Showing papers in "Molecular Crystals and Liquid Crystals in 2001"
TL;DR: In this paper, the effects of 1-d photonic band-gap materials on the fluorescence spectrum of dyes dissolved in cholesteric liquid crystal, as well as on the fluorescent spectrum of the liquid crystal itself were studied.
Abstract: Cholesteric liquid crystals, due to their periodic structure and large birefringence, are 1-d photonic band-gap materials. Circularly polarized light of the same handedness as the cholesteric structure cannot propagate in the reflection band. We have studied the effects of this band-gap structure on the fluorescence spectrum of dyes dissolved in the cholesteric liquid crystal, as well as on the fluorescence spectrum of the liquid crystal itself. We have found that emission is enhanced at the band edges, and, above a certain pump threshold, lasing occurs both in dye-doped and in pure liquid crystals. Extremely low lasing thresholds and high efficiencies were observed in dye-doped systems. The results suggest an active energy transfer mechanism between the LC host and the fluorescent dye molecules.
104 citations
TL;DR: In this paper, electron and hole mobility data were measured by the time-of-flight method over a wide temperature range along five different crystallographic directions, and full tensor data (principal axes mobilities, tensor orientations and tensor rotations with temperature) have been determined.
Abstract: Using ultrapurified, highly perfect anthracene crystals electron and hole mobility data were measured by the time-of-flight method over a wide temperature range along 5 different crystallographic directions. From these results full tensor data (principal axes mobilities, tensor orientations and tensor rotations with temperature) have been determined. The highest principal electron mobility component is close to the a axis, reaching 35cm2/Vs at 20K, the highest hole component along b, reaching 50cm2/Vs at 35K. For electrons moving along the crystallographic c direction a transition from a nearly temperature-independent mobility (400K-100K) to a mobility increasing upon further cooling has been found, with very similar absolute values and transition temperature as previously reported for naphthalene and explained as a hopping to band transition. A non-Ohmic, sublinear velocity–electric field relation, reflecting hot charge carriers, has been obtained between 20 and 36K for electrons drifting along ...
84 citations
TL;DR: In this paper, a variety of anions of pharmaceutical interest, such as salicylate, citrate, glutamate and aspartate, were intercalated using two different synthesis methods, a direct one (coprecipitation) and an indirect one (anion exchange of dodecylsulfate).
Abstract: Layered double hydroxides (LDHs) are a class of materials with increasing interest due to their singular properties, which make possible their application in a wide variety of fields. Because LDHs can intercalate anions within their interlayer region, they can be used in medicine as drug supports or matrices. Once encapsulated, the drug could be released at a rate dependent on the pH value. The aim of the present work was to intercalate a variety of anions of pharmaceutical interest, such as salicylate, citrate, glutamate and aspartate, using two different synthesis methods, a direct one (coprecipitation) and an indirect one (anion exchange of dodecylsulfate). The materials obtained were characterised by PXRD, simultaneous TG-DTA, chemical analysis and FT-IR spectroscopy. Furthermore, the buffer effect of the intercalated LDHs was evaluated.
56 citations
TL;DR: In this paper, a theoretical investigation of molecular ordering of smectic C liquuid crystals was carried out by the method based on the Rayleigth-Schrodinger perturbation theory and the mutual arrangement of two molecules corresponding to the minimum of the total interaction energy between them Upair has been established.
Abstract: A theoretical investigation of molecular ordering of smectic C liquuid crystals C8H17O-C6H4-C6H 4-COO-C6H4-OC8H17 { A }, C9H19O-C6H4-COO- C6H4-COO-C6H4-NO2 { B } and its binary mixtures { A+B } has been carried out by the method based on the Rayleigth-Schrodinger perturbation theory. The mutual arrangement of two molecules corresponding to the minimum of the total interaction energy between them Upair has been established. The curves of the dependence of Upair and its various contributions on displacement of molecules from each other along the molecular long X-axis and angle θ between molecules' long axes has been also computed in the minimum point vicinity. The obtained results make it possible to determine the peculiarities of the structural organization of molecules, as well as to construct a model of the structure of A and B compounds in different phases taking into account the most probable packing of molecules.
52 citations
TL;DR: In this paper, the interaction between two rigid molecules for three mesogenic compounds is calculated by atom-atom potential method taking into account Van der Walls' and electrostatic interactions, and the most favorable arrangements for the dimers are found.
Abstract: Interaction between two rigid molecules for three mesogenic compounds are calculated by atom-atom potential method taking into account Van der Walls' and electrostatic interactions. Stacking, in-plane and overall minimum energy configurations are studied and the most favourable arrangements for the dimers are found. The Van der Waals' interactions are found to be determinant in the dimer stabilization. The stable configurations are compared with the previous data obtained from crystal structure analysis and from X-ray scattering studies in mesophases.
50 citations
Abstract: The liquid crystals have been discovered in 1888. Until about 1900 only few mesogenic compounds were known. Daniel Vorlander started his work on liquid crystals about one century ago and he may be called “father of liquid crystal chemistry”. By systematical syntheses of compounds with predetermined geometrical shape he was able to derive his rule of the most extended molecular shape. He initiated many special areas of LC chemistry, most of which are still topical. The historical development of the some topics of LC research will be sketched. Among the topics are: the role of impurities and polymorphism, inclusion of different chemical moieties (aromatic, cycloaliphatic and heterocyclic compounds, metallomesogens, supramolecular systems), compounds with unconventional molecular shape (twins, banana shaped compounds, lateral substituted compounds), glassy mesogens, oligomeric and polymeric compounds, discotic compounds, dendrimers. materials for applications.
49 citations
TL;DR: In this paper, a new photosensitive chiral dopant based on cinnamic acid and isosorbide was synthesized and the helical twisting power of the chiral dioxide was calculated.
Abstract: A new photosensitive chiral dopant based on cinnamic acid and isosorbide was synthesised. UV light action leads to E-Z photoisomerization of the dopant about C=C double bond of cinnamoyl fragment. Mixtures of the dopant with nematic and cholestric polymers were studied. It was shown, that planarly-oriented films of mixtures possess a selective light reflection in ultraviolet, visible and infrared regions of spectrum depending on the dopant concentration. The helical twisting power of the chiral dopant was calculated. UV light action on mixture films leads to a shift of selective light reflection peak to the long wavelength region of spectrum, which is explained by photoisomerization and decrease of the helical twisting power of the dopant. Kinetics of the process of photoinduced untwisting of the cholesteric helix was studied. Advantages of the use of the synthesised dopant and of mixtures based on it were demonstrated in comparison to earlier studied chiral-photochromic materials for optical dat...
48 citations
TL;DR: In this paper, the energies of pair interaction of molecular systems: first excited state and ground state of molecules pentyl - and pentyloxy - cyanobiphenyls was calculated and equilibrium configuration of these molecular systems was determinate.
Abstract: The energies of pair interaction of molecular systems: first excited state and ground state of molecules pentyl - and pentyloxy - cyanobiphenyls was calculated and equilibrium configuration of these molecular systems was determinate Pair orientated destabilization energies evaluation is given at the absorbtion and emission processes
48 citations
TL;DR: Microcystin is a bacterial hepatotoxin generated from the cyanobacteria causing waterblooms and its direct detection range using an immobilized aptamer and a surface plasmon resonance biosensor was 50 μ/ml to 1000μ/ml.
Abstract: Microcystin is a bacterial hepatotoxin generated from the cyanobacteria causing waterblooms. A DNA aptamer binding to microcystin was screened by the in vitro selection method of twelve rounds in order to obtain a sorbent suitable for a microcystin detection system. The obtained DNA aptamer showed specific binding to microcystin. The direct detection range using an immobilized aptamer and a surface plasmon resonance biosensor was 50 μ/ml to 1000 μ/ml.
44 citations
TL;DR: In this paper, small angle Neutron scattering (SANS) was used to study the structure of the adsorbed polymer layers of monodisperse discotic laponite clays.
Abstract: Normally synthetic well defined monodisperse discotic laponite clays are known to form a gel phase at mass concentrations as low as a few percent in distilled water. Hydrosoluble polymer polyethylene oxide was added to this intriguing clay system, it was observed that it either prevents gelation or slows it down extremely depending on the polymer weight, concentration or the laponite concentration. Small Angle Neutron scattering (SANS) was used to study these systems because only by isotopic labeling can the structure of the adsorbed polymer layers be determined. The contrast variation technique is specifically used to determine separately the different partial structure factors of the clay and polymer. In this way the signal of the adsorbed chains is separated from the signal of the free chains in the dilute regime. Attempts have also been made to characterize the structure in the concentrated regime of laponite with polymer.
43 citations
TL;DR: In this paper, a search method for investigating the extent to which chloro-methyl interchange is exhibited by crystal structures deposited with the Cambridge Crystallographic Database is presented, and 105 pairs were identified for which there was a common molecular framework, differing only in the presence of a chloro or methyl group.
Abstract: A search method for investigating the extent to which chloro-methyl interchange is exhibited by crystal structures deposited with the Cambridge Crystallographic Database is presented. Using the October 1998 release with 190,307 data entries, 105 pairs were identified for which there was a common molecular framework, differing only in the presence of a chloro or methyl group. Approximately 30% appear to demonstrate isotructural packing arrangements. Two examples within the database that were found not to show Cl-Me interchange were subsequently examined from the viewpoint of structural mimicry and the formation of mixed crystals.
TL;DR: In this paper, the authors have observed extremely efficient optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by photoexcited dopant molecules.
Abstract: In nematic liquid crystal films doped with methyl-red dye or azobenzene liquid crystal, we have observed extremely efficient optically induced director axis reorientation effects, and refractive index change coefficients >> 1 cm2/W. The basic mechanisms involved are determined to be optically induced space charge fields and flow reorientation, order parameter modification and molecular torques by the photo-excited dopant molecules. These supra-optical nonlinearities (SONs) enable nonlinear image conversion, optical limiting and holographic grating formation with μW power lasers.
TL;DR: In this article, a mesogenic polymer stabilized (MPS)-V-FLCD has been fabricated, which exhibits symmetric electrooptic characteristic called V-shape EO performance by choosing an appropriate photocuring condition such as applied voltage waveforms and the concentration of photocurable mesogenic monomers.
Abstract: We have succeeded in fabricating a novel FLCD, which exhibits symmetric electrooptic (EO) characteristic called V-shape EO performance, by choosing an appropriate photocuring condition such as applied voltage waveforms and the concentration of photocurable mesogenic monomers. Our mesogenic polymer stabilized (MPS)-V-FLCD shows high contrast ratio owing to zig-zag defect free, continuous grayscale without accompanying switching domains, and good switchability with TFTs due to free from the value of the spontaneous polarization.
TL;DR: In this paper, photo-aligned optical thin-film elements on plastic films are presented; colorless nematic wide-view films for twisted nematic (TN)-LCDs, ideally bright polarisation interference color filters which meet PAL/SECAM standards, as well as photoaligned and photo-patterned linear polarizers.
Abstract: Photo-alignment and photo-patterning of monomeric- and polymeric liquid crystals by the linear photo-polymerisation (LPP) technology enables novel molecular configurations in liquid crystal displays (LCDs) as well as a plethora of new thin-film optical elements on single plastic substrates. A molecular building block design of LPP photo-co-polymers with display-specific functional groups is depicted. Contrast and brightness of twisted nematic (TN)-LCDs are shown to markedly improve by combining photo-aligned multi-domain TN-LC-configurations with liquid crystal polymer (LCP) wide-view films. To illustrate the diversity of the LPP/LCP technology, the following photo-aligned new optical thin-film elements on plastic films are presented; colorless nematic wide-view films for TN-LCDs, ideally bright polarisation interference color filters which meet PAL/SECAM standards, as well as photo-aligned and photo-patterned linear polarizers. Photo-generated monomer and polymer liquid crystal devices are key t...
TL;DR: In this article, a series of intermolecular hydrogen bonding complexes have been synthesized by using p-nalkoxybenzoic acids (alkyl chain length varies from propyl- to decyl and dodecyl-) and a non-mesogenic moiety, 2-amino-5-chloro-pyridine (ACP), the thermal and phase behaviour of these complexes are studied by thermal microscopy (TM) and differential scanning calorimetry (DSC) techniques.
Abstract: A novel series of intermolecular hydrogen bonding complexes have been synthesized by using p-n-alkoxybenzoic acids (alkyl chain length varies from propyl- to decyl and dodecyl-) (ABA) and a non-mesogenic moiety, 2-amino-5-chloro-pyridine (ACP), The thermal and phase behaviour of these complexes are studied by thermal microscopy (TM) and differential scanning calorimetry (DSC) techniques. The stabilization of intermolecular hydrogen bonding is studied by IR spectra. A detailed IR spectral investigation in solid and solution states suggest that both acid and pyridine nitrogens are complementary to one another by acting as proton donor and acceptor respectively. The comparative thermal studies with free benzoic acids and the corresponding analogous hydrogen bonded complexes reveal the inducement of a monotropic smectic-G phase with simultaneous quenching of nematic and smectic-C phases.
TL;DR: The first liquid crystalline (LC) binaphthyl derivatives were presented in this article, in which the rigid rod-like substituents containing phenylcyclohexyl mesogenic cores were introduced into the 2, 2′, 6, and 6′ positions of the chiral binphthyl rings.
Abstract: We present the first liquid crystalline (LC) binaphthyl derivatives, in which the rigid rod-like substituents containing phenylcyclohexyl mesogenic cores are introduced into the 2, 2′, 6, and 6′ positions of the chiral binaphthyl rings. The binaphthyl derivatives show enantiotropic smectic A phase, but neither chiral nematic nor chiral smectic phase. The present quadri-substituted binaphthyl derivatives gave larger values in twisting power (βM), compared with the non-substituted and di-substituted binaphthyl derivatives.
TL;DR: In this paper, a series of ferroelectric liquid crystalline materials containing methyl and methoxy lateral groups on the aromatic ring of the core unit, as well as non-substituted materials have been synthesized.
Abstract: New series of ferroelectric liquid crystalline materials containing methyl and methoxy lateral groups on the aromatic ring of the core unit, as well as non-substituted materials have been synthesised. All compounds exhibit a wide temperature range of the ferroelectric chiral SmC* phase. A low temperature ferroelectric phase was detected for some of the investigated compounds probably composed of the SmC* phase and a crystalline phase. Measurements of spontaneous polarisation and tilt angle, helical pitch and dielectric permittivity has been performed.
TL;DR: In this article, a cholesteric liquid crystal (CLC) film doped with a mixture of laser dyes has been observed under UV excitation with 35ps pulses at 355nm.
Abstract: Lasing in cholesteric liquid crystal (CLC) films doped with a mixture of laser dyes has been observed under UV excitation with 35ps pulses at 355nm. To probe optical interactions between molecules ...
TL;DR: In this paper, a hemicyanine with a disulfide group was synthesized to construct a self-assembled monolayer array on the Au surface with simple immersion of the Au substrate in its ethanol solution.
Abstract: In order to construct hemicyanine self-assembled monolayers (SAMs), we have synthesized a hemicyanine with a disulfide group. Surface plasmon resonance spectroscopy and reflection absorption spectroscopy revealed formation of the hemicyanine monolayer array on the Au surface with simple immersion of the Au substrate in its ethanol solution. While many works have reported the association of H-aggregates in hemicyanine monolayers, we could not observe the H-aggregation in the present case.
TL;DR: Uracil-fused tetrathiafulvalenes (TTF) as discussed by the authors are compounds easily synthesised from barbituric acid and its derivatives, they can be easily oxidised, they form cation-radical salts and at the same time intermolecular hydrogen bonds with complementary components enhancing the electroconductivity of crystalline samples.
Abstract: Uracil-fused and isocytosine-fused tetrathiafulvalenes (TTF) are compounds easily synthesised from barbituric acid and its derivatives. The compounds can be easily oxidised, they form cation-radical salts and at the same time intermolecular hydrogen bonds with complementary components enhancing the electroconductivity of crystalline samples. The formation of hydrogen bonds produces changes in the ultraviolet absorption spectra of compounds and in their redox properties: it may be used for the recognition complementary components of nucleic acids. An oligonucleotide containing a TTF group has been synthesised and its redox activity studied.
TL;DR: In this paper, the molecular requirements for phase biaxiality in three types of rod-plate nematics were studied, i.e., binary mixtures of hard rods and discs, single component systems with interconverting rod-like and plate-like conformations.
Abstract: A nematic consisting of rod-like and plate-like molecular species is expected to exhibit phase biaxiality over a range of concentrations. Phase separation, however, prevents the achievement of such concentrations in mixtures of conventional, low molar mass, thermotropic calamitic and discotic mesogens. We present a theoretical study of the molecular requirements for phase biaxiality in three types of rod-plate nematics, namely in (i) binary mixtures of hard rods and discs, (ii) mixtures of rods and discs exhibiting molecular association, either by direct rod-disc interaction or through selective interactions with a third molecular species and (iii) single component systems with interconverting rod-like and plate-like conformations.
TL;DR: In this paper, a cautious outlook on possibilities of synthesizing low-molecular weight thermotropic biaxial nematic mesogens is discussed, indicated by lateral interactions or substitution.
Abstract: Reflections on the design and remarks on the identification of low-molecular weight thermotropic biaxial nematic mesogens demonstrate unsolved problems in this field of work on liquid crystals of current international interest. However, a cautious outlook on possibilities of syntheses of such highly desirable materials, indicated by lateral interactions or substitution, is discussed.
TL;DR: In this article, anisotropy is generated in films of photochromic LCPs upon irradiation with linearly polarized light and Annealing of these films above the glass transition temperature results in a significant amplification of the photo-induced anisotropic effect.
Abstract: Optical anisotropy is generated in films of photochromic LCPs upon irradiation with linearly polarized light. Annealing of these films above the glass transition temperature results in a significant amplification of the photo-induced anisotropy. The amplification takes place even in the case of copolymers with only 10% of azobenzene side groups and if only 7% of the saturation value of anisotropy are induced. Photoorientation and photoinduced alignment are restricted by thermal pretreatment and ageing at room temperature.
TL;DR: In this paper, asymmetric isomers of the liquid crystals mentioned above, keeping the same banana-shape, were synthesized and their properties were investigated, including spontaneous polarization in the absence of any chiral group in the molecules.
Abstract: In the last years bent banana-shaped mesomorphic molecules found an increasingly great interest among theoreticians and experimentalists due to the nature of this type of liquid crystals to exhibit a spontaneous polarization in the absence of any chiral group in the molecules. Similar property was found in mesomorphic 1,3,4-oxadiazoles containing as a central unit the five-membered heterocycle with the symmetric positions of heteroatoms and as the link units - the ester groups [1]. In the present work 1,2,4-oxadiazoles - asymmetric isomers of the liquid crystals mentioned above, keeping the same banana-shape, were synthesized and their properties were investigated.
TL;DR: In this paper, the properties of antiferroelectric liquid crystals were investigated by means of dielectric, densitometric, refractometric, and X-ray measurements, and effective values of molecular dipole moment μ have been calculated from electric permittivity e in the 1 phases of these compounds, using Maier and Meier equation.
Abstract: Two investigated antiferroelectric liquid crystals, MHPB(H)PBC and MHPB(F)PBC, were synthesized [1] and some their physical properties were described earlier [2]. Apart of the above, MHPB(H)PBC and MHPB(F)PBC have been studied by means of dielectric, densitometric, refractometric and X-ray measurements. Spontaneous polarization P s and tilt angle θ in ferro- (SmC*) and antiferroelectric (SmCA*) phases of MHPB(H)PBC and MHPB(F)PBC have also been investigated. Transverse α1 and longitudinal α1 molecular polarizabilities have been calculated from the ordinary n 0 and extraordinary n e refractive indices and density ρ of MHPB(H)PBC and MHPB(F)PBC, using Neugebauer's, Vuks and Lorentz equations with both Haller and Subramhanyama extrapolation procedures. Effective values of the molecular dipole moment μ have been calculated from electric permittivity e in the 1 phases of these compounds, using Maier and Meier equation.
TL;DR: In this article, a periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported, characterized by a periodic modulation of the director in the xy-plane.
Abstract: A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.
TL;DR: RefReflective holographic polymer dispersed liquid crystals (HPDLC) have been fabricated by irradiating Ar-ion laser(λ=514 nm) of various intensities on LC/monomer mixtures sandwiched between two ITO coated glass plates as mentioned in this paper.
Abstract: Reflective holographic polymer dispersed liquid crystals(HPDLC) have been fabricated by irradiating Ar-ion laser(λ=514 nm) of various intensities on LC/monomer mixtures sandwiched between two ITO coated glass plates. Gratings were written for various monomer compositions and laser intensities. UV-visible spectra have been analyzed. Eventually, reflective efficiency-irradiation intensity-monomer composition relationships have been obtained.
TL;DR: In this article, a trifluoromrthoxy group was added to 3-ring phenylacetylene (3PA) liquid crystals to investigate the effects of substitution on physical properties.
Abstract: We have synthesized new high Δ n 3-ring phenylacetylene(3PA) liquid crystals which have substituent groups with core center phenyl ring and investigated the effects of substitution on physical properties. Nematic range and viscosity were improved remarkably with keeping high Δ n by introducing trifluoromrthoxy group. The Δ n of 3PAs were also descussed with relationship of polarizability and order parameter.
TL;DR: Reflective holographic polymer dispersed liquid crystals (HPDLCs) have been fabricated with a ternary monomer system composed of dipentaerythritol hydroxy penta acrylate (DPHPA)/trimethylol propane triacylate (TMPTA)/N-vinylpyrrolidone(NVP)=7/2/1 by weight.
Abstract: Reflective holographic polymer dispersed liquid crystals(HPDLCs) have been fabricated with a ternary monomer system composed of dipentaerythritol hydroxy penta acrylate(DPHPA)/trimethylol propane triacylate(TMPTA)/N-vinylpyrrolidone(NVP)=7/2/1 by weight. Gratings were written by irradiating with an Ar-ion laser(λ = 488 nm) at various intensities(20-200 mW/cm 2 ) on LC/monomers composite films of various composition(25/75, 30/70, 35/65, 38/62, 40/60) Polarized optical micrography(POM) images of gratings and UV-visible spectra were taken and reflection efficiency-irradiation intensity-film composition relationships are subsequently obtained in three dimensional plot which showed that maximum reflection moves from high LC content (38wt%) at low irradiation intensity (20 mW/cm 2 ) to low LC content (25wt%) at high irradiation intensity (200 mW/cm 2 ).
TL;DR: In this article, two homologous series are synthesized by fixing a rigid 4-methyl phenyl azo group to resorcinol moiety, one phenolic -OH group is alkylated whereas, in series II both the phenolic-OH groups are alkylation.
Abstract: Two homologous series are synthesized by fixing a rigid 4-methyl phenyl azo group to resorcinol moiety. In series I one phenolic -OH group is alkylated whereas, in series II both the phenolic -OH groups are alkylated. Monoethers of series I having free lateral hydroxy group due to strong hydrogen bonding, exhibit nematic mesophases whereas, diethers of series II are non-mesogenic.