were used to confirm their chemical structures and thermal stabilities. Their phase transition behaviors and phase structures were systematically investigated by a combination of analysis methods such as differential scanning calorimetry, polarized optical microscopy, and X‐ray. Results revealed that attributing to the decisive role of the polarity interaction, all the polymers only developed a monolayer interdigitated SmA phase in which the period arrangement was determined by the cyano‐terminated side chains, the increased content of cholesterol‐terminated chiral side chains (X_chol) just expanded the distance between neighboring molecules within a layer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1765–1772     

Thermal switching characteristics based on smectic-A↔chiral nematic phase transitions of (liquid crystals/chiral dopant) composites,with and without side chain-type liquid crystalline polymer

The light switching characteristics induced by a thermal smectic A (SmA) ? chiral nematic (N*) phase transition were studied for homeotropically aligned [smectic A liquid crystal (SmA-LC)/nematic liquid crystal (N-LC)/chiral dopant] and [side chain type smectic A liquid crystalline polymer (SmA-LCP)/N-LC/chiral dopant] composites. A drastic change from a transparent SmA phase to a light-scattering N* phase occurred in both composites upon heating. In the case of the heat-induced N* phase for the (SmA-LC/N-LC/chiral dopant) composite, the N* phase exhibited weak light scattering due to formation of a scroll texture. On the other hand, in the case of the heat-induced N* phase for the (SmA-LCP/N-LC/chiral dopant) composite, the N* phase showed strong light scattering due to formation of a focalconic texture. The existence of a SmA-LCP was responsible for a higher contrast ratio between the transparent SmA phase and the light scattering N* phase for the (SmA-LCP/ N-LC/chiral dopant) composite than for the (SA-LCN/N-LC/chiral dopant) composite.     

Synthesis of (2R,3S)-isobutyl phenylisoserinate,the Taxol® side chain,from ethyl benzoylacetate

Reduction of ethyl 2-chloro-3-phenyl-3-oxopropionate with borohydride affords predominately the syn-chlorohydrin. Resolution of this ester with the lipase MAP-10 gives (2S,3R)-2-chloro-3-hydroxypropionic acid which after esterification with MeOH/HCl is converted to the cis-epoxide with potassium carbonate and DMF. Aminolysis of the epoxide with aqueous ammonia results in ring opening and amide formation. The amide is converted to an ester upon treatment with isobutyl alcohol and HCl(g) at 100°C. Neutralization then affords the Taxol^® side chain as the free amine.     

Raman study of a liquid crystalline gel consisting of photochromic 4‐methyl‐N‐(4‐n‐heptyloxysalicylidene)aniline and a gelling agent (R,R′)‐1,2‐bis(dodecanoylamino)cyclohexane

A liquid crystalline physical gel has been prepared from the mixture of a nematic liquid crystal and a low molecular mass gelling agent containing a hydrogen‐bonding moiety. The newly synthesized liquid crystalline compound exhibited photochromism in the crystalline solid phase. Although photochromism was not observed in the nematic gel state of the mixture, the lifetime of photochromism in the solid phase became longer, compared with that of a single liquid crystalline compound. Some Raman bands of the mixture showed a marked change in both intensity and frequency through the phase transitions. These bands have been assigned to the vibrational modes related to the core part of molecule.     

Investigation of folding patterns in homo-oligomers of (R)-β2,2-amino acids with carbohydrate side chain

The study describes the synthesis of new β^2,2-peptides made from geminally disubstituted β^2,2-amino acid and their folding propensities. The (R)-C-linked carbo-β^2,2-amino acid [(R)-β^2,2-Caa] was prepared from d-glucose and converted into the homo-oligomeric di-, tetra-, and hexapeptides. The conformational studies were carried out using NMR (in CDCl₃), CD, IR, and MD calculations. These β^2,2-peptides were interestingly stabilized by five-membered (mr) inter-residue H-bonds NH(i)?O(i-1) (furanoside) and a 6-mr intra-residue H-bond between amide proton (NH(i)) and the oxygen of –OMe(i) at the C3 carbon of the carbohydrate side chain. These results amply demonstrate that the ‘epimerization’ at the spiro carbon center has an effect on the conformational behavior of these peptides. Finding of these, H-bonding patterns, which are not so common to stabilize the folds in this class of β^2,2-Caa derived peptides would further facilitate augmentation in the domain of foldamer.