A comparison of ferroelectric liquid crystals containing diastereomeric propionic acids derived from natural ethyl lactate

Two diastereomeric carboxylic acids, 2(S)-[2(R)-methylhexyloxy]propionic acid and 2(S)[2(S)-methylhexyloxy]propionic acid, were prepared from ethyl (S)-lactate and (R)-1-iodo2-methylhexane or (S)-1-iodo-2-methylhexane in the presence of Ag₂O. From these acids two liquid crystals, 2 and 3 , whose configurations are (S , R ) and (S, S) were synthesized and their liquid crystal properties investigated. Although both LCs have the same phase sequence Cr-SmC*-N*-I as well as a wide SmC* phase range, the influence of the relative stereochemistry on their physical properties is clear. The liquid crystal with (S, S)-configuration possesses better properties: lower SmC* phase transition temperature, wider SmC* phase range and higher P_s value. The P_s value difference between the ferroelectric LCs 2 and 3 (97 and 131nCcm^-2, respectively, at T_c - T = 10°C) is unexpectedly large. The consideration, alone, of a zigzag conformation at the chiral molecular part of 2 and 3 is insufficient to explain such a difference.     

Chiral Liquid Crystals Synthesized from 2(S)‐[2(R)‐Methylhexyloxy]Propanol and its (S,S)‐Diastereomer

Two new chiral alcohols, 2(S)‐[2(R)‐methylhexyloxy]propanol ( 5 ) and 2(S)‐[2(S)‐methylhexyloxy]propanol ( 6 ), were prepared from the corresponding propionic acid ethyl ester 1 and 2 in the presence of sodium borohydride. They were used as the chiral moiety for the synthesis of two diastereomeric liquid crystals 7 and 8 . Both of them exhibit the phase sequence I‐SmA‐SmC*‐SmX‐Cr. The mesogenic properties of the (S,S)‐diastereomer 8 are more unique in comparison with those of the (S,R)‐diastereomer 7. It possesses not only lower SmA and SmC* phase transition temperature, 103 °C vs. 112 °C for SmA phase and 31 °C vs. 65 °C for SmC* phase, but wider SmA and SmC* phase range, 40 °C vs. 31 °C for SmA phase and 72 °C vs. 47 °C for SmC* phase. The diastereomer 8 also has a larger Ps value than that of 7, 24 vs. 15 nC cm^?2 measured at Tc ‐ T = 10 °C. The difference in these mesogenic properties is discussed by comparing their conformation difference at the molecular part of benzoate.     

Effect of the naphthalene moiety on the mesogenic properties of ferroelectric liquid crystals containing diastereomeric propionic acids

Two ferroelectric liquid crystals containing the naphthalene moiety were synthesized from 2(S)-[2(R)-methylhexyloxy]propionic acid and its (S, S)-diastereomer. Their mesogenic properties are compared with those of the analogous mesogens containing the biphenyl ring.     

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 p/15/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 crystals derived from (S)‐2‐(6‐methoxy‐2‐naphthyl)propionic acid with non‐fluorinated or semi‐fluorinated alkanes at a chiral terminal chain

Two series of ferroelectric liquid crystals derived from (S)‐2‐(6‐methoxy‐2‐naphthyl)propionic acid, with non‐fluorinated or semi‐perfluorinated alkanes positioned at a chiral terminal chain, have been synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and electro‐optical measurements. The non‐fluorinated compounds, 1‐hexyl (S)‐2‐{6‐[4‐(4‐alkanoyloxyphenyl)benzoyloxy]‐2‐napthyl}propionates exhibit rich mesomorphism—the BP_II, N*, TGB_A*, SmA* and SmC* phases. The fluorinated compounds display only the SmA* and SmC* phases, suggesting that the fluorination promotes the formation of smectic phases. In addition, the SmA* and SmC* phases of the fluorinated compounds have enhanced thermal stability as compared with the corresponding phases of the non‐fluorinated compounds. The spontaneous polarization (P _s values) for the non‐fluorinated compounds are higher than those of the fluorinated compounds at any reduced temperature below the SmA*–SmC* transition. The electro‐optical responses measured for these compounds in the ferroelectric phase displayed thresholdless, V‐shaped switching.     

Effect of the variation of swallow-tailed groups on the mesomorphic and electro-optical properties of chiral compounds derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid

Three homologous series of chiral swallow-tailed compounds, alkyl (S)-2-{6-[4-(4′-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionates, (S)HNP(p,n,q) derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid in conjugation with a variety of swallow-tailed groups, attached to the external side of the chiral centre, have been synthesized and their mesomorphic and electro-optical properties studied. Both (S)HNP(p,1,2) and (S)HNP(p,1,3) exhibited an enantiotropic antiferroelectric SmC*_A phase. This implys that the swallow-tailed groups in the molecules favour zigzag pairing of the molecules in the smectic phase. The maximum P _S values of compounds (S)HNP(p,1,2) in the antiferroelectric phase were measured in the range 21–30 nC cm^-2; those of compounds (S)HNP(p,1,3) were in the range 15–23 nC cm^-2, indicating that these chiral compounds possess low polarity. The electro-optical response of the compounds in the antiferroelectric SmC*_A phase displayed thresholdless V-shaped switching.     

From mushroom alcohol to liquid crystals: a useful platform molecule

The syntheses and liquid crystal properties of two novel esters derived from 4-(4-(decyloxy)phenyl)thiophene-2-carboxylic acid and either (±)-oct-1-en-3-yl 4?-hydroxybiphenyl-4-carboxylate or (S)-(+)-oct-1-en-3-yl 4?-hydroxybiphenyl-4-carboxylate are reported. Within the synthesis of the (S)-(+)-oct-1-en-3-yl 4?-hydroxybiphenyl-4-carboxylate, mushroom alcohol, a natural source of chiral oct-1-en-3-ol and a platform molecule, was employed. The phases present within these compounds have been characterised by thermal optical polarising microscopy and differential scanning calorimetry and assigned as SmA, SmC and SmC Alt for the racemic compound and; SmA, SmC* and SmC*_A for the enantiomerically pure compound. This is first reported occurrence of a liquid crystalline ester derived from mushroom alcohol, and potential platform molecule, exhibiting SmA*, SmC* and SmC*_A phases.     

A convenient synthesis and the mesomorphic properties of new chiral ferroelectric thiobenzoates

A secondary chiral (R)-(?)-2-alcohol underwent the Mitsunobu reaction with triphenylphosphine, diethyl azodicarboxylate and ethyl 4-hydroxybiphenylcarboxylate, resulting in the desired (S)-(+)-product with high enantiomeric purity (>99% ee), with the chiral branched chain attached to the biphenyl. This method is operationally simple and provides the very important chiral precursor in good yields (62% in dry THF and 72% in dry Et₂O). The condensation of the (S)-(+)-acid chloride from this material and a suitable 4-n-alkylthiophenol in toluene in the presence of pyridine or triethylamine furnishes the chiral (S)-(+)-thiobenzoate liquid crystals in good yields (80–83% in pyridine and 65–68% in Et₃N). (S)-(+)-4-(1-Methylheptyloxy)biphenyl 4-alkylthiobenzoates are abbreviated (S)-MHOBSn , where n varies from 4 to 10 and denotes the number of carbon atoms in the alkyl chain. DSC, polarizing microscopy and X-ray diffraction showed that the (S)-MHOBSn series possesses a rich phase polymorphism: two highly ordered tilted phases CrG* and SmI*, as well as the ferroelectric smectic C (SmC*) and chiral nematic (N*) phase. In this series, the seldom observed transition between the chiral phases SmI*–SmC* is seen. All the compounds possess stable enantiotropic SmC* and N* phases. The existence of weak intermolecular hydrogen-bonding in (S)-MHOBSn was confirmed by FTIR spectroscopy.     

Ferroelectric liquid crystals with axially chiral 2,2′-spirobiindan-1,1′-dione cores

The axially chiral mesogens 5-alkoxy-5′-[(4-alkoxybenzoyl)oxy]-2,2′-spirobiindan-1,1′-dione (QL7-n) were synthesised as racemic mixtures and resolved as single enantiomers by preparative chiral phase HPLC. The shorter homologues (R)-QL7-8 and QL7-9 form enantiotropic N* and SmC* phases, whereas the longer homologues (R)-QL7-10 and QL7-12 form an enantiotropic SmC* phase only. Texture analysis showed that the helical pitch of the N* phase is below the wavelength range of visible light; a helical pitch of 187 nm was measured at T – T_NI = –5 K by selective reflection using enantiomerically enriched mixtures of (R)- and (S)-QL7-8 (0.35 ≤ ee ≤ 0.80). Spontaneous polarisations were measured as a function of temperature in the SmC* phase by the triangular wave method. P_S values at saturation range from 102 nC cm^–2 for (R)-QL7-8 to 120 nC cm^?2 for (R)-QL7-9, which are up to three times greater than the highest polarisation previously reported for an axially chiral SmC* mesogen. Optical tilt angles θ were measured as a function of temperature and showed a sharp rise consistent with first-order N*–SmC* or I–SmC* transitions, with values at saturation ranging from 42 to 44°.     

Dielectric and optical rotatory power investigations of an antiferroelectric liquid crystal 12OF1M7 in a homeotropic cell: implications for models of the structure of ferrielectric phases

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smectic phases. Measurements of the optical rotatory power (ORP) of these phases have elucidated the quality of this method for finding the phase transitions between several phases and for investigating their helical structure. The optical rotatory power as a function of temperature for seven wavelengths of light in the range 441 to 665 nm was measured for different phases of the AFLC material investigated, (R)-1-methylheptyl 4-(4'-n-dodecyloxybiphenyl-4-yl-carbonyloxy)-3-fluorobenzoate, with acronym 12OF1M7. The values of the pitch and the optical anisotropy in the plane of the smectic layers for the FiLC (SmC*_FI3) phase and SmC*_Aphase have been calculated from the ORP data. The results of the ORP rule out the simple clock model for describing the structure of the SmC* (SmC*_I1) and AF (SmC*_FI2) phases. The results for these phases can be explained either in terms of the modified Ising model or the highly distorted clock model. The ORP measurements establish the existence of SmC*_FI3 found already from dielectric, polarization and polarized IR spectroscopy.     

Chiral 2,2-disubstituted oxirane ethers and carboxylic esters. New ferroelectric liquid crystals and dopants for induced ferroelectric phases

Abstract

The synthesis, phase behaviour and spontaneous polarization of a new class of chiral LCs and chiral dopants for induced ferroelectric phases of general structure A, possessing a (2S)-2-hydroxymethyloxirane unit, and B, possessing a (2R)-2-oxirane carboxylic acid unit connected to mesogenic building blocks are described. One of these new compounds exhibits a S*_c phase. A carbonyl group adjacent to the oxirane ring does not increase the spontaneous polarization which is in contrast to the results obtained for 2,3-disubstituted oxiranes. A comparison with analogous species containing a (2S)-2-hydroxymethyloxetane, a (2R)-2-hydroxymethylthiirane or a (2S,3R)-2-hydroxymethyloxirane unit is given.     

Ferroelectric Liquid Crystals: Synthesis and Thermal Behavior of Optically Active,Three‐Ring Schiff Bases and Salicylaldimines

The chiral ferroelectric smectic C (SmC*) phase, characterized by a helical superstructure, has been well exploited in developing high‐resolution microdisplays that have been effectively employed in the fabrication of a wide varieties of portable devices. Although, an overwhelming number of optically active (chiral) liquid crystals (LCs) exhibiting a SmC* phase have been designed and synthesized, the search for new systems continues so as to realize mesogens capable of meeting technical necessities and specifications for their end‐use. In continuation of our research work in this direction, herein we report the design, synthesis, and thermal behavior of twenty new optically active, three‐ring calamitic LCs belonging to four series. The first two series comprise five pairs of enantiomeric Schiff bases whereas the other two series are composed of five pairs of enantiomeric salicylaldimines. In each pair of optical isomers, the configuration of a chiral center in one stereoisomer is opposite to that of the analogous center in the other isomer as they are derived from (3 S)‐3,7‐dimethyloctyloxy and (3 R)‐3,7‐dimethyloctyloxy tails. To probe the structure–property correlations in each series, the length of the n‐alkoxy tail situated at the other end of the mesogens has been varied from n‐octyloxy to n‐dodecyloxy. The measurement of optical activity of these chiral mesogens was carried out by recording their specific rotations. As expected, enantiomers rotate plane polarized light in the opposite direction but by the same magnitude. The thermal behavior of the compounds was established by using a combination of optical polarizing microscopy, differential scanning calorimetry, and powder X‐ray diffraction. These complementary techniques demonstrate the existence of the expected, thermodynamically stable, chiral smectic C (SmC*) LC phase besides blue phase I/II (BPI or BPII) and chiral nematic (N*) phase. However, as noted in our previous analogous study, the vast majority of the Schiff bases show an additional metastable, unfamiliar smectic (SmX) phase just below the SmC* phase. Notably, the SmC* phase persists over the temperature range ≈80–115 °C. Two mesogens chosen each from Schiff bases and salicylaldimines were investigated for their electrical switching behavior. The study reveals the ferroelectric switching characteristics of the SmC* phase featuring the spontaneous polarization (P_S) in the range 69–96 nC cm^?2. The helical twist sense of the SmC* phase as well as the N* phase formed by a pair of enantiomeric Schiff bases and salicylaldimines has been established with the help of circular dichroism (CD) spectroscopic technique. As expected, the SmC* and the N* phase of a pair of enantiomers showed mirror image CD signals. Most importantly, the reversal of helical handedness from left to right and vice versa has been evidenced during the N* to SmC* phase transition, implying that the screw sense of the helical array of the N* phase and the SmC* phase of an enantiomer is opposite.     

V-shaped switching in ferroelectric liquid crystal mixtures induced by an achiral swallow-tailed material

A new ferroelectric liquid crystal, 1-ethylpropyl (S)-2-[2-fluoro-4-(4′-decyloxybiphenylcarbonyloxy) benzoyloxy] propanoate, F, was synthesized and mixed with an achiral swallow-tailed material, 2-propylpentyl 4-(4′-nonyloxybiphenyl-4-carbonyloxy) benzoate,P, for the preparation of binary mixtures for the study. The binary mixtures gave a phase sequence SmA* -SmC* -SmX*. The electro-optic response of the mixtures in the ferroelectric SmC* phase was investigated. V-shaped switching was observed as the amount of the achiral swallow-tailed material became greater than 20 wt %. This result suggests that thresholdless, V-shaped switching in ferroelectric liquid crystal mixtures can be achieved by mixing a ferroelectric liquid crystal with an achiral swallow-tailed compound.     

Photoinduced phase transitions and helix untwisting in the SmC* phase of a novel cinnamoyl-based liquid crystal

A photoinduced phase transition and helix untwisting in a new liquid crystal forming the SmC* phase were studied in detail. The compound consists of a cinnamoyl photosensitive fragment with C?=?C double bond capable of photoisomerisation and photocycloaddition. It was shown that ultraviolet (UV) irradiation (365 nm) induces an extreme decrease in phase transitions temperatures (SmC*–SmA*, SmA*–N*, N*–I). Vertically aligned samples in the SmC* phase cause selective light reflection in the visible spectral range. The light action results in a noticeable helix untwisting that causes a shift in the selective light reflection peak to the long-wavelength spectral region. The temperature dependence of spontaneous polarisation P _s was measured and it was found that UV irradiation induces a decrease in the values of P _s. Photo-optical phenomena taking place in the liquid crystal are attributed to the formation of photoproducts having low anisometry, which disrupts mesophases.     

From the SmA to the Hexatic,Including the SmC*, SmC*A and SmC*re Phases: A 2H NMR Relaxation Study

The molecular dynamics of a ferroelectric liquid crystal, denoted ZLL 7/* , is investigated by means of ²H NMR relaxation. The spin–lattice (T_1Q and T_1Z) and spin–spin (T₂) relaxation times of two isotopomers of ZLL 7/* , labeled on the phenyl and biphenyl fragments, are measured and their behavior upon passing from the SmA to the hexatic phase, through the ferroelectric SmC*, antiferroelectric SmC*_A, and re‐entrant ferroelectric SmC*_re phases, is discussed. A comparison between the measured T₂ and T₂*, directly related to the experimental linewidth, provides information on the heterogeneity of the system, thus allowing confirmation of previous hypotheses concerning the structural and ordering properties of the SmC*_A and SmC*_re phases. The possibility to look at different sites of the core of the ZLL 7/* smectogen reveals a peculiar sensitivity of the phenyl moiety with respect to the biphenyl fragment, which may be justified by its vicinity to the chiral centers. Interestingly, the trend of the longitudinal relaxation times is characterized by a minimum that corresponds to the SmC*_A and SmC*_re phases, which is reproducible for the two isotopomers and at several Larmor frequencies. A quantitative analysis of T_1Q and T_1Z is performed in the SmA and SmC* phases, for which the narrowing regime approximation is valid. A multifrequency approach is applied to self‐consistently determine the diffusion coefficients for the overall molecular motions, namely spinning and tumbling, and the internal rotations around the para axes of the phenyl and biphenyl fragments. The effect of the magnetic field in unwinding the helical structure of the SmC* phase (for H>9 T) allows observation of a sensitive change in the rotational diffusion coefficients in the frustrated unwound SmC* phase with respect to the SmC* phase.     

Synthesis and analytical properties of (S)-(+)-2-methoxy-2-(9-phenanthryl)propionic acid

2-Methoxy-2-(9-phenanthryl)propionic acid was synthesized as a novel chiral resolving agent. The absolute configuration of (+)-2-methoxy-2-(9-phenanthryl)propionic acid was determined to be S by using X-ray structural analysis of the (1R,2S,5R)-menthyl ester. In the crystal, the methoxyl and carbonyl groups of the ester are in a syn-periplanar position. The syn-periplanar conformations of (1R,2S,5R)-menthyl esters were also observed by the NMR analyses in CDCl₃. The utility of (S)-(+)-2-methoxy-2-(9-phenanthryl)propionic acid was exemplified by the resolution of (±)-3-octanol.     

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     

New side chain ferroelectric liquid crystalline polymers based on (2S, 3S)-2-chloro-3-methylpentanoic acid: synthesis and phase behaviour

The synthesis of four new chiral mesogenic monomers (M₁–M₄) and side chain ferroelectric liquid crystalline polymers containing (2S, 3S)-2-chloro-3-methylpentanoate is described. The chemical structures and phase behaviour of the monomers and polymers obtained in this study were characterised by Fourier transform infrared, proton nuclear magnetic resonance, polarising optical microscopy, differential scanning calorimetry, thermogravimetric analysis and X-ray diffraction. The selective reflection of light was investigated with ultraviolet/visible (UV/Vis). Their structure–mesomorphism relationships were discussed. M₁ and P₁ all showed a chiral smectic C (SmC*) phase. M₂ and M₃ revealed a SmC* phase and cholesteric phase, while their corresponding polymers P₂ and P₃ revealed a SmC* phase and smectic A (SmA) phase. M₄ only exhibited a cholesteric phase, whereas the corresponding polymers P₄ showed a SmA phase. Moreover, the selective reflection of light shifted to the long-wavelength region at the SmC* phase range and to the short-wavelength region at the cholesteric range with increasing temperature, respectively. The results seemed to demonstrate that the tendency towards melting temperature (T_m), glass transition temperature (T_g), isotropic temperature (T_i) and mesophase range for the monomers and polymers increased by increasing the mesogenic core rigidity or the number of phenyl ring. The polymerisation effect could lead to higher liquid crystalline to isotropic phase transition temperature, wider mesophase range and more ordered smectic phase formed. In addition, all the obtained polymers had a very good thermal stability and the corresponding T_d increased by increasing the number of phenyl ring.     

Influence of the position and number of fluorine atoms and of the chiral moiety on a newly synthesized series with anticlinic properties

A series of trifluoro-substituted benzoate derivatives: (S)-1-ethylheptyl 4-[4-(4-alkyloxy-3- fluorobenzoyloxy)-3-fluorobenzoyloxy]-2-fluorobenzoates is reported. The short chain members (n = 8 to n = 11) display a direct SmC*_A-SmA transition, whereas for longer chains a SmC* phase appears, but no ferrielectric phases are present, and a direct SmC_A*-SmC* transition is obtained. The mesomorphic properties were studied by optical microscopy and DSC, and by electro-optical, helical pitch and optical rotatory power measurements. The effect of the number and position of the fluoro substituents, and the influence of the chiral moiety on the mesomorphic behaviour are 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.