Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic trans-4-substituted-cyclohexyl (E)-alk-2-enoates

The effect on the liquid crystal transition temperatures of introducing various groups (for example incorporating C=C, O, CO₂ and CO) into the terminal alkyl chain of a weakly polar model compound 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]pentane has been investigated systematically. Only the compound containing both an ester function and a trans-carbon-carbon double bond exhibited a wide-range nematic mesophase at elevated temperatures. Therefore, a wide variety of trans-4-substituted-cyclohexyl (E)-alk-2-enoates incorporating a carbon double bond with a trans-configuration (E) in the terminal alkyl chain has been synthesized. Nearly all the two-ring esters prepared exhibit a nematic phase over a wide temperature range (≤ 100°C) at elevated temperatures (≤ 200°C). The tendency to form smectic mesophases is often low. Comparisons with the corresponding derivatives incorporating either just a carboxy group (COO) or just a carbon-carbon double bond (C=C) in the same positions indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. The new compounds are easily prepared from known starting materials.     

Liquid-crystalline side chain polymethacrylates II. Variation of spacer and central linkage in nitro-terminated polymers

The thermal behaviour and the mesogenic properties of liquid-crystalline side chain polymers with the structure are described with n = 3, 6, 8 or 11 and Z = -COO-, -CONH- or -N=N-. The polymers were made by radical chain polymerization. Most of them exhibit smectic phases, nematic phases were not observed. The amide linkage which has not yet been described as a structural element in such polymers gives rise to polymers of high polarity and high glass transition temperatures (T_g). In all cases the amide polymers produced the highest T_g values, the ester polymers showing the lowest. The azo polymers exhibited the largest mesophase ranges and the highest clearing temperatures (T_c1) of all the materials investigated. Whereas the T_c1 values for the azopolymers are nearly independent of the length of the spacer those of the amide and ester polymers show rising clearing points with increasing spacer length.     

Mesogenic Unsymmetric dimers containing cholesteryl ester and tolane moieties

Among unsymmetric oligomesogens, chiral dimers formed by connecting a cholesteryl ester fragment with various aromatic mesogenic cores through a polymethylene spacer have been attracting much attention due to their remarkable thermal behaviour. In particular, dimers containing a diphenylacetylene segment having an alkoxy chain have shown interesting mesomorphic behaviour. In view of this a new series of unsymmetric dimers consisting of a diphenylacetylene moiety having an alkyl chain and a cholesteryl ester unit joined through a paraffinic spacer have been synthesized and their liquid crystalline properties characterized. The lengths of the central methylene spacer (C₃, C₄, C₅ and C₇) as well as that of the alkyl chain (n-butyl, n-pentyl, n-hexyl and n-heptyl) have been varied to establish structure-property relationships. These investigations have revealed that all the dimers exhibit smectic A, twist grain boundary and chiral neamtic (N*) phases with the exception of one of the dimers for which only the N* phase was observed. Some differences in the mesomorphic properties of the unsymmetric dimers containing odd or even parity methylene spacers have been observed. The majority of dimers having an even (C₄) parity paraffinic spacer show a blue phase while the dimers with odd (C₃, C₅ and C₇) parity spacers exhibit the chiral smectic (SmC*) phase. In some cases, the SmC* phase exists well below (-60°C) and above room temperature.     

Methylene- and ether-linked liquid crystal dimers II. Effects of mesogenic linking unit and terminal chain length

Four series of liquid crystal dimers have been prepared containing either ether-linked or methylene-linked spacers. Changing the spacer from being ether-linked, i.e. O(CH₂) _n O, to methylene-linked, i.e. (CH₂) _{n +2}, results in decreased nematic-isotropic transition temperatures, and this reduction is more pronounced for odd-membered spacers. By contrast, the entropy change associated with the nematic-isotropic transition is higher for an even-membered methylene-linked dimer than for the corresponding ether-linked material. This trend is reversed for odd members. These observations are completely in accord with the predictions of a theoretical model developed by Luckhurst and co-workers in which the only difference between the dimers is their shape. For the highly non-linear pentamethylene-linked dimers, only those with a short terminal chain exhibited fluid smectic behaviour, specifically, a monotropic alternating SmC structure which allowed for the efficient packing of the bent molecules. Once the terminal chain reached a value of m = 9, a modulated ordered smectic phase was observed. For even-membered dimers, which exhibit only nematic phases upon melting for short terminal chain lengths, smectic phase behaviour was promoted with increasing terminal chain length, as is conventionally observed. Even-membered ether-linked dimers exhibited a SmC phase whereas even-membered methylene-linked dimers exhibited an ordered smectic G/J phase. Thus, it would appear that the differences in the transitional properties of ether- and methylene-linked dimers can be accounted for largely in terms of geometrical factors.     

Synthesis and characterization of a series of side chain chiral smectic liquid crystalline elastomers

A series of new chiral smectic liquid crystalline elastomers was prepared by graft polymerization of a nematic monomer with a chiral and non-mesogenic crosslinking agent, using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by FTIR and ¹H NMR. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X-ray diffraction. Monomer M₁ showed a nematic phase during heating and cooling. Polymer P₀ exhibited a smectic B phase; elastomers P₁-P₃ showed the smectic A phase, P₄-P₆ showed a chiral smectic C(SmC*), and P₇ displayed stress-induced birefringence. Elastomers containing less than 15 mol % M₂ displayed elasticity, reversible phase transitions with wide mesophase temperature ranges, and high thermal stability. With increasing content of the crosslinking unit, glass transition temperatures first increased, then fell, then increased again; isotropization temperatures and mesophase temperature ranges steadily decreased.     

Synthesis and thermal behaviour of salicylaldimine-based liquid crystalline symmetrical dimers

A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C₈) while the number of methylene units in the central spacer was varied from C₃ to C₁₁. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dimers containing 4, 6 and 8 methylene units in the central spacer, a unique filament growth pattern has been observed in the smectic A phase while cooling from the isotropic phase. The dimers containing of C₉ to C₁₁ methylene groups exhibit the nematic phase in addition to smectic modifications. This observation indicates that when the terminal chains are shorter than the spacer, the tendency to form smectic phases is not fully extinguished but is perhaps reduced.     

Synthesis, micro structure, and thermal stability of side chain liquid crystalline polysiloxane polymers with an oligo (ethylene oxide) unit in the side chain

A series of new alkene monomers [MS3BDBEn, n=1-3] containing 4-oligo (ethylene oxide) monomethyl ether 4-biphenyl ether carboxyl benzoate as terminal groups were synthesized. These polymers were prepared by grafting these monomers onto the poly (methylhydrosilox-ane) (PMHS) backbone. The transition temperatures, liquid crystalline textures, and thermal stability of the polysiloxane polymers have been determined by thermal data, by optical texture, and by X-ray diffraction patterns. Polymers PS3BDBEn showed smectic or smectic and nematic phases which were not analogous to their precursor nematic monomers. The terminal length of the polymers affects not only the mesophase transition temperatures but also the layer-spacing length (d₁) and the side-chain distance (d₂). The long- and short-range orders can remain to some extent above the isotropization temperature and below the melting point. The polymer PS3BDBE3 decomposed in air 20°C above the isotropization temperature and lost its short range orders as detected by the X-ray diffraction analysis.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

Synthesis and characterization of halogen-containing ferroelectric liquid crystals and side chain liquid crystalline polymers

A new series of ferroelectric liquid crystals and side chain liquid crystalline polymers based on halogen-containing chiral centres has been synthesized. Chemical structures were analysed by NMR. Liquid crystal phases were characterized by differential scanning calorimetry, optical polarizing microscopy, and X-ray diffractometry. The behaviour of the liquid crystalline phases was investigated as a function of spacer units and differing terminal asymmetric moieties. It was found that phase transition temperatures decreased with increasing length of the oligooxyethylene spacer unit. Differing terminal asymmetric moieties led to differing mesophase phenomena. Furthermore, a wide temperature range (including room temperature) of a chiral smectic C phase was achieved.     

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

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

Effect of alkyl chain length in the terminal ester group on mesomorphic properties of new chiral lactic acid derivatives

Two series of new liquid crystalline lactic acid derivatives with a terminal ester group have been synthesised. The effect of this ester unit and the length of its alkyl chain on the mesomorphic and dielectric properties of the compounds exhibiting a broad temperature range of chiral smectic phases have been studied. We found that the mesomorphic behaviour and phase transition temperatures are strongly affected by the molecular architecture. Depending on the alkyl chain length in the terminal ester unit, the studied materials exhibited paraelectric smectic A*, ferroelectric tilted smectic C* and antiferroelectric smectic C_A* phases over a broad temperature range. The physical properties of the compounds have been studied by optical polarising microscopy, differential scanning calorimetry, electro-optic measurements, small-angle X-ray scattering and dielectric spectroscopy. Furthermore, the homologues with short terminal alkyl chains showed a very small layer shrinkage at the transition from the orthogonal SmA* to the tilted SmC* phase, which is a characteristic feature of ‘de Vries-type’ behaviour.     

The first series of ferroelectric steroidal fluorinated liquid crystals

A series of cholesteryl p-perfluoroalkylphenyl carbonate (CPC) [C₂₇H₄₅OCO₂C₆H₄(CF₂)_nF, n=1,4,6,8] liquid crystals was synthesized and their phase transition behaviours were studied. The results show that the compounds containing a not too long perfluoroalkyl chain (n=6,8) have monotropic chiral smectic C phases.     

Thermal behaviour of lanthanum(III) alkanoates

The mesophase behaviour of the lanthanum(III) alkanoates [La(C_xH_2x+1COO)₃] (x =3-19) has been investigated by hot-stage polarizing optical microscopy, differential scanning calorimetry and high-temperature X-ray diffraction. Lanthanum(III) butyrate monohydrate shows no mesomorphism, whereas for the remaining short chain homologues (x = 4-9) a highly viscous mesophase M and a smectic A phase were observed. The longer chain lanthanum(III) soaps (x = 10-19) exhibit only a smectic A phase. However, the chain length has a pronounced effect on the transition temperatures. The thermal behaviour of lanthanum(III) alkanoates is compared with that of other lanthanide(III) alkanoates.     

Novel 2-(4-octylphenyl)pyridin-5-yl alkanoates and alkenoates: Influence of dipoles and chain conformation on smectic C formation

A series of 2-(4-octylphenyl)pyridin-5-yl alkanoates has been synthesized and found to exhibit smectic mesomorphism including the smectic C phase. The influence on the transition temperatures of introducing a carbon-carbon double bond in the terminal alkanoyloxy (ester) chain of the alkanoates to produce the corresponding alkenoates has also been investigated. The position and configuration of the double bond has been changed systematically in order to determine the optimal configuration and conformation of the terminal chains for smectic C formation. The observed results are consistent with a linearly-extended (alternately cis and trans) conformation of the chain. The dependence of the transition temperatures on chain length was studied for one homologous series each of the alkanoates and the (E)-alk-2-enoates. The new esters are constitutional isomers of the 5-(4-octylphenyl)pyridin-2-yl alkanoates and alkenoates previously synthesized (differing only in the position of the nitrogen atom). Comparisons revealed consistently higher smectic C transition temperatures and lower ordered smectic tendencies for the new esters. The dependence of S_c formation on the position and number of dipoles associated with oxygen atoms, nitrogen atoms and carboxy groups was also investigated. Several of the new esters exhibit remarkably low viscosity values (i.e. short response times) in an optically active base mixture used for evaluation and comparison purposes.     

Synthesis and liquid crystalline properties of novel laterally connected twins

Several laterally connected twin mesogens incorporating rigid p-terphenyl units have been synthesized. Their liquid crystalline behaviour was investigated by polarizing microscopy and differential scanning calorimetry. If one compares the twins with their respective halves, the mesophase stabilizing influence of the lateral fixation becomes evident. It obviously pre-organizes the calamitic units and therefore stabilizes smectic phases. The mesophase behaviour of these twins depends strongly on the length and the structure of the spacer. Short as well as highly polar and rigid spacer units give rise to smectic phases with high transition temperatures. By increasing the spacer length the mesophase stability is drastically diminished. Dimesogenic 4,4'-didecyloxyterphenyl derivatives containing spacers consisting of two connecting atoms exhibit smectic C phases exclusively. Elongation of the spacer to three connecting atoms gives an S_A-S_c dimorphism, and in the case of compounds with longer spacers mostly the S_A phase and in one case the nematic phase were found.     

The Effect Of The Position And Configuration Of Carbon-Carbon Double Bonds On The Mesomorphism Of Thermotropic, Non-Amphiphilic Liquid Crystals

The influence on their mesomorphic behaviour of introducing a carbon-carbon double bond into the chain, central linkage and alicyclic rings in the core of nematic and smectic liquid crystals (LCs) is discussed. Mesogens incorporating a trans-carbon-carbon double bond conjugated with an aromatic ring exhibit high mesophase-isotropic transition temperatures (T_c). However, they are photo-sensitive and can convert to the non-linear, non-mesogenic cis-isomers under the action of light. Non-conjugated double bonds in the terminal chain of mesogens can also lead to higher nematic and smectic C transition temperatures than those of the corresponding materials without a double bond, although the effect is not nearly as great. The position and trans-cis-configuration (E/Z) of the double bond are seen to be decisive. The combination of a hetero-atom (dipole effect) and the added rigidity imposed by the carbon-carbon double bond (steric effect) with a trans-configuration (E) in the terminal alkyl chain attached to the core of a liquid crystal molecule can give rise to a broad nematic phase. The double bond in the terminal chain of nematogens advantageously modifies the elastic constant ratios, as well as other properties of relevance to LCDs, especially for supertwisted TN-LCDs. The double bond in a central linkage gives rise to a broad spectrum of effects, sometimes suppressing undesired smectic phases and widening the nematic phase temperature range. Non-conjugated double bonds in the molecular core in the form of cyclohexene rings generally lead to lower transition temperatures, although smectic phases are sometimes suppressed and a nematic phase is observed. A conjugated double bond in a cyclohexene ring gives rise to a slightly higher T_NI. The effect on the transition temperatures of the double bond in steroid systems is complex.     

Synthesis and study of 4-(4'-n-alkoxybenzoyloxybenzoyl)-4'-n-butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n-propyl derivatives exhibit nematic phases, n-butyl to n-decyl derivatives exhibit smectic and nematic mesophases, whereas n-dodecyl to n-octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd-even effect for nematic-isotropic transition temperatures. Nematic-isotropic and smectic-cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin-layer chromatography and spectral data.     

X-ray diffraction study of smectic A layering in terminally fluorinated liquid crystal materials

We report an X-ray study of smectic A layering for mesogenic compounds with fluorinated substituents in terminal positions. The measurements were carried out using diffractometers with one- and two-coordinate proportional chambers. It was found that in contrast to -CN or -NO₂ terminated smectics, the polar -OCF₃ compounds form only a monolayer smectic A₁ phase. The ratio of the intensity of the second harmonic to the first in smectic A phases formed by molecules with lengthy perfluorinated chains was found to be two orders of magnitude higher than is commonly reported for low molar mass thermotropic mesogens, indicating deviations of the density distribution function p(z) from a pure sinusoidal form. The layer periodicity d for these mesogens exceeds the molecular length L: d/L ≃ 1·1, which corresponds to a smectic A_d phase consisting of parallel or antiparallel dimers. We have observed that lateral fluorine substitution in the benzene ring adjacent to the perfluorinated chain leads to the disturbance of uniform smectic A layering and to the formation of a defect-modulated smectic A structure of a chequer-board type. For polyphilic compounds containing both hydrogenous and perfluorinated units in the terminal chain, the in-plane structure factor displays double-peaked liquid-like profiles indicating the existence of nearest-neighbour molecular stacking at different distances. The peculiarities of smectic A layering in fluorinated mesogens are discussed in terms of steric coupling and enhanced conformational rigidity of fluorine containing moieties.     

The synthesis and liquid crystal properties of some 4-n-nonylphenyl esters of 3-(4'-n-alkoxybiphenylyl)- and 3-(4'-n-alkoxybiphenylyl)-3-methyl-propanoic acids and their laterally fluorinated analogues

A number of 4-n-nonylphenyl esters and mono- and di-fluorinated 4-n-nonylphenyl esters derived from 3-(4'-n-alkoxybiphenylyl)propanoic acids (II) have been synthesized and their thermotropic liquid crystal properties assessed with a view to obtaining tilted smectic phases for possible use in ferroelectric display devices. Many of these compounds exhibit wide temperature range S_C and S_I/F phases, but none gave the ideal phase sequence S_C-S_A-N-I mainly because the structure of these compounds was not conducive to the formation of the nematic phase. To try to alleviate this problem a series of 4-n-nonylphenyl esters based on 3-(4'-n-alkoxybiphenylyl)-3-methylpropanoic acid was prepared, where a lateral methyl group was incorporated in the β-position of the -CH₂CH₂CO₂- linkage. Incorporation of the lateral methyl group encouraged the formation of a nematic phase at the expense of both the smectic A phase and tilted smectic phases.