Synthesis, properties and crystal structure of chiral semiperfluorinated liquid crystals with ferro and anticlinic smectic phases

A new chiral and semiperfluorinated series with ferro and anticlinic properties has been synthesized and characterized. The mesomorphic behaviour has been established on the grounds of both microscopic observations and DSC measurements. The non-chiral intermediate ethyl 4-semiperfluorinated alkyloxybenzoates exhibit SmA phases, unusual for compounds with a single phenyl ring. The final derivatives display SmA, SmC* and in several cases SmC*A phases. The longer fluorinated chains favour the SmA and SmC* phases at the expense of the SmC*A phase. Electro-optical measurements were carried out with the classical SSFLC geometry. The spontaneous polarization and tilt angle at saturation are higher than those of the hydrogenous homologues, around 140 nCcm-2 at 40 degrees C. One compound of the series, the 4,4,5,5,6,6,7,7,8,8,8-nonafluoroheptyloxy derivative, C36H35O7F9, Mx=750.6 g mol-1, crystallizes in the triclinic system, space group P1 with four independent molecules in the asymmetric unit (Z=4). The molecules are arranged in a head-to-tail fashion with two molecules oriented in the same direction and the two others in the opposite direction. They give rise to sheets with a smectic C-like arrangement. The final reliability factors were R=0.117 and wR=0.134; the goodness of fit was S=1.366.     

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral mesogenic series

ABSTRACT

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral mesogenic series (2F3R and 3F3R) have been investigated. While both the compounds exhibit SmC* phase over a wide temperature range, 2F3R forms orthogonal SmA* from tilted SmC* on heating but 3F3R melts directly to the isotropic phase. The SmA* phase of 2F3R is found to have de vries characteristics with small effective layer contraction. Both the samples on cooling form hexagonal SmF* phase below SmC* phase. On further cooling soft crystal like hexagonal SmJ* phase is formed in 2F3R, undergoing a change in the tilt direction, but in 3F3R, SmG* phase is formed without any change in the tilt direction. A coexistence phase of (SmC*+SmF*) is also observed in a certain temperature range. Slight differences in the dipole moment and molecular conformation of the two molecules give rise to a subtle change in the intermolecular interaction and play an important role in the appearance of different phases in the two compounds. Cell parameters of SmF*, SmG*, SmJ* phases have also been determined. Layer spacings, tilt angles, average intermolecular spacings and correlation lengths have been measured. How some of these properties compare with other members of the series has been discussed.     

Synthesis, mesomorphic properties and crystal structure studies of a chiral ferroelectric liquid crystal series

A new series of chiral mesogens: (R) 4-(1-methylheptyloxycarbonyl)phenyl 4-(4-alkyloxyphenyl)ethylylbenzoates has been synthesized. All materials are mesomorphic and the liquid crystalline properties were characterized by optical observation, DSC and electro-optical measurements. They display SmA and ferroelectric SmC* phases at low temperatures. One of them, the nonyloxy derivative, exhibits the SmC*alpha phase, present in different 'antiferroelectric' series. This behaviour is confirmed by mixture studies. Furthermore one compound of the series, the heptyloxy derivative, C37H48O5 crystallizes in the P21 space group (Z=2) with the following parameters: a=8.497 (1), b=5.490 (1), c=36.148 (4)A with beta 92.72 (1) . The final R and wR reliability factors were equal to 0.109 and 0.124, respectively; the goodness of fit was equal to 3.0. The whole molecule is L-shaped with the long chiral chain almost perpendicular to the core moiety as observed for several compounds showing an antiferroelectric mesophase. In the crystal, the molecules related through the 2 1 axis, located at z=0.5, give an antiparallel smectic C-like arrangement. The thickness of the sheets is equal to the c parameter, with a tilt angle close to 35 . There are strong intersheet interactions between the chiral chains related through the 2 1 axes located at z=0 and 1.     

Dielectric and electro-optical response of a room temperature tri-component antiferroelectric mixture

Frequency- and temperature-dependent dielectric and switching parameters of a room temperature tri-component antiferroelectric liquid crystal mixture W-287 have been determined. Dielectric, optical texture and thermodynamic studies show wide room temperature range antiferroelectric SmC*_a (?91.1°C to <–25°C) phase in addition to high temperature paraelectric SmA* (?2.6°C) and ferroelectric SmC* (?4.4°C) phases. The dielectric studies carried out in the frequency range of 1–35 MHz under planar anchoring condition of the molecules show five different relaxation modes appearing in the SmA*, SmC* and SmC*_a phases. Using Curie–Weiss law fit, ferroelectric SmC* to paraelectric SmA* transition temperature has been found to be 91.8°C. The dielectric response of SmC*_a phase exhibits unusually three relaxation modes due to collective as well as individual molecular processes in addition to phason mode in the SmC* phase and amplitudon mode in the SmA* phase. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value of P_S is ?300 nC/cm², whereas viscosity is moderate. Switching time is of the order of few milli seconds.     

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.     

Chiral two ring compounds exhibiting antiferroelectric phases

Novel chiral two ring compounds possessing phenylethenoate moieties were synthesized and their liquid crystalline properties determined. Some of these compounds were found to exhibit SmC* γ, AF and SmC* A phases, whereas in other compounds only SmA* and SmC* phases appear. The influence of the molecular structure on the appearance of SmC* γ, AF and SmC* γ phases is discussed. The basic electro-optical properties of the homologous series of ( S )-( E )4-(1-methylheptyloxycarbonyl)phenyl 4-alkoxycinnamates, in which SmC*, SmC* γ, AF and SmC* A phases appear, were also investigated. High values of spontaneous polarization, low threshold voltages and fast switching times at room temperature were observed.     

Temperature and bias-field dependences of dielectric behaviour in the antiferroelectric liquid crystal, (R)-MHPOBC

Temperature and bias-field dependences of dielectric behaviour in the antiferroelectric liquid crystal, R-MHPOBC, were investigated (see also previous paper). There are mainly two relaxation modes in the SmC*alpha and SmC* phases: one behaves as the soft-mode, which shows significant slowing down in the SmA* and SmC*alpha phases; the other one appears at lower frequencies and changes the dielectric strength remarkably, especially in the SmC* phase, which is considered to relate to the azimuthal phase-fluctuation of molecules in the parallel tilt sequences of the smectic layers. These two modes show different bias field dependences in different C* subphases. In the SmC*A phase, two other types of relaxation mode were observed, which are probably due to the in-phase and anti-phase azimuthal angle fluctuations of molecules in the anti-tilt pairs.     

Synthesis and characterization of a novel liquid crystal series with tribenzoate cores and monofluoro-substitution on the phenyl ring near the chiral chain

Two new chiral series, with benzoate cores and monofluoro-substitution in positions 2 and 3 of the first phenyl ring near the chiral chain, have been synthesized and characterized. The mesomorphic properties have been analysed by optical microscopy, differential scanning calorimetry and electro-optical measurements. The first series (I_c) displays a very rich polymorphism including SmA, SmC*_α, SmC*, SmC*_FI, SmC*_A phases, whereas the second (I_b) does not exhibit the SmC*_A phase, and moreover only displays the SmA phase for short alkoxy chains. The effect of the position of the fluoro substituent and the influence of the alkoxy chain length on the mesomorphic behaviour are discussed.     

Electroclinic effect in the chiral smectic A and cholesteric phases at the proximity of a N*–SmA–SmC* multicritical point

We studied the electro-optic and dielectric properties of three pure ferroelectric liquid crystal materials (C10, C11 and C12) of the same series exhibiting cholesteric (N*), smectic A (SmA) and chiral smectic C (SmC*) phases. From electro-optic investigations, the tilt angle and spontaneous polarisation were determined as a function of temperature. In the dielectric measurements carried out without a dc bias field, we studied the soft-mode relaxation in the SmA phase. From experimental data and using the results of a Landau model, we evaluated the soft-mode rotational viscosity and the electroclinic coefficient in the SmA phase. A soft-mode like mechanism was also observed in the N* phase for compounds with shorter chains (C10 and C11). This relaxation process is not detected for the homologue with a longer chain (C12). The observation of this mechanism is related to smectic order fluctuations within N* phase whose amplitude is increased when approaching the SmC*–SmA–N* multicritical point.     

Dielectric and thermodynamic studies of Smectic-A–Smectic-C* phase transition in antiferroelectric liquid crystal: thermal hysteresis

We present the experimental evidence and theoretical studies of the para–ferroelectric phase transition in the chiral liquid crystal C12HH. The investigated compound presents a large SmC* range. Both microscopic observation and dielectric measurements show thermal hysteresis and coexistence phases at paraelectric (SmA)–ferroelectric (SmC*) phase transition. The thermal hysteresis is analysed in samples with different planar cell thickness. The SmA–SmC* phase transition under electric field is also investigated. Finally, the Landau free energy density is used to analyse the experimental measurements, showing a satisfactory agreement with experimental data.     

Ferroelectric liquid crystals derived from isoleucine I. Synthesis and characterization

A series of ferroelectric liquid crystals (FLCs), 4 (3-methyl-2-halopentanoyloxy)- 4-hexyloxybiphenyls (3M2XPHOB, X=F for fluorine, C for chlorine, B for bromine) and their racemates (3M2XPHOB-R), were synthesized and characterized. The FLCs contain a chiral tail comprised of alpha -halo acids which are derived from L-isoleucine (DL-isoleucine for the racemates). The mesogens were characterized by high-resolution 1H and 13C NMR and their phase behaviour was studied by optical microscopy and differential scanning calorimetry. The chloro and bromo derivatives show both chiral smectic C (SmC*) phases and smectic A (SmA) phases, while the fluoro derivatives exhibit only a SmA phase. The spontaneous polarization of 3M2CPHOB and 3M2BPHOB were measured in the respective SmC* phases; that of the fluoro derivative was inferred by extrapolating its concentration dependent polarization in an achiral SmC solvent, a racemic mixture of 3M2BPHOB.     

Synthesis and mesomorphic properties of chiral liquid crystals having thiolactate groups

A homologous series of chiral thiolactate liquid crystals, alkyl 2-[4-(4-decyloxyphenyl)benzoyloxyphenyloxy]thiopropionates, ADBPT-n (n=2-6), was prepared from (R)-2-(4-hydroxyphenoxy)propionic acid. Structural effects on the mesomorphic and physical properties were investigated in terms of variation in the length of alkyl chain attached to the thiolactate group. The mesophases and their corresponding transition temperatures were identified by polarizing optical microscopy and differential scanning calorimetry. Compound ADBPT-4 exhibited SmA* and SmC* phases, whereas other compounds in the series exhibited the SmA* phase along with unidentified SmX₁* and SmX₂* phases. The maximum P_s value measured for ADBPT-4 in the SmC* phase was 17.5 nC cm^-2, suggesting the polarization of the material is rather low. The tilt angles were also measured, reaching a maximum of 48 degree.     

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.     

On the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal mixture

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic C_a* (SmC_a*) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC* phase, the distribution is biased such that the molecular tilt already present in SmA* now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmC_a* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmC_a* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.     

On the origin of high optical director tilt in a partially fluorinated orthoconic antiferroelectric liquid crystal mixture

We have investigated the orthoconic antiferroelectric liquid crystal mixture W107 by means of optical, X-ray and calorimetry measurements in order to assess the origin of the unusally high tilt angle between the optic axis and the smectic layer normal in this material. The optical birefringence increases strongly below the transition to the tilted phases, showing that the onset of tilt is coupled with a considerable increase in orientational order. The layer spacing in the smectic A* (SmA*) phase is notably smaller than the extended length of the molecules constituting the mixture, and the shrinkage in smectic C* (SmC*) and smectic C_a* (SmC_a*) is much less than the optical tilt angle would predict. These observations indicate that the tilting transition in W107 to a large extent follows the asymmetric de Vries diffuse cone model. The molecules are on average considerably tilted with respect to the layer normal already in the SmA* phase but the tilting directions are there randomly distributed, giving the phase its uniaxial characteristics. At the transition to the SmC* phase, the distribution is biased such that the molecular tilt already present in SmA* now gives a contribution to the macroscopic tilt angle. In addition, there is a certain increase of the average tilt angle, leading to a slightly smaller layer thickness in the tilted phases. Analysis of the wide angle scattering data show that the molecular tilt in SmC_a* is about 20° larger than in SmA*. The large optical tilt (45°) in the SmC_a* phase thus results from a combination of an increased average molecule tilt and a biasing of tilt direction fluctuations.     

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.     

Molecular and collective modes in ferroelectric liquid crystals studied by dielectric spectroscopy

The complex dielectric permittivity has been measured for a ferroelectric liquid crystal in the range 102-109Hz. Six different relaxations have been obtained and characterized: soft mode (SmA* and SmC* phases), Goldstone mode (SmC* phase), rotation around molecular long axis, rotation around molecular short axis, ferroelectric domain mode (SmC* phase) and an internal motion associated with a polar group. Strengths and frequencies of these modes have been obtained for the different phases for different bias fields. Using these results together with spontaneous polarization and molecular tilt measurements we have also obtained the rotational viscosities associated with the soft mode and the Goldstone mode. We explain the results in the light of the so-called Landau extended model, concluding that the biquadratic coupling between polarization and tilt is quite important with regard to the bilinear coupling. This fact has been used to explain the noticeable increase of the activation energy of the frequency of the mode related to the rotation around the molecular long axis at the SmA*-SmC* phase transition.     

New series of ferroelectric liquid crystals with two or three chiral centres exhibiting antiferroelectric and hexatic phases

New series of liquid crystals with a chiral chain containing two asymmetric carbon atoms in two lactate groups have been synthesized. Some compounds have a third asymmetric centre in an (S)-2-methylbutyl alkyl chain. All the new compounds have SmA, SmC* and hexatic phases. Two of the homologues with three chiral centres also exhibit very wide SmC_A phases and one of them a re-entrant SmC* phase. The phases are characterized by DSC, dielectric, and X-ray studies. Spontaneous polarization and tilt angle are determined for the polar phases.     

Systematic study of the chiral smectic phases of a fluorinated compound

ABSTRACT

Physical properties of the partially fluorinated compound 3F5FPhF, with hockey stick-like molecules, were studied by complementary methods. Apart from the already reported paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmC*_A phases, the presence of the smectic C*_α subphase in the phase sequence was proved by differential scanning calorimetry, polarising optical microscopy, electro-optic and dielectric spectroscopy methods. The temperature dependence of the smectic layer thickness and correlation length of the lateral short-range order was determined by X-ray diffraction. Based on dielectric measurements three relaxation processes were revealed in the antiferroelectric SmC*_A phase (two collective: P_L, P_H and one molecular: s-process), two collective ones (Goldstone and soft modes) were found both in the ferroelectric SmC* phase and SmC*_α subphase while one relaxation process (soft mode) in the paraelectric SmA* phase. The results were compared with that obtained for other structurally similar compounds, and it was shown that even addition of one methylene group to the side chain influences much on the physical properties.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.