New chiral liquid crystal monomers and cholesteric polyacrylates: synthesis and characterisation

The synthesis of new chiral monomers (M₁ ?M₃ ) based on menthol and the corresponding polyacrylates (P₁ ?P₃ ) is described. The chemical structures, formula and phase behaviour of the obtained monomers and polymers were characterised with FT-IR, ¹H-NMR, elemental analyses, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). The effect of the mesogenic core rigidity, spacer length and menthyl steric effect on the phase behaviour of M₁ ?M₃ and P₁ ?P₃ is discussed. The expected mesophase of the compounds based on menthol can be obtained by inserting a flexible spacer between the mesogenic core and the terminal groups. For the chiral monomers and polyacrylates, their corresponding melting temperature (T _m), glass transition temperature (T _g) and clearing temperature (T _i) increased with an increase of the mesogenic core rigidity; while the T _m, T _g and T _i decreased with increasing the spacer length. M₁ and P₁ showed no mesophase, while M₂ and M₃ all revealed a SmC* and cholesteric phases. P₂ and P₃ only showed a cholesteric phase.     

Synthesis and liquid crystal properties of new cyclic carbonate monomers functionalised with cholesteryl moiety

ABSTRACT

Four new liquid crystal cyclic carbonate monomers M₁–M₄, with cholesteryl moiety and flexible spacer of different lengths, were synthesised through coupling reaction. The chemical structures, mesophase properties and thermal behaviour of the monomers were characterised with Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy (¹H NMR), polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. Based on these results, the relationship between the number of methylene groups into flexible chain and the mesomorphism of the monomers was investigated. It was found that all the monomers showed a focal conic texture of a smectic A phase and exhibited an interdigitated molecular arrangement. Moreover, the glass transition temperature and the isotropisation temperature of the monomers except M₁ decreased, and the mesophase range narrowed as the number of methylene units into the flexible chain increased.     

Synthesis and mesomorphism of the liquid crystal based on diosgenyl end-capped polycarbonate

ABSTRACT

To enhance the activity of the hydroxyl group of diosgenin, the diosgenyl derivative Dios–(CH₂)₆–OH with an alkyl spacer and end primary hydroxyl group was synthesised via the chain extension reaction between diosgenin and 1,6-hexanediol. Then the liquid crystal (LC) compounds Dios–(CH₂)₆–(trimethylene carbonate [TMC])_n with different TMC content were obtained via the ring-opening polymerisation of TMC with Dios–(CH₂)₆–OH as initiator and mesogenic unit, importantly, there was no catalyst added. The chemical structures of the Dios–(CH₂)₆–OH and Dios–(CH₂)₆–(TMC)_n were characterised by Fourier transform infrared (FT-IR) spectra and ¹H NMR. The mesomorphism was characterised by polarising optical microscopy and X-ray diffraction measurement, and the phase transition was investigated by differential scanning calorimetry. The results showed that Dios–(CH₂)₆–(TMC)_n displayed an enantiotropic smectic phase, and TMC content played an important role in the LC properties of Dios–(CH₂)₆–(TMC)_n. The higher the TMC content, the lower the phase transition temperature of Dios–(CH₂)₆–(TMC)_n.     

First examples of monodisperse discotic liquid crystal pentamers: synthesis and mesomorphism

The synthesis and mesomorphism of the first examples of ‘discotic liquid crystal pentamers’ are reported. Structural characterization of these difunctional pentamers has been achieved by FTIR, NMR, and elemental analysis. The mesophase behavior exhibited by these ‘star-shaped’ molecules has been studied by polarizing optical microscopy and differential scanning calorimetry. The columnar hexagonal structure of the mesophase was established with the help of X-ray diffraction studies.     

Main-chain biodegradable liquid crystal based on cholesteryl end-capped polycarbonate copolymers

Main-chain biodegradable liquid crystal (LC) based on cholesteryl end-capped polycarbonate copolymers was investigated. The novel LC was synthesised through ring-opening copolymerisation of trimethylene carbonate with ε-caprolactone (CL) initiated by cholesterol, without adding any catalyst. The chemical structure of the resulting polymers was confirmed by ¹H NMR. The liquid crystalline properties were validated by X-ray diffraction, differential scanning calorimetry and polarising optical microscopy. The results showed that the synthesised polycarbonate copolymers Chol-(TMCL)x + y exhibited liquid crystallinity in particular temperature ranges because of the incorporation of the cholesterol moieties. Furthermore, the effect of CL content on the mesomorphism properties of Chol-(TMCL)_x+y was also investigated; the higher the CL content, the lower the mesomorphism properties. It might be attributed to the hindered orientation of LC caused by the crystallinity of the poly(ε-caprolactone) (PCL) segments in the polymer chain.     

Synthesis,structure, and properties of chiral liquid crystal monomers and polymers based on menthol

To study structure–mesomorphism relationships of the monomers and polymers based on menthol, four new chiral monomers ( M₁ – M₄ ) and the corresponding homopolymers ( P₁ – P₄ ) with menthyl group were synthesized. Their chemical structures, formula, phase behavior, and thermal stability were characterized by FTIR, ¹H NMR, ¹³C NMR, elemental analyses, differential scanning calorimetry, polarizing optical microscopy, X‐ray diffraction, and thermogravimetric analysis. The selective reflection of light was investigated with ultraviolet/visible spectrometer. The influence of the mesogenic core rigidity, spacer length, and menthyl steric effect on the mesomorphism of M₁ – M₄ and P₁ – P₄ was examined. By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, four target monomers and polymers could form the expected mesophase. Moreover, their melting temperature (T_m), glass transition temperature (T_g), clearing temperature (T_i), and mesophase range (ΔT) increased with increasing the mesogenic core rigidity; whereas the T_m and T_g decreased, T_i and ΔT increased with an increase of the spacer length. M₁ and M₂ showed monotropic and enantiotropic cholesteric phase, respectively, whereas M₃ and M₄ all revealed chiral smectic C (SmC*), cholesteric and cubic blue phases. In addition, with increasing temperature, the selective reflection of light shifted to the long wavelength region at the SmC* phase range and to the short wavelength region at the cholesteric range, respectively. P₁ and P₂ only showed a smectic A (SmA) phase, whereas P₃ and P₄ exhibited the SmC* and SmA phases. All the obtained polymers had very good thermal stability. © 2012 Wiley Periodicals, Inc. J. Polym. Sci. Part A: Polym Chem, 2012     

Main-chain biodegradable liquid crystal derived from cholesteryl derivative end-capped poly(trimethylene carbonate): synthesis and characterisation

Main-chain biodegradable liquid crystal derived from cholesteryl derivative end-capped poly(trimethylene carbonate) was investigated. The novel liquid crystal was synthesised via ring-opening polymerisation of trimethylene carbonate initiated by cholesteryl derivative with an alkyl spacer and end primary hydroxyl group, without any catalyst. The chemical structure of resulting polymers was verified by ¹H NMR. Liquid crystalline properties were validated by X-ray diffraction, differential scanning calorimetry, and polarising optical microscopy. The results showed that all the synthesised polymers Chol-(CH₂)₂-(TMC)_n exhibited mesomorphism in particular temperature ranges because of the introduction of the cholesteryl derivative moiety.     

Synthesis and mesomorphism of diacetylene-bridged triphenylene discotic liquid crystal dimers

Connecting two discotic mesogens via a spacer not only stabilizes the columnar mesophase but also leads to the formation of glass columnar phase, and therefore improves the physical properties of discotic liquid crystals as organic semiconductor. Here, we report the synthesis of eight diacetylene-bridged triphenylene discotic liquid crystal dimers, [C18H6(OCnH2n+1)4(OMe)O2C-C8H16-C≡≡ C-]2, 3(n), (n = 4-8), [C18H6(OC6H13)5O2C-C8H16-C≡≡ C-]2, 6 and [C18H6(OC6H13)5O-(CH2)m-C≡≡ C-]2, 8(m), (m = 1, 3) by Eglinto...     

Molecular design of flexible liquid crystal oligomers stabilising the chiral frustrated phases

ABSTRACT

Chirality induces structural frustration in liquid crystal systems, producing various kinds of chiral frustrated phases, for example, twist grain boundary (TGB) phases, blue phases (BPs) and dark conglomerate (DC) phases. Almost all molecules exhibiting these frustrated phases have a rigid shape. Especially, a bent–core unit is regarded as a key structure for BPs and DC phases. This paper describes that some flexible liquid crystal oligomers being far from a rigid bent–core molecule stabilise these phases. The LC oligomers have a supermolecular structure in which mesogenic units are connected via flexible spacers. By designing intermolecular interactions, they can exhibit various molecular packing structures in the liquid-crystalline phases as follows: chiral dimers inducing TGB phases, U-shaped and T-shaped oligomers stabilising BPs and achiral liquid crystal trimers exhibiting DC phases. I discuss how the designed liquid crystal oligomers produce the chiral frustrated phases.     

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, structure and mesomorphism of new cholesteric monomers and smectic comblike polymers

Seven new cholesteric monomers (M-1−M-7) and the corresponding smectic comblike polymers containing cholesteryl groups (P-1−P-7) were synthesized. The chemical structures and purity were characterized by FT-IR, ¹H NMR, and elemental analyses. The specific optical rotations were evaluated with a polarimeter. The mesomorphism was investigated by polarizing optical microscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction. The specific optical rotation values of these monomers and polymers with the same numbers of phenyl ring and terminal groups were nearly equal, however, they decreased with increasing the aryl numbers in the mesogenic core. M-1−M-7 showed oily streak texture and focal conic texture, or fingerprint texture, or spiral texture of cholesteric phase. P-1−P-7 showed the smectic A phase. The melting, clearing, and glass transition temperatures increased, and the mesophase temperature ranges widened with increasing the aryl number in the mesogenic core. Surprisingly, although the molecular structures of M-6 and M-7 were similar to those of M-4, namely the mesogenic cores contained three phenyl rings, their phase behavior had a considerable difference, and T_m and T_i of M-6 and M-7 were less than those of M-4. In addition, M-6 and M-7 also showed an obvious glass transition. TGA showed that all the polymers had good thermal stabilities.     

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.     

Stability of liquid crystal micro-droplets based optical microresonators

Liquid crystal (LC) based tuneable optical microresonators are potential for being used as crucial components in photonic devices. In this article, we report experimental studies on LC micro-droplets dispersed in several dispersing media. We find that the size of the micro-droplets formed in a low refractive index and optically transparent perfluoropolymer are most stable with time than commonly used dispersing media. Using a negative dielectric anisotropy nematic liquid crystal, we show that the whispering gallery mode optical resonance properties such as the quality factor and the free spectral range of stable micro-droplets are independent of the strength of the applied electric field. The optical resonance properties under applied field are significantly different than that of the liquid crystals with positive dielectric anisotropy and are explained based on the elastic deformation of the micro-droplets.     

Synthesis and properties of liquid crystal monomers containing a reactive group in the lateral substituent

A series of nematic liquid crystal (LC) monomers containing a reactive group (double bonds) in the lateral substituent was designed and synthesised. Length of the lateral substituted groups that have one double bond varied from 1 to 4 methylene units. Length of the terminal substituted groups varied from 2 to 5 methylene units. The molecular structures of the intermediates and the LC monomers were characterised by Fourier transform infrared spectroscopy (FT-IR), elemental analysis and nuclear magnetic resonance (NMR) spectroscopy. The thermal phase behaviour of the monomers was investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM) coupled with hot stage. Some molecules (V₁₅, V₂₅) with high aspect ratio exhibit enantiotropic nematic mesophase. The other compounds (V₁₂, V₂₂, V₄₂, V₄₃) show monotropic nematic mesophase during cooling. The relationship between the structure and mesomorphic property is also discussed.     

Facile synthesis of gold nanoparticles capped with an ammonium-based chiral ionic liquid crystal

Herein, we describe a facile synthesis of stable chiral ionic liquid crystal (ILC)-capped gold nanoparticles. A new ILC containing a chiral cholesterol moiety having a terminal triethylammonium group was synthesised which exhibited an enantiotropic lamellar mesophase. Stable, monodisperse citrate-stabilised gold nanoparticles having a size of ~60 nm were prepared and the citrate ligands on the gold nanoparticles were replaced with chiral ILC through a two-phase ligand exchange process. The resulting chiral ILC-stabilised particles were characterised using UV–visible (UV–Vis) and transmission electron microscopy (TEM) studies. Different from the citrate-stabilised nanoparticles, the ligand exchanged gold nanoparticles were dispersible in organic solvent and resulting dispersion was stable for more than observed period of 3 months. Furthermore, the chiral ILC-decorated gold nanoparticles were found to be well dispersible in a nematic host without any aggregation and induced a vertical alignment of the nematic director.     

Design of filtering tunable liquid crystal phase shifter based on coplanar waveguide and split-ring resonators

ABSTRACT

In this paper, the designing method for microwave filtering tunable liquid crystal phase shifter (LCPS) is proposed for the first time. The proposed microwave filtering tunable LCPS integrated tunable phase shifter and passband filter can simultaneously provide tunable differential phase shifts and self-embedded stable filtering function. This filtering tunable LCPS is based on the coplanar waveguide (CPW) magnetically coupled with a series of split-ring resonators (SRRs). On this basis, the equivalent circuit model is utilized to analyse the relationship between stable filtering characteristic and tunable differential phase shifts. Finally, the prototype samples are fabricated and tested. The measured results not only show that the proposed filtering tunable LCPS realizes the stable passband from 7.9 to 8.2 GHz but also indicate that the differential phase shifts and figure of merit are, respectively, more than 55° and 9.6°/dB in the passband, which provide a good verification for the theoretical concepts.     

Phase modulation using different orientations of a chiral nematic in liquid crystal over silicon devices

Modulation of the intensity of light by high quality reflective micro-displays is predominantly carried out by liquid crystal over silicon (LCoS) spatial light modulator (SLM) technology for applications such as pico-projectors. Wider use of these devices, in applications such as computer-generated holography and optical correlation, is limited by their phase modulation ability and illumination polarisation state dependence. These devices rely on planar or twisted nematic liquid crystals to modulate the light, but due to their viscoelastic properties they are inherently slow. Research into the use of the polymer stabilised blue phase has already shown that it can offer high speed phase modulation. However, other chiral nematic orientations are yet to be compared in LCoS devices. In this article, we demonstrate that polymer-stabilised chiral nematic liquid crystal electro-optical effects can offer phase modulation in silicon backplane devices. The uniform standing helix and focal conic textures are shown to be independent of the input light polarisation state and the uniform lying helix is shown to be polarisation dependent. These optical responses are then compared with that of the blue phase to identify a suitable orientation for further development in LCoS technology in order to find a high-speed, full phase modulating material.     

Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACT

A series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, ¹H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase.     

Synthesis and optical activity of isosorbide chiral derivative containing fluorocarbon group as chiral dopant in liquid crystal materials

Novel isosorbide derivative containing perfluorocarbon group,bi（perfluorooctanesulfonyl）isosorbide ester as chiral dopant in liquid crystal,was synthesized.Chemical structure was characterized by elemental analysis,FT-IR,¹H NMR and ¹⁹F NMR.The optical texture of the mixture was observed by polarized optical microscopy（POM）.Novel chiral dopant containing perfluorocarbon group had excellent optical activity.Its specific rotation and molar rotation were noticeable higher than those of bi（4-chloromethylbenzenecarbonic）isosorbide ester.The fluorocarbon group improved the molar rotation of chiral compound and did not affect optical rotation direction.The texture of the mixture added isosorbide derivative with fluorocarbon group showed the oily streak texture.