Selective Light Reflection Research Articles

Photoinduced phase transitions and helix untwisting in the SmC* phase of a novel cinnamoyl-based liquid crystal

A photoinduced phase transition and helix untwisting in a new liquid crystal forming the SmC* phase were studied in detail. The compound consists of a cinnamoyl photosensitive fragment with C = C double bond capable of photoisomerisation and photocycloaddition. It was shown that ultraviolet (UV) irradiation (365 nm) induces an extreme decrease in phase transitions temperatures (SmC*–SmA*, SmA*–N*, N*–I). Vertically aligned samples in the SmC* phase cause selective light reflection in the visible spectral range. The light action results in a noticeable helix untwisting that causes a shift in the selective light reflection peak to the long-wavelength spectral region. The temperature dependence of spontaneous polarisation P s was measured and it was found that UV irradiation induces a decrease in the values of P s. Photo-optical phenomena taking place in the liquid crystal are attributed to the formation of photoproducts having low anisometry, which disrupts mesophases.

Synthesis and properties of new chiral mesogenic monomers and side-chain smectic homopolymers containing (−)-menthyl groups

The synthesis of new chiral monomers ( M 1 − M 5 ) and the corresponding smectic homopolymers ( P 1 − P 5 ) containing menthyl groups is described. 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 phase behavior and mesomorphism were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The selective reflection property of light was studied with UV/visible/NIR. The monomers M 2 − M 5 formed a chiral smectic C ( S C ∗ ) , and cholesteric or blue phase when a flexible linkage chain was inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect. M 1 showed no mesomorphism, while M 2 − M 5 showed enantiotropic S C ∗ and cholesteric phases. Moreover, M 5 also exhibited a cubic blue phase on cooling. With increasing temperature, the selective reflection of light shifted to the long wavelength region at the S C ∗ phase range, and to the short wavelength region at the cholesteric phase range, respectively. The homopolymers P 1 − P 5 all exhibited the batonnet textures of a 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.

Mesomorphic behavior and optical properties of liquid‐crystalline polysiloxanes bearing different chiral groups

AbstractA series of side‐chain liquid crystalline polysiloxanes bearing different chiral groups were synthesized with a cholesteric liquid crystalline monomer cholesteryl 2‐(4‐allyloxy‐benzoxy)‐succinate and a chiral monomer 2‐isopropyl‐5‐methyl‐cyclohexyl 4‐allyloxy‐benzoate. All the chiral polymers showed cholesteric mesophase with very wide mesophase temperature ranges. Along with phase morphology varied from solid state to isotropic state, the sample exhibited varied spectroscopic images, indicating different phase behaviors between mesophase and isotropic state. Optical properties of the polymers were characterized with circular polarization spectra and optical rotation analysis. The specific rotations of the chiral liquid crystalline polymers were all negative values and increased with increase of menthyl component in the polymer systems. As compared with corresponding monomers, the polymers showed significantly lower specific rotations, suggesting that the existence of polysiloxane main chains affect the molecule polarity. The selective light reflection maximum wavelength of the polymers at the same temperature shifted to short wavelength with decrease of cholesteryl component, indicating that the helical pitch becomes shorter due to induction of menthyl groups with high chiral properties. The temperature dependence of selective light reflection maximum wavelength was shifted to a longer wavelength region as the temperature decreased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

New steroid α,β-unsaturated ketones as chiral components of induced cholesteric liquid crystal systems

New (E)-16-arylidene derivatives of 3β-hydroxyandrost-5-en-17-one and their acetates containing different substituents in the arylidene fragment were synthesized. The ability of the synthesized chiral compounds to induce helical supramolecular ordering (their helical twisting power) upon the introduction into the nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) and into multicomponent mixtures E63 and LC-1289 characterized by a wide mesophase interval. The dependence of the helical twisting power of the studied chiral additives (CAd) on their molecular structure was analyzed. The highest helical twisting power (44.6–67.1 μm−1) was revealed for the synthesized acetates. It was found that the composites based on LC-1289 and E63 containing the studied CAd in very low concentrations (≤10–11 mol.%) have selective light reflection in the visible spectral region. The helical twisting power of the studied α,β-unsaturated ketones is determined by the combined influence of the anisotropy of polarizability of CAd molecules and specific features of their molecular shape.

Chirooptical and photooptical properties of a novel side-chain azobenzene-containing LC polymer

A novel chiral–photochromic side-chain polyacrylate with azobenzene fragments in the side groups has been synthesised. It was shown that the polymer forms a smectic phase and a cholesteric supramolecular helical structure with selective light reflection in IR spectral range. Thin spin-coated films of the polymer were prepared and their photooptical and chirooptical properties were studied in detail. It was found that UV irradiation of the films led to E–Z isomerization of the azobenzene moieties with high conversion, which is dependent on thermal prehistory of the films. Subsequent action of visible light results in partial recovery of the E-isomer content, whereas annealing leads to the full back conversion. Circular dichroism (CD) measurements revealed formation of the helical supramolecular structure even in the initial spin-coated polymer films. The E–Z isomerization induces complete disruption of helical order in non-annealed films of the polymer, whereas in the smectic phase of the annealed film only a significant decrease in CD values was found. In addition, the photoorientation phenomena induced by polarized light were studied. It was shown that polarized light induces linear dichroism in the films provided by azobenzene group orientation and the dichroism is stable at room temperature for a prolonged time. These combined chirooptical and photooptical features of this novel polymer enable one to consider this multifunctional compound as a promising material for photonics and for optical applications.

Novel type of combined photopatternable and electro-switchable polymer-stabilized cholesteric materials

A novel type of photopatternable polymer network-stabilized cholesteric materials for creation of electrooptical switching was elaborated. For this purpose cholesteric composites based on a commercial low-molar-mass nematic mixture, a chiral dopant, different amount of polymerizable mesogenic diacrylate (1–6 wt%) and a chiral-photochromic substance consisting cinnamoyl and isosorbide fragments (∼4 wt%) were prepared. A small amount of photoinitiator sensitive mostly to light with wavelength of 365 nm was also added. UV irradiation of the complex mixture with this wavelength light results in the polymerization of the diacrylate and formation of the polymer network stabilizing the initial planar texture in the electrooptical cell. Electrical field application induces a transition from the transparent planar cholesteric texture to a scattered focal conic one; at higher voltage the selective light reflection peak disappears. Irradiation of the cell with shorter wavelength light (313 nm) leads predominantly to the E–Z isomerization of the chiral-photochromic dopant followed by irreversible helix untwisting and to the shift of the selective light reflection from blue to green or red spectral regions. In this case, the competing photopolymerization process almost does not take place. Subsequent irradiation of the cell by longer-wavelength UV light (365 nm) allows one to obtain a polymer network and stabilize the new texture. Electrooptical properties of cells with different values of helix pitch were studied in detail. These data demonstrate the new opportunities for creation of photopatternable electrooptically switchable devices, which can be used in photonics and electrooptics.

Molecular Structure and Liquid-Crystalline Characteristics of Chitosan Phenylcarbamate

Chitosan phenylcarbamate (CtsPC) samples were synthesized to have different degrees of substitution (DS) ranging from approximately 2.7 to approximately 3.7, and the lyotropic liquid crystallinity was mainly characterized by spectrophotometry. The products of DS>2.8 formed a cholesteric type of mesophase in concentrated solutions of >44 wt % with polar aprotic solvents such as N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO), and some of the solutions imparted vivid colorations because of selective visible light reflection. The cholesteric helical pitch increased with increasing temperature and with decreasing polymer concentration, but the helical sense remained left-handed under the adopted measurement conditions. It was also found that the helical pitch increased in proportion to the increment of DS, whereas an average degree of phenylcarbamoyl polyaddition in the side chains (DPs) rather affected the pitch distribution measuring an orderliness in the cholesteric structure. With a small increase in DPs (e.g., from 1.01 to 1.04), the cholesteric orderliness decreased and the temperature sensitivity of the pitch turned sluggish. Wide-angle X-ray diffractometry was also used for evaluating the distance and azimuth difference between adjacent nematic thin layers in the cholesteric mesophase.

Synthesis and characterization of new cholesteric monomers and smectic polymers containing menthyl groups

New cholesteric monomers (M2−M5) and the corresponding smectic homopolymers (P2−P5) based on menthyl groups were synthesized. The chemical structures were characterized by Fourier transform infrared and 1H NMR. The specific optical rotations were evaluated with a polarimeter. The structure–property relationships of the new compounds are discussed. The mesomorphism was investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The selective reflection property of light was studied with UV/Visible/NIR. The monomers M2−M5 formed the cholesteric or blue phase when a flexible link chain was inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect. M1 showed no mesomorphism, while M2−M5 revealed enantiotropic cholesteric phase. In addition, M2 and M3 also showed a cubic blue phase on cooling. The selective reflection of light for M2−M5 shifted to the short reciprocal wavelength region with increasing the temperature or intramolecular spacer length. P2−P5 exhibited the smectic A phase. The melting, clearing, and glass transition temperatures increased when increasing the aryl number in the mesogenic core or decreasing the intramolecular spacer length.

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.

Selective reflection of light from a layer of cholesteric liquid crystal with induced inhomogeneity of helix pitch

A possibility to induce a planar defect in a plane-layer structure of a cholesteric liquid crystal via an external electric field is shown experimentally. The results of studies of behavior of the selective reflection (transmission) coefficient of light for a layer of the cholesteric liquid crystal, where planar defects are induced, are presented

Cholesteric Liquid-Crystalline Thermosets Derived from Side-Chain Liquid-Crystalline Epoxy Oligomers

A series of side-chain liquid crystalline (LC) oligomers bearing glycidyl ether groups were synthesized with cholest-5-en-3-ol(3 β)-4-(2-propenyloxy) benzoate, octyl 4-(4-allyloxy-benzoyloxy)-benzoate and epoxy monomer allyl glycidyl ether. The chemical structures and LC properties of the monomers and oligomers were characterized by use of various experimental techniques. All the chiral LC oligomers showed cholesteric mesophase with very wide mesophase temperature ranges. All the synthesized LC oligomers were cured with 4,4'-diaminodiphenylmethane at the same curing conditions. The thermal behaviors, mesomorphic phase and optical properties of the thermosets were characterized. The synthesized thermosets have high thermal stability. The cholesteric phases of LC oligomers were frozen in the polymer networks. Compared with those of corresponding oligomers, the reflection located at wide angles of thermosets became more diffuse, and the peak intensity decreased in X-ray diffraction analysis. The selective light reflection maximum λmax of the thermosets at room temperature shifted slightly to short wavelengths as the LC monomer bearing cholesteryl component was increased, indicating that the helical pitch P becomes shorter due to more chiral groups in the polymer network systems. The temperature dependences of both reflection wavelength and optical rotation under the influence of the crosslinking reaction were also investigated.

Synthesis and properties of high tilted antiferroelectric esters with partially fluorinated alkoxyalkoxy terminal chains

AbstractNovel chiral esters with partially fluorinated alkoxyalkoxy terminal chains are described. Their phase transition temperatures, enthalpies, and electrooptical properties are reported. A helical pitch in pure compounds and their mixtures based on selective reflection of light is also characterized.

Dielectric properties of LC mixture with induced antiferroelectric phase

AbstractMixture of two liquid crystalline components exhibits the antiferroelectric phase in a broad temperature range at room temperatures, though the two components separately do not show an antiferroelectric phase in a temperature range applied. The dielectric spectroscopy technique combined with measurements of the selective light reflection was used for identification and characterization of the phases and subphases existing in the mixture. In the SmC*A phase, the low frequency mode characteristic of antiferroelectric phase has been detected. In the broad temperature range between SmC*A and SmA* phases, no relaxation mode has been detected. The soft mode registered near the phase transition to SmA* phase follows the Curie-Weiss law.

Photoinduced Chiral Nematic Organization in an Achiral Glassy Nematic Azopolymer

AbstractA liquid crystalline homopolymer that has photoisomerizable methoxyazobenzene groups in the side chain has been synthesized and characterized. Thin films of the nematic glassy phase of this polymer have been processed in order to study the absorption spectra and the vibrational and electronic circular dichroism responses by irradiation with 488 nm circularly polarized light (CPL). Selective reflection of visible light demonstrates that the irradiation of this glassy nematic azopolymer induces a helix as a consequence of the chiral arrangement of the azobenzene units. Moreover, a wedge cell with an aligning layer for planar orientation was filled with the polymer with the aim of investigating the change in the macroscopic optical properties and optical textures of the azopolymer on irradiation with CPL. The transfer of chirality from CPL to azopolymer through chiral conformations is proposed as a model for explaining the supramolecular chirality.

Sculptured Layer‐by‐Layer Films

The precise assembly of nanostructured materials into two (2D) and three-dimensional (3D) periodic microscopic arrays is achieved by employing various assembling technologies such as microparticle self-organization, photolithography, holographic lithography, selective chemical etching, ink printing, laser-based polymerization, selective responsive grafting, and inversion of bilayers. Soft lithography, such as microcontact printing has been widely applied to tackle this challenge in a few steps, with a submicrometer resolution, and at low cost. It has been demonstrated that the most popular fabrication of well-defined nanostructured materials with layer-by-layer (LbL) assembly can be combined with photolithography and microprinting to fabricate complex 2D and 3D structures with modulated distribution of different components. Examples of the micropatterned assembly of nanoparticles, microchannels, antireflective coatings, and Raman arrays have already reported. A variety of functional materials have been utilized in the LbL construction to make these structures suitable for prospective applications as antiwetting coatings, sensitive films for solar cells, fuel cells, ultra-strong nanomaterials, microcapsules, and membranes for controlled drug release. On the other hand, microstructural arrays based upon the principles of either diffraction or refraction with in-printed microscopic modulations are extensively used as optical components such as grating, beam splitters, microlenses, displays, and mirrors. A vast majority of LbL structures have been fabricated on solid planar or curved (microparticle) supports and represent essentially 2D planar structures with vertical (along the surface normal) modulations of chemical composition. Micropatterning of LbL films has been recently reviewed by Hammond. Few examples of free-standing LbL structures have been reported either in the form of uniform, micropatterned, or curved shell structures. Therefore, although potential for the fabrication of interesting modulated structures, which provides for optical effects, is inherently present in the LbL technology, few attempts have been made to investigate the feasibility of the fabrication of modulated LbL structures for light control. For example, in a recent study, Rubner et al. have demonstrated that a properly matched LbL coating can serve for constructive–destructive light interferences with efficient antireflective ability. In a related study, Rubner and Cohen have demonstrated that a proper modulation of the reflective index within LBL films might lead to a pronounced structural color effect with selective reflection of visible light controlled by the reflective index modulation rather than the presence of conjugated molecules with proper electronic structures. Although these initial studies showed some potential in LbL technology to generate nanostructured materials with the ability to control visible light diffraction/reflection, they have been limited to one-dimensional modulation of the refractive properties. In this communication, an example of 3D LbL grating structures with the ability to diffract light because of the modulation of local LbL film shape with microscopic periodicity and nanometer-scale vertical modulations is reported. In these freely suspended 3D LbL films, the effective modulation of the refractive properties is caused by the topological variation of the local film shape, thus representing a purely structural color effect. A simple and economical spin-assisted LbL assembly of conjugated polyelectrolytes on a sacrificial microimprinted modulated substrate was employed here to generate a robust, free-standing sculptured LbL structure with an effective thickness of 60 nm and a 160 nm peak-to-peak vertical modulation on a square lattice with 2.5 lm lateral periodicity (Figure 1). These films demonstrate efficient optical grating properties and bright structural colors in a reflective mode controlled by the in-plane spacing and the angle of incidence. The conjugated polyelectrolyte, poly(2,5-methoxypropyloxy sulfonate phenylene vinylene) (MPS-PPV) applied in the LbL assembly synthesized here was critical in providing a mechanically strong structure as has been tested in our previous work to fabricate mechanically robust planar ultrathin LbL films. MPS-PPV is a well known water-soluble conjugated polymer with a highly charged, stiff backbone and strong fluorescence properties. Planar LbL films of MPS-PPV and poly(allylamine hydrochloride) (PAH) demonstrate excellent mechanical properties combined with high fluorescence as has been demonstrated in our previous work. C O M M U N IC A IO N

New (S)‐1‐phenylethylamine N‐arylidene derivatives as chiral dopants to liquid crystalline systems

New chiral N‐arylidene (S)‐1‐phenylethylamines with a 4,4″‐biphenylene group in the central path of the rigid core were synthesized and their mesomorphic properties investigated. The helical twisting power and the temperature dependences of the helical pitch and the selective light reflection were analysed for several liquid crystalline systems based on 4‐pentyl‐4′‐cyanobiphenyl, E63 (Merck) and 4‐(n‐hexyloxy)phenyl 4′‐(n‐butyl)benzoate containing new chiral components. The chiral compounds investigated induce helical ordering effectively in all the host materials and have significant potential for applications. The results obtained for the new chiral compounds are compared with those for some previously studied similar chiral additives.

Circular Differential Scattering of Light in Films of Chiral Polyfluorene

Circular differential transmission in thick films (1.1 mum) of poly{9,9-bis[(3S)-3,7-dimethyloctyl]-2,7-fluorene} is investigated. The vitrified liquid crystalline film obtained after annealing shows high circular differential transmission of light in the wavelength range where the polyfluorene does not absorb (lambda > 450 nm). Using a specifically designed chiroptical setup, we show that circular selective reflection of light in which the circular polarization of the light is retained after reflection, a process which is characteristic for cholesteric liquid crystalline films, makes a negligible contribution to the differential transmission. Using an integrating sphere, we show that circular differential scattering can account for the observed circular differential transmission for lambda > 450 nm.

Photo-optical properties of photopolymerizable cholesteric compositions

Compositions consisting of nematogenic diacrylate monomer and chiral dopants capable of forming the cholesteric mesophase and of photopolymerizing were obtained and studied. For the first time, the change of optical properties and cholesteric helix pitch during photocrosslinking of diacrylate under UV irradiation (365 nm) was investigated. The kinetics of photopolymerization was studied and several stages of this process were observed. Cholesteric photopolymerizable blends containing chiral photosensitive dopant capable of E–Z isomerizing under UV irradiation (313 nm) were studied. The decrease of the helix twisting power of the dopant during photoisomerization was observed resulting in untwisting of the helix and shift of the selective light reflection peak into the long-wavelength region of the spectrum. The possibility of the photoregulation of optical properties of such blends with following fixation of structure and of these properties by means of photopolymerization under UV irradiation (365 nm) was demonstrated.

Cristais líquidos colestéricos: a quiralidade revela as suas cores

Chirality deeply impacts the macroscopic properties on all scales of the universe from elemental particles through biological structures up to astronomic systems. Specifically, chiral liquid crystals give rise to self-organized macrostructures with unparalleled properties. Here we present easy experiments with a special class of chiral liquid crystals that show temperature-induced selective reflection of light (thermochromic effect). Understanding this thermochromic effect allows undergraduate students better realize important properties of super and supramolecular systems.

Preparation of Cholesteric (Hydroxypropyl)cellulose/Polymer Networks and Ion-Mediated Control of Their Optical Properties

(Hydroxypropyl)cellulose (HPC)/vinyl polymer networks were synthesized in film form from liquid-crystalline solutions of HPC in a mixed solvent of methacrylate monomer/methanol/water (2:1:2 in weight) containing cross-linking agents, via photopolymerization of the methacrylate monomer. Di(ethylene glycol) monomethyl ether methacrylate (DEGMEM) or 2-hydroxypropyl methacrylate (HPMA) was used as the polymerizing monomer, and tetra(ethylene glycol) diacrylate and glutaraldehyde were the cross-linkers for the monomers and HPC, respectively. The polymer composite films, HPC/PDEGMEM and HPC/PHPMA, prepared at ca. 60-70 wt % concentrations of HPC in the starting solutions, were iridescently colored due to the selective light reflection, originating from the cholesteric helical arrangement carried over successively into the network system. When the cholesteric films were immersed and swollen in water containing an inorganic neutral salt, their coloration and optical turbidity varied according to a strength of 'chaotropicity' of the impregnant ions. This ionic effect may be interpreted as essentially identical with that found formerly in the coexistent salt-sort dependence of the cholesteric pitch and lower critical solution temperature for HPC aqueous solutions. It is also demonstrated that visual appearance of the swollen networks can be changed by application of an electric potential of practical magnitude between both edges of the samples of rectangular shape.