Synthesis and mesomorphic properties of a series of dimers derived from thioether-terminated and cholesteryl

ABSTRACT

The synthesis and material properties of a series of new liquid crystalline compounds containing thioether and cholesteryl, these homologues with different alkyl chain lengths of 2–8, are reported. Thermal analysis shows that all oligomers have wide mesophase temperature ranges with high thermal stability. The oligomers were determined by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The molecule not only successfully exhibits strong optical properties and rainbow colours, but also the cholesteric helical pitch decreased with increasing temperature. The mesogenic incidence and tendency were found to be strongly dependent on the numbers of carbon in the flexible alkyl chain. Even members formed widely mesophase compared to odd members that showed narrower ones. The reflection wavelengths of 6S8Ch are almost across the entire visible region when they are heated, which offer tremendous potential for various optical applications. Also, it not only shows a lower transition temperature but also has a narrower cholesteric phase compared to analogues with alkoxy groups. These results not only provide practical design principles for the synthesis of new sulphur-containing LC materials with optical applications, also make a significant contribution to use as thermally sensitive liquid crystal devices requiring fast response.     

Optical and mesomorphic properties characterisation of chiral liquid crystalline copolysiloxane exhibiting cholesteric and blue phases

A series of liquid crystalline polymers (LCPs) have been synthesised by two cholesteric monomers M₁, M₂ and a nematic monomer M₃. The chemical structures and liquid crystalline properties of the monomers and polymers have been characterised by FTIR, ¹H-NMR, differential scanning calorimetry, thermogravimetric analyses, X-ray diffraction measurements and polarising optical microscopy. All LCPs show a high thermal stability with wide mesophase temperature ranges. For polymer P₁ bearing only cholesteric LC monomers component, it shows a cholesteric phase, whereas others display a blue phase besides a cholesteric phase. The formation of the blue phase is based on the structures of the polymers and the produced biaxial helix. The glass transition temperature and isotropic temperature of the polymers decrease on heating cycle with increasing the content of M₃ in the polymers. The specific rotation values of the polymers are temperature-sensitive. The reflection spectra of polymers P₁–P₆ show that the maximum reflected wavelengths shift to long wavelength with increasing the content of M₃ in the polymer systems. The frequency and intensity of the bands change sharply at the temperature where cholesteric phase changes to blue phase, but they show a weak dependence on temperature in the blue phase.     

Förster energy transfer in cholesteric mixtures: a new type of phototunable fluorescent material

A new approach to the creation of cholesteric glass‐forming materials with photovariable fluorescent properties is suggested. This approach is based on Förster type energy transfer from a photochemically active donor to a highly fluorescent acceptor. For this purpose, a cholesteric mixture containing two fluorescent dopants based on anthracene (Dianthr) and stilbene (DCM) was prepared and studied. The absorbance peak of DCM molecules overlaps the emission peak of Dianthr. The possibility of using energy transfer in cholesteric mixtures containing a photoactive energy donor capable of photobleaching is demonstrated. It is shown that UV irradiation of planarly oriented films of the mixture leads to photodimerization of the Dianthr dopant. This photoreaction results in a significant decrease in the emission intensity of the DCM dopant. In all cases the emitted light is strongly circularly polarized, and the degree of polarization does not change during photoreaction. Such types of photo‐patternable glass‐forming cholesteric materials combining fluorescent properties, the possibility of energy transfer between two fluorescent dyes and a photoactivity of one fluorescent component, provide new opportunities for optical data recording and storage.     

Cholesteric liquid crystal displays as optical sensors of barbiturate binding

The influence on the optical properties of cholesteric liquid crystal displays (LCDs) was examined for neutral molecule binding by mesogen/receptors in the mesomorphic phase. The motivation was to prepare neutral molecule sensors that use a colour change to signal analyte binding. A receptor that binds barbiturate analytes was modified with two or one cholesteryl groups to yield compounds 2 and 3, respectively. LCDs were prepared by incorporating one of the receptor/mesogen compounds into a cholesteric LC blend along with a potential H‐bonding guest. The optical properties of the LCDs were then determined by measuring the absorbance of the displays. For various LCDs, the colour of the display depended upon several factors: the amount of guest molecule used, the number of cholesteryl side chains on the receptor and the mole concentration of receptor/mesogen in the blend. In particular, complementary host/guest binding of H‐bonding analytes by the bis(cholesteryl) receptor 2 in a cholesteric LCD caused a change of up to +70 nm, which was observed by the naked eye as a blue‐to‐orange colour change. Control experiments confirm that the colour of an LCD is a consequence of molecular recognition in the mesomorphic phase.     

Side chain cholesteric liquid crystalline elastomers: synthesis and phase behaviour

A series of new side chain cholesteric liquid crystalline elastomers (P-2–P-6) containing the nematic crosslinking monomer 4-(10-undecen-1-yloyloxy)benzoyl-4′-allyloxybenzoyl-p-benzenediol bisate (M-1) and the cholesteric monomer 4-cholesteryl 4-(10-undecen-1-yloyloxy)benzoate (M-2) were synthesized. The chemical structures of the monomers and elastomers obtained were confirmed by FTIR and ¹H NMR spectroscopy. Their liquid crystalline properties and phase behaviour were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The effect of the crosslinking units on phase behaviour is discussed. Elastomers containing less than 20?mol?% of the crosslinking units showed elasticity, reversible phase transitions and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition and isotropization temperatures of P-2–P-6 increased with the increasing concentration of crosslinking unit M-1.     

Preliminary Communication. Refractive index of a sheared cholesteric polymer

Abstract

After unwinding a cholesteric solution of a mesomorphic polymer by shear, we investigate the evolution of the refractive index. Two mechanisms with different time constants are observed, one corresponding to the cholesteric rewinding, the other to the reorientation of the cholesteric axes.     

Optical properties and photonic density of states in one-dimensional and three-dimensional liquid-crystalline photonic crystals

ABSTRACT

Complex optical investigations were performed in one-dimensional (cholesteric) and three-dimensional (Blue Phase II) liquid-crystalline photonic crystals. Spectra of optical transmission and luminescence in the range of the photonic stop band contain information about the local anisotropy, characteristics of the photonic stop band, photonic density of states. We determine the photonic density of states in one-dimensional and three-dimensional liquid-crystalline photonic crystals employing measurements of polarised luminescence. The width of the photonic band gap, density of states and the optical characteristics related to the density of states essentially change with temperature in one-dimensional cholesteric photonic crystal. Drastic transformation of the density of states was found at the transition from one-dimensional to three-dimensional (Blue Phase II) photonic crystal. The results of our investigations demonstrate the possibility to employ the applied method for various types of photonic structures.     

Optical reorientation in cholesteric nematic mixtures

Abstract

The optical Fréedericksz transition for linearly polarized light at normal incidence is studied in mixtures of nematic E7 and cholesteric C15 in cells coated for homeotropic alignment. The reorientation process is found to be dramatically different from the case of pure nematic samples showing the phenomenon of optical phase locking and large hysteresis. These effects are ascribed to the occurrence of self-induced stimulated light scattering, which does not occur in pure nematics.     

Nematic liquid crystal gyroids as photonic crystals

ABSTRACT

We demonstrate nematic and cholesteric liquid crystal (LC) gyroids and show their photonic properties as photonic crystals by using numerical modelling. The LC gyroids are designed as composite optical materials, where we take one labyrinth of passages to be a solid dielectric, whereas the other (complementing) labyrinth of passages is taken to be filled by chiral or achiral nematic LC, with the intermediate gyroid surface imposing homeotropic (perpendicular) surface anchoring. The nematic inside the gyroid matrix is shown to exhibit a variety of possible orientational profiles which are characterised by complex networks of topological defects – from ordered, semi-ordered, to completely disordered. The diversity of possible nematic states is shown to lead to a rich structure of photonic bands, which can be tuned by the LC volume fraction and the cholesteric pitch, including control over full – direct and indirect – band gaps.     

Wide temperature range of an electrically tunable selective reflection of light by oblique helicoidal cholesteric

ABSTRACT

Oblique helicoidal cholesteric liquid crystals (Ch_OH) offer an unprecedented opportunity to tune selective reflection of light in a broad spectral range from ultraviolet to infrared by an electric field. The major problem is that the temperature range of stable Ch_OH is typically above the room temperature and is relatively narrow, a few degrees. In this work, we demonstrate that by using a mixture of flexible dimeric and rod-like molecules, one can significantly expand the temperature range of intense Bragg reflection, from 16°C to 27°C. We demonstrate that the selective reflection peak, reflection intensity, bandwidth and threshold electric field are all temperature dependent and discuss the associated mechanisms. The results show that both the electric field and temperature can be used to tune the structural colour of oblique helicoidal cholesteric structures. The proposed material can be used in switchable optical devices based on liquid crystals, such as light modulators, indoor smart windows, and filters.     

Preparation and characterization of side chain cholesteric liquid crystalline polysiloxanes containing two chiral groups

A series of liquid crystalline (LC) polysiloxanes containing diosgeninyl and menthyl groups (from monomers M ₁ and M ₂, respectively) were synthesized. The chemical structures of the monomers and polymers obtained were confirmed by elemental analysis, Fourier transform infrared spectroscopy, proton NMR and carbon‐13 NMR. The LC properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction. Monomer M ₁ showed cholesteric oily‐streak and spiral textures. Copolymers P ₂–P ₅ exhibited cholesteric phases. With increasing concentration of M ₂ units, the glass transition and clearing temperatures decreased. Experimental results demonstrated that a flexible polymer backbone and a long flexible spacer tended to favour a lower glass transition temperature, higher thermal stability, and wider mesophase temperature range.     

Potassium-ion-selective sensing based on selective reflection of cholesteric liquid crystal membranes

It is well-known that cholesteric liquid crystals have an optical property, selective reflection, due to changes in the pitch of their helical structure. This unique property of cholesteric liquid crystals can be used to attain a visual sensing system showing color changes as the detection signal. In this paper, we report a visual sensing membrane comprising cholesteric liquid crystals, in which a 15-crown-5 derivative was incorporated as ion recognizing sites, for K⁺ in aqueous solution. The resulting CLC membrane showed a shift of the reflection peak sensitive to K⁺ in water. We have also designed polymer-dispersed liquid crystal membranes that showed ion-selective reflection peak shifts with improved response time.     

Synthesis,structure and properties of new chiral liquid crystalline monomers and homopolysiloxanes containing menthyl groups

The synthesis is described of four new chiral liquid crystalline monomers (M₂–M₅ ) and their corresponding side‐chain homopolysiloxanes (P₂–P₅ ) containing menthyl groups. Chemical structures were characterised using FT‐IR or ¹H NMR spectra, and specific optical rotations were evaluated with a polarimeter. The phase behaviour and mesomorphic properties of the new compounds were investigated by differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy, UV/visible/NIR spectrocopy and X‐ray diffraction. The monomers and homopolymers with more aryl segments showed noticeably lower specific optical rotation value. The monomers M₂–M₅ formed a cholesteric or blue phase when a flexible spacer was inserted between the rigid mesogenic core and the terminal menthyl groups by reducing the steric effect. M₂–M₅ revealed enantiotropic cholesteric phase. Moreover, M₂ also exhibited a monotropic smectic A (SmA) phase, and M₄ also exhibited a cubic blue phase on cooling. The selective reflection of light shifted to the long wavelength region with increasing rigidity of the mesogenic core for M₂–M₅ . P₂–P₅ exhibited SmA phases, and the mesogenic moieties were ordered in smectic orientation with their centres of gravity in planes. Melting or glass transition temperature and the clearing temperature increased, and the mesophase temperature range widened with increasing rigidity of the mesogenic core.     

Mesomorphic properties of side-chain cholesteric liquid-crystalline elastomers

New monomer cholesteryl 4-(10-undecylen-1-yloxybenzoyloxy)-4′-ethoxybenzoate (M₁), crosslinking agent biphenyl 4,4′-bis(10-undecylen-1-yloxybenzoyloxy-p-ethoxybenzoate) (M₂) and a series of side-chain cholesteric elastomers were prepared. The chemical structures of the monomers and elastomers obtained were confirmed by element analyses, FT-IR, and ¹H NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. M ₁ showed cholesteric phase, and M ₂ displayed nematic phase. The elastomers containing less than 12 mol% of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability.     

Anomalous light scattering in photonic cholesteric liquid crystals

ABSTRACT

We have carried out polarisation and angle-resolved measurements of the light scattered from photonic cholesteric liquid crystals. Both in samples doped with laser dyes and in inactive (non-doped) samples we have observed pronounced directional dependences of the scattered light, finding angular ranges where the scattering is greatly enhanced and regions where the effect is almost suppressed. Moreover, the total amount of scattered light has also been found to depend strongly on the polarisation and direction of the incident beam. All the results have been interpreted successfully in terms of a simple expression proposed for the scattering cross section, in which the density of states of the ingoing and outgoing beams plays a major role. The expression would be applicable not only to cholesteric liquid crystals but to any one-dimensional photonic material.     

Effect of composition on the pressure-induced re-entrant cholesteric phase behaviour of mixtures of non-polar cholesteryl n-alkanoates

Abstract

Recently we reported a pressure-induced re-entrant cholesteric phase for ternary systems of non-polar cholesteryl n-alkanoates. In these systems one component contains a shorter n-alkyl chain than the other two. We now show that the former produces a positive excess volume in the smectic phase, which is probably responsible for the pressure-induced re-entrant phase behaviour. The maximum temperature of the cholesteric/smectic A phase boundary is found to decrease drastically with decrease of the n-alkyl chain length of this particular component.     

Polarization holographic grating recording in the cholesteric azobenzene‐containing films with the phototunable helix pitch

The recording of polarization gratings in films of a cholesteric liquid crystalline polymer with different helix pitch was studied in detail. For this purpose, the cholesteric mixture of the nematic azobenzene‐containing copolymer with a chiral‐photochromic dopant was prepared. The utilization of such mixture has made possible to realize dual optical photorecording in one system, first due to the phototuning of the helix pitch by UV light and second the polarization grating recording process by exposure with polarized visible light. The diffraction efficiency strongly depends on the cholesteric helix pitch and films thickness: the increase of the confinement ratio d/p (where d, film thickness; p, helix pitch) results in growth of the diffraction efficiency. Comparison of the induction of polarization gratings in cholesteric, nematic (copolymer without chiral dopant), and amorphous (nonannealed) cholesteric films revealed that only the cholesteric films were characterized by significant oscillations in the diffraction efficiency signal as well as by the presence of the maximum in the first‐order diffraction efficiency in the initial stage of the grating recording process. It was found that in addition to the polarization grating surface relief gratings (SRGs) were also formed in the studied systems, however, the amplitude of the SRG inscribed in the cholesteric films was lower (～20 nm) compared to the grating amplitude obtained in nematic films (～60 nm). Moreover, increasing helix pitch resulted in a decrease of the SRG amplitude. The obtained experimental data demonstrate the great potential of cholesteric LC mixtures of such type for different applications as photoactive materials for photonics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 773–781     

Visualization of enantiomers in cholesteric solvents through deuterium NMR

Abstract

Proton decoupled deuterium NMR of mixtures of enantiomers in homogeneously oriented cholesteric solvents produces simple spectra with linewidths of 10 to 50 Hz in cases where the proton spectra would give second order patterns so complicated as to defy analysis. The chiral solvent orders each of a pair of enantiomers differently which results in a difference in the residual quadrupolar coupling constant yielding well resolved spectra for each enantiomer. That the technique constitutes a new tool for measurement of enantiomeric ratios is illustrated using several chiral benzylic alcohols.     

Optical determination of the pitch-dependent diffusion constant in a cholesteric liquid crystal

Abstract

The concentration profile of a cholesteric compound diffusing into a nematic liquid crystal is observed with optical methods. We propose new experimental initial and boundary conditions which allow a more accurate mathematical solution of the differential equation involved. A numerical approach is applied to compute the concentration profiles with a concentration dependent diffusion constant. Diffusion is found to decrease with the shorter pitch length.     

Optical and liquid crystalline properties of dicholesteryl derivatives having different configurations of the hydrocarbon linkages

The liquid crystalline and optical properties of four dicholesteryl derivatives having various linkage configurations, but with the same number of connecting carbons (18), have been investigated, using differential scanning calorimetry, polarizing optical microscopy, Fourier transform infrared (FTIR) spectroscopy and UV-Vis spectroscopy. Quite different phase behaviours and optical properties have been found for these compounds, while all of them exhibited the behaviour specific for a cholesteric phase. By rapid cooling from the cholesteric phase to 0°C, the iridescent colours of some of the dicholesteryl esters were fixed in the cholesteric glassy state. It was difficult but possible to fix various stable colours in the cholesteric glassy state of one compound with a polymethylene (CH₂)₁₈ linking chain. It was much easier to fix less stable colours for samples with the linking groups (CH₂)₆ and C=C double bonds between them. Very unstable colours could also be fixed for the compound with the (CH₂)₇ chains and a diyne group. The fourth compound with a branched linking chain gave a cholesteric phase which showed colours at room temperature. FTIR spectroscopy measurements provided interesting results concerning the average change in molecular configuration between the blue cholesteric glass and the crystal for the compound with the (CH₂)₇ polymethylene chains which crystallized within a few hours; also frequency changes associated with some bands were found and reported.