Mesogens with central naphthalene core substituted at various positions

We have prepared liquid crystalline materials based on hydroxynaphthalene carboxylic acids. The mutual location of both functional groups has been varied, which resulted in compounds possessing a bend angle of 120° and 60°, therefore in bent-core and V-shaped compounds, respectively. The mesomorphic properties have been established with polarizing optical microscopy, DSC measurements and X-ray investigation. The properties of the studied have been compared with analogous derivatives to establish the role of ester orientation in the vicinity of the central naphthalene core.     

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.     

[2]Benzothiophene bent-shaped liquid crystals

New liquid crystalline (LC) compounds containing the [2 Weissflog, W., Nádasi, H., Dunemann, U., Pelzl, G., Diele, S., Eremin, A. and Kresse, H. 2001. J. Mater. Chem., 11: 2748–2758. [Crossref], [Web of Science ®] , [Google Scholar]]benzothiophene chromophore unit have been synthesised and their mesomorphic properties studied. Mesomorphic and fluorescent properties have been modified by various chemical moieties. Among others, terminal fluoroalkyl and/or chiral chains were introduced to stabilise mesophases and support the formation of polar phases. In spite of the bent shape due to the central 1,3-diphenylbenzo[c]thiophene unit, all compounds exhibit phases typical for the rod-like LC: nematic, smectic A and smectic C or their chiral analogues using the chiral (S)-C₂H₅CH(CH₃)(CH₂)₆CO or (S)-C₁₀H₂₁OCH(CH₃)CO chain. The orientational order in the nematic phase was studied by polarised infrared measurements. The fluorescent properties of the studied compounds have been measured and their parameters established.     

Lateral substituted phenyl biphenylcarboxylates ‒ non-chiral analogues of ferroelectric liquid crystals

ABSTRACT

We report mesomorphic properties of non-chiral liquid crystalline molecules and study the effect of lateral halogen (fluoro, chloro) substitution in the molecular core and the length of terminal chains. We have prepared nine homologue series with the molecular core substituted at different positions with respect to the ester linkage group. Additionally, we have modified the length of both terminal alkyl chains (hexyl, octyl, decyl and dodecyl) symmetrically for both terminal chains. The effects of the lateral substitution and the chain length are analysed with respect of the possibility to reduce the transition temperatures and tune the properties of presented liquid crystalline molecules.     

New liquid crystal based on 2-phenylthiophene central core

Six series of bent-shaped liquid crystals based on phenylthiophene central unit and differing in lateral substitution, the number and orientation of ester linkages and character of the terminal chain have been synthesised and studied. In comparison with the previously investigated biphenyl-based analogues, the presence of the thiophene unit results in a strong increase of transition temperatures and appearance of calamitic phases, the character of which depended on the structural changes of mesogens.     

The effect of terminal epoxy modification on the mesomorphic and thermal stability of biphenyl ester liquid crystals

ABSTRACT

A series of biphenyl ester compounds with alkenyl, alkyl, oxirane, and 1,3-dioxolane as the terminal groups have been synthesised for investigating the mesomorphic and thermal stability. Based on the same carbon skeleton, the allyl-terminated compound V1OLC has no mesophase, while the oxirane-terminated compound POLC not only has wider multiple smectic mesophase compared with the propyl-terminated compound 3OLC but also exhibits the best thermal stability. However, when the terminal groups change from oxirane to 1,3-dioxolane, EOLC shows neither mesophase nor the good thermal stability. In addition, with the extension of the terminal chains, the butyl-terminated compound 4OLC gives the widest smectic mesophase. These imply that the mesomorphic and thermal stability of these biphenyl ester compounds are dependent on the suitable terminal chain lengths and epoxy sizes. Meanwhile, Density Functional Theory (DFT) calculations were conducted to assist in analysing the experimental results.     

Orientational ordering of a nematic liquid crystal and its mixture with its chain-perfluorinated analogue

The orientation of different segments of 4'-cyanophenyl 4-heptylbenzoate (7CPB) has been investigated using ¹³C NMR. The method of proton-encoded local field (PELF) spectroscopy in combination with off-magic-angle spinning (OMAS) of the sample was used. High resolution 2D spectra were obtained, from which the order parameters were calculated. Linear relations between the obtained order parameters and anisotropic chemical shifts determined by 1D ¹³C NMR were established and semi-empirical parameters were achieved. A 1:2 mixture of 7CPB and its chain-perfluorinated analogue (7PFCPB) showed interesting phase behaviour with the change of temperature. It was studied by the use of ¹³C NMR and polarizing optical microscopy. The order parameters of 7CPB in the smectic A phase of the mixture were calculated using the semi-empirical parameters obtained from the 2D NMR method.     

Bent-core liquid crystals based on 6-substituted 3-hydroxybenzoic acid: the role of substitution and linkage group orientation on mesomorphic properties

We have prepared and studied new series of bent-core liquid crystals based on laterally substituted 3-hydroxybenzoic acid. The physical properties of the materials were tuned by the type of lateral substitution in the vicinity of the carboxylic unit. Further modification was achieved by reorientation of connecting ester linkages in the elongating side arms and by changing the length of terminal alkyl chains. The physical properties of the materials were determined by differential scanning calorimetry (DSC), polarising optical microscopy and for selected compounds by x-ray diffraction analysis. The introduction of fluorine as lateral substituent leads to the appearance of a lamellar or columnar mesophase. On contrary, methyl- or chlorine-substituted compounds are mostly crystalline. We have found a strong correlation between the size of the lateral substituent and the mesogenicity of the studied materials.     

Synthesis and characterisation of asymmetrical mesogenic materials based on 2,5-disubstituted-1,3,4-oxadiazole

New mesogenic homologous series bearing 1,3,4-oxadiazole ring with a nitro terminal group, 4-(5-(4-nitrophenyl)-1,3,4-oxadiazol-2-yl)phenyl 4-((4-methoxybenzylidene)amino)benzoate (G₁–G₁₁), were synthesised. Their chemical structures are identified by fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR) and elemental analysis. The liquid crystalline properties of the series G_n and their precursory F_n were screened by differential scanning calorimetry and optical polarising microscopy (OPM). The compounds of the series G_n were screened by thermogravimetric analysis to observe their thermal stability. The target compounds (G_n) in this study, were displayed different liquid crystalline mesophase, the first two homologous (G₁ and G₂) did not show any liquid crystalline behaviour, the homologous (G₃–G₁₀) which have an alkoxy terminal group (n = 3–10) exhibited nematic phase, whilst the last derivative of the series (G₁₁), n = 12, displayed SmA phase. The mesomorphic properties of these derivatives were affected by the presence of the nitro group at the end of the molecules which was classified as a strong polar group. Also, the role of alkoxy terminal chain and the bent heterocyclic ring (1,3,4-oxadiazole) in the liquid crystalline properties of these molecules were debated.     

Synthesis of achiral four-ring unsymmetrically substituted toluene derived liquid crystals with a polar end group

Achiral four-ring unsymmetrically substituted toluene-derived liquid crystals have been designed and synthesised by known and straightforward methods. All these compounds exhibit nematic phase over wide temperatures.     

A new family of four-ring bent-core nematic liquid crystals with a highly polar end-group

A new family of four-ring achiral bent-core compounds derived from 2-methyl 3-amino benzoic acid with the methyl group in the bent direction incorporated into the central core have been designed and synthesised. These compounds possess an alkoxy chain attached at only one end of the bent core molecule, while the other arm consists of a biphenyl moiety possessing a highly polar cyano-group. The molecular structure has been confirmed by elemental analysis and spectroscopic data, and the thermal behaviour and phase characterisation has been investigated by differential scanning calorimetry and polarising microscopy. All the compounds exhibit a wide-range enantiotropic nematic phase. A comparison with non-mesomorphic unsubstituted and 4-methyl-substituted homologues is also presented.     

New azobenzene containing bent-core liquid crystals based on disubstituted resorcinol

New bent-core molecules with 4,6-dichlororesorcinol or 4-chloro-2-methylresorcinol as the central unit, and azobenzene with different alkyloxy chain length as side arms were synthesised. The mesophase behaviour of the new compounds was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical measurements. It is found that 4,6-dichlororesorcinol is more conducive towards mesomorphism than 4-chloro-2-methylresorcinol. The liquid crystalline properties of all of the prepared compounds are greatly affected by the lateral substitution on the outer ring. 4,6-Dichlororesorcinol-based compounds without lateral substitution show nematic phases with cybotactic cluster of the SmC-type (N_CybC). Moreover, depending on the chain length, the nematic phase appears as enantiotropic phase for the shortest homologue and as monotropic phase for the higher homologues.     

Effect of orientation of extra fused benzene ring and lateral methyl substituent on the mesophase behaviour of three-ring azo/ester molecules

ABSTRACT

Two homologous series of the three-ring azo/ester compounds 2-(or1-) naphthyl 4?-(4?’-alkoxy phenylazo) benzoates (In_a and IIn_a) were synthesised. A lateral methyl group was introduced in different positions of the alkoxy phenyl moiety, in position-2 to give series In_b and IIn_b and in position-3 to give series In_c and IIn_c. Molecular structures were characterised via elemental analyses, infrared and ¹H-NMR. Their mesophase characteristics were investigated by differential scanning calorimetry (DSC) and their phases identified via polarised light microscopy (PLM). Transition temperatures were correlated with the alkoxy-chain length (n) that varies between 6, 8, 10, 12, 14, and 16 carbons. Comparative studies were first made to investigate the effect of including the extra fused benzene ring, and its orientation, into the previously investigated three-ring compounds, 4-phenyl 4?-(4′′-alkoxyphenylazo) benzoates (IIIn_a). Investigation of the mesophase behaviour was extended to cover the effect of introducing the lateral methyl group and its position. The comparison between the present six series and their corresponding phenyl analogues IIIn_a,b,c, indicated that the 2-naphthyl analogues, In_a,b,c, exhibit the highest mesophase stability. Whereas, the steric effect of the protruded naphthalene group destabilises all mesophases and appear only monotropically. The results were discussed in terms of polarisability effect.     

Supermolecular liquid crystals with a six-armed cyclotriphosphazene core: from columnar to cubic phases

New liquid crystals having a non‐conventional structure have been synthesised from a six‐armed cyclotriphosphazene core, [N₃P₃(OC₆H₄OH‐4)₆], which was condensed with polycatenar acids. Reactions were monitored by ³¹P{¹H} and ¹H NMR spectroscopy and the chemical structure of the resulting materials was confirmed by different spectroscopic techniques and mass spectrometry (MALDI‐TOF). Results were in accordance with monodisperse, fully functionalised cyclotriphosphazenes. Thermal and mesomorphic properties were studied by optical microscopy, differential scanning calorimetry and X‐ray diffraction. All of the synthesised phosphazenes, substituted with benzyl ether chains, show a high thermal stability and exhibit mesomorphic properties, which depend on the number and type of alkyl terminal chains located at the periphery of the mesogens. Mesomorphic properties range from Col_h for cph‐A1 and cph‐A2 to a cubic phase detected for cph‐A3 , which has the larger number of alkyl chains. Furthermore, helical order was detected on X‐ray data of cph‐A2 , which has chiral branched chains. Circular dichroism spectra of annealed films at mesophase temperature show a signal attributed to the chiral helical arrangement of the mesogenic chromophores.     

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.     

Intercalated smectic a phases in banana-shaped liquid crystals with carbonate end groups.

Novel, symmetrical, bent-core mesogens, namely substituted 1,3-phenylenebis{4'-[(ethoxycarbonyl)oxy]-1,1'-biphenyl}-4-carboxylates, are prepared with carbonate groups at both ends of the molecules. The mesophase properties are investigated for different electron-donating and -withdrawing substituents connected to the central ring at different positions. These compounds exhibit nematic and/or smectic A intercalated (SmA(int)) mesophases. Electro-optical studies show a very significant electric-field-induced biaxiality that is fast (<1 ms) and large (0.03) associated with a high birefringence (0.33 at 500 nm, 10 V mum(-1) field) for the unsubstituted analogue. For one substance this effect is observable from above 100 degrees C down to room temperature. Such electric-field-induced biaxiality may find practical applications in fast electro-optic modulation devices.     

High birefringent reactive discotic liquid crystals based on asymmetrical triphenylene with phenyl-acetylene moieties

Triphenylene (TP)-based liquid crystals, a category of discotic liquid crystals (DLCs), are easy to synthesise, thermally stable and can undergo self-assembly. A new DLC compound, 2,7,10-tris[4-(6-acryloyloxyhexyloxy)benzoate]-3,6,11-tris[(4-hexylphenyl)ethynyl]-triphenylene, was obtained with a 49% yield using catechol as a starting material and features acetylene groups on its TP core. This material can form films with high birefringence through its cross-linkable acrylate ends. Here, we synthesised this new compound and characterised it using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. The thermal behaviour was also investigated using differential scanning calorimetry and polarised optical microscopy. This new asymmetric TP-based DLC compound exhibited a nematic liquid-crystalline phase between ?20 and 170°C and formed an optical anisotropic film with a high birefringence (Δn = 0.21–0.25).     

The fabrication of novel optical diffusers based on UV-cured polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) with different sizes of the LC droplets are prepared based on the ultraviolet (UV) light curable acrylate monomers/LCs composites to fabricate the optical diffuser films. To acquire light diffusers with high optical performance, the effects of the monomer structure and the UV light intensity on the micro-structure of the PDLC films are studied. Results show that the PDLC films could exhibit a strong light scattering at the premise of maintaining high transmittance in the visible region. As the LC droplets are spherically dispersed in the polymer networks, when the size of LC droplets is about 3.0 μm, the haze can reach 88.5% and the transmittance is nearly 90.0%, which can be used as a bottom diffuser film. While when the size of LC droplets is about 10.0 μm, the haze and transmittance are 39.2% and 90.2%, respectively; hence, it can be a good choice for a top diffuser film. With the advantages of simple preparation, roll-to-roll industrial production and tunable optical properties, it is supported that the films based on UV-cured PDLC films can be applied as outstanding optical diffuser films in the liquid crystal display industry.     

Star-shaped molecules as functional materials based on 1,3,5-benzenetriesters with pendant 1,3,4-thiadiazole groups: liquid crystals,optical, solvatofluorochromic and electrochemical properties

New molecular structures consisting of a C₃ star-shaped 1,3,5-benzene unit as a central core, and three pendant 2-phenyl-5-(di-, and/or tri-n-alkoxyphenyl)-1,3,4-thiadiazole arms, containing ester connecting groups, variable number and positions of linear alkoxy chains (2an, 2b10 and 2c10) were synthesised. The mesomorphic properties were studied by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction. Compounds bearing nine peripheral n-decyloxy chains (2a10) and nine peripheral n-dodecyloxy chains (2a12) form an enantiotropic hexagonal columnar phase, whereas their homologue 2a14, bearing nine peripheral n-tetradecyloxy chains, does not show mesomorphic behaviour. Compound bearing six peripheral n-decyloxy chains, which are in the 3,4-positions of the terminal benzene rings (2c10) does not show mesomorphic properties, whereas its analogous compound 2b10, bearing six n-decyloxy chains in the 3,5-positions of the terminal benzene rings, is liquid. The electrochemical behaviour of the liquid crystalline compounds 2a10 and 2a12 was investigated by cyclic voltammetry which revealed the hole-blocker character of these compounds. The ultraviolet–visible absorption and emission properties of the compounds 2an, 2b10 and 2c10 have been investigated. Photophysical studies were carried out in solution and in film. In addition, solvatofluorochromism studies were performed with compound 2a12 as a representative member of the compounds of series 2.