Chiral dopant induced twist grain boundary phases

A diarylethane alpha-chloro ester was mixed with a chiral dopant of high helical twisting power at variable concentration to study its capability for induction of twist grain boundary phases. With increasing concentration of the chiral dopant, TGBA* and TGBC* phases were observed with a rather broad region of existence. In contrast to homeotropic alignment, planar boundary conditions seem to enhance the phase stability of the TGB phases, resulting in metastable TGB states and phase coexistence with the respective smectic phases. The phase growth of SmA*/SmC* out of the TGBA*/TGBC* state was studied under isothermal conditions as a function of temperature and concentration of the chiral dopant.     

Chiral dopant induced twist grain boundary phases

A diarylethane alpha-chloro ester was mixed with a chiral dopant of high helical twisting power at variable concentration to study its capability for induction of twist grain boundary phases. With increasing concentration of the chiral dopant, TGBA* and TGBC* phases were observed with a rather broad region of existence. In contrast to homeotropic alignment, planar boundary conditions seem to enhance the phase stability of the TGB phases, resulting in metastable TGB states and phase coexistence with the respective smectic phases. The phase growth of SmA*/SmC* out of the TGBA*/TGBC* state was studied under isothermal conditions as a function of temperature and concentration of the chiral dopant.     

Twist-grain-boundary transitions in a chiral tolane compound

High resolution calorimetric studies have been carried out on the chiral compound methylheptyloxydifluorooctyloxybenzoyloxytolane (8BTF2O1M7). The tilt tendency is greater in this compound than in several other structurally similar fluorinated tolane liquid crystals, and it exhibits tilted chiral smectic C(SmC*) and tilted twist-grain-boundary (TGBC) phases but not the untilted SmA or TGBA phases. The data confirm the presence of two tilted TGBC phases denoted TGB1 and TGB2. The TGB1-TGB2 first-order transition exhibits considerable hysteresis and a very small latent heat. There is no rounded excess heat capacity peak in the cholesteric N* phase associated with the non-transitional evolution of a chiral line liquid N*L, although such a feature has been observed in other fluorinated tolanes with a smaller tilt tendency.     

New compounds with a TGBA-TGBC-SmC* phase sequence

A new series of lactic acid derivatives has been prepared with molecular cores consisting of two laterally substituted biphenyls connected by an ester group. The compounds exhibited the TGBA phase in a broad temperature range. In addition, for some compounds, a narrow blue phase was found above the TGBA phase on cooling from the isotropic phase. Except for the compound with the shortest chain, the TGBC phase appeared below the TGBA phase. The low temperature SmC* phase existed within the interval up to 80 K, including the overcooled state, which was down to room temperature. In the compounds studied a strong pre-transitional phenomenon took place manifested as a broad peak on the differential scanning calorimetry plot in the isotropic phase. The phases were established on the basis of a planar sample and free-standing film microscope observation. Small angle X-ray diffraction measurements provided information about layer spacing, d, and confirmed the phase assignment. Dielectric spectroscopy revealed the soft and Goldstone modes in the respective TGBA and SmC* phases.     

Blue phases and twist grain boundary phases (TGBA and TGBC) in a series of fluoro-substituted chiral tolane derivatives

A series of fluoro-substituted tolane derivatives: ( R )-1-methylheptyl 3-fluoro-4-(3-fluoro-4- n alkoxybenzoyloxy)tolane-4-carboxylates is reported. Some members of this series exhibit the phase sequence: Cr-SmC*-TGBC-TGBA-BPI-BPII-BPIII-I. The blue phases, the TGBA and TGBC phases and the SmC* phase were characterized in detail by microscopic observation, differential scanning calorimetry, helical pitch measurements, X-ray structural analysis and electro-optical study. The blue phases directly next to the TGBA phase were shown to be a new type of blue phase exhibiting smectic ordering. A commensurate TGBC phase with constant number of slabs per pitch was observed.     

Dielectric spectroscopy of unsymmetrical liquid crystal dimers showing wide temperature range TGBA and TGBC* phases

The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4‐n‐decyloxy‐4′‐(cholesteryloxycarbonyl‐1‐butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC^* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (～200–300 Hz). Activation energies for DC conductivity have also been determined for various phases of the material.     

Variety of mesophases in compounds with an increasing number of lactate units in the chiral chain

Liquid-crystalline compounds with different numbers of lactate units, n, in the chiral part were synthesised and mesomorphic properties studied. Physical properties were compared with respect to n. In the compound with one lactate unit in the chiral part the TGBA–TGBC–SmC* phase sequence was detected. For two lactate units the antiferroelectric SmC*_A phase occurs. Finally, three-lactate material exhibits the tilted hexatic SmI*(F*) phase below the ferroelectric SmC* phase. Dielectric spectroscopy and spontaneous tilt and polarisation were measured. For the three-lactate compound the temperature dependences were analysed in the vicinity of the SmC*–hexatic phase transition, and these properties compared with the theoretically predicted behaviour.     

A review of textures of the TGBA* phase under different anchoring geometries

Polarizing microscope textures of the twist grain boundary A* (TGBA*) phase are reviewed for two different compounds in different geometries with different surface treatments giving monostable planar and homeotropic boundary conditions. The textures are discussed in the light of the helical structure of the TGBA* phase. Depending on the compound, the underlying phase is either SmA* or SmC*, whereas the adjacent phase at higher temperature is cholesteric (N*). Sample preparations in wedge-shaped cells subjected to a slight temperature gradient exhibit TGBA* textures much more typical for the cholesteric than for the ordinary SmA* phase. For instance, Grandjean steps and fingerprint textures are observed for planar and homeotropic boundary conditions, respectively. Preparation of smectic droplets clearly reveals the helical axis of the TGBA* phase to be perpendicular to the helical axis of the helielectric SmC* phase. For thin samples, a suppression of the TGBA* helix leading to a surface-induced structure corresponding to a conventional bulk SmA* phase is observed. Under certain conditions, a cholesteric phase in the vicinity of a twist inversion point may exhibit very similar textures to the TGBA* phase near the transition to the SmA* phase. On exemplified textures similarities are discussed and differences pointed out.     

Twist grain boundary states in non-chiral smectogen-chiral dopant

The liquid crystalline systems studied consisted of non-chiral mesomorphic esters [the eutectic mixture of 4-n-hexyloxyphenyl 4-n-octyloxybenzoate and 4-n-octyloxyphenyl 4-n-hexyloxybenzoate] and structurally similar chiral dopants (N-arylidene derivatives of S-1-phenyl- and S- 1-benzyl-ethylamine). Twist grain boundary phases occur between the cholesteric and smectic C* or smectic A phases in all the investigated systems. The different structures of these TGB phases (TGBA and TGBC) are proved by small angle X-ray scattering and textural studies. The concentration and temperature ranges of the TGB phases are defined by the twisting power of the chiral dopants and their own mesomorphic peculiarities. The experimental dependences of TGB phase temperature range on cholesteric helical twist are influenced by a ratio change of the optically active and racemic forms of the dopant at a constant total concentration. The results obtained are discussed within Renn's theory.     

Twist Grain Boundary Liquid‐Crystalline Phases under the Effect of the Magnetic Field: A Complete 2H and 13C NMR Study

A liquid crystal ( HZL 7/* ) containing an (S)‐2‐methylbutyl‐(S)‐lactate unit in the chiral chain, is investigated by means of ²H and ¹³C NMR spectroscopy in order to obtain information on its orientational order, its molecular structure and the effect of external magnetic fields on the supramolecular structure of its phases. This mesogen presents very peculiar mesomorphic properties and exhibits frustrated TGBA* and TGBC* phases in a wide temperature range up to 60 °C, as well as an additional phase transition from TGBC₁* to TGBC₂*. ²H NMR measurements show, for the first time, a peculiar magnetic field effect in unwinding the supramolecular structure of both the TGBA* and TGBC* phases. This effect is particularly evident at higher magnetic fields, while different behaviour is observed at lower magnetic fields. This indicates that the supramolecular structure is very sensitive to magnetic fields of the order of 1 Tesla. Moreover, the analysis of the ²H and ¹³C NMR spectra of HZL 7/* allow us to obtain several structural properties, such as the tilt angle of the TGBC* phases and the local orientational order parameters referred to the phenyl and biphenyl fragments. This is the first structural characterization of the frustrated phases of these complexes by means of NMR.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4'-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8-12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmC_A*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmC_A* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmC_A* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm^?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmC_A*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol^?1 in the SmC* phase.     

Optical textures in TGBA mesophases

In this paper, we present the results of polarized optical microscopy observations performed on different series of compounds exhibiting twist grain boundary mesophases (TGB). From all the compounds investigated, it appears there are two main typical optical textures which characterize TGBA mesophases in general. One type of texture is characteristic of a planar helical structure, while the other is a cylindrical (or marginally cone-like) domain texture, resembling the developable domains observed in columnar systems. Moreover, both textures can coexist in the same sample over the whole temperature range of the TGB phases. The detection, at the same temperature, of these two types of textures could then be considered as one of the signatures of the TGBA mesophases.     

Optical textures in TGBA mesophases

In this paper, we present the results of polarized optical microscopy observations performed on different series of compounds exhibiting twist grain boundary mesophases (TGB). From all the compounds investigated, it appears there are two main typical optical textures which characterize TGBA mesophases in general. One type of texture is characteristic of a planar helical structure, while the other is a cylindrical (or marginally cone-like) domain texture, resembling the developable domains observed in columnar systems. Moreover, both textures can coexist in the same sample over the whole temperature range of the TGB phases. The detection, at the same temperature, of these two types of textures could then be considered as one of the signatures of the TGBA mesophases.     

Dielectric and electro-optic properties of two biphenylyl benzoate-based ferroelectric mesogens with tilted hexagonal phases

ABSTRACT

Two partially fluorinated ferroelectric liquid crystals (2F3R, 3F3R) with biphenylyl benzoate core have been investigated by frequency domain dielectric spectroscopy and electro-optic method. Both the compounds exhibit SmC* phase over a wide range of temperature along with tilted hexatic phase SmF* and soft crystal phases SmJ* (in 2F3R) and SmG* (in 3F3R) and one coexistence phase of (SmF*+SmC*). Both BOO phason and tilt phason relaxations are observed in SmF* phase and cooperative relaxation behaviour is also observed in crystal-like smectic phases. Goldstone mode relaxation is observed in SmC* phase and Maxwell Wagner mode is observed in all the phases. In 2F3R soft mode is detected in both SmC* and SmA* phases but not in 3F3R which melts directly to isotropic phase from SmC* phase. Compounds possess moderate values of spontaneous polarisation and exhibit quite sharp electrical response especially in 2F3R. Rotational viscosities, pitch and elastic constant are also measured. Effect of chain length and fluorination on various physical parameters has been discussed.     

Synthesis and mesomorphic properties of chiral liquid crystals derived from ( S )-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4′-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8?12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.     

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 (T_SmC*-SmA*/T_SmA*-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.     

Effect of lateral methoxy substitution on mesomorphic and structural properties of ferroelectric liquid crystals

A new series of materials with a chiral fragment derived from lactic acid and a methoxy group as lateral substituent in different positions of the molecular core was synthesised and investigated. Derivatives with ester or ether linkages of the non‐chiral chain were also studied. Depending on the molecular structure, cholesteric, twist grain boundary smectic A (TGBA*), chiral smectic A (SmA*) or chiral smectic C (SmC*) phases were detected. In derivatives with the ester linkage and a methoxy group at the nearest and the next nearest phenyl ring to the non‐chiral chain these phases completely disappear. On the other hand, a methoxy group on the phenyl ring close to the chiral chain provides a compound with low layer shrinkage at the SmA*–SmC* phase transition (“de Vries” behaviour). The temperature dependence of the spontaneous polarisation, the tilt angle, the layer spacing as well as the complex permittivity were studied and the results discussed in terms of molecular structure.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.     

Chiral two ring compounds exhibiting antiferroelectric phases

Novel chiral two ring compounds possessing phenylethenoate moieties were synthesized and their liquid crystalline properties determined. Some of these compounds were found to exhibit SmC* γ, AF and SmC* A phases, whereas in other compounds only SmA* and SmC* phases appear. The influence of the molecular structure on the appearance of SmC* γ, AF and SmC* γ phases is discussed. The basic electro-optical properties of the homologous series of ( S )-( E )4-(1-methylheptyloxycarbonyl)phenyl 4-alkoxycinnamates, in which SmC*, SmC* γ, AF and SmC* A phases appear, were also investigated. High values of spontaneous polarization, low threshold voltages and fast switching times at room temperature were observed.