Aromatic Heterocycles Used as Terminal Groups of Calamitic Liquid Crystals

Three series of compounds 1 ‐ 21 were prepared by using 2‐, 3‐, and 4‐pyridinemethanol as terminal group of calamitic liquid crystals. These compounds have some similarities in their phase behavior. They all exhibit SmA phase. A tilt smectic phase, SmC, will be formed when their flexible alkoxy chain is longer than hexoxy group. The profound difference between them is that the compounds 15 ‐ 17 possess a short temperature range of N phase. The compounds 3 and 6 have the widest SmA phase, up to 94 °C, and SmC phase, up to 46 °C, respectively.     

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.     

Liquid crystalline behaviours of some 3-perfluoroalkylbenzoate esters

A series of 3-perfluoroalkylbenzoate esters are synthesised. They tend to exhibit enantiotropic SmA and SmC phases. The clearing points are decreased with increasing the alkoxy chain length and show odd-even effect. With increasing the fluorocarbon chain length, the melting and clearing points increase; however, the SmC phase is suppressed.     

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.     

Polarized infrared spectroscopic study on changes in molecular orientation and interaction during phase transitions of a ferroelectric liquid crystal with a naphthalene ring

Temperature-dependent polarized infrared spectra were measured over the temperature range 105-30°C for a ferroelectric liquid crystal with a naphthalene ring (FLC-1) in the isotropic, smectic A (SmA), and chiral smectic C (SmC*) phases to investigate its molecular conformation, interactions, and alignment in each phase. It has been found, from the temperaturedependent spectral changes in the 1610-1600 cm^-1 region, that the degree of twist between the naphthalene and benzene rings of FLC-1 changes with temperature. The peak intensity of the band at 1606 cm^-1 containing contributions from both the benzene and naphthalene ring stretching modes begins to decrease, not suddenly but gradually, upon going from the SmA phase to the SmC* phase, suggesting that the molecular orientation of the two rings changes gradually between the two phases. The frequencies of two CH₂ stretching bands suggest that the disorder of the alkyl chain of FLC-1 is similar for the liquid crystal phase and the isotropic liquid phase. The splitting of the core C=O stretching band indicates that the resonance system consisting of the benzene ring and the C=O group in the core part of FLC-1 is involved in two kinds of intermolecular interaction between adjacent molecules in the liquid crystal phase.     

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 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.     

Oxadiazole-based unsymmetrical chiral liquid crystal dimers: synthesis and mesomorphic properties

The synthesis and liquid crystalline properties of novel chiral Schiff's base dimers containing the 1,3,4-oxadiazole ring are reported. The length of the terminal S-alkyl chain has been varied. All the compounds synthesised were thermally stable and exhibited enantiotropic mesomorphism, showing either SmC*–SmA–TGB–N*–BP or SmC*–SmA phase sequence.     

Polarized infrared spectroscopic study on changes in molecular orientation and interaction during phase transitions of a ferroelectric liquid crystal with a naphthalene ring

Temperature-dependent polarized infrared spectra were measured over the temperature range 105-30°C for a ferroelectric liquid crystal with a naphthalene ring (FLC-1) in the isotropic, smectic A (SmA), and chiral smectic C (SmC*) phases to investigate its molecular conformation, interactions, and alignment in each phase. It has been found, from the temperaturedependent spectral changes in the 1610-1600 cm^-1 region, that the degree of twist between the naphthalene and benzene rings of FLC-1 changes with temperature. The peak intensity of the band at 1606 cm^-1 containing contributions from both the benzene and naphthalene ring stretching modes begins to decrease, not suddenly but gradually, upon going from the SmA phase to the SmC* phase, suggesting that the molecular orientation of the two rings changes gradually between the two phases. The frequencies of two CH₂ stretching bands suggest that the disorder of the alkyl chain of FLC-1 is similar for the liquid crystal phase and the isotropic liquid phase. The splitting of the core C=O stretching band indicates that the resonance system consisting of the benzene ring and the C=O group in the core part of FLC-1 is involved in two kinds of intermolecular interaction between adjacent molecules in the liquid crystal phase.     

Induction of ionic smectic C phases: a systematic study of alkyl-linked guanidinium-based liquid crystals

A series of ionic liquid crystals with an alkoxy biphenyl unit tethered via an alkyl spacer to a guanidinium head group were synthesised and the mesomorphic properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction (XRD; WAXS and SAXS). Whereas all symmetrical guanidinium chlorides with the same chain lengths in alkyl tail and spacer displayed enantiotropic SmA₂ phases, monotropic SmC₂ phases with 1–2 K temperature range were only formed for chain lengths ≥ C₁₀. Shifting the calamitic core more closely to the ionic head group by decreasing the tether length and simultaneously increasing the terminal alkyl chain improved the stability of both SmA and SmC phases considerably and led to enantiotropic SmC phases for the guanidinium chloride with C₁₄ alkyl tail and C₆ spacer. An even more pronounced effect was detected during anion exchange. Bromide, iodide, hexafluorophosphate, thiocyanate and triflate suppressed any SmC phase, whereas tetrafluoroborate behaved similar to chloride maintaining the SmC phase. However, acetate stabilised the SmC phase at the expense of the SmA phase. Based on temperature-dependant XRD measurements, a bilayer structure was proposed.     

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 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.     

Thermal behaviour under pressure of the thermotropic cubic mesogen 4'- n -alkoxy-3'-nitrobiphenyl-4-carboxylic acids

The thermal behaviour of members of a homologous series which exhibits the optically isotropic cubic phase, the 4'- n -alkoxy-3'-nitrobiphenyl-4-carboxylic acids having alkoxy chains containing 16, 20 and 22 carbon atoms (referred to as ANBC-16, -20 and -22, respectively) was investigated under pressures up to 200-400 MPa by high pressure differential thermal analysis. In the phase diagram of ANBC-16 obtained on heating, a triple point was estimated at 54 ±1 MPa and 205 ±1°C for the SmC, Cub and SmA phases. It was found that the X phase is formed on cooling under all pressures, while appearing on heating at high pressures above about 54 MPa. Thus the X phase appears monotropically between the SmA and Cub phases in the low pressure region and enantiotropically between the SmA and SmC phases under higher pressures. It is strongly suggested that the X phase is a columnar mesophase. For ANBC-20 and -22, the cubic phase tends to be destabilized with increasing pressure. The temperature region of the cubic phase of ANBC-20 becomes narrower with increasing pressure and a triple point for the SmC, Cub and I phases is estimated to be at about 309 MPa. On the other hand, the cubic phase of ANBC-22 is still observed at the highest pressure examined.     

Synthesis and characterisation of two homologous series of LC acrylic monomers based on phenolic and resorcinic azobenzene groups

The synthesis and phase characterisation of two homologous series of monomers of acrylic azo-compounds has been presented. The characterisation comprises polarising optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffractometry (PXRD). All studied monomers exhibit a narrow nematic phase with decrease in temperature range while increasing the alkoxy tail length. For the resorcinic azo benzenes at higher tail lengths, SmA and SmC phases are present. Phenolic azo compounds show only SmA phase. The resorcinic family showed more stable phases, with broader liquid crystalline temperature range than its phenolic equivalent.     

Mesomorphic and photophysical behaviour of 1,3,4-oxadiazole based hockey stick reactive mesogens

New hockey stick mesogens derived from 1,3,4-oxadiazole as a bent-core unit have been synthesised. The molecules resemble hockey stick shape due to the presence of two arms containing a different number of phenyl rings attached with the 1,3,4-oxadiazole bending unit. The shorter arm of the molecule consists of one phenyl ring and 4-n-alkyloxy terminal chains whereas the long arm of the molecule possesses containing two phenyl rings which are linked via imine linkage and reactive 4-n-undecenyloxy as a terminal chain. The thermal stabilities of the newly synthesised compounds were carried out by thermogravimetric analysis (TGA). The mesomorphic behaviour was investigated by polarising optical microscopy (POM) and differential scanning calorimetry (DSC). All the compounds exhibit enantiotropic nematic phase along with smectic phases (SmA and SmC phases). Interestingly, the compounds with lower 4-n-alkyloxy terminal chains (n = 4 and 6) exhibit a wide range of optically isotropic DC phase. On increasing, the terminal 4-n-alkoxy chain length the DC phase disappears. The photophysical properties of the compounds were investigated in different solvents and in the solid state. It was observed that the compound exhibit absorption in UV region and emission in the green region.     

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.     

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.     

Phase transitional behaviour of S-shaped oligomers incorporating biphenylene and cholesterol entities

A new series of symmetrical S-shaped oligomers 4,4′-bis[(5-cholesteryloxycarbonylpentyl alkoxy)hexyloxy]biphenyl consisting of outer spacers –(COCH₂)_n– as well as an inner spacer –(CH₂)₆– has been synthesised. Their liquid crystalline properties and phase transition temperatures with associated enthalpy changes are recorded. The outer spacers are varied from n = 5–8 to 10 and 11. The compounds with even spacer exhibit enantiotropic phase and oligomers with odd parity display monotropic phase. The oligomers with odd membered n = 5, 7 and 11 exhibit N* and SmC* phases upon cooling. Whilst upon heating, the homologues with even-numbered member n = 6 and 10 show N* phase and upon cooling, both compounds exhibit N* and SmC* phases. However, oligomer with outer spacer n = 8 displays enantiotropic N* and SmC* phases. The temperature range of N* phase for even and odd membered decreased as the outer spacers are increased. The odd–even effect has been found in the I–N* transition temperatures where the odd-parity oligomers exhibit lower values when compared to compounds of an even-parity series. The X-ray diffraction measurements reveal the appearance of SmC* phase that can be associated with the monolayer ordering of these oligomers.     

Calamitic liquid crystals of 1,2,3-triazole connected to azobenzene: synthesis,characterisation and anisotropic properties

Azobenzene-based calamitic liquid crystals, 4-((4-(4-methoxymethyl-1,2,3-triazol-1-yl)phenyl)diazenyl)phenylalkanoates have been isolated and their structures were characterised. The structure–property correlation with respect to the different alkanoyloxy terminal chain (–COOC_nH_2n+1 where n adopts odd numbers ranging from 3 to 15) has also been given attention in the present study. In this series, all compounds exhibit smectogenic properties. The lower homologues shows enantiotropic SmA phase as well as monotropic SmC phase. The higher homologues exhibit homeotropic alignment of smectic phases. These compounds possess very high anisotropic inclination in which the mesomorphic region covers nearly 80°C.     

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.