Synthesis and mesomorphic behaviors of a series of heterocyclic pyridine-imine-ester based liquid crystals

ABSTRACT

A series of new calamitic liquid crystals, 4-{[(5-methylpyridin-2-yl)imino]methyl}phenyl 4-alkoxybenzoates were synthesized. This series consists of nine members wherein they are differed by the length of alkoxy chain. Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviors were analyzed with Differential Scanning Calorimetry (DSC), Polarizing Optical Microscopy (POM) and powder X-ray diffraction techniques. First member with the shortest alkoxy chain (n?=?2) is a non-mesogen. As the alkoxy chain increased to n?=?4, the monotropic nematic phase appeared. An enantiotropic mesophase (nematic) was observed for the following three members (n?=?6, 8, 10). As the alkoxy chain increased to n?=?12, enantiotropic nematic phase exhibited together with monotropic smectic A (SmA) phase. As the alkoxy chain continuously increased to n?=?14 and 16, enantiotropic phases were observed for both N and SmA. When moving to n?=?18, the nematic phase disappeared and this compound only exhibited a single mesophase (SmA).     

Synthesis and phase transition behaviours of laterally substituted liquid crystals containing methylhydroquinone: emerging of smectic C phase for higher homologues

A series of laterally substituted low-molar-mass liquid crystals with molecular geometry were constructed with three phenyl rings bridged through ester central groups as the rigid core and a lateral methyl group as the flexible part of a molecule, with a view to understanding and establishing the effect of molecular structure on liquid crystal behaviour. Low-molar-mass mesogens known as 1,4-bis[(4′-n-alkyloxybenzoyl)oxy]toluene with different number of carbon (n) at the alkyl chain have been prepared. Their molecular structures were proposed via physical measurements and spectroscopic techniques. Mesomorphic properties were studied by using differential scanning calorimetry, optical polarizing microscopy and powder X-ray diffraction techniques. The results showed that the melting points as well as the clearing temperatures decreased upon lengthening of the terminal alkyloxy chain lengths. Members with the shorter chain (n = 2–10) exhibited nematic phase. As for higher homologues, members with n = 12, 14, 16 and 18 showed polymorphism, whereby these compounds displayed both smectic and nematic properties.     

Effect of fluorine groups and different terminal chains on the electro-isomerization of azobenzene liquid crystals

A series of azobenzene liquid crystals with one or two terminal acrylate groups were synthesized and their polymers were fabricated.The azobenzene liquid crystals and their polymers achieved the photoisomerization from the liquid crystalline trans-isomer to the isotropic cis-isomer with UV irradiation.Then,the cis to trans isomerization induced by an electric field was studied,the time required for electro-isomerization was measured,the texture change and absorption variation from cis to trans form induced by the electric field were observed clearly,and the time required for electro-isomerization was much shorter than that for thermal relaxation.The influence of the polar group(fluorine),terminal acrylate group,and flexible alkyl chain on the time of electro-isomerization was studied.The results show that the compounds with polar fluorine group require shorter time for electro-isomerization and the polymerization of terminal acrylate group delays the electro-isomerization.     

Azo substituted V-shaped liquid crystalline compounds: synthesis and mesophase characterisation

The first examples of V-shaped liquid crystalline materials made of non-symmetrical molecules are reported. They belong to a homologous series of azo substituted compounds, wherein the two arms attached to the 1,2-positions of the benzene ring are different. The new compounds are characterised by the spectroscopic methods. The liquid crystalline properties are investigated using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction studies. All the compounds synthesised here are found to be liquid crystalline. They exhibit nematic and smectic A mesophases, which are uniaxial in nature. The preliminary qualitative experiments showed that the compounds are photosensitive. Based on the X-ray results, the molecular arrangement of these compounds in their mesophases is illustrated. A relationship between molecular structures to mesomorphic properties in such systems has also been predicted.     

Hydrazide‐based non‐symmetric liquid crystal dimers: synthesis and mesomorphic behavior

Hydrazide‐based non‐symmetric liquid crystal dimers were synthesized. The liquid crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder X‐ray diffraction (XRD). These non‐symmetric liquid crystal dimers are evidenced to display the monolayer smectic C phase. The effects of the lateral intermolecular hydrogen bonding as well as the length of the terminal alkyl chains and the spacers on the mesophase are discussed. Our studies reveal that intermolecular hydrogen bonding between the hydrazide groups and microsegregation effect is the driving force for the formation of the monolayer smectic C structure. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis/structural and mesomorphic properties of substituted hydrazide based calamitic molecules

A set of rod-shaped molecules with central hydrazide moiety has been synthesized and characterized using standard spectroscopic techniques. The thermal behaviors of the final compounds were established by the combination of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and high-temperature powder X-ray method (XRD). The structure and conformation of the molecules were achieved by FTIR, ¹H NMR, ¹³C NMR spectroscopy and DFT calculations. It is found that two compounds are non-liquid crystalline and two compounds are liquid crystalline (LC). The structure property correlations with alkyl chain, connecting position, hydrogen bonding and without hydrogen bonding were established. The studies envisaged that LC properties are induced due to hydrogen bonding and elongation of the molecular length.     

Synthesis and phase transition studies on non-symmetric liquid crystal dimers: N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-benzylidene)-4-chloroanilines

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)benzylidene)-4-chloroaniline containing benzothiazole and benzylideneimine units connected by a flexible alkyl spacer, –(CH₂) _n –, with n ranging from 4 to 12 in even parity have been prepared. All five members of this homologous series exhibit an enantiotropic nematic phase. The compounds with greater n of 8–12 exhibit both nematic and smectic phases upon cooling. A notable feature among this series is that for the member with n?=?10, the smectic–nematic transition is also present. The nematic–isotropic transition temperatures and associated entropy changes with respect to all compounds in this series exhibit a dramatic dependence on the length of the flexible spacer. A comparison of the transitional properties of this series with those of α-(4-benzylidenechloroaniline-4′-oxy)-ω-[4-(thiophene-2-carboxyl)benzylideneaniline-4′-oxy]alkanes reveals that replacing benzothiazole moiety at one side of the flexible alkyl spacer reduces the nematic–isotropic transition temperature.     

Structure and frustration in liquid crystalline polyacrylates II. Thin-film properties

We report on the thin-film behaviour of a polyacrylate with phenyl benzoate mesogenic side groups and a narrow polydispersity. Depending on the degree of polymerisation, these polymers show a nematic, a smectic-A _d, a re-entrant nematic and a C phase with a two-dimensional monoclinic lattice. X-ray reflectivity and atomic-force microscopy have been used to characterize the structure and surface morphology. The system exhibits two stable side-chain packing configurations with incommensurate spacings that can be both stabilized at a free surface. Thin films in the nematic phase show a structural dewetting induced by the growth of surface domains of the C phase. Additionally, surface-induced ripples with a nanoscale lateral period form at the air-film interface. We attribute these patterns to a coupling between the local liquid crystalline ordering of the mesogenic side groups and the surface curvature energy. Received 28 February 2001 and Received in final form 6 August 2001     

Nonlinear optical properties of stilbene and azobenzene derivatives containing azaphosphane groups

We have studied the nonlinear optical (NLO) properties of some donor–acceptor molecules with stilbene and azobenzene molecules as backbone. We have used the nitro group as the acceptor and azaphosphane (R₃P=N–) as the donor group. To study the effect of variation of NLO properties, we have replaced the substituents (Rs) connected to the phosphorus atom by methyl, amine and phenyl groups. We find that both first-order polarizability and hyperpolarizabilities are larger for stilbene derivatives and is maximum for the phenyl substitution. Second-order polarizability is higher for methyl substitution. We have also obtained the two-photon absorption cross-section for these molecules. We find that both one-photon and two-photon absorption cross-sections are maximum for the same excited state (first excited state in the case of stilbene and second excited state in the case of azobenzene derivatives).     

Supramolecular assemblies of azobenzene‐β‐cyclodextrin dimers and azobenzene modified polycaprolactones

Click chemistry is employed to couple two β‐cyclodextrins at both ends of azobenzene moiety yielding dumbbell‐shaped amphiphiles (AZO‐β‐CD dimer) constructed by rigid aromatic building blocks as “body”, and hydrophilic cyclodextrins as “head” with almost quantitative yield and purity. Bulk aggregates formed by the self‐assembly of the supraamphiphiles through π–π stacking and hydrophobic effect are observed. Meanwhile, the rationally designed polyesters, named as AZO‐PCL with controllable molecular weights and low polydispersities, are successfully synthesized by ring‐opening polymerization of ε‐caprolactone in the presence of p‐aminoazobenzene as initiator. In the aqueous phase, very stable spherical particles are formed by host–guest interactions between AZO‐β‐CD and AZO‐PCLs; the spherical aggregates inherit the photo‐responsiveness of azobenzene. The detailed aggregation and disaggregation behaviors are fully investigated by TEM, SEM, NMR, 2D NOESY, IR, UV and XRD measurements. Compared to the previous works, our newly developed system can be fabricated with more readily manners, avoiding tedious synthetic process; the reversible and dynamic nature of the non‐covalent interactions also can be modulated alternatively by UV or visible light. Thus, such dumbbell‐shaped supra‐amphiphiles are of great potential applications in the controlled delivery systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Spatial reorientation of azobenzene side groups of a liquid crystalline polymer induced by linearly polarized light

The photoinduced 3D orientational structures in films of a liquid crystalline polyester, containing azobenzene side groups, are studied both experimentally and theoretically. By using the null ellipsometry and the UV/Vis absorption spectroscopy, the preferential in-plane alignment of the azobenzene fragments and in-plane reorientation under irradiation with linearly polarized UV light are established. The uniaxial and biaxial orientational order of the azobenzene chromophores are detected. The biaxiality is observed in the intermediate stages of irradiation, whereas the uniaxial structure is maintained in the photosaturated state of the photo-orientation process. The components of the order parameter tensor of the azobenzene fragments are estimated for the initial state and after different doses of irradiation. The proposed theory takes into account biaxiality of the induced structures. Numerical analysis of the master equations for the order parameter tensor is found to yield the results that are in good agreement with the experimental dependencies of the order parameter components on the illumination time. Received 23 April 2001 and Received in final form 1 August 2001     

Electro-optic properties of nematic and ferroelectric liquid crystalline nanocolloids doped with partially reduced graphene oxide

Flakes of partially reduced graphene oxide (PRGO) were doped in nematic liquid crystals (NLCs) and ferroelectric liquid crystals (FLCs), respectively. The dielectric and electro-optical properties of NLCs doped with those flakes have been investigated. Threshold voltage and switching times are reduced by 30%–50%. This is primarily due to the decrease of the elastic properties of the nanocolloids compared to the non-doped nematics. The influence of the PRGO flakes on the spontaneous polarization, tilt angle and switching time of FLCs was investigated too. Such flakes reduce the response time by 40%–60%, increases spontaneous polarization by 20%–25% and increase the tilt angle by 15%–20%.     

Structure and frustration in liquid crystalline polyacrylates I. Bulk behaviour

The phase behaviour and structure are reported of a new type of frustrated side-chain liquid crystalline (LC) polymer, a polyacrylate with phenylbenzoate mesogenic side groups and a narrow polydispersity. At a high degree of polymerisation the LC polymers show a nematic, a smectic-A_d, a re-entrant nematic and a C phase, for shorter chains only a nematic and a C phase. This constitutes a new example of nematic re-entrance for which the driving field is the length of the polymer chain. The smectic-A_d layers consist of partially overlapped side groups while in the C phase the side chains are rearranged into chevron-like blocks of bilayers. We propose an explanation of the frustrated phase behaviour in terms of these two different competing length scales and their coupling to the backbone conformations. Received 28 February 2001 and Received in final form 6 August 2001     

Synthesis of novel fluorinated bis(biphenyl)diacetylene derivatives as highly birefringent nematic liquid crystalline materials

Three novel fluorinated bis(biphenyl)diacetylene (1,4-bis(biphenyl-4-yl)buta-1,3-diyne) derivatives, with alkoxy tails at the terminal positions, were prepared for use as high-birefringent nematic materials. To lower the thermal transition temperatures and determine the origin of the nematic phase of these compounds, fluorine atoms were introduced at lateral positions. Thermal transition temperatures and mesomorphic properties were investigated by differential scanning calorimetry and polarized optical microscopy. In addition, birefringence was measured using a microscopic method based on the interference method. Both the nematogenic properties and optical performances depended not only on the number of fluorine substituents, but also on the position of the substituents. The compound having the highest fluorine content exhibited the lowest birefringence, and the compound bearing fluorine atoms only on the inner benzene rings of the biphenyl cores exhibited an enantiotropic nematic phase with a maximum birefringence value of 0.48 at 550 nm and 140 °C.     

Syntheses and mesomorphic properties of laterally fluorinatedphenyl isothiocyanatotolanes and their high birefringent mixtures

Syntheses and mesomorphic properties of di-, tri- and tetrafluorosubstituted 4′-(4-alkylphenyl)-4-isothiocyanatotolanes are described. Four fluorine substituted compounds in the position 2′, 3′, 3, and 5 show stronger nematic character than difluorosubstituted ones in the position 3, 5 or 2′, 3′. Example of high birefringence nematic mixture and its properties are given.     

Synthesis and phase-transition behaviors of new symmetrical dimer liquid crystals containing benzothiazole unit

This article reports the synthesis and characterization of a series of homologous symmetrical dimers α,ω-bis[4-(6′-ethoxybenzothiazol-2′-yl)iminomethylphenoxy]alkane. Five members with different lengths of alkyl spacer groups of even parity varying from butyl (C₄H₈) to dodecyl (C₁₂H₂₄) were synthesized. Spectroscopic analysis (IR, ¹H and ¹³C NMR) along with electron-ionization mass spectrometric techniques were employed to verify the molecular structures of the dimers. Differential scanning calorimeter was used for determination of the phase-transition temperature and associated enthalpy changes. Optical studies and mesophase identification were carried out using a polarizing optical microscope attached to hotstage. A diversity of phase-transition behavior was observed as the length of the alkyl spacer increased from C₄H₈ to C₁₂H₂₄. Almost all title compounds exhibited nematic phase except the dimer containing butyl spacer in which the mesomorphic property was absent.     

Synthesis and mesomorphic properties of novel three-armed star-shaped mesogens of phloroglucinol (1,3,5-trihydroxybenzene) containing Schiff-base moiety

A series of symmetrical three-armed star-shaped mesogens based on phloroglucinol (1,3,5-trihydroxybenzene) as a core unit containing (4-benzylidene-substituted-aniline-4′-oxy)-6-bromohexane as peripheral arm where the 4-position at one end consisting different terminal substituent, X (X?=?fluoro, chloro, bromo, iodo and ethyl) was synthesized and characterized. The chemical structure of the title compounds were studied using FT-IR, ¹H- and ¹³C-NMR while their thermal behavior and mesomorphic properties were investigated via differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The results indicate that the three-armed compounds containing F, Cl, Br, and C₂H₅ substituents exhibit mesophase except for the analog having iodo substituent which is found to be non-mesogenic. The absence of liquid crystalline properties for iodo-substituted analog could be due to the influence of the substituent of which the lamellar packing of these compounds will be disrupted when the size of substituent is increased.     

On some liquid crystalline phases exhibited by compounds made of bent-core molecules and their mixtures with rod-like molecules

In most homologous series of compounds made of bent-core (BC) molecules, the B₂ B₁ and B₆ phases occur as the chain length decreases. We have studied binary mixtures of the compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4-n-dodecylbiphenyl 4’-carboxylate (BC12) which exhibits the B₂ phase with the compound 4-biphenylyl 4’-n-undecyloxybenzoate (BO11) made of rod-like (R) molecules. We find the above sequence of occurrence of the B phases with increasing concentration of BOH. In this paper we describe the physical origin for the formation of these phases in both pure compounds and in the mixtures. We have also found the occurrence of the biaxial smectic A phase when the BO11 concentration is increased to 87–95.5 mol%. We also report on another binary system composed of BC12 and 4-n-octyloxy 4’- cyanobiphenyl (8OCB) made of R molecules. This system exhibits the biaxial smectic A phase down to 30°C. Using polarized infrared spectroscopy we find that the mutual orientation of the R and BC molecules in the SmA_db liquid crystal is such that the arrow axes of the BC molecules are along the layer normal of the partial bilayer smectic structure formed by the rods. We also describe unusual growth patterns obtained when the nematic phase transforms to the SmA_db phase in a mixture with 24 mol% of BC12.     

Influence of hydrogen bonding on the generation and stabilization of liquid crystalline polyesters,poly(esteramide)s and polyacrylates

Induction and stabilization of liquid crystallinity through hydrogen bonding (HB) are now well-established. Interesting observations made on the influence of HB on LC behaviour of amido diol-based poly(esteramide)s, poly(esteramide)s containing nitro groups and azobenzene mesogen-based polyacrylates will be discussed. The use of amido diol as an important precursor for the synthesis of novel PEAs containing inbuilt di-amide linkage enabled generation of extensive hydrogen bondings between the amide-amide and amide-ester groups which stabilized the mesophase structures of the PEAs. The contributions of hydrogen bonding to the generation and stabilization of mesophase structures were plainly evident from the observation of liquid crystallinity even in PEAs prepared from fully aliphatic amido diols. Replacement of terephthaloyl units by isophthaloyl moiety totally vanquished liquid crystalline phases while biphenylene and naphthalene units did only reduce the transition temperatures as expected. The occurrence of the smectic phases in some of the polymers indicated possibly self-assembly through the formation of hetero intermolecular hydrogen bonded networks. A smectic polymorphism and in addition, a smectic-to-nematic transition, were observed in the monomers and polymers based on 1,4-phenylene[bis-(3-nitroanthranilidic acid)] containing nitro groups. A smectic polymorphism was also observed as a combined effect of hydrogen bonded carboxyl groups and laterally substituted alkyl side chains in the case of azobenzene mesogen containing side chain polyacrylates. It was further shown that the presence of the mesophase enhances the non-linear optical (NLO) response of these polymers.