Non-symmetric chiral isoflavone dimers: synthesis,characterisation and mesomorphic behaviour

A new series of non-symmetric chiral isoflavone-based liquid crystalline dimers, α-(2-methylbutyl-4′-(4″-phenyloxy)benzoate)-ω-(3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy)alkanes, with 3–12 carbon atoms in the alkyloxy spacer, have been synthesised. A pronounced odd–even effect for the phase transition temperatures upon varying the spacer length was observed. The short dimers exhibited monolayer smectic A (SmA) and smectic C (SmC*) phases while for longer homologues a chiral nematic (N*) phase was found. The temperature range of the nematic phase was broadened with elongation of the alkyl spacer. Stabilisation of the nematic phase resulted from competition between the monolayer and intercalated smectic structures. The SmA–SmC* phase transition was second order for all studied compounds with a cross over to the de Vries type behaviour for the shortest homologue.     

Optically-active comb-branch liquid crystalline polymers having the chiral center in the flexible spacer unit. III. Copolymers of (S)-2-(4′-cyano-4-biphenylyloxy)-1-methylethoxy ethyl methacrylate and di[6-(4-methoxy-4′-oxybiphenyl) hexyl]-2-methylene butane-1,4-dioate

A new series of copolymers with optically-active liquid crystalline side chain units has been synthesized from the comonomers (S)-2-[2-(4′-cyano-4-biphenylyloxy-1-methylethoxy]ethyl methacrylate ( 1 ) and di[6-(4-methoxy-4′-oxybiphenyl)hexyl]-2-methylene butane-1,4-dioate ( 4 ). Chiral nematic phases were exhibited by two members of the series, rich in monomer 1 , while a smectic phase was exhibited in copolymers rich in 4 . While it was thought possible that ordered chiral liquid crystalline phases may be induced by copolymerizing chiral mesogenic monomers with mesogenic derivatives of itaconic acid where the high side chain density encourages greater ordering in the system, no evidence of smectic C^* phases could be found in the present systems. © 1993 John Wiley & Sons, Inc.     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Design and self-assembling behaviour of comb-like stereoregular cyclolinear methylsiloxane copolymers with chiral lactate groups

Several new comb-like stereoregular cyclolinear methylsiloxane copolymers exhibiting the self-assembling behaviour have been designed and studied. For the resulting macromolecular materials, the chiral mesogenic unit, namely (S)-(–)-4-[1-ethoxycarbonyl)ethoxycarbonyl]phenyl-4′-[11-tetramethyldisiloxyl)-undecenyloxy)-biphenyl-4′-carboxylate, has been used as a side group to assure the mesomorphic behaviour. The mesomorphic and structural properties were studied by the polarising optical microscopy, differential scanning calorimetry and small-/wide-angle X-ray diffraction techniques. The designed copolymers form the orthogonal smectic A* and the tilted smectic C* phases over relatively broad temperature range, down to room temperatures. The obtained results are discussed in order to contribute to better understanding of the molecular architecture–nano-organisation relationship for a specific type of macromolecular system based on the lactic acid derivatives used as a flexible side groups.     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Dimesogenic compounds consisting of cholesterol and azobenzene-based moieties: dependence of liquid crystal properties on spacer length and fluorination of the terminal chain

The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds consisting of cholesterol and azobenzene-based moieties interconnected by ω-oxyalkanoyl spacers of varying length are compared: one series (AOC-n) has an octyloxy chain attached to the azobenzene mesogen unit while the other (AOCF-n) has a perfluoroheptylmethyloxy chain. In general, compounds bearing the fluorinated alkoxy chain exhibited LC properties over a much broader temperature range than those with the alkoxy chain. In addition, the AOC-n series exhibited the chiral smectic C (SmC*), smectic A (SmA) and cholesteric (N*) phases depending on the length of the central spacer, whereas the AOCF-n series favoured the formation of only the SmA phase with the N* phase completely suppressed. Both series showed an odd-even dependence of the isotropization temperature on spacer length.     

Synthesis and characterization of fluorine-containing cholesteric liquid crystalline polysiloxanes bearing trifluoromethyl-substituted mesogens

A series of side-chain liquid crystal (LC) polysiloxanes were synthesised with Poly(methylhydrogeno)siloxane, 4?-(undec-10-enoyloxy) biphenyl – 4 – yl 4- (trifluoromethyl) benzoate (M_th) and a chiral nematic (N*) LC monomer 1-allyl 10-(cholesteryl)-decanedioate (M_ch). The chemical structures and LC properties of the monomers and polymers were characterised by FTIR, ¹H-NMR, differential scanning calorimetry, thermogravimetric analysis, POM and X-ray diffractometer. M_ch is monotropic N* LC. The homopolymer derived from monomer M_ch is enantiotropic N* LC. Monomer M_th is a smectic A liquid crystal. The copolymers derived from M_ch and M_th are N* LCs. The temperatures at which 5% weight loss occurred are greater than 300°C for all the fluoro-containing polymers, and the residue weights of the samples at 600°C increased slightly as the content of trifluoromethyl mesogens increased in the polymers. The glass transition temperatures of the polymers increased as trifluoromethyl mesogens increased, too. The N*–I phase transition temperatures show a negative deviate from ideal or linear behaviour. The values of the enthalpy changes for the cholesteryl containing polymers are rather low and this is attributed to the biaxiality of cholesteryl moiety which tends to reduce the change in the orientational order at the N*–I transition. Compared to the monomers, the polymers show wider mesophase region.     

Effect of terminal perfluorocarbon chain containing mesogens on phase behaviors of chiral comb-like liquid crystalline polymers

A novel perfluorinated liquid crystal 4′-(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctanoyloxy)biphenyl-4-yl undec-10-enoate (PFOBU) was synthesized, which exhibited smectic C phase. Several liquid crystalline polymers (PI–PVI) were synthesized by use of poly(methylhydrogeno)siloxane, PFOBU, and cholesteryl 3-(4-allyloxy-phenyl)-acryloate. The chemical structures and liquid crystalline (LC) properties of the monomers and polymers, and some ferroelectric properties of the chiral smectic C (S_C^*) phase were characterized by use of various experimental techniques. The effect of perfluorocarbon chains on phase behaviors of the fluorinated LC polysiloxanes was studied as well. PI and PII showed single chiral nematic (N^*) mesophase when they were heated and cooled, but PIII, PIV, PV, and PVI containing more perfluorocarbon chain units exhibited S_C^* phase besides N^* mesophase. Introduction of perfluorocarbon chain containing mesogens to the chiral cholesteryl-containing polymer systems resulted in a S_C^* mesophases, indicating that the fluorophobic effect could lead to microphase segregation and modifications of smectic mesophases from the chiral nematic phase.     

The helical structure of highly ordered smectic phases

Optical studies of smectic phases have been performed in homogeneously oriented samples of chiral 4-(2′-methylbutyl) phenyl-4′-n-octylbiphenyl-4-carboxylate (CE8). The helix structure has been found in smectic phases C, I and J, but not in the smectic G phase. Two chiral phases have been found between S_I* and S_G phases. Up to now one of them has not been observed. The pitch of the helix has been measured in all of the twisted smectic phases, including the S_J* phase. The existence of the helix in this phase suggests that the correlations between smectic layers are not very strong.     

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

Seven 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.     

Dielectric studies of a ferroelectric liquid crystal

Measurements of the complex dielectric permittivity in the frequency range 30 Hz-13MHz have been made for the chiral smectic C and smectic A phases of the mixture ZLI-3654. In the S*_C phase a large contribution to the electric permittivity coming from the Goldstone mode was found. For the pretransitional region S*_C-S_A, only the soft mode has been detected. It is found that the molecular relaxation originating from the reorientation around the short axis is well separated from the soft mode even in the pretransitional region. The influence of different relaxation modes on the dielectric anisotropy is presented.     

The thermodynamic parameters of sorption and enantioselectivity of the chiral smectic liquid crystal 2-methylbutyl ester of 4-(4-decyloxybenzylideneamino)-cinnamic acid

The thermodynamic characteristics of sorption of n-alkanes, arenes, aldehydes, monoatomic alcohols, and optical isomers of camphene and butanediol-2,3 by a chiral smectic liquid crystal, 2-methylbutyl ester of 4-(4-decyloxybenzylideneamino)-cinnamic acid, from the gas phase were studied over the temperature range including the S*_C and S*_A mesophases and isotropic phase. The standard and excess thermodynamic functions of sorption were determined for 26 sorbates of the classes of substances specified. The S*_C and S*_A mesophases exhibited selectivity with respect to the separation of para and meta xylenes (α _p/m = 1.06–1.07, 90–108°C) and pronounced enantioselectivity (α_R/S = 1.05–1.09, 87–108°C). The helically twisted structure of the smectic liquid crystal was shown to play an important role in the mechanism of the chiral recognition of optical isomers of polar and low-polarity compounds under gas-liquid chromatography conditions.     

Shear flow and magnetic field effects on smectic C,C*, CM and C*M liquid crystals

Abstract

The continuum equations of Leslie et al. [1] for smectic C, and the extension of this theory for chiral smectic C* [2], are applied to problems involving simple planar layer configurations which accommodate uniform layer thickness constraints. The chiral smectic C*_M and non-chiral smectic C_M [3] are considered as either biaxial smectic A phases or antiferroelectric smectic C phases and are therefore included as interesting degenerate cases of the smectic C* and C phases, respectively. The effects of static and time dependent magnetic fields on these materials are compared with related deformations occurring in nematics [4] and cholesterics [5,6]. Their reaction to applied shears is also investigated yielding examples of flow alignment, induced secondary flows and unwinding of the chiral helix and testing the validity of enforcing a constant layer thickness.     

Studies on the synthesis and properties of ferroelectric side chain liquid crystalline polyoxetanes

Two series of novel ferroelectric liquid crystalline (FLC) monomers were derived from 3-(hydroxymethyl)-3-methyloxetane, used as the backbone unit, and 2-(S)-[2-(S)-methylbutoxy]propionic acid, as a chiral moiety. The corresponding polyoxetanes were prepared by ring-opening polymerization using BF₃ · OEt₂ as an initiator. In addition to the structure identification, their liquid crystal phase behavior and electrical properties are also studied. Before their connection to the chiral molecular moiety, two series of carboxylic acids, 4-(6-[(3-methyloxetan-3-yl)methoxy]alkoxy)-benzoic acids and 4,4′-[6-(3-methyloxetan-3-yl)alkoxy]biphenylcarboxylic acids, show the phase sequence K Sc I and K Sc N I, respectively. After connection, the phase behavior of the corresponding chiral monomers is changed from K Sc I to K Sc* N* I as well as from K Sc N I to K Sc* Sa I. Only the phase sequence K Sc* Sa I is observed in both series of polyoxetanes. All of the synthesized monomers exhibited enantiotropic chiral smectic C(Sc*) phase. The monomers, with the biphenyl unit linked directly with a chiral center, possessed higher spontaneous polarization (P_s) values. Polyoxetanes possess a wide temperature range for the liquid crystal phase, about 120°C, and the Sc* phase range can be up to 95°C. However, the position of the biphenyl unit will not affect the spontaneous polarization of the synthesized side chain FLC polyoxetanes. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2843–2855, 1997     

Effect of alkyl chain length in the terminal ester group on mesomorphic properties of new chiral lactic acid derivatives

Two series of new liquid crystalline lactic acid derivatives with a terminal ester group have been synthesised. The effect of this ester unit and the length of its alkyl chain on the mesomorphic and dielectric properties of the compounds exhibiting a broad temperature range of chiral smectic phases have been studied. We found that the mesomorphic behaviour and phase transition temperatures are strongly affected by the molecular architecture. Depending on the alkyl chain length in the terminal ester unit, the studied materials exhibited paraelectric smectic A*, ferroelectric tilted smectic C* and antiferroelectric smectic C_A* phases over a broad temperature range. The physical properties of the compounds have been studied by optical polarising microscopy, differential scanning calorimetry, electro-optic measurements, small-angle X-ray scattering and dielectric spectroscopy. Furthermore, the homologues with short terminal alkyl chains showed a very small layer shrinkage at the transition from the orthogonal SmA* to the tilted SmC* phase, which is a characteristic feature of ‘de Vries-type’ behaviour.     

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

ABSTRACT

A 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 (S_mA*) 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.     

Hydrogen bond-induced ferroelectric liquid crystals

Abstract

Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (C _n OSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.     

Helical pitch of ferroelectric liquid-crystalline polymers and copolymers

Ferroelectric liquid-crystalline polymers and copolymers were synthesized from acrylate polymers with side chains consisting of a flexible spacer unit, a mesogenic unit and an optically active substituent. In the chiral smectic C (S*_c) phases typical fan-shaped textures with equidistant lines (caused by the helical structures) were observed. It was found that the helical pitch increased with increasing polymer molecular weight. The helical pitch also increased in copolymers containing mixtures of right- and left-handed chiral side chains.     

Synthesis of new smectic C* liquid-crystalline block copolymers

A series of smectic C^* liquid-crystalline (LC) block copolymers were successfully synthesized via the living anionic polymerization of polystyrene with optically active methacrylate monomers containing (S)-2-methylbutyl 4-(4-hydroxyphenylcarbonyloxy)-biphenyl-4′-carboxylate mesogens. These materials are the first reported smectic C^* block copolymers. Anionic polymerization in tetrahydrofuran (THF) at −70°C leads to LC block molecular weights of approximately twenty repeating units. The number-average molecular weight of the polystyrene block was varied from 7000 to 20000 to adjust the composition in the block copolymers. Differential scanning calorimetry and optical microscopy indicate that the smectic C^* phase is present in the systems over broad temperature ranges.     

A proton spin relaxation study of ferroelectric liquid crystals

Here we present and analyse N.M.R. measurements of the Larmor frequency dependence (dispersion) of the longitudinal proton spin relaxation time, T ₁(v), for two chiral ferroelectric mesogens (Merck IS-1912 and DOBAMBC) in the isotropic, smectic A and smectic C* phases, making use of fast field cycling techniques. Although in the low frequency range the relaxation times of IS-1912 are much shorter than those of DOBAMBC, the form of the dispersion profiles is not basically different for the two materials. This reveals contributions by smectic order fluctuations, self-diffusion and molecular rotations. The order fluctuation term, which means relaxation by collective molecular reorientations, is clearly seen by characteristic dispersion profiles in the kHz regime (T ₁ ～ v ¹ or T ₁ ～ v ^1/2), which disappear in the isotropic phase. Our results do not indicate significant dissimilarities between the main relaxation processes in the S_C and S*_C mesophases.