A chiral material with a new phase sequence: twist grain boundary smectic C phase-smectic blue phases

A new phase sequence: twist grain boundary smectic C (TGBC) to smectic blue phases (BP_Sm) is observed in a chiral compound (S)- or (R)-1-methyloctyl 3'-fluoro-4'-(3-fluoro-4-hexadecyloxybenzoyloxy)tolane-4-carboxylate. It is the first time that a TGBC phase has been found to occur under smectic blue phases in the absence of the twist grain boundary smectic A (TGBA) phase. These phases are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering.     

Twist grain boundary and frustrated liquid crystal phases

Research into the structures and properties of frustrated liquid crystal phases continues apace. A number of new twist grain boundary phases have been discovered, of particular interest are those with their rod-like molecules inclined to the layer planes, i.e. the TGBC phases. A new anticlinic TGBC phase has also been discovered complimenting the synclinic TGBC phases. In addition a number of smectic blue phases (BP_Sm) have recently been discovered.     

A chiral material with a new phase sequence: Twist grain boundary smectic A phase—blue phases

A new phase sequence: twist grain boundary S_a phase to blue phases was observed in a chiral compound (R)- or (S)-1-methylheptyl 3'-fluoro-4'-(3'-fluoro-4'-octadecyloxybenzoyloxy)tolan-4-carboxylate. It is the first time that blue phases have been found to occur above the TGBA phase in the absence of a cholesteric phase. These phases were characterized by microscopic observation, differential scanning calorimetry, the contact method and study of the racemic system.     

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.     

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.     

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.     

Broad liquid crystalline temperature range of ferroelectric liquid crystals

Novel ferroelectric liquid crystalline compounds, containing the (S)-2-methyl-l -butyl (4-hydroxybiphenyl-4'-carbonyloxy)biphenyl-4-carboxylate mesogenic group and an oligooxyethylene spacer, were synthesized. The mesomorphic properties of these materials were investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurement. The results indicate that all members of this series exhibit a very broad temperature mesophase range (reaching a maximum around 210°C) including a blue phase (BP), cholesteric (Ch), twist grain boundary A (TGB_A), chiral smectic C (S*_c), and smectic X (S_x) phases. The mesomorphic properties are discussed and a comparison is made with three phenyl rings of ester core analogues.     

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.     

Novel phase behaviour in two (smectic/cholesteric) liquid crystal mixtures

The phase behaviours of mixed liquid crystal systems having either Sm/N or Sm/Ch properties have been studied. The (smectic/nematic) binary system formed smectic phases over a wide and much enhanced range of temperature (42 C) and a broad concentration range (0-90 wt %). The ternary smectic/cholesteric system, in appropriate concentration ranges, exhibited the smectic A phase, a TGBA-like twist grain boundary A phase, the cholesteric phase and blue phases. The TGBA-like phase appeared in the cholesteric-smectic phase transition range. Three textures (chiral pitch, fan-shaped and scale-like) for the cholesteric phase of the ternary smectic/cholesteric mixtures were observed in the ranges 0-7, 7-43 and 43 wt % respectively, of cholesteric CB15, in a binary Sm/N mixture.     

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* (UTGB_C*) 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 TGB_A 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.     

Effect of a lateral substituent on the mesomorphic properties of ferroelectric side chain liquid crystalline polysiloxanes

The synthesis of side chain liquid crystalline polysiloxanes containing oligooxyethylene spacers and ( S )-2-methylbutyl 4-\[(4-oxybiphenyl-4-yl)carbonyloxy]-3-fluorobenzoate mesogenic side groups is presented. Differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction measurements reveal liquid crystalline properties for all synthesized monomers and polymers. All three precursor olefinic monomers reveal cholesteric and smectic A phases. The olefinic monomer which contains two oligooxyethylene units in the spacer is the only one which reveals a twist grain boundary A phase and a blue phase, besides the cholesteric and smectic A phases. All three polysiloxanes present enantiotropic smectic A and chiral smectic C phases. The mesomorphic behaviours of the monomers and polymers are compared with those of the corresponding monomers and polymers without the lateral fluoro substituent. The results seem to demonstrate that incorporating a lateral fluoro substituent in the mesogenic cores of the monomers affects not only the mesophase thermal stability, but also the nature of the mesophases formed. However, incorporating a lateral fluoro substituent in the mesogenic cores of the polymers affects only the thermal stability of the mesophases formed. The lateral fluoro substituent has a more profound effect on the mesomorphic behaviour for the monomers than that for the polymers.     

TGBA phase in a series of propionates containing two stereocenters

A homologous series of chiral materials, (R)-2-pentyl (S)-2-(6-(4-(4'-alkoxyphenyl)benzoyloxy)-2-naphthyl)propionates (R,S)PmPBNP (m = 7-14), derived from a naphthalene ring as part of the core structure in conjunction with a chiral tail containing two stereocentres has been synthesized for the investigation. The twist grain boundary smectic A (TGB_A) and smectic C (TGB*_C) phases were characterized by the microscopic textures of compounds packed in two untreated glass slides, Cano wedge cell, or homogeneously aligned cell. It was found that the occurrence of these phases depend remarkably on the nature of alkyl chain length m; in the case of shorter alkyl chain length (m = 7-10), the TGB_A phase behaves as a metastable-like phase mediated between N and S_A phases in a short temperature range, whereas in the longer chain length (m' 11-14), both the TGB_A and TGB_C phases become thermodynamically stable phases with a wide temperature range. Consequently, this series of chiral materials resulted in two different mesophase sequences: N-TGB_A-S_A-S_Cand N-TGB_A-TGB_C. A kind of parquet texture displaying two types of domain with different relative directions of the smectic layer normal was found in the S phase from the materials (m = 9-14) packed in 2 μm homogeneously aligned cells and cooled down from the isotropic liquid without applying an electric field. The magnitudes of spontaneous polarization (P_s) in the S_C and TGB_C phases showed that the P_s values are nearly the same for all compounds at the same temperature below the Curie point. Dielectric measurements revealed no significant occurrence of soft mode switching in the TGB_A phase.     

Synthesis and characterization of new ferroelectric liquid crystals containing a (2S)-2-[6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'- naphthyl] propionate mesogenic group and oligo(oxyethylene) spacers

Three series of ferroelectric liquid crystals containing a (2S)-2-(6-(4-hydroxybiphenyl-4'-carbonyloxy)-2'-naphthyl)propionate mesogenic group and oligo(oxyethylene) spacers were synthesized. These obtained liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC), optical polarized microscopy (POM), and X-ray powder diffraction measurements. Some of these materials containing four phenyl rings of ester cores (i.e. -Ph-Ph-COO-naph-) and chiral heptyl tail exhibited a rich mesomorphic behaviour, a blue phase (BP), a cholesteric phase (Ch), a smectic A (S_A), a twist grain boundary A (TGB_A), and a chiral smectic C (S_C*) phase. Another series containing four phenyl rings of ester cores and chiral butyl and pentyl chain tails revealed only a S_A phase and a S_C* phase. Moreover, a crystal E phase was observed in the short spacer chain (n = 0 or 1) homologues of three series of compounds. Also, the mesomorphism properties were discussed as a function of spacer units, numbers of aromatic rings of core, and different terminal asymmetric moieties.     

A novel liquid crystal showing antiferroelectric smectic C* and twist grain boundary phases

A novel antiferroelectric liquid crystal, 4-[5-(4-octyloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(4-methoxyphenyl)butanoate (TFMB) showing twist grain boundary phases was found and investigated. In optically active TFMB, a diffuse liquid-liquid transition was observed above the clearing point. TFMB exhibited a stable antiferroelectric smectic C* phase. The relationship between antiferroelectricity and the molecular structure is discussed.     

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.     

Mesomorphic properties of troponoid esters and amides with cholest-5-ene-3β-carboxylic acid moiety

Troponoid esters and amides connected with cholest-5-ene-3β-carboxylic acid have been synthesized to characterize their mesomorphic properties and to compare them with those of the corresponding benzenoids. 5-Alkoxytroponyl esters and amides with a long alkoxy group exhibited a twist grain boundary A* phase, as well as chiral nematic and smectic A* (SmA*) phases. The corresponding benzenoid esters exhibit SmA* and blue phases and the benzenoid amides only a SmA* phase. The differences between the mesomorphic properties are discussed in terms of the structural features of the core.     

A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGB_C*) phase. In the UTGB_C* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGB_A-TGB_C-SmC* sequence. As the chiral strength is increased this goes to the sequence TGB_A-UTGB_C*-SmC*. Such sequences have been observed experimentally.     

Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

Spacer length dependence of TGBC* and SmA phases in cholesteryl and dihydrocholesteryl-containing trimer liquid crystals

Two series of trimer liquid crystals were investigated that contain a biphenylyl central group and two cholesteryl or dihydrocholesteryl terminal mesogenic groups. Only compounds with even spacers were investigated. The dihydrocholesteryl-containing trimers show a triply intercalated smectic A (SmA) phase when the spacer lengths are greater than 8, whereas the cholesteryl-containing trimers exhibit this triply intercalated SmA phase when the spacer lengths are more than 6. With shorter spacers, a twist grain boundary C* (TGBC*) phase was found. This is revealed by the formation of a typical dotted square grid pattern upon cooling from the chiral nematic (N*) phase in the planar texture. The dots are spaced by a distance of about 1.5-1.8 µm. Upon cooling from the N* phase in the focal conic texture a striped pattern is observed with the same spacing. X-ray diffraction revealed a repeat distance for the TGBC* phase that corresponds with a monolayer ordering. The results show that the weaker interaction between the dihydrocholesteryl groups compared with cholesteryl groups or longer spacers destabilize the monolayer TGBC* phase.     

Liquid crystal dimers possessing chiral rod-like anisometric segments: synthesis, characterization and electro-optic behaviour

Several new optically active liquid crystal dimers comprising pro-mesogenic cholesterol and a chiral diphenylacetylene (tolane) segment, covalently linked in an end-to-end fashion through a flexible spacer, have been synthesized and investigated for their mesomorphic behaviour with the aid of optical, calorimetric and X-ray diffraction studies. Five unsymmetrical dimers, designed on the basis of recent work, involve molecular structural variations of the tolane mesogenic entity with a view to stabilizing a wide thermal range smectic A (SmA) phase featuring the electroclinic effect. Three different chiral chains, namely, (S)-1-methylheptyloxy, (S)-2-methylbutyloxy, (3S)-3,7-dimethyloctyloxy, with or without polar (nitro or fluoro) lateral substituents, were incorporated, while keeping the length (C₆) of the spacer constant. As expected, all the dimers exhibited a SmA phase. A few also showed chiral nematic (N*) and/or twist grain boundary and/or chiral smectic C (SmC*) phases. Remarkably, some of these oligomesogens, upon melting, had a stable SmA phase over a wide thermal interval (100-150°C); this state seems to be stable for a long period of time. Electro-optic studies, including optical tilt angle as well as temporal response as a function of temperature, were carried out in the SmA phase. The SmC* phase was also investigated for its electrical switching and optical tilt angle, as well as spontaneous polarization as a function of temperature. These studies showed that the mesophase response to an applied field is weak and is independent of variations in the dimer investigated.