Design,synthesis and mesomorphic properties of chiral benzoates and fluorobenzoates with direct SmCA*-Iso phase transition

Novel chiral (S)-(+)-4?-(1-methylalkoxycarbonyl)biphenyl-4-yl 4-[ω-(2,2,3,3,4,4,4-hepta?uoro-butoxy)alkoxy]benzoates and 4-[ω-(2,2,3,3,4,4,4-hepta?uorobutoxy)alkoxy]-2-fluorobenzoates were prepared using different optically active alcohols: (S)-(+)-2-hexanol, (S)-(+)-2-heptanol, (S)-(+)-2-nonanol, (S)-(+)-2-decanol, (S)-(+)-2-undecanol and (S)-(+)-5-methylhexan-2-ol. Properties, such as the sequence of phases, transition temperatures and enthalpies, were tested by polarising optical microscopy and differential scanning calorimetry. Extremely low melting compounds were found (below 10°C) for derivatives of (S)-(+)-2-hexanol. The antiferroelectric smectic phase (SmC_A*) with a direct transition from the antiferroelectric to isotropic phase (SmC_A*-Iso) was observed, usually for propoxy-(CH₂)₃- spacing group. Initially, multicomponent mixtures with broad temperature ranges of antiferroelectric phase and direct SmC_A*-Iso transition were formulated.     

Influence of the bias field on dielectric properties of the SmCA* in the vicinity of the SmC*–SmCA* phase transition

In a large class of smectic mixtures prepared at our University, the phase transition between chiral ferroelectric smectic C (SmC*) and chiral antiferroelectric smectic C (SmC_A*) phases can be observed on cooling. Under bias field the temperature of the phase transition SmC*?SmC_A* decreases (ca. 100°C in the investigated mixture). The transition is called: unwound SmC*?twisted SmC_A* phase transition. The Goldstone mode in SmC* phase is reduced by a direct current field while two modes (P_H and P_L) in the SmC_A* phase are amplified. The amplitude of the fast X mode observed in the SmC_A* phase is reduced. The aim of this paper is to show how parameters of the modes in SmC_A* phase (calculated from Cole–Cole model) change with bias voltage—when twisted structure in SmC_A* phase is gradually unwound. The character of the modes observed in SmC_A* is discussed. A new effect is shown: a high value of dielectric loss is detected in the unwound SmC* phase, which is very close to SmC_A*.     

Synthesis and properties of antiferroelectric and/or ferroelectric compounds with the –CH2O group close to chirality centre

ABSTRACT

Novel chiral three-ring compounds with the –CH₂O group close to chirality centre were synthesised and their properties were studied. The phase transitions and phase sequences were observed using a polarising optical microscope. The phase transition temperatures and enthalpies were checked by differential scanning calorimetry. A broad-frequency dielectric spectroscopy was also used to confirm the phase transition temperatures as well as the phase sequence. The helical pitch was measured by the spectrophotometry method. It was found that the compounds differing only in one lateral substituted fluorine atom create two different chiral tilted smectic phases, one a ferroelectric phase (SmC*) and the other an antiferroelectric phase (SmC_A*).     

Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric–ferrielectric transition

We report the dielectric relaxation behaviour in the antiferroelectric SmC_A* and ferrielectric SmC_γ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmC_A* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmC_γ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.     

Novel chiral swallow-tailed amide liquid crystals possessing antiferroelectricity

Novel chiral swallow-tailed amide materials, N,N-dipropyl-(S)-2-{6-[4-(4-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionamides, DPmPBNPA (m=9-13), have been designed and synthesized for the investigation of mesomorphic properties. The materials DPmPBNPA (m=9-11) display a monotropic phase sequence of I-SmA*-SmC_A*-Cr. The antiferroelectric SmC_A* phase for the materials was characterized by microscopic texture, switching behaviour, dielectric permitivity and electro-optical response. The measured maximum P _s values in the SmC_A* phase of the materials are in the range 80–87 nC cm^-2.     

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.     

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmC_A*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmC_A* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmC_A* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm^?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmC_A*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol^?1 in the SmC* phase.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.     

Synthesis and properties of ferro- and antiferroelectric esters with a chiral centre based on (S)-(+)-3-octanol

A homologous series of chiral three ring esters were synthesised and their properties were studied. The materials possess an antiferroelectric phase (SmC_A*) or/and a ferroelectric phase (SmC*), which was confirmed by observations of microscopic textures using a polarising optical microscope. The phase transition temperatures and enthalpies were checked by differential scanning calorimetry. The helical pitch was measured by a spectrophotometry method. For all cases the helix is right-handed, in the antiferroelectric phase the helical pitch increases with temperature, and in the ferroelectric phase the helical pitch is short.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.     

Electric field‐induced birefringence,optical rotatory power and conoscopic measurements of a chiral antiferroelectric smectic liquid crystal

Using a photoelastic modulator‐based novel set‐up, the electric field‐induced in‐plane birefringence and the optical rotatory power (ORP) were measured of an antiferroelectric liquid crystalline compound (12OF1M7) in its various phases using 30 µm homeotropic cells. Some specific signatures of the in‐plane birefringence and of the ORP for the various phases are being established. A relatively small threshold field is needed for the unwinding process of the antiferroelectric phase with a unit cell of four layers [SmC_A*(1/2)] compared with that for two layers [SmC_A*(0)]. On application of the electric field on the high temperature side of the SmC_A*(1/2) phase (80.1–81.5°C), a field‐induced phase transition is shown to occur directly to the SmC* phase, whereas on the lower temperature side (79.4–80.1°C) the transition takes place to SmC* via the SmC_A*(1/3) phase. The in‐plane birefringence exhibits a critical power law dependence for the SmC*–SmA transition. The ORP changes sign within the temperature range of the phase with a unit cell of three layers, reflecting a change in the handedness during this phase. Using tilted conoscopy, the results for the biaxiality and the apparent tilt angle for a smectic liquid crystal with a tilt angle greater than 18° in the ferroelectric phase are reported. The biaxiality implies the difference in the refractive indices between the two minor axes of the refractive index ellipsoid. The optical transmittance at visible and IR wavelengths for free‐standing films reveal characteristic reflection bands for these phases. The modulated structures of the reflected bands appear just above the SmC_A* phase and below SmC_A*(1/3); these are possibly due to an easy deformation of the phase by the surfaces.     

Novel liquid crystals based on [1]benzothieno[3,2- b][1]benzothiophene

Four series of new [1]benzothieno[3,2- b][1]benzothiophene derivatives have been synthesized. In the non-chiral series a SmA phase occurs, while the chiral series exhibits a rather wide antiferroelectric SmC^A* phase just below the SmA phase. The SmA-SmC^A* phase transition has been studied using DSC and dielectric spectroscopy. In the SmC^A* phase the spontaneous quantities have been measured. The tilt angle shows a typical temperature dependence and the values of spontaneous polarization are rather moderate. The length of the helical pitch increases on increasing the length of the non-chiral alkyl chain.     

Chirality driven mesomorphic behaviour difference: dichiral compounds containing two lactate groups

ABSTRACT

Four compounds containing two lactate groups and one perfluorocarbon chain are designed and synthesised, whose chirality is tuned by changing the chirality of the lactic acid residues. (R,S)- and (S,R)-2 stereoiosomers exhibit an enantiotropic SmA phase, while (R,R)- and (S,S)-2 stereoisomers exhibit an enantiotropic SmA phase and an enantiotropic SmC_d* one. Therefore, the chirality of the compounds plays an important role in the mesomorphic behaviours of the compounds. The optical activity of these liquid crystals is dominated by the chirality of the lactate group near the core. (R)- and (S)-1 with one lactic acid residue and one perfluorocarbon chain exhibit only an enantiotropic SmA phase.     

The substituent effect on mesomorphic properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy)benzoates and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates

Abstract

The thermal properties of 4-octyloxyphenyl 4-(4-R-3-nitrobenzoyloxy) benzo-ates (1) and 4-(4-octyloxybenzoyloxy)phenyl 4-R-3-nitrobenzoates (2) have been examined, where R = hydrogen, halogens, alkyl and alkoxy groups. The derivatives of compound 1 incorporating hydrogen, halogens, methoxy and nitro groups show a smectic A phase having a bilayer arrangement, and the others with a long alkoxy group show the S_A phase with the monolayer arrangement. The derivatives of compound 2 incorporating halogens, and the nitro group show the S_A phase with the monolayer arrangement. The alkoxy derivatives show a smectic C phase as well as the nematic phase. The nitro group at the lateral position tends to increase the ratio of the S_A-N transition temperature to the N-I. The effect of the nitro group on the smectic properties has been discussed in terms of the structural and electrostatic nature of the nitro group.     

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.     

Dielectric investigation of the liquid crystal compound with the directSmA*–SmCA* phase transition

New compound showing a direct SmA*–SmC_A* phase transition was synthesised. As far as authors know there are a few pure compounds showing para- and antiferroelectric phases without SmC* between them. Direct current (DC) field applied into a planar-oriented cell induces ferroelectric SmC* phase in an investigated compound. Typical for SmC*, Goldstone mode starts to be detectable. DC field also shifts down the temperature of a SmC_A* phase creation. Moreover, modes in the appearing antiferroelectic phase are enhanced by DC field. This paper shows and discusses relations between modes detected in SmA*, SmC_A* and SmC* (SmC* phase – nucleated by DC field) phases. Parameters of observed modes are calculated using the Cole–Cole relaxation model and a calculation procedure useful especially for high frequency relaxations (higher than 200 kHz).     

Side chain liquid crystalline composites,occurrence of interdigitated bilayer smectic C phases

We report on the results of X-ray investigations in two series of polymer monomer composites, PM6Rm-33 and PMnR12-33, which consist of mixtures of achiral liquid crystalline side chain polymers and their monomers. These mixtures present a unique integration of monomer in the polymeric base which assists in modifying their properties and forming homogenous composites. X-ray measurements for all the investigated composites indicate the existence of bilayered smectic C phases (SmC₂). In several composites, the interlayer distance of the SmC₂ phase abnormally increases with cooling; this is associated with the aliphatic interdigitation at the tail-to-tail interface being more prominent when longer aliphatic tails are present.     

Metallkomplexe mit biologisch wichtigen Liganden. CLVII [1] Halbsandwich‐Komplexe mit Isocyanoacetylaminosäureestern und Isocyanoacetyldi‐ und tripeptidestern („Isocyano‐Peptide”︁)

Metal Complexes of Biologically Important Ligands, CLVII [1] Halfsandwich Complexes of Isocyanoacetylamino acid esters and of Isocyanoacetyldi‐ and tripeptide esters (?Isocyanopeptides”?) N‐Isocyanoacetyl‐amino acid esters CNCH₂C(O) NHCH(R)CO₂CH₃ (R = CH₃, CH(CH₃)₂, CH₂CH(CH₃)₂, CH₂C₆H₅) and N‐isocyanoacetyl‐di‐ and tripeptide esters CNCH₂C(O)NHCH(R¹)C(O)NHCH(R²)CO₂C₂H₅ and CNCH₂C(O)NHCH(R¹)C(O)NHCH (R²)C(O)NHCH(R³)CO₂CH₃ (R¹ = R² = R³ = CH₂C₆H₅, R² = H, CH₂C₆H₅) are available by condensation of potassium isocyanoacetate with amino acid esters or peptide esters. These isocyanides form with chloro‐bridged complexes [(arene)M(Cl)(μ‐Cl)]₂ (arene = Cp*, p‐cymene, M = Ir, Rh, Ru) in the presence of Ag[BF₄] or Ag[CF₃SO₃] the cationic halfsandwich complexes [(arene)M(isocyanide)₃]⁺X^? (X = BF₄, CF₃SO₃).     

Switching and electrical properties of ferro‐ and antiferroelectric phases of MOPB(H)PBC

Switching and dielectric relaxation phenomena were investigated for an antiferroelectric liquid crystal, 4,4‐(1‐methyloctyloxycarbonyl)phenyl]‐4′‐[3‐(butanoyloxy)prop‐1‐oxy]biphenyl carboxylate, exhibiting chiral smectic A (SmA*), smectic C (SmC*) and antiferroelectric (SmC_A*) phases. Spontaneous polarisations, rotational viscosities, relaxation frequencies, dielectric strengths and distribution parameters were determined as a function of temperature. The electric field required for saturation of the spontaneous polarisation increased with a decrease in temperature. In the SmA* phase, only one relaxation mechanism was observed that behaves as soft mode. Two relaxation processes were detected in the SmC* phase. A high‐frequency relaxation process invariant at 2.2 kHz was due to a Goldstone mode, but the origin of low‐frequency relaxation process (1–20 Hz) is unclear; however, it may belong to an X‐mode. The dielectric spectrum of the SmC_A* phase exhibits two absorption peaks separated by two decades of frequency. The low‐frequency peak is related to the antiferroelectric Goldstone mode, whereas the high‐frequency peak originates from the anti‐phase fluctuation of the directors in the anti‐tilt pairs of the SmC_A* phase.     

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.