Influence of halogen substituent on the mesomorphic properties of five-ring banana-shaped molecules with azobenzene wings

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B₆ phases, a B₄-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B₆-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-induced trans–cis-isomerisation on the prepared compounds.     

Banana‐shaped molecules with 4,6‐dichlorinated central core: effect of lateral substituents and terminal chains on the formation of antiferroelectric smectic mesophases

Six banana‐shaped compounds with a central core based on a 4,6‐dichloro‐1,3‐phenylene group were synthesized by varying the terminal chains (R = OC₁₀H₂₁ or OC₁₁H₂₁) and the lateral substituents (X = H, F or Cl). Their mesophases were characterized by a combination of differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X‐ray diffractometry. Mesomorphic properties of the banana‐shaped mesogens with an olefinic group (R = OC₁₁H₂₁) as a terminal chain are sensitive to lateral halogen substituents as much as those of the analogues with a saturated group (R = OC₁₀H₂₁). The compounds with X = F showed an antiferroelectric switchable smectic phase, which has been designated a B₂ phase. The compounds without a lateral halogen substituent only formed a nematic phase, while the compounds with X = Cl did not exhibit a mesophase in the melt.     

Banana-shaped molecules with 4,6-dichlorinated central core: effect of lateral substituents and terminal chains on the formation of antiferroelectric smectic mesophases

Six banana-shaped compounds with a central core based on a 4,6-dichloro-1,3-phenylene group were synthesized by varying the terminal chains (R = OC₁₀H₂₁ or OC₁₁H₂₁) and the lateral substituents (X = H, F or Cl). Their mesophases were characterized by a combination of differential scanning calorimetry, polarizing optical microscopy, triangular wave method, and X-ray diffractometry. Mesomorphic properties of the banana-shaped mesogens with an olefinic group (R = OC₁₁H₂₁) as a terminal chain are sensitive to lateral halogen substituents as much as those of the analogues with a saturated group (R = OC₁₀H₂₁). The compounds with X = F showed an antiferroelectric switchable smectic phase, which has been designated a B₂ phase. The compounds without a lateral halogen substituent only formed a nematic phase, while the compounds with X = Cl did not exhibit a mesophase in the melt.     

Dependence of Mesomorphic Behaviour of Methylene‐Linked Dimers and the Stability of the NTB/NX Phase upon Choice of Mesogenic Units and Terminal Chain Length

Twelve symmetrical dimeric materials consisting of a nonamethylene (C9) spacer and either phenyl 4‐(4′‐alkylphenyl)benzoate, phenyl 4‐(4′‐alkylcyclohexyl)benzoate or phenyl 4‐(4′‐alkylbicyclohexyl)carboxylate mesogenic units were prepared and their mesogenic behaviour characterised by POM, DSC and XRD. All of the materials exhibited nematic phases with clearing points in excess of 200 °C. Four compounds were found to exhibit the twist‐bend nematic phase, with one material exhibiting a transition from the N_TB phase into an anticlinic smectic ‘X’ phase. Across all three series of compounds the length of terminal chain is seen to dictate, to some degree, the type of mesophase formed: shorter terminal chains favour nematic and N_TB mesophases, whereas longer terminal aliphatic chains were found to promote smectic phases.     

The effect of a lateral aromatic branch on the orientational ordering of laterally alkoxy substituted nematics

Two new mesogenic series have been synthesized. They have the same main core which contains four aromatic rings, with two lateral substituents on the same side of one of the inner rings. One of the substituents is a 4-chlorobenzyloxy group and the second is an alkoxy chain. The positions of the lateral substituents are different in the two series. Despite the large aromatic branch, an enantiotropic nematic phase is obtained for most of the compounds. The orientational ordering behaviour of these new compounds has been investigated by C-13 NMR, and two isomeric compounds containing a lateral dodecyloxy chain were chosen for the study. The order parameters of the chain were obtained by a 2D C-13 NMR technique with variable angle spinning. The temperature dependence of the order parameters was determined using the C-13 chemical shifts with slow spinning of the sample parallel to the magnetic field. The first methylene fragment in each lateral substituent of each compound has a positive C-H bond order parameter, implying that this fragment adopts a cis conformation in the nematic phase. The two lateral substituents are folded back along the mesogenic core in opposite directions. The lateral chain is found to be roughly aligned along the molecular long axis imposed by the core, whereas the para -axis of the phenyl ring in the lateral aromatic branch makes a considerable angle with the molecular long axis. It was also found that the different patterns of substitution do not affect the position of the molecular long axis to any large extent.     

Effect of lateral bromo substituent on the phase behavior of four-ring azo/ester/azo liquid crystalline materials

ABSTRACT

In order to study the influence of lateral Br substitution on mesophase behaviour, five homologous series of 4-substituted phenylazo phenyl 4?-(3?-bromo-4?-alkoxyphenylazo) benzoates (In_a–e) have been synthesised. Within each homologous series, the alkoxy group varies from 6 to 16 carbons, while other terminal group substituents, X, are CH₃O, CH₃, H, Br and NO₂ groups; the mesophase behaviour of these series is compared with previously prepared laterally neat analogues, 4-substituted phenylazo phenyl 4?-(4?-alkoxyphenylazo) benzoates (IIn_a–e) and laterally methyl analogues, 4-substituted phenylazo phenyl 4?-(3?-methyl-4?-alkoxyphenylazo) benzoates (IIIn_a–e). Similar to lateral methyl analogues, the present series, lateral Br substitution showed that, independent of the polarity of the substituent X or the alkoxy-chain length, the nematic phase is predominant with relatively high stability and broad temperature ranges. The mesophase stability varies between 204.0°C and 335.0°C for the nematic phase and 169.6°C and 281.0°C for the SmA phase. Their total mesophase temperature ranges vary between 87.2°C and 201.4°C. All compounds were found to be thermally stable within the mesophase temperature range, except the lower homologue of the nitro and Br substituted derivatives. The obtained results are discussed in terms of molecular polarisability.     

Cybotactic nematic phases of photoisomerisable hockey-stick liquid crystals

New five-ring hockey-stick liquid crystalline materials with 4-bromoresorcinol as the central core unit and an azobenzene-based side arm were synthesised and their mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and under a triangular wave electric field. Additional structural modification was done by introducing a lateral fluorine atom in the terminal ring of one of the side arms. It is found that regardless of the alkyl chain length or the lateral fluorine substitution, all of the prepared materials are liquid crystalline exhibiting nematic phases composed of cybotactic clusters of the SmC-type (N_CybC) in addition to a monotropic SmC phase for the longest homologue.     

Synthesis of novel nematic liquid crystals containing 3,3'-dimethyl-2,2'-bipyridyl

Liquid-crystalline 3,3′-dimethyl-2,2′-bipyridyl derivatives with long 4-(alkoxyphenyl)ethynyl (-C≡C-C₆H₄-OR) groups in the 5,5′-positions were synthesized by palladium-catalysed crosscoupling reactions. The compounds exhibit exclusively nematic behaviour; for example, the hexyl derivative showed a nematic phase over the temperature range 145.2–205.0°C. On increasing the length of the terminal chain, the transition temperatures were lowered; for example, the hexadecyl derivative was nematic in the range 117.0–126.8°C.     

Synthesis and characterization of new liquid crystalline monomers for non-linear optics. X-ray study of re-entrant nematic behaviour with smectic-like fluctuations of C-type

The syntheses and structural properties of four liquid crystalline monomers are presented. They were prepared by two different reaction pathways. Transition temperatures and phase characterization were done by DSC, polarization microscopy and X-ray diffractometry. The dependence of phase transition temperatures on substitution of the aromatic core are discussed. Two of the compounds investigated highly ordered smectic phases. The improvement of the conjugation by means of a lateral hydroxy group disturbs the occurrence of the E re-entrant phase with smectic-like fluctuations of the C-type was observed in a compound with a lateral hydroxy group. The existence of this re-entrant nematic phase was confirmed by X-ray measurements both on cooling and heating of the sample. the alkyl chain lengths and the lateral show and B phases. The presence of a nematic     

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.     

Bent-core liquid crystals with a 2-substituted 3-hydroxybenzoic acid central core

We report the synthesis and study of the mesomorphic behaviour for a series of bent-core materials based on 2-substituted 3-hydroxybenzoic acid as a central unit. Properties of the studied compounds are tuned by lateral substitution (F, Cl, CH₃, NO₂ and CN) in the position 2 of 3-hydroxybenzoic acid and by the length of the terminal alkyl chain. All studied homologues exhibit at least one mesophase. Mesomorphic properties are established and compared with other 3-hyhroxybenzoic derivatives studied previously, with analogous orientation of ester linkages, laterally substituted at different position on the central core. The role of the substitution and the length of the terminal alkyl chain are discussed.     

Conformational behaviour of laterally dialkoxy branched mesogens. Part one

The crystal structures of two mesogenic compounds having two neighbouring lateral alkoxy chains in the central part of the molecules have been solved. In both structures, the common four ring central core is extended and quite rigid with a total length close to 28.5 Å. All the molecules are strictly parallel because of the P1 space group; the molecular arrangement is characteristic of a nematogenic system. In these structures, the lateral alkoxy chains are folded back to the core; they exhibit slightly different conformations and are quite disordered. The ¹³C chemical shift of the first -OCH₂- within the lateral chains can probe the mean conformation of the chain in the nematic phase. This chemical shift is independent of the compound. Nevertheless, in the solid phase, this chemical shift is dependent on small geometrical changes due to the influence of the oxygen in the neighbouring lateral chain. In addition, temperature cycling of the sample leads to different crystalline solid forms as evidenced by DSC and NMR studies.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Hydroxy-functionalized liquid crystalline polyazomethines II. Study of new central cores and synthesis of coordination polymers

A series of hydroxy-functionalized semi-flexible polyazomethines has been synthesized and characterized to investigate the effect of the structure of the diamine monomer on the thermal and mesogenic properties. The diamine monomer introduces structural modifications such as flexibility, lateral substitution, kinks or alteration of chain coaxiality. The mesomorphic phase of the mesogenic polymers was characterized as nematic in nature, but an evolution of the nematic into a smectic mesophase was observed for the polyazomethine derived from 3,3dimethoxybenzidine by annealing in the mesophase temperature range. Polyazomethines derived from aliphatic diamines were complexed with copper(II) to study the dependence of type of complexation on length of the aliphatic diamine.     

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.     

Heterocyclic pyridine-based liquid crystals: synthesis and mesomorphic properties

A series of new calamitic liquid crystals, 4-{[(pyridin-4-yl)methylidene]amino}phenyl 4-alkoxybenzoates comprising a heterocyclic (pyridine) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were synthesised and characterised. This series consists of nine members wherein the members differ by the length of alkoxy chain (C_nH_2n+1O–, where n = 2, 4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviours were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first three members of the series (n = 2, 4 and 6). As the alkoxy chain increased to n = 8 and n = 10, the nematic phase appeared together with an additional smectic A (SmA) phase. When moving from n = 12 until the highest members (n = 18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).     

Effect of the surfactant alkyl chain length on the stabilisation of lyotropic nematic phases

Lyotropic quaternary mixtures of potassium alkanoates (KCx) and sodium alkyl sulphates (NaCxS), where x is the number of carbon atoms in their alkyl chains, were prepared to investigate the effect of the surfactant alkyl chain length on the stabilisation of lyotropic nematic phases. The lyotropic mixtures investigated were formed by the dissolution of KCx (NaCxS) surfactants in the mixture of Rb₂SO₄/1-decanol/water (Na₂SO₄/1-decanol/water), separately. The uniaxial-to-biaxial nematic phase transitions were identified from the temperature dependence of the birefringences of the nematic phases by means of laser conoscopy. The micelle dimensions were obtained from small-angle X-ray scattering measurements. It was observed that the increase in the surfactant alkyl chain length causes the micellar growth in the plane perpendicular to the main amphiphile bilayer. The surfactant alkyl chain length plays a key role on the shape anisotropy of micelles, which triggers the orientational fluctuations that are responsible for the stabilisation of the different lyotropic nematic phases.     

Effect of small additives of organic compounds and inorganic salts on chain photosubstitution for halogen in halogenated naphthalenes

It was found that various organic compounds and inorganic salts added in small concentrations to aqueous solutions increase the quantum yields of photochemical chain ion-radical reactions of the sulfo group substitution for a halogen in halogenated naphthylamine (4-chloro-1 -naphthylamine) and hydroxynaphthalenes (1-bromo-2-hydroxynaphthalene and 4-chloro-2-hydroxynaphthalene). An increase in the quantum yield of the photosubstitution chain reaction of the sulfo group for a halogen on the aromatic ring in the presence of low concentrations of organic compounds and inorganic salts is a phenomenon common to various substrates in both direct and sensitized photolysis. Neither organic compounds nor inorganic salts in low concentrations affect the fluorescence of substrates and the chain length of the substitution reaction. It is suggested that the main effect of small additives on the photochemical chain reaction is due to a change in the reactivity of ionradical pairs, the primary products of a photoinduced electron transfer reaction at the initiation step.     

Kerr effect of low melting nematic liquid crystals with negative dielectric anisotropy

The two low melting nematic liquid crystals, 2-chloro-4-heptylphenyl 4-pentylbicyclo[2,2,2]octane-1-carboxylate (7CP5BOC) and 2-chloro-4-heptylphenyl 4-heptylbicyclo[2,2,2]octane-1-carboxylate (7CP7BOC) have been investigated to determine their electro-optical behaviour and third order non-linearity by the static Kerr effect method. Both liquid crystals are laterally substituted by a single chlorine atom located close to the ester linking group. The temperature dependence of the electric Kerr constant in the isotropic phase and the pretransitional behaviour have been investigated for these low birefringence nematic liquid crystals in the isotropic phase. Both the compounds, with negative dielectric anisotropy, have a positive Kerr constant. The Landau-de Gennes model was obeyed for these compounds.     

Mesomorphic properties of achiral halogen-substituted banana-shaped compounds

Three banana-shaped achiral compounds, derivatives of 4,6-dichloro-1,3-phenylene bis-[4-(4-n-octyloxyphenyliminomethyl) benzoate], were synthesized by varying the substituent (X = H, F and Cl). Their mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray analysis. The compound with X = H exhibited an enantiomeric nematic phase. The compounds with X = F and Cl formed a nematic phase on heating, while on cooling they formed a nematic phase at high temperature and a smectic A phase at a lower temperature (monotropic). A schematic representation of the mesophase structures for the compound with X = F is also illustrated.