Three ring dioxanes as dopants enhancing the stability of the smectic C phase

Searching for compounds which could be useful as modifiers of smectic C mixtures, we have synthesized four homologous series of three ring dioxanes, 2BBD, 5BBD, 2CBD, and 5CBD. Their phase transition temperatures and enthalpies were measured and their liquid crystal phases identified. Compounds belonging to series n-BBD form smectic B_cr phases for shorter alkyl chains, and smectic B_cr and A phases, for longer chains. Compounds belonging to the n-CBD series exhibit the smectic A phase, but those with longer alkyl chains have exclusively smectic B phases and those with short tails have other low temperature, highly ordered smectic phases. The compounds were added to smectic C mixture and it was found that some can be useful as dopants. Compounds with longer alkyl tails in the molecule are more suitable for this purpose; the type of ring in the core is less important.     

Asymmetric dimeric liquid crystals The preparation and properties of the α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes

Abstract

The first eleven members of the homologous series of α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes have been synthesized. The compounds are all enantiotropic nematogens and, with the exception of the heptyl, octyl and nonyl homologues, exhibit smectic phases. The thermal stability of the smectic A phase initially increases with the length of the terminal alkyl chain, passes through a maximum and then falls dramatically before disappearing. The smectic A phase subsequently reappears with the decyl homologue which has the highest smectic A-nematic transition temperature of the series. In order to understand this unusual behavior we have determined the entropies of transition for the compounds and we have measured the layer spacing of the smectic A phase for three of them.     

Binary mixtures of smectic A1S with abnormal behaviour

Binary mixtures are composed of compounds belonging to the 4'cyanobiphenyl-4-yl 4'-n -alkylbiphenyl-4 carboxylate homologous series and halogenophenyl 4-(trans-4-n-decylcyclohexyl)benzoate (10XPCHB) have been studied. Compounds CBnAB are smectic A₁ for n ≤ 7 and smectic A_d for n ≥ 8; compounds 10XPCHB are smectic A, only. It was found that the S_Ad phase of compounds CBnAB exhibits ideal miscibility with the S_A1 phase of compounds 10XTCHB, and that the S_A1 phases of both compounds show abnormal behaviour. The S_A1 phase of compounds CBnAB is destabilized and the range of the S_A1 phase is enhanced on the side of 10XPCHB. The reasons for this behaviour are discussed.     

Synthesis and mesomorphic properties of compounds exhibiting the undulated twist grain boundary smectic C* phase

The synthesis and characterization of three homologous series of compounds exhibiting the undulated twist grain boundary smectic C* (UTGB_C*) phase are reported. The chiral mesophases have been obtained using cholesterol as the chiral moiety. Cholestanol and [S]-[+]-octan-2-ol have also been used as the chiral moiety for comparitive purposes. In addition to this novel phase, cholesteric, smectic A, smectic C* and TGB_A phases have also been observed. The mesophases were characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and measurement of helical pitch.     

Mesomorphic properties of a homologous series of chiral liquid crystals containing the α-chloroester group

The mesomorphic properties of the N*, S*_c and higher ordered smectic phases have been investigated for a homologous series of 4-[(S)-2-chlor-3-methylbutanoyloxy]-4'-(4-n-alkyloxy-benzoyloxy)biphenyls. They have been characterized by optical texture observation, differential scanning calorimetry (DSC), small angle X-ray scattering and electro-optic measurements. The compounds exhibit a strongly twisted cholesteric phase and smectic phases with large spontaneous polarization and tilt angle values. One or two higher ordered, monotropic smectic phases were found which significantly differ in their rotational viscosity.     

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.     

Chiral ordering in the nematic and an optically isotropic mesophase of bent-core mesogens with a halogen substituent at the central core

Two new homologous series of bent-core compounds have been synthesized. Their mesophase behaviour has been investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction, NMR spectroscopy, and by dielectric and electro-optical measurements. It was found that, with one exception, all the chlorine-substituted compounds form a nematic phase and an optically isotropic 'banana phase'. The latter phase shows spontaneously chiral domains of opposite handedness. This phase may be considered as a type of smectic blue phase. The mesophase behaviour of the homologous bromine-substituted compounds is more complicated. Depending on the chain length, B₆, columnar, nematic or the isotropic 'banana phase' occur.     

TGBA phase in a series of propionates containing two stereocenters

A homologous series of chiral materials, (R)-2-pentyl (S)-2-(6-(4-(4'-alkoxyphenyl)benzoyloxy)-2-naphthyl)propionates (R,S)PmPBNP (m = 7-14), derived from a naphthalene ring as part of the core structure in conjunction with a chiral tail containing two stereocentres has been synthesized for the investigation. The twist grain boundary smectic A (TGB_A) and smectic C (TGB*_C) phases were characterized by the microscopic textures of compounds packed in two untreated glass slides, Cano wedge cell, or homogeneously aligned cell. It was found that the occurrence of these phases depend remarkably on the nature of alkyl chain length m; in the case of shorter alkyl chain length (m = 7-10), the TGB_A phase behaves as a metastable-like phase mediated between N and S_A phases in a short temperature range, whereas in the longer chain length (m' 11-14), both the TGB_A and TGB_C phases become thermodynamically stable phases with a wide temperature range. Consequently, this series of chiral materials resulted in two different mesophase sequences: N-TGB_A-S_A-S_Cand N-TGB_A-TGB_C. A kind of parquet texture displaying two types of domain with different relative directions of the smectic layer normal was found in the S phase from the materials (m = 9-14) packed in 2 μm homogeneously aligned cells and cooled down from the isotropic liquid without applying an electric field. The magnitudes of spontaneous polarization (P_s) in the S_C and TGB_C phases showed that the P_s values are nearly the same for all compounds at the same temperature below the Curie point. Dielectric measurements revealed no significant occurrence of soft mode switching in the TGB_A phase.     

Competition between micro-segregation and anti-parallel alignment of an amphiphilic rod-like liquid crystal

Recently, we reported that a rod-like molecule, 4-[4-(7-hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3-difluorobenzene, exhibited a nematic phase with a layered structure and smectic C phases consisting of three states. We prepared a homologous series of the rod-like molecules in which a 2,3-difluoro-1,4-diphenylbenzene unit and a hydroxyl unit are connected via a flexible methylene spacer. We investigated their physical properties using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Although they showed nematic, smectic A and smectic C phases, the phase structures were found to depend on the flexible spacer length. Those compounds possessing a shorter spacer length than a heptyl unit exhibited bilayered smectic A and smectic C phases, whereas those possessing a longer spacer length than a heptyl unit showed conventional monolayered smectic A and smectic C phases. A nematic phase with a smectic-like layer ordering was observed for the compound possessing an octyl unit. The structure–property relations of the amphiphilic compounds are discussed.     

Broad liquid crystalline temperature range of ferroelectric liquid crystals

Novel ferroelectric liquid crystalline compounds, containing the (S)-2-methyl-l -butyl (4-hydroxybiphenyl-4'-carbonyloxy)biphenyl-4-carboxylate mesogenic group and an oligooxyethylene spacer, were synthesized. The mesomorphic properties of these materials were investigated by differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurement. The results indicate that all members of this series exhibit a very broad temperature mesophase range (reaching a maximum around 210°C) including a blue phase (BP), cholesteric (Ch), twist grain boundary A (TGB_A), chiral smectic C (S*_c), and smectic X (S_x) phases. The mesomorphic properties are discussed and a comparison is made with three phenyl rings of ester core analogues.     

Orthogonal hexatic smectic phase-rare or common?

The hexatic smectic B (Hex B) phase is commonly encountered among hydrogen-bonded liquid crystal materials. Among enaminoketone and Schiff's base compounds, twenty homologous series (c. 10² compounds) exhibiting orthogonal mono-or bi-layer hexatic phases were identified by combined microscopy and DSC. Phase transitions from crystal B (CrB) of S_f to Hex B as well as from Hex B to S_f, S_a or S_c phases were observed within the range 50 to 190°C. Temperature ranges of the Hex B phase detectable by DSC varied from 0.2 to 20°C. Within two groups of three-ring enaminoketones, the formation of the Hex B phase was found to be controlled by both inductive and mesomeric components of the Hammett constants of the terminal substituents of the molecules.     

Three ring dioxanes as dopants enhancing the stability of the smectic C phase

Abstract

Searching for compounds which could be useful as modifiers of smectic C mixtures, we have synthesized four homologous series of three ring dioxanes, 2BBD, 5BBD, 2CBD, and 5CBD. Their phase transition temperatures and enthalpies were measured and their liquid crystal phases identified. Compounds belonging to series n-BBD form smectic B_cr phases for shorter alkyl chains, and smectic B_cr and A phases, for longer chains. Compounds belonging to the n-CBD series exhibit the smectic A phase, but those with longer alkyl chains have exclusively smectic B phases and those with short tails have other low temperature, highly ordered smectic phases. The compounds were added to smectic C mixture and it was found that some can be useful as dopants. Compounds with longer alkyl tails in the molecule are more suitable for this purpose; the type of ring in the core is less important.     

N.M.R. study of segmental molecular interactions in liquid crystals

A simple statistical model of interacting non-rigid molecules, based on a perturbation expansion of the pair correlation function and the additivity of segmental interactions, is applied to the study of orientational order as measured by N.M.R. in the nematic and S_A phases of 4-n-alkyl-4'-cyanobiphenyls (N-CB, N = 5 to 8), the nematic and S_C phases of 4-n-alkyloxybenzoic acids (N-OBA, N = 7, 8) and the D_h0 columnar discotic phase of hexa-alkyloxytriphenylenes (N-THE, N = 5 to 8). The order parameters of each homologous series are correctly described in terms of two isotropic and two anisotropic segmental coupling constants. The model predicts certain relations among coupling constants pertaining to different homogous series. These predictions are supported by the results obtained for the three types of compounds studied.     

Non-symmetrical bent-shaped liquid crystals based on a laterally substituted naphthalene central core with four ester groups

In the continuation of our study of the role of direction of the linking ester group and lateral substitution, we present three series of bent-core liquid crystals based on 7-hydroxynaphthalene-2-carboxylic acid. The mesomorphic properties were investigated by polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical methods. For non-substituted and laterally methyl-substituted compounds, columnar phases of broken-layer type and a smectic A phase were found. With the increasing length of the terminal alkyl chain, an increasing stability of the lamellar smectic A phase was observed. Substitution with a methyl group led to substantial narrowing of the mesophase range and the chloro-substituted materials exhibited no mesophases. The results are also discussed in context with the earlier studied naphthalene based mesogens.     

A detailed study of hydrogen bonded ferroelectric mesogens formed between alkyl and alkyloxy benzoic acids with carbamyl glutamic acid

A new ferroelectric homologous series of hydrogen-bonded ferroelectric liquid crystals is designed and synthesised with 14 compounds comprising two homologous series. The hydrogen bond is formed between dicarboxylic acid, viz. Carbamyl glutamic acid (CGA) with p-n-alkyl/alkyloxy benzoic acids (BA/BAO) and is confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) studies. These two series referred as the CGA+nBAO and the CGA+nBA yields 14 mesogenic homologues and the mesogenic nature is detected by optical textures through polarising microscope which is confirmed through Differential Scanning Calorimetry (DSC). CGA+nBAO series exhibit cholesteric, smectic X*, smectic C* and smectic G* phases while CGA+nBA exhibits cholesteric and smectic G*phases. The enthalpy values of the corresponding temperatures are elucidated. Odd–even effect is evinced at smectic C* to smectic G* phase transition temperatures of CGA+nBAO series. The order of the individual phase transition and thermal stability factor of the phases is also calculated. Optical tilt angle in smectic C* phase is obtained by optical extinction method. Dielectric relaxation and their corresponding frequency shift observed in different ferroelectric phases are measured and a dielectric molecular modelling is presented. Crystallisation kinetic study of smectic G* phase in CGA+nBAO is computed. An interesting phenomenon – ‘Parachromatism’ – is elaborated.     

Synthesis and characterization of new ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (S_A), a chiral smectic C (S_C) and a crystal E (C_r E) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n = 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic S_A and Cr E phases.     

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.     

Sulphur containing mesogens. The mesophasic behaviour of di(4-alkyloxybenzoates) of 4,4'-dimercaptobiphenyl

A homologous series of di(4-alkyloxybenzoates) of 4,4'-dimercaptobiphenyl: CH₃(CH₂)_n-1O-C₆H₄-COS-C₆H₄-C₆H₄-SOC-C₆H₄-O(CH₂)_n-1CH₃,n=1-7, has been synthesized and the thermotropic liquid-crystalline behaviour investigated. All compounds exhibit enantiotropic mesomorphism over a remarkable temperature range. While the mesophase thermal stability is moderately higher than that found for the corresponding oxygenated analogues, the smectic stability is definitely lower. In fact, all the compounds are nematic but smectic mesomorphism (S_C) is observed for n = 7. Compounds with n = 6 or 7 exhibit enantiotropic highly ordered smectic (or disordered crystal) phases, probably S_G in type.     

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.     

Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4'-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4'-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0-23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.