TGBA Phase Research Articles

Effect of lactate group in the chiral chain of new compounds exhibiting short-pitch cholesteric or TGBA phase

ABSTRACTWe have synthesised and studied lactic acid derivatives based on chlorine substituted molecular core, which is created from two biphenyls linked by an ester, and terminated with one or two lactic units in a chiral chain. The compounds with one lactate group exhibit cholesteric phase with rather short helix pitch (200 nm) in a broad temperature range. On contrary, compound with two lactate units reveals a stable TGBA phase, with transition temperatures substantially lower than those for one-lactate derivatives. We have studied mesophase behaviour and electro-optical properties, mostly based on the texture observation in polarising microscope. Additionally, we have used AFM and x-ray techniques to confirm mesophase identification and establish structural properties.

Frustrated smectic liquid crystalline phases in lactic acid derivatives

ABSTRACTWe have prepared and studied a series of compounds with different types of molecular core and lactate unit in the chiral terminal chain. We draw a survey and comparison of their mesomorphic properties with respect to the occurrence of twist grain boundary (TGB) phases. The materials exhibit extremely wide TGBA phase more than 60K broad, unique TGBA-TGBC-SmC*-SmCA* phase sequence and unique re-entrant TGBA phase below the SmA phase. TGB phases have been induced in binary mixtures of molecules with different molecular shape and chirality (chiral lactic acid derivative and non-chiral hockey-stick mesogen). Unique effect is observed for compounds with TGBA phase, where the applied electric field transforms the planar texture into the homeotropic one, homogeneously dark in crossed polarizers. The process is analogy of the Frederiks transition so far known only for nematics. This effect, changing the bright state to the dark one, is promising for applications.

Eutectic behaviour of binary mixtures composed of two isomeric lactic acid derivatives

ABSTRACTMixing of structurally similar self-assembling materials is a powerful tool to tune their mesomorphic, orientational or dielectric properties. We present design and study of binary mixtures composed of two newly designed chiral lactic acid derivatives with a similar structure but different geometry of the molecular core. One compound possesses a monotropic ferroelectric SmC* phase while another shows a direct isotropic – crystalline phase transition. It has been shown that the binary mixtures possess the eutectic behaviour and the ferroelectric phase temperature range has been extended. Moreover, for certain mixtures induced liquid crystalline phases, namely the cholesteric or TGBA phases, were detected.

Unique effect of an electric field on a new liquid crystalline lactic acid derivative.

A new chiral lactic acid derivative is presented, exhibiting a frustrated liquid crystalline phase, namely the orthogonal twist grain boundary TGBA phase in a broad temperature interval. A unique effect is observed that the applied electric field reversibly transforms the planar TGBA texture to the homeotropic one, homogeneously dark in crossed polarizers. The transformation is analogous to the Frederiks transition known in nematics, in which switching under electric field is driven by the positive dielectric anisotropy. A similar effect is established also in the SmA phase of the racemic mixture, where the field induced transformation is irreversible. A positive dielectric anisotropy in both the chiral compound and the racemic mixture is detected up to the frequency of about 10 kHz, above this frequency the anisotropy is negative. The unusual behavior of the TGBA phase under the electric field can be explained by the specific packing of molecules within the smectic layers, resulting in a relatively high layer compressibility which lowers the energy of the structural defects and thus facilitates the structure transformation. The perfectly dark state of the studied compounds, induced by the electric field, either stable or reversible, is appealing for specific applications. The change of the sign of the dielectric anisotropy, known in nematics as the dual frequency effect, might be important for photonics such as adaptive or diffractive optics.

Effects of chain branching and lateral fluorine substitution on mesomorphism of cholesteryl benzoates

The mesogenic cholesteryl 4′-alkoxyphenyl-4-carboxylates possessing terminal normal/branched/saturated/unsaturated alkyl chains with laterally ortho/meta substituted electronegative fluorine atom are described. All the homologues exhibited enantiotropic mesomorphism. Smectic A phase, chiral nematic, blue phase (BP) and TGBA phases were observed in different homologues. All the novel compounds were characterised by spectroscopic and elemental analysis. Thermal investigations and mesophase characterisations for all the compounds were carried out by the combination of DSC, POM and X-ray analysis. The effects of the various terminal normal/branched/saturated/unsaturated alkyl chains and the position of the substituted fluorine atom with its structurally related compounds have been discussed.

Anomalous phase sequence in new chiral liquid crystalline materials

We have prepared and studied a series of rod-like liquid crystalline compounds with the molecular core laterally substituted by bromine atom and with the lactate unit in the chiral terminal chain. Microscopic observation under the polarising microscope, x-ray and dielectric spectroscopy results enable us to establish the TGBA–SmA–TGBARE–TGBC phase sequence. The presence of the re-entrant TGBA phase below the regular SmA phase, found for three homologues, is quite uncommon. Results are discussed and compared with the previously studied compounds with regular phase sequences.

Frustrated phases induced in binary mixtures of hockey-stick and chiral rod-like mesogens

Two liquid crystalline compounds with different shapes and chirality aspects have been used to prepare binary mixtures. An achiral hockey stick mesogen exhibiting a nematic–smectic C synclinic–smectic C anticlinic phase sequence has been blended with a chiral rod-like mesogen with a SmA*–ferroelectric (FE) SmC*–antiferroelectric (AF) SmCA* phase sequence. Several frustrated phases (blue phases, TGBA and TGBC phases) have been induced in broad temperature and concentration ranges. By tailoring the composition of the chiral compound we succeeded in preparation of mixtures with unusual mesomorphic behaviour. We demonstrate a new system showing chiral nematic and antiferroelectric phases in its phase sequence.

Twisted Grain Boundary Phases in Binary Mixture of Smectic and Cholesteryl Compounds

A binary mixture of cholesteryl ethyl corbonate (CEC) and terephthal-bis-p-n-tetradecylaniline (TB14A) exhibits a twisted grain boundary (TGB) phase and smectic polymorphic phases. The mixture exhibits TGB-A and TGB-C phases at different concentrations and at different temperatures. Mixtures with higher concentrations of TB14A exhibit I – Ch – TGB-A – TGB-C – Sm A – Sm I – Sm F phases sequentially when the specimen is cooled from its isotropic melt. The existence of different TGB phases and polymorphic smectic phases are confirmed by differential scanning calorimetric (DSC) and optical microscopic studies. The phase diagram is in accordance with the Renn and Lubbensky model.

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.

Dipolar phases in liquid crystals with the chiral part based on the lactic acid

New lactic acid derivatives exhibit paraelectric SmA and TGBA phases, ferroelectric SmC* and TGBC phases and antiferroelectric phases, the TGB and antiferroelectric phases being unusually broad. The polymorphism depends on the number of lactate units in the chiral chain and on the length of the non-chiral chain. The dielectric spectroscopy revealed the characteristic modes in all these phases. In phases with the dipolar order the spontaneous polarization and the tilt angle have been determined.

Twist Grain Boundary Phases Giving Developable Domain Textures as Those Exhibited by Columnar Phases

This paper reports experimental observation of TGBA, TGBC* phases in a mixture of Liquid Crystals. The phases exhibit developable domain textures as those observed for columnar systems. Under polarizing microscope they appear as cylindrical and cone-like domain texture. In such domains the smectic blocks, the grain boundaries and the layers of helical structure of TGB phase are coiled in a double twist manner around an axis. A peculiar texture is obtained in the intermediate region between the TGBA and Cholesteric phase while cooling the sample from isotropic phase. This phase is identified as ‘Twist Grain Boundary phase of Hexatics’ (TGBH) proposed theoretically by Kamien. The phase reappears as TGBA is heated. The Twist grain boundary phase of Hexatics consists of twisted N + 6 regions separated by grain boundaries. Within each region the bond order twists with pitch 2π/q and the director is aligned in a nematic state. The director is rotated by some finite angle across a grain boundary moving along the pitch axis that is perpendicular to the nematic director.

Phase diagram of new lactic acid derivatives exhibiting ferro‐ and antiferroelectric phases

Rich polymorphism has been found in a new series of lactic acid derivatives. The length of a non‐chiral n‐alkyl chain plays an important role in changing the sequence and temperature range of phases. Except for one homologue with the shortest chain, all compounds exhibit a rather wide antiferroelectric chiral smectic C (SmC*A) phase below the ferroelectric (SmC*) and paraelectric smectic A (SmA) phases. The spontaneous polarisation and tilt angle were measured in the SmC* and SmC*A phases. For several compounds, small‐angle X‐ray diffraction measurements provided information about layer spacing, d. Temperature dependences, d(T), exhibit a rather large jump at the SmC*–SmC*A phase transition, but no anomaly was observed in the temperature dependences of the tilt angle and spontaneous polarisation. For one compound studied, a TGBA phase was established on the basis of the planar sample and free‐standing film observation under a polarising optical microscope. For two compounds with the longest non‐chiral chain, a direct phase transition from isotropic to the SmC*A phase was observed. All materials studied by dielectric spectroscopy revealed soft and Goldstone modes in the respective SmA and SmC* phases and two high‐frequency modes in the SmC*A phase.

Synthesis and mesomorphic properties of new compounds exhibiting TGBA and TGBC liquid crystalline phases

We synthesised a series of rod‐like mesogens with a (S)‐2‐methylbutyl‐(S)‐lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free‐standing films, by Grandjean‐Cano texture and by NMR studies on a deuterium‐labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20–30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in 2H‐NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases.

Twisted-grain-boundary (TGB) phases: nanostructured liquid-crystal analogue of Abrikosov vortex lattices

Twisted-grain-boundary (TGB) phases shown by some liquid-crystalline materials have properties common to those of both smectic and cholesteric phases. Following analogy between liquid crystals and super conductors proposed by de Gennes [Solid State Commun., 10, 753 (1972)], Renn and Lubensky [Phys. Rev. A, 38, 2132 (1988)] theoretically predicted a chiral analog of the smectic A (SmA*) phase, which is now known as TGBA phase. The TGBA phase was experimentally observed for the first time by Goodby et al. [Nature, 337, 449 (1989)] in the chiral homologous series of ferroelectric liquid-crystal material R- and S-1-methylheptyl 4′-[(4′′-n-alkoxyphenyl)propionoyloxy]-biphenyl-4-carboxylates (nP1M7), with n = 13,14,15. Since then, more than a hundred pure and mixed systems showing TGBA phase have been found. Later, Renn derived a mean-field phase diagram based on the chiral Chen-Lubensky model, and predicted two more TGB phases, namely TGBC and TGBC*. These two phases have also been experimentally observed in many systems, and in a few, the phase diagram is similar to that predicted by Renn. Unlike the TGBA phase, several theoretical models have been proposed for TGBC and TGBC* phases, and it remains to be ascertained whether all the types of proposed TGBC/TGBC* structures experimentally exist. A review of the theoretically predicted and experimentally observed TGB phases is given in the present article. Some recently observed novel optical textures of the TGB phases are also reported.

Dielectric spectroscopy of a binary mixture of liquid crystals showing wide temperature range twisted grain boundary phase with re-entrant cholesteric phase

There are only few materials, which have shown long temperature range twisted grain boundary (TGB) phases. One such material is the chiral binary mixture of 7OCB and 5* CBB (mole ratio 0.8 and 0.2) which shows unique phase sequence of cholesteric ( N*) , a wide temperature range TGBA (approximately 31 degrees C) and reentrant cholesteric (N*re) phases. In the present work we are reporting the dielectric spectroscopy of the above mixture with a chiral analog of earlier reported nematic (N) , smectic- A (SmA) , and reentrant nematic (Nre) phase sequences [Phys. Rev. A 46, 7733 (1992)] for different conditions of molecular orientations. Two modes of dielectric relaxations have been detected in a homeotropically aligned sample with unusually low relaxation frequencies for one of them. Planar oriented molecules in the TGBA phase show a soft mode of relaxation and support the recently proposed theory for the soft mode relaxation of the TGBA phase [Phys. Rev. E 65, 11701 (2001)]. By applying the dc electric field on planar oriented molecules in the TGBA phase, it has been possible to obtain a helix free homeotropically aligned TGBA phase.

INFLUENCE OF CONFINING GEOMETRY ON THE DIELECTRIC BEHAVIOR OF A FLC SAMPLE HAVING TGBA PHASE

We report on the dielectric permittivities in 1 kHz to 4 MHz frequency region of the ferroelectric liquid crystal (FLC) material CB500 having a TGBA phase. Molecular and collective relaxation processes have been studied in bulk and in confined environment. In the bulk sample, a Goldstone mode (GM) and a molecular mode were observed in the SmC* phase and the softmode (SM) was observed in the TGBA phase. The Goldstone mode was also present in the FLC sample confined in 0.2 μm Anopore membranes. The relaxation frequency of this mode was shifted to the higher-frequency region in comparison to that observed in the bulk sample. In the Anopore confined FLC sample, another relaxation process (AP) was observed, which possibly arises due to some collective movement of the molecules attached to the cavity walls. The relaxation frequency of this process is almost independent of temperature and this process is only observed in the SmC* phase.

Rheological properties of chiral liquid crystals possessing a cholesteric–smectic A transition

We have measured the rheological properties of two cholesterol derivatives (cholesteryl myristate and cholesteryl nonanoate) in the vicinity of their cholesteric–smecticA transitions. The results for the two compounds differ qualitatively, and are in agreement with results based on optical observations of new defects in cholesteryl nonanoate showing that this material, traditionally considered as a typical cholesteric, in fact exhibits a TGBA phase between the cholesteric and smectic A phases.

Synthesis and mesomorphic properties of compounds exhibiting the undulated twist grain boundary smectic C* phase

The synthesis and characterization of three homologous series of compounds exhibiting the undulated twist grain boundary smectic C* (UTGBC*) phase are reported. The chiral mesophases have been obtained using cholesterol as the chiral moiety. Cholestanol and [S]-[+]-octan-2-ol have also been used as the chiral moiety for comparitive purposes. In addition to this novel phase, cholesteric, smectic A, smectic C* and TGBA phases have also been observed. The mesophases were characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and measurement of helical pitch.

Thermodynamical and Optical Studies of Ternary Mixtures of 4-n-Heptyloxy Benzoic Acid, 4-n-Decyloxy Benzoic Acid, and 5-Cholesten-3ß-ol 3-octanoate

Binary eutectic mixture of 4-n-heptyloxybenzoic acid (HOBA) and 4-n-decyloxybenzoic acid (DOBA) shows smectic C (SmC) and nematic (N) phases over a wide temperature range. 5-cholesten-3β-ol 3-octanoate(ChO), which has monotropic chiral nematic (N∗) phase, has been added as third component in the eutectic bi-component mixture (BCM) of HOBA-DOBA in different mole ratios. Transition temperatures and enthalpies of the ternary mixtures have been determined by using differential scanning calorimeter (DSC) and mesophases have been identified with the help of polarizing microscope. It has been observed that the thermal stability of SmC phase of BCM decreases rapidly due to the addition of ChO molecules and SmC phase is completely suppressed for about 10–12 mole percent of ChO. A narrow temperature range TGBC phase has been observed for ChO concentrations 2 to 8 mole percent. For higher concentrations of ChO molecules, TGBA and SmA phases have been induced over a wide temperature range. Ternary system of ChO and BCM shows N∗ phase over a wide temperature range in the heating and cooling cycles at about eutectic composition (12.5 mole percent of ChO) of ChO-BCM system.

Synthesis and Mesogenic Properties of Enantiopure Trioxadecalin Derivatives Bearing an n-Alkylphenyl Substituent

A number of chiral trioxabicyclo[4.4.0]decanes and boratrioxabicyclo[4.4.0]decanes have been prepared. The mesogenic properties of these compounds have been studied. They show short cholesteric pitch, blue phases and TGBA phases depending on the chain length of the n-alkyl substituent, with quite large differences between the trioxa- and the boratrioxaderivatives.