(S)-Isoleucine and (R)-2-octanol as chiral precursors of new chiral liquid crystalline thiadiazoles: synthesis, mesomorphic and ferroelectric properties

New chiral Schiff bases with a 1,3,4-thiadiazole unit in the rigid core were synthesized (SB1-SB4). These compounds contain a chiral chain derived from (S)-isoleucine and (R)-2-octanol. Their liquid crystalline properties were studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. Thereby it was found that most of the new compounds exhibit a chiral Smectic C phase. A study of the ferroelectric properties is described.     

Liquid-crystalline properties of chiral malolactonate monomers and their comb-shaped polymers

A series of optically pure mesogenic ester precursors, and the malolactonate monomers containing mesogenic alcohols obtained from them were prepared and polymerized to homopolymers and copolymers. The liquid-crystalline properties of the precursors, monomers and polymers were investigated by differential scanning calorimetry, polarized light microscopy and wide angle X-ray diffraction. The liquid-crystalline properties of the chiral mesogenic precursors and monomers were only slightly influenced by the chemical structure, configuration and the alkyl spacer length of the mesogenic alcohol substituent of the ester. The precursor, 1-[6-(4'-hexyloxy-4-biphenylyl)oxyhexyl] hydrogen (S)-O-mesylmalate (V-(S)-6) formed smectic E and smectic A phases on melting, while the monomer, 6-(4'-hexyloxy-4-biphenylyl)oxyhexyl (R)-malolactonate (II-(R)-6) showed only a crystal-isotropic phase transition. In contrast, the homopolymer of the latter exhibited a chiral smectic C phase. Copolymers from 2-(4'-hexyloxy-4-biphenylyl)oxyethyl (R)-malolactonate (II-(R)-2) also formed chiral smectic C phases.     

Synthesis and characterization of two isomeric liquid crystal series with reactive double bonds

Two series of potentially cross-linkable liquid crystal materials derived from p-phenylenebisacrylic (series I, n(P.FB.P), n = 2-10) and p-hydroxycinnamic acid (series II, n(HC.T.HC), n = 2-10) have been synthesized and their thermal and mesomorphic properties studied. All these compounds show enantiotropic mesomorphism over a wide range of temperatures. Compounds with short terminal chains are nematic and when the terminal chain length is increased they show smectic polymorphism-smectic A and C. Most of the compounds are thermally stable over their mesophase ranges.     

Unsymmetrical dimers possessing a cholesteryl ester moiety and a difluoro-substituted biphenyl core: synthesis and mesomorphic behaviour

Some new unsymmetrical dimers consisting of a cholesteryl ester moiety, covalently linked to either a 4'-(2,3-difluoro-4-n-octyloxy) biphenyloxy or a 4'-(2,3-difluoro-4-n-decyloxy) biphenyloxy through odd-even parity paraffinic central spacers, have been synthesized and investigated for their mesomorphic behaviour. Except for one, all the dimers exhibit enantiotropic smectic A, twist grain boundary (TGB) and chiral nematic mesophases. Five of the eight unsymmetrical dimers synthesized show a chiral smectic C (SmC*) phase. Interestingly in some of the compounds the SmC* exists over a wide temperature range including room temperature. Among the eight compounds, a dimer having a C₇ central paraffinic spacer and a C₈ alkoxy terminal chain shows an enantiotropic twist grain boundary with SmC* blocks (TGB_C*) phase. It appears that the variation in the length of the spacer has a remarkable influence on the phase transition temperatures as well as on the mesomorphic behaviour.     

Cholesteric helix inversion: Novel nitro compounds showing unusual changes of the cholesteric helical pitch

We present a homologeous series of liquid crystalline trioxadecalin compounds having a terminal alkoxy chain and a nitro group. The (1S, 3R, 6R,8R)-3-(4'-nitrophenyl)-8-(4'-octoxyphenyl)-2,4, 7-trioxabicyclo [4.4.0]decane 2 c shows a temperature-dependent inversion of the cholesteric helix at lower temperatures. For higher temperatures, the reciprocal helical pitch reaches a minimum, then it increases, tending to a second inversion point just above the clearing point. An additional chiral centre in the side chain leads for nitro compounds 3 to non-mesomorphic behaviour. For the cyano compounds 4, the change in the cholesteric helix is suppressed, for the R configuration, but for the S configuration helical inversion occurs at high temperatures and selective reflection above the transition to the TGB_A phase.     

Mesogenic cinnamates with a substituted ethyl terminal chain

Two new mesogenic homologous series of liquid crystalline cinnamates with substituted ethyl tails, β-methoxyethyl [4-(4'-n-alkoxycinnamoyloxy)benzoates (I) and β-chloroethyl [4-(4'-n-alkoxycinnamoyloxy)benzoates (II), have been synthesized and characterized by a combination of elemental analysis and standard spectroscopic methods. In series I, lower-chain members exhibit nematic mesophase, middle members exhibit enantiotropic nematic as well as smectic A (SmA) mesophases, whereas higher members exhibit only an enantiotropic SmA mesophase. In series II, methoxy to n-butyloxy derivatives exhibit a monotropic nematic mesophase. The SmA mesophase commences from n-propyloxy derivative as monotropic and persists up to the last member synthesized. The mesomorphic properties of present series were compared with each other and with a structurally related mesogenic homologous series to evaluate the effects of substituted ethyl tail and cinnamoyloxy central linkage on mesomorphism.     

Novel amides and Schiff's bases derived from 1,3,4-oxadiazole derivatives: synthesis and mesomorphic behaviour

The synthesis and liquid crystalline properties of novel achiral amides (Ia–g, IIa–g and IVa,b), achiral Schiff's bases (IIIa–g and Va–g), chiral amides (VI, VII) and chiral Schiff's bases (VIII–XI) incorporating a 1,3,4-oxadiazole ring are reported. All amides of the series I and II display an enantiotropic smectic A phase. The amide IVa,b did not show mesomorphic properties. Amides of the series Ia–g and IIa–g contain a flexible n-tetradecylthio chain, the other terminal substituent is an n-alkoxy chain and n-alkyl chain, respectively (n?=?4–10) and the 1,3,4-oxadiazole is in the terminal rigid core. Amides Ia–g have broader mesomorphic range and higher thermal stability than the corresponding amides IIa–g. Amides IVa,d contain the 1,3,4-oxadiazole ring in the centre of the rigid core and two flexible alkoxy chains as flexible terminal substituents. Thus, the mesomorphic properties are favoured if 1,3,4-oxadiazole is shifted to a terminal position of the rigid core. Schiff's bases IIIa–g display an enantiotropic dimorphism smectic C–smectic A. Schiff's bases IIIa–g have a broader mesomorphic range than the analogous amides Ia–g. Schiff's bases Va–g exhibit a dimorphism smectic A–nematic, and in contrast to this the analogous amide IVa,b did not show mesomorphism. The chiral amides VI and VII and chiral Schiff's bases X and XI did not show mesomorphic properties and only the chiral Schiff's bases VIII and IX display a chiral smectic C phase in a short mesomorphic range. A density functional theory theoretical study at the B3LYP/6–311++G(d,p) level was performed in order to analyse the structural features that must be related with the mesomorphic behaviour of the reported compounds.     

Synthesis and transition temperatures of novel fluorinated chiral liquid crystals containing 1,4-tetrafluorophenylene units

To continue the search for novel series of fluorinated ferroelectric liquid crystals, an additional two series of 4-[(S)-2-methylbutoxy]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] benzoates (C) and 4-(n-alkoxy)phenyl 4-[(4-(S)-2'-methylbutoxy-2,3,5,6-tetrafluorophenyl)] benzoates (D) have been synthesized. Polarizing microscopic textural observations and DSC measurements of the phase transitions of these novel compounds showed that compounds C were liquid crystals with a chiral nematic (N) phase and a monotropic chiral smectic C phase (S_c), and compounds D exhibited a chiral nematic (N) phase.     

Dimesogenic liquid crystal consisting of cholesterol and Schiff base moieties: dependence of LC properties on the spacer length and fluorination of the alkoxy tails

The liquid crystalline properties of two series of non-symmetric liquid crystal dimers consisting of cholesterol and Schiff base moieties interconnected by ω-oxyalkanoyl spacers of varying length are compared: one series (SBOC- n ) carry the octyloxy tail on the Schiff base mesogen, and the other (SBOF- n ) a perfluoroheptylmethyloxy tail. In general, compounds with the fluorinated alkoxy tail exhibited mesophases over a much wider temperature range than those with the alkoxy tail. The latter series favoured the formation of more diverse mesophases than the former. SBOC-4, -5 and -7, and SBOF-4, -5 and -10 formed the chiral smectic C 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.     

Structural characterization of the smectic polymorphism of chiral liquid crystalline polysiloxanes containing biphenyl mesogens

A structural characterization of chiral side chain siloxanes with different average degrees of polymerization, DP_n, was performed by X-ray diffraction experiments on powder and oriented fibre specimens. Polymers (DP_n = 35) and oligomers (DP_n = 4) contained the 4,4'-biphenylene unit with either an (S)-2-methylbutoxy (An, Bn) or an (S)-2-chloro-3-methylbutanoyloxy substituent (Cll). The spacer segment connected to the siloxane backbone had a variable number, n, of methylene groups (n = 5, 8, or 11). Independent of the spacer length and the chiral tail nature, the polysiloxanes underwent the same sequence of phases: C-S_F1 (or S_I1)-S_C1-S_A1-I, whereas in the oligosiloxanes the sequence C-S_B1-S_A1-I (B11) or C-S_F1-S_C1-I (B5) occurred. The influence of the structure of the polysiloxanes on the formation of the smectic (tilted or orthogonal) mesophases was elucidated. The rather large number of reflections (three or four) detected in the X-ray patterns at low angles, allowed a drawing of the projection of the electron density profiles along the layer normal, p(z), and deduction of the most physically acceptable electron density profiles from among the numerous possibilities for each smectic phase. The electron density profiles were in agreement with monolayer smectic phases presenting a microphase separation between the siloxane backbones and the side chains, so constraining the polymer backbones within a thin layer.     

Ferroelectric liquid crystals induced by atropisomeric biphenyl dopants: the effect of chiral perturbations on achiral dopants

The addition of the achiral biphenyl dopant 2,2',6,6'-tetramethyl-4,4'-bis(4-n-nonyloxybenzoyloxy)biphenyl (3) or its dithionoester or dithioester analogue (4, 5) to a 4 mol % mixture of the atropisomeric biphenyl dopant (R)-2,2',6,6'-tetramethyl-3,3'-dinitro-4,4'-bis(4-n-nonyloxybenzoyloxy)biphenyl, (R)-1, in the phenylpyrimidine SmC host PhP1 produces a significant amplification of the spontaneous polarization induced by (R)-1. This amplification may be due to a chiral perturbation by (R)-1 which causes a shift in the equilibrium between enantiomeric conformations of the achiral dopant. The degree of polarization amplification afforded by the achiral dopant, as expressed by the polarization amplification factor PAF, varies with the nature of the linking group. This may be ascribed to different rotational distributions of the core transverse dipole moments relative to the polar axis of the SmC* phase and/or to differences in lateral bulk of the polar linking groups. The latter may affect the degree of chiral molecular recognition achieved by 3-5 in the binding site of the SmC* phase.     

Synthesis of di-and tetrasaccharide containing 6-deoxytalose from the O-antigenic polysaccharide of B. pseudomallei strain 304b

Synthesis of the disaccharide β-D-Glup-(1→3)-6-deoxy-α-L-Talp Ⅱ,and its dimer Ⅲ from the O-antigenic polysaccharide of Burkholderia pseudomallei strain 304b,were achieved through assembly of suitably synthesized building blocks,4-methoxylphenyl 6-deoxy-2,4-di-O-benzoyl-α-L-talopyranoside (7),3-O-allyl-2,4,6-tri-O-benzoyl-α,β-D-glucopyranosyl trichloroacetimidate (8) and 2,3,4,6-tetra-O-benzoyi-α,β-D-glucopyranosyl trichloroacetimidate (Ⅱ).The total yield of Ⅲ from 4-methoxyiphenyl 2,3-O-isopropylidene-α-L-rhamnopyranoside (1) was 18%.     

V-shaped switching in binary mixtures of an achiral swallow-tailed material with the antiferroelectric liquid crystal (S)-MHPOBC

An achiral swallow-tailed material, 2-propylpentyl 4-(4'-decyloxybiphenyl-4-carbonyloxy)benzoate, p, showing SmA and SmC_alt phases was prepared for mixing (by weight percentage) with an antiferroelectric liquid crystal, (S)-MHPOBC, m, for the study. The binary mixture p15/m85 using (S)-MHPOBC (85%) as a host doped with achiral material (15%) resulted in a phase sequence SmA-SmC*-SmC*_A. The electro-optic response of this mixture in the ferroelectric SmC* phase displayed V-shaped switching, while that in the antiferroelectric SmC*_A phase displayed a double hysteresis switching. The mixture p85/m15 possessed SmA* and SmC*_A phases; V-shaped switching was found in the antiferroelectric SmC*_A phase of this mixture. These optical phenomena implied that a binary mixture containing a larger amount of achiral swallow-tailed material and/or possessing relatively lower polarization favours the occurrence of V-shaped switching in the antiferroelectric phase. The results of this work also suggested that thresholdless V-shaped switching in chiral smectic liquid crystals can be achieved by mixing an achiral swallow-tailed material with an antiferroelectric liquid crystal.     

Ferroelectric liquid crystal materials synthesized from 2(S)-[2(S)-methylbutyloxy]propanol

A new chiral alcohol, 2(S)-[2(S)-methylbutyloxy]propanol (3), containing two chiral centres has been synthesized from ethyl lactate and (S)-1-iodo-2-methylbutane. It was used as a chiral building block for the preparation of ferroelectric liquid crystal materials. Several of the new materials exhibit an enantiotropic S*_c phase with a wide temperature range. The results indicate that the molecular structure of 3 is useful for synthesizing ferroelectric liquid crystal materials.     

The influence of terminal chlorine on the mesomorphic properties of new thiobenzoates containing two and three benzene rings

New liquid crystalline chloro-substituted thioesters containing two and three benzene rings have been synthesized. 4-Chlorophenyl 4-n-alkoxythiobenzoates and 4-chlorophenyl 4-n-alkoxybenzoyloxy-4'-thiobenzoates are referred to as nO.SCl and nO.OSCl, respectively, where n varies from 4 to 16 for nO.SCl, from 4 to 10 for nO.OSCl and denotes the number of carbon atoms in the alkyl chain. Their mesomorphic properties were investigated by means of polarizing optical microscopy, differential scanning calorimetry, transmittance light intensity and X-ray diffraction measurements. The nO.SCl homologous series possesses smectic A (SmA) and nematic (N) phases for n = 4, 5, 6 while higher homologues have only an enantiotropic SmA phase. Those from the nO.OSCl homologous series have enantiotropic N and SmA phases and higher transition temperatures. The range of the N phase decreases, and of the SmA increases, with the elongation of the alkoxy chain in the nO.OSCl homologous series. The effect on mesomorphic behaviour of terminal alkoxy chain lengthening and replacement by chlorine on the other side of the molecule 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.     

Structure-property investigation of 2- and 3-thienylacrylates bearing laterally fluorinated azobenzene moieties

The synthesis, transition temperatures and structure-property relationships of a variety of thiophene-containing azobenzene esters derived from either 3-(2-thienyl)acrylic acid (series I, IV, VI, VIII, X and XII) or 3-(3-thienyl)acrylic acid (series II, V, VII, IX, XI and XIII) and appropriate fluoro- and non-fluoro-substituted 'azophenols' are reported. For comparative purposes, the non-heterocyclic counterparts, i.e. cinnamates (series III), were also prepared and are reported. All 70 final esters are mesomorphic, exhibiting the nematic phase alone. Their mesomorphic properties are dependent on the disposition of the terminal thiophene moiety. In general, 3-thienyl-substitution gives thermally more stable compounds than 2-thienyl-substitution. The influence of mono- (series IV, V, VI and VII) and di-lateral (series VIII, IX, X, XI, XII and XIII) fluoro-substitution on mesomorphic properties is investigated in detail. Lateral fluorination lowers mesophase thermal stability and its extent is dependent on the number and disposition of the lateral fluoro-substituents. Di-lateral fluorination across the long molecular axis is more detrimental to mesophase thermal stability than along the long molecular axis.     

Stereochemical studies of optically active 2-hydroxy-5-alkyl-δ-valerolactone derivatives as chiral dopant for ferroelectric liquid crystals

Both (2S, 5R)- and (2R, 5R)-2-hydroxy-5-alkyl-δ-valerolactone derivatives, cis and trans, respectively, show almost the same magnitude of spontaneous polarization (P_s) when added to a non-chiral smectic C mixture. The stereochemistry of these chiral dopants was studied using ¹H NMR. Trans derivatives seem to have a half-chair conformation with the 2, 5-diequatorial substituents and the cis derivatives have rather a flat conformation in solution. However in the liquid-crystalline phase, the cis and trans derivatives appear to change their conformation or the distribution of their conformations as the alkyl chain length is varied. The difference in the effect as a chiral dopant depends upon lateral interactions between chiral molecules through the solvent liquid crystal phase.     

Novel fluorinated liquid crystals. II. The synthesis and phase transitions of a novel type of ferroelectric liquid crystals containing 1,4-tetrafluorophenylene moiety

4-[(S)-2-Methylbutoxycarbonyl]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl]benzoates have been prepared from the starting material 1-pentafluorophenyl-2-trimethylsilylacetylene. Polarizing microscope textural observation and DSC measurements of the phase transitions of these novel compounds showed that they were liquid crystals with chiral smectic C phase (S*_C), smectic A(S_A) and cholesteric (Ch) phases. The effects of the alkoxy chain length on the transition temperatures and enthalpies were also studied.