Fluorene‐based liquid crystalline networks with linearly polarized blue emission

A series of fluorene‐based luminophores containing methacrylic end groups have been prepared and incorporated into uniaxially oriented liquid crystalline films by in situ photopolymerization. Various structural modifications on the 2‐(4‐cyanophenyl)fluorene core, which include alkyl chains at the 9‐position and elongation of the rigid core with one additional phenyl ring, have been investigated to generate emitters with adjusted liquid crystal compatibility, improved luminescence and dichroic properties. Polarized blue‐emitting films were produced that had an acceptable photostability, and it was found that the polarization emission was better for samples with low (5%) cross‐linker contents. Polarization of the luminescence was favored by the liquid crystalline properties of the luminophore. In addition, the detrimental effect of the alkyl substituent at the fluorene core on the mesomorphism and on the emission polarization can be overcome by lengthening the π‐system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4804–4817, 2007     

Dielectric behaviour of two dimeric liquid crystalline siloxanes

The temperature and frequency dependence of their dielectric constants have been investigated for two dimers consisting of swallow-tailed terminal groups connected via siloxane central parts of different length. The results have been compared with those of the respective monomer. The central part contributes in different ways to the static dielectric constants in the isotropic phase. From relaxation measurements, it can be concluded that the reorientation process about the short axis of the terminal groups is comparable with the respective process in monomers. The relaxation frequencies as well as intensities are almost uninfluenced by the siloxane part which suggests a very flexible structure of such dimers. Unfortunately, the maintenance of the antiparallel arrangement of swallow-tailed parts of the dimers could not be proved because of insufficient material for experimental work on the SA phase.     

Cholesteric liquid crystalline materials reflecting more than 50% of unpolarized incident light intensity

Cholesteric liquid crystals (CLCs) selectively reflect light when the wavelength matches the helical pitch. The reflectance is limited to 50% of ambient, unpolarized light because only circularly polarized light of the same handedness as the helix is reflected. Here the elaboration procedure and the properties of a CLC gel whose optical characteristics go beyond the 50% reflectance limit are reported. Photopolymerizable monomers are introduced into the volume of a CLC exhibiting a thermally induced helicity inversion and the blend is then cured with UV light when the helix is right-handed. The reflectance exceeds 50% when measured at the temperature assigned at a cholesteric helix with the same pitch but a left-handed sense before reaction. The reflection properties are investigated in the infrared region. From scanning electron microscopy investigations, it is shown that the organization of the mesophase is transferred onto the structure of the network. The gel structure is discussed as consisting of a polymer network with a helical structure containing two populations of low molar mass LC molecules. Each of them is characterized by a band of circularly polarized light which is selectively reflected. The monitoring of the optical response with temperature offers the opportunity to discriminate the respective contributions of the bound and free fractions of LC molecules to the reflectance, and to give evidence of the progressive increase of the reflected flux when the temperature decreases from the curing temperature. Novel opportunities to modulate the reflection over the whole light flux range are offered. Potential applications are related to the light management for smart windows or reflective polarizer-free displays with a larger scale of reflectivity levels.     

Emission of polarized light from liquid crystalline segmented poly(arylenevinylene)s

Polyesters with substituted bis(styryl)benzene segments, biphenylene segments, and linear aliphatic spacers in the main chain were obtained by polycondensation. These polyesters show intensive photoluminescence and exhibit thermotropic behavior. UV dichroism, polarized photoluminescene, and polarized electroluminescence were observed when films of the polyesters were cast from solution on rubbed polyimide and subsequently annealed. LC phases get disorders with increasing amount of biphenylene moieties in the main chain. The macroscopic order parameters derived thereof are a nonlinear function of the order of the LC‐phase.     

The effect of an electric field on the selective reflection bands of liquid crystalline blue phases

The effect of an electric field on the Bragg reflection of the liquid-crystalline cubic blue phases BPI and II has been studied. The results can be explained by a reorientation of the cubic BP lattice in the electric field. A BP II/I phase transition cannot be induced by the field in the systems under examination.     

Photoelectron spectroscopy of above-threshold ionization of xenon with circularly and linearly polarized light

Above-threshold multiphoton ionization (photon energy 1.17 eV and 2.33 eV) of xenon was studied with circularly and linearly polarized light. Pronounced differences in shape have been observed for photoelectron spectra taken with linearly and circularly polarized light. With circularly polarized light a strong suppression of low-energy electrons was observed; the total electron yield was reduced by factors of approximately 4 (photon energy 2.33 eV) and, depending on the laser intensity, between 13 and 80 (photon energy 1.17 eV).     

Thermal stability of alignment of a nematic liquid crystal induced by polyimides exposed to linearly polarized light

The thermal stability of alignment of a nematic liquid crystal (LC) on three polyimide (PI) films exposed to linearly polarized light at 366 nm was investigated. Polarizing optical microscopy analysis indicates that the thermal stability of the LC alignment on the PI film without significant structural change was higher than that with obvious structural change.     

Photoelectron-spectroscopy of multi-photon ionization of rare gases with circularly and linearly polarized light

Above-threshold multiphoton ionization of xenon, krypton, and argon was studied with circularly and linearly polarized light (photon energy 1.17 eV and 2.33 eV). With linearly polarized light photoelectrons are preferentially ejected along the direction of the polarization vector. With circularly polarized light a strong suppression of ejected photoelectrons was observed; the measured yield of photoelectrons was reduced by factors of up to 80 depending on the laser intensity and the photon energy. The experimental results are compared with theoretical calculations based on a multiphoton-detachment model.     

Thermal stability of alignment of a nematic liquid crystal induced by polyimides exposed to linearly polarized light

The thermal stability of alignment of a nematic liquid crystal (LC) on three polyimide (PI) films exposed to linearly polarized light at 366 nm was investigated. Polarizing optical microscopy analysis indicates that the thermal stability of the LC alignment on the PI film without significant structural change was higher than that with obvious structural change.     

Stretching effect of linearly polarized Ar+ laser single-beam on azo polymer colloidal spheres

This work shows that a linearly polarized Ar+ laser single-beam irradiation can cause stretching deformation of azo polymer colloidal spheres along the polarization direction of the laser beam. An epoxy-based polymer, containing 4-amino-4'-carboxyazobenzene at each repeat unit, was used to construct the colloidal spheres. The colloidal spheres were prepared by gradual hydrophobic aggregation of the polymeric chains in a THF/H2O dispersion medium, which was induced by a steady increase in the water content. When the obtained colloidal spheres were exposed to the spatially filtered and collimated Ar+ laser beam (488 nm, 150 mW/cm2), the colloids were stretched along the polarization direction of the laser beam. In the testing period (20 min), the colloids were deformed continuously as the irradiation time increased. When 2D close-packed arrays of the colloidal spheres were irradiated by the polarized laser single-beam, the colloidal spheres were all uniformly stretched along the polarization direction of the laser beam. On the contrary, when the arrays were irradiated by the interfering p-polarized laser beams, only the colloidal spheres in the bright regions of the interference pattern were significantly deformed.     

Alignment of guest-host liquid crystals with polarized laser light

Surface-mediated alignment of nematic liquid crystals with polarized laser light was reported recently [1]. In this communication we describe the alignment of a guest-host liquid crystal medium with polarized laser light. Liquid crystals in the illuminated region orient perpendicular to the direction of the laser polarization and remain aligned in the absence of laser radiation. The liquid crystals can be reoriented again by subsequent illumination. The kinetic feature of this surface-mediated liquid crystal orientation is characterized by the presence of coexisting liquid crystal regions of directors pointing away from the initial alignment.     

New ferroelectric liquid crystalline materials with an azo group in the molecular core

Six series of new liquid crystalline materials with an azo group (N=N) located in different parts of the mesogenic core of the molecule have been synthesized and their physical properties studied. The chiral segments of these materials are based on alkoxypropionate or alkyllactate units. It has been found that lateral methyl substitution on a phenyl ring in the molecular core disturbs the packing of the molecules. As a result smectic phases disappear and the phase transition temperatures decrease. In addition, shifting the N=N group closer to the chiral centre of the molecule leads to the disappearance of the ferroelectric SmC* phase. For the compounds which show the ferroelectric SmC* phase, the temperature dependence of the spontaneous polarization, the spontaneous tilt angle, the helix pitch length and the complex permittivity has been studied. The effect of shifting the azo group in the molecular core on the physical properties is discussed.     

Photo-induced alignment of a photo-crosslinkable liquid-crystalline copolymer film by linearly polarized UV light

The anisotropic photoreaction of a liquid-crystalline (LC( copolymer comprising one co-monomer being anisotropically photo-crosslinkable by linearly polarized (LP( -UV light and the other being insensitive to UV-irradiation is presented. Polarized UV and IR studies suggested a photo-induced alignment of the mesogenic side groups during the LP-UV photoreaction at the LC temperature range of the copolymer. The direction of the aligned mesogenic groups is parallel to the electric vector of the incident LP-UV light and the induced birefringence is 0.07.     

Variation in molecular stacking resulting from the different polarity of liquid crystalline molecules: synthesis and study of azo dye compounds

To investigate the effect of molecular polarity on the packing of liquid crystalline molecules, two liquid crystals, N,N-disubstituted aminophenylazo-4-alkylbenzenes, were synthesized and studied by single crystal structure determination. A comparison of the resulting molecular stacking with that of N,N -disubstituted aminophenylazo-4-butylbenzoate was made.     

Variation in molecular stacking resulting from the different polarity of liquid crystalline molecules: synthesis and study of azo dye compounds

To investigate the effect of molecular polarity on the packing of liquid crystalline molecules, two liquid crystals, N , N -disubstituted aminophenylazo-4-alkylbenzenes, were synthesized and studied by single crystal structure determination. A comparison of the resulting molecular stacking with that of N , N -disubstituted aminophenylazo-4-butylbenzoate was made.     

Optical anisotropy of a photoreacted side-chain liquid-crystalline polymer induced by linearly polarized UV light

Linearly polarized (LP) UV photoreaction of a photo-crosslinkable side-chain liquid-crystalline polymer (SLCP) containing photoreactive cinnamoyl and biphenyl mesogenic groups ( 1 ) was studied. The optical anisotropy of the polymer film was induced by the LP-UV photoreaction and was investigated by the temperature-controlled polarized UV absorption spectroscopy and polarized FT-IR measurements. The reorientation of the nonreacted mesogenic groups along to the Ê direction of the incident LP-UV light during the photoreaction occurred at the LC temperature range of the polymer, and the induced birefringence Δn was about 0.02. Because of the high-density photo-crosslinking, the LP-UV photoreacted film showed orientational stability up to 160°C. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1521–1526, 1998     

Asymmetrical phenyldiacetylenes liquid crystalline compounds with high birefringence and characteristics of selective reflection

A series of asymmetrically disubstituted liquid crystalline compounds with high birefringence based on phenyldiacetylenes was synthesised by coupling of intermediate molecules with a phenylacetylene fragment. The structures of the intermediates and resulting compounds were confirmed by Fourier transform infrared, ¹H nuclear magnetic resonance and Matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Their liquid crystalline behaviour, including transition temperatures and phase sequences, was investigated by differential scanning calorimetry and polarising optical microscopy. As an important parameter, the birefringence was also measured, using polarising light interferometry. The results indicated that the compounds exhibited different liquid crystalline phases in the high temperature region, with high birefringence values. In addition, the selective reflection characteristics associated with birefringence under conditions which maintained the matrix nematic liquid crystal and the concentration of chiral dopant were studied.