Synthesis and mesomorphic properties and optical behaviour analysis of homologous series azobenzene liquid crystal monomer with polar substituents

ABSTRACT

Twenty novel azobenzene liquid crystal micromolecular compounds named ω-[4-(p-substituted azobenzeneoxy carbonyl]acid (X-ABCnA) have been designed and synthesised, followed by studies on the thermal performance and mesomorphic properties of the compounds. The liquid crystal compounds were divided into five homologous series based on the terminal substituents R (R = CH₃O, CH₃, H, Cl, NO₂). In each series, the number of carbons on flexible chain was 4, 6, 8 and 10, respectively. Fourier-transform infrared, proton nuclear magnetic resonance and elementary analysis demonstrated that the structure of the synthesised azobenzene liquid crystal compounds was consistent with the molecular design. The mesomorphic properties were tested, analysed and characterised by using differential scanning calorimetry and polarised optical microscopy. The melting transition (T _m) of all the compounds in homologous series with different substituents appeared to decrease with the increase of carbon numbers on flexible chains. The same held true for the temperature of isotropic-mesophase/crystalline transition. The compounds with stronger polarity of terminal substituents were more likely to form broader mesogenic ranges. The liquid crystal compounds discussed in this work can be regarded as a reference for the synthesis of mesogenic arms participating in the synthesis of novel multi-arm liquid crystalline macromolecules and polymers.     

Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties

Two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles were synthesised and characterised. Their molecular structures differed wherein the latter comprised a lateral hydroxyl group, unlike the former. Spectroscopic techniques such as FT-IR, ¹H & ¹³C NMR and mass spectrometry together with elemental analysis were employed to elucidate the molecular structures. The transition temperatures and their mesophases were determined by differential scanning calorimetry, optical polarising microscopy and X-ray diffraction techniques. Members with decanoyloxy till hexadecanoloxy chain in the series without the lateral hydroxyl group each exhibited a smectic A phase, while those in the series with the lateral hydroxyl group were non-mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure–mesomorphic properties relationship.     

LC diketones: synthesis,transformations and mesomorphic properties

The synthesis and chemical transformations of various diketones into liquid crystalline compounds are discussed. The diketones were prepared by the interaction of cyclohexene with acid chlorides and benzene in the presence of aluminium chloride (Nenitzescu method) or by Friedel Crafts reaction of trans-4-alkanoyl-1-phenylcyclohexanes or trans-2-alkyl-5-phenylcyclohexanones with acid chlorides, or by the condensation of corresponding Mannich salts with 2-substituted acetoacetates or acetylacetone.     

LC diketones: synthesis, transformations and mesomorphic properties

The synthesis and chemical transformations of various diketones into liquid crystalline compounds are discussed. The diketones were prepared by the interaction of cyclohexene with acid chlorides and benzene in the presence of aluminium chloride (Nenitzescu method) or by Friedel Crafts reaction of trans-4-alkanoyl-1-phenylcyclohexanes or trans-2-alkyl-5-phenylcyclohexanones with acid chlorides, or by the condensation of corresponding Mannich salts with 2-substituted acetoacetates or acetylacetone.     

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).     

Synthesis,mesomorphic behaviour and optoelectronic properties of phosphorus-based thermotropic liquid crystalline dendrimers

A new series of thermotropic phosphorus-based liquid crystalline (LC) dendrimers based on a thiophosphoryl-phenoxymethyl(methylhydrazono) core (thiophosphoryl-PMMH) up to the fifth generation has been synthesised by solution condensation of aldehyde groups, surface-functionalised thiophosphoryl-PMMH dendritic substrates of generation numbers G_0.5 to G_5.5, with the appropriate molar equivalents of the pro-mesogenic n-hexadecylaniline mono-functional building block. Their chemical composition has been confirmed by ¹H/¹³C/³¹P nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, elemental analysis. Optical properties have been studied by ultraviolet-visible absorption, photoluminescence spectroscopy and polarised optical microscopy, and thermal characteristics by differential scanning calorimetry. Electrical studies have been made using the current-voltage characteristics of organic light-emitting diodes consisting of multi-layered indium tin oxide/dendrimer/aluminium tris(8-hydroxyquinoline)Al architecture. It has been demonstrated that the molecular engineering approach adopted can successfully lead to phosphorus-containing dendritic organic semiconductors (OSCs) which show tunable mesomorphic behaviour (extension of the observed smectic mesophase) and (opto) electronic properties, owing to their peripheral decoration with a tunable number of azomethine-based optically active chromophoric units. This rare combination of ‘tunable by design’ properties makes this series of thermostable thiophosphoryl-PMMH-based LC dendrimers a particularly appealing class of OSCs for use in optically and/or electronically active layers of (opto)electronic devices such as light-emitting diodes, field-effect transistors, solar cells and lasers.     

Discotic liquid crystalline hydrazone compounds: synthesis and mesomorphic properties

[reaction: see text] 1,3,5-Trisacetoacetamidobenzene with three 1,3-diketo groups was synthesized by the reaction of 1,3,5-triaminobenzene with diketene. Discotic hydrazone compounds were prepared by the diazo coupling reaction between 1,3,5-trisacetoacetamidobenzene and diazonium salts of 4-alkyloxyphenylamines. The compounds existed in hydrazone forms exclusively, being stabilized by the intramolecular hydrogen bonds, and showed discotic nematic or columnar hexagonal mesophases.     

Heterocyclic columnar pyrimidines: synthesis, characterization and mesomorphic properties

The synthesis, characterization, and mesomorphic properties of two series of heterocyclic compounds derived from a pyrimidine core are reported. These series, 1a and 1b, are substituted with a variety of functional groups (X=NHSO₂CF₃, F, Cl, Br, I, OCH₃, CH₃, C₂H₅) at the C^3' (meta)- or C^4' (para)-position of the terminal phenyl ring, and the substituent effect on mesophase formation was studied. The compounds were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis, and the mesomorphic behavior of the compounds was characterized and studied by differential scanning calorimetry, polarizing optical microscopy and powder X-ray diffraction. Most of the compounds were mesogenic at room temperature, and the mesophases were assigned as lamellar columnar (Col_L) phases, as expected for disk-like molecules. The results also revealed that compounds with a para-substituent (1a; except for -OCH₃) at the C^4'-position, exhibited higher clearing temperatures and wider temperature mesophase ranges than those of compounds with a meta-substituent (1b) at the C^3'-position. The higher clearing temperatures may be attributed to stronger dipolar interactions resulting from a greater resonance effect with the central core for para-substitution than for meta-substitution. The results also indicated that the columnar mesophases observed show a correlation with the electronic properties of the substituents; compounds containing electron-withdrawing substituents (X=F, Cl, Br, I, NHSO₂CF₃) also have higher clearing temperatures than compounds containing electron-donating substituents (X=Me, Et, OMe).     

Chiral liquid crystalline m-nitrotolans and tolans: synthesis and mesomorphic properties

Two homologous series, 4'-(4-n-alkoxybenzoyloxy)-4-substituted 3-nitrotolans (Ia-d) and 4'-(4-n-alkoxybenzoyloxy)-4-substituted tolans (IIa-c), with substituents (S)-2-methyl-1-butyl and n-alkyl, were synthesized by Sonogashira's coupling. Their mesomorphic behaviour is reported. The thermal stability of the series II is higher than that of series I. Series I melting points and clearing points are lower than those of series II. None of the chiral tolans or m-nitrotolans have an enantiotropic smectic C phase. When the chiral chains are changed to n-alkyl groups in compounds Ia,b and IIa, an enantiotropic smectic C phase is seen. All compounds have a nematic or cholesteric phase, and two homologues of series II present a monotropic smectic C phase with mosaic texture. The mesophases were characterized using optical polarized light microscopy and differential scanning calorimetry.     

Mesogenic polymers. III. Thermal properties and synthesis of three homologous series of thermotropic liquid crystalline “backbone” polyesters

Details of the thermal behavior, including transition temperatures, transition heats and transition entropies, are reported for three series of nematic liquid-crystalline “backbone” polyesters having the general structure All polysters described above were examined by differential scanning calorimetry and were found to have reproducible thermal behavior once the polymer had been annealed by heating to the isotropic phase and then subsequently cooled. Enantiotropic nematic phases were found for all 30 polysters studied. Many polymers showed multiple endotherms on melting to the nematic phase. Plots of solid–nematic and nematic–isotropic transition temperatures versus number of carbon atoms in the diacid segment (x) for each (y) reveal an even–odd alternation reminiscent of trends in homologous series of small-molecule liquid crystals. Enthalpies for the solid → nematic and nematic → isotropic transitions do not show such a precise alternation.     

The synthesis and properties of some mesomorphic cyclohexene derivatives

This paper describes the synthesis and mesomorphic properties of some new liquid crystalline cyclohexene derivatives.     

Polymerizable liquid crystalline twin molecules: synthesis and thermotropic properties

The synthesis of 14 novel low molar mass liquid crystalline twin molecules is described and experimental details are given. The twin monomers contain two mesogenic units which are connected by a flexible spacer. Two terminal acrylate groups make these twins suitable for photopolymerization. The insertion of lateral groups into the mesogen leads to glassforming properties. We tested several substituents (-OCH₃, -CH₃) in different positions of the mesogenic unit and investigated their thermotropic properties as well as their crystallization behaviour by polarizing microscopy and DSC experiments. Some of the novel twin molecules with lateral substituents in the mesogenic core have unusually broad mesophases of about 150^oC. Below T _g stable LC glasses are formed. At room temperature a slow, kinetically hindered crystallization starts after about three hours. The broad mesophases of the twin molecules allow investigations of the photopolymerization kinetics over a wide temperature range. The addition of chiral non-liquid crystalline comonomers and subsequent photopolymerization leads to cholesteric networks with interesting optical properties. Last but not least, the twins are suitable mixing agents which suppress the crystallization of classical mono-rods.     

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.     

Synthesis and mesomorphic behaviours of a homologous series of 2-(4-alkyloxybenzylidenamino)benzothiazoles

The synthesis and characterisation of a series of new rod-shaped liquid crystal (LC), 2-(4-alkyloxybenzylidenamino)benzothiazoles possessing even number of carbon atoms at the alkyloxy chain (C_nH_2n?+?1O-, n = 2, 4, 6, 8, 10, 12, 14, 16, 18) are reported. The molecular structures of title compounds were elucidated using FTIR and nuclear magnetic resonance spectroscopic techniques. The phase behaviour of these new compounds was characterised and studied by differential scanning calorimetry and polarising optical microscopy. Lower members (n = 2, 4 and 6) did not exhibit mesophase. Octyloxy derivative was monotropic LC. Enantiotropic smectic A phase was observed from n-decyloxy derivatives onward to the last homologous synthesised.     

Synthesis and mesomorphic properties of the homologous series of 4'-bromophenyl 4'-[(4-n-alkoxy-2,3,5,6 tetrafluorophenyl)ethynyl]benzoates

A homologous series of 4'-bromophenyl 4'-[(4-n-alkoxy-2,3,5,6-tetrafluoro-phenyl)ethynyl]benzoates has been synthesized. The transition temperatures were studied by polarizing optical microscopy. The compounds exhibit nematic and smectic A phases.     

Preparation and mesomorphic properties of a homologous series of N,N'-(2-chloroterephthalylidene)bis(4-n-alkylaniline)s

A series of N,N'-(2-chloroterephthalylidene)bis(4-n-alkylaniline)s (CTBAAs) were prepared and their mesomorphic behaviour characterized. None of the homologues showed smectic behaviour, but nematic behaviour with broad mesophases and rather low melting points was seen. The mesomorphic behaviour was compared with that of N,N'-(terephthalylidene) -bis(4-n-alkylaniline)s (TBAAs) and 4,4-bis-(n-alkylbenzal)-2-chloro-1,4-phenylenediamines (BACPDs). An X-ray diffraction analysis of N,N'-(2-chloro-terephthal)bis(4-n-butylaniline) (CTBBA) showed that the molecules were packed in crystals forming a staggered arrangement called an 'imbricated structure'.     

New star-shaped triarylamines: synthesis, mesomorphic behavior, and photophysical properties

A total of 18 compounds 1-6 derived from triphenylamine as core group were prepared and characterized, and their mesomorphic properties were also investigated. Compounds 1-4 and 5,6 were prepared from p,p′,p″-triformyltriphenylamine and p,p′-diformyltriphenylamine with appropriate alkoxyphenylamines. The phase behavior of these mesogenic compounds was characterized and studied by differential scanning calorimetry, polarized optical microscopy, and powder XRD diffraction. Compounds 1-3 exhibited columnar mesophase, however, compounds 4-6 were nonmesogenic. The mesophases observed in compounds 1-3 were found to be side dependent. Compounds 1a, 2a, and 3a appended with one, two, or three side chains exhibited lamellar columnar (Col_L) phases, and compounds 2b and 3b with four or six side chains formed hexagonal columnar (Col_h) phases. The formation of the mesophases, lamellar or columnar mesophases, was probably induced by H-bonding formed between -CH₂NH groups. The oxidation process determined by cyclic voltammetry showed two redox waves, one appeared at 220-255 mV and the other one at 503-677 mV, which gave energy to HOMOs range of 5.02-5.36 eV. The fluorescent properties of the compounds were examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds occurred at ca. 307-392 nm and 368-456 nm, respectively. Compound 4a has a larger red shift due to a better conjugation linked by CC double bonds instead of -CH₂NH in other compounds.     

Synthesis and mesomorphic properties of several series of fluorinated ester liquid crystals

Eight series of fluorosubstituted three-ring ester liquid crystals have been synthesized. Polarizing microscopic textural observations and DSC measurements of their phase transitions show that most are thermotropic liquid crystals with nematic and smectic A phases; furthermore, several show monotropic high order smectic phases. The results showed that the SmA phase is enhanced with the increasing number of fluoro-substituents at the para - and meta -positions of the terminal phenyl groups. The mesomorphic properties of these compounds are also affected by the direction of the ester bonds. The effect of the triple bond is also discussed.     

Ionic liquid crystals: synthesis and mesomorphic properties of dimeric 2,4,6-triarylpyrylium tetrafluoroborates

The synthesis of some dimeric pyrylium salts is described. The link between the two cationic parts of the molecule can be flexible or rigid. Surprisingly, the starting dimeric chalcones with a rigid link exhibit monotropic columnar hexagonal phases. Results concerning the spectroscopic and mesomorphic properties of the pyrylium salts and the chalcones are given.     

Ionic liquid crystals: synthesis and mesomorphic properties of dimeric 2,4,6-triarylpyrylium tetrafluoroborates

The synthesis of some dimeric pyrylium salts is described. The link between the two cationic parts of the molecule can be flexible or rigid. Surprisingly, the starting dimeric chalcones with a rigid link exhibit monotropic columnar hexagonal phases. Results concerning the spectroscopic and mesomorphic properties of the pyrylium salts and the chalcones are given.