Molecular organization and aggregation in Langmuir and Langmuir-Blodgett films of azo dye/liquid crystal mixtures

A study of azo dye/liquid crystal mixtures in monolayers formed at air-water (Langmuir films) and air-solid substrate (Langmuir-Blodgett films) interfaces was performed. Three azo dyes with various molecular structure and two liquid crystal materials: 4-octyl-4′-cyanobiphenyl (8CB) and trans-4-octyl(4′-cyanophenyl)-cyclohexane (8PCH) were used. The morphology of Langmuir films was monitored by means of a Brewster angle microscope (BAM). Moreover, a surface pressure and electronic absorption spectra of the monolayer spread on the water surface of dye/liquid crystal mixtures as a function of a mean molecular area were simultaneously recorded. In addition, the absorption for Langmuir-Blodgett films deposited on quartz plates was measured. Information about intermolecular interactions in the mixtures of the nonamphiphilic dye and the liquid crystal with polar terminal group was obtained. Conclusions about the formation of self-aggregates between dye molecules have been drawn. The influence of the dye molecular structure and its concentration on aggregates’ geometry was found.     

Self-aggregates formation of 3,4,9,10-tetra-(n-alkoxy-carbonyl)-perylenes in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films formed of some 3,4,9,10-tetra-(n-alkoxy-carbonyl)-perylenes and of their binary mixtures with the liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) have been studied. Absorption and fluorescence studies were carried out. Both absorption and fluorescence spectra have revealed the formation of self-aggregates of dye molecules in monomolecular layers. Moreover, information about the molecular organization at the air-solid substrate interface has been obtained.     

Comparative Investigations of Light Transmission in Aligned and Non-aligned Textures of Homogeneous Mixtures of 4-n-alkyl-4′-Cyanibiphenyl Mesogens at Direct and Reverse <Emphasis Type="Italic">smectic A</Emphasis> ↔ <Emphasis Type="Italic">isotropic liquid</Emphasis> Phase Transitions

The thermo-optical properties of various types of textures (the homeotropic, planar, and tilted aligned and non-aligned textures) in liquid crystalline materials with smectic A mesophase have been investigated. Investigations have been carried out for large temperature interval and at the direct smectic A mesophase–isotropic liquid (SmA–I) and isotropic liquid–smectic A mesophase (I–SmA) phase transitions that have been carried out. Homogeneous mixtures of 4-n-octyl-4′-cyanobiphenyl with 4-n-decyl-4′-cyanobiphenyl were the objects of the investigations. Temperature dependences of the optical transmission for aligned and non-aligned textures have been measured. Temperature widths of the heterophase regions for the SmA–I and I–SmA phase transitions have been determined. The temperature shift in the optical transmission dependences to low temperatures for the reverse I–SmA phase transition temperatures and the thermal hysteresis has been found for the aligned and non-aligned textures.     

Spectral properties of naphthalimide dyes mixed with 4-heptyl-4′-cyanobiphenyl (7CB) in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films formed of some fluorescent dyes, namely derivatives of 4-aminonaphthalimide, and their binary mixtures with the liquid crystal 4-heptyl-4′-cyanobiphenyl (7CB) have been studied. The electronic absorption and fluorescence spectra were recorded. On the basis of these spectra, the spectral properties of the dyes and the intermolecular interactions in ultrathin ordered films were determined. The conclusions about the tendency to the creation of aggregates by dye molecules in LB films have been drawn. The measurements of absorption by using linearly polarised light have allowed us to determine the arrangement of the dye and liquid crystal molecules on the air-solid substrate interface.     

Optical and Dielectric Properties of ZnO Nematic Liquid Crystals Prepared by the Chemical Precipitation Method

Journal of Applied Spectroscopy - Mesogenic 4-pentyl-4′-cyanobiphenyl (5CB) is a commonly used dielectric material for display devices and liquid crystal biosensors. A small concentration of...     

Critical behavior in the vicinity of nematic to smectic A (N–SmA) phase transition of two polar–polar binary liquid crystalline systems

Phase diagram, critical behavior and order of the nematic (N)–smectic A (SmA) phase transition of two polar–polar binary systems (i) 4-n-heptyloxy-4′-cyanobiphenyl (7OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB); (ii) 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-n-nonyloxy-4′-cyanobiphenyl (9OCB) by means of a high-resolution temperature scanning measurement of birefringence have been reported in this work. A simple power law analysis has been adopted to extract the specific heat critical exponent (α′) at N–SmA transition from birefringence data. The α′ for N–SmA transition indicates a uniform crossover behavior and has appeared to be non-universal in nature. With increasing concentration of the higher homologues for both the binary systems, the N–SmA transition reveals a strong tendency to be driven towards the tricritical nature. The 3D-XY limit (i.e. α′ = ?0.007) for N–SmA transition reaches at the concentration x_8OCB = 0.28 corresponding to the McMillan ratio 0.914, whereas the tricritical point has been found to appear near x_9OCB = 1.0 corresponding to McMillan ratio 0.992.     

Thermal and Optical Properties of Some Hydrogen-Bonded Liquid Crystal Mixtures

Phase transition properties of the mixtures of hydrogen-bonded nematic liquid crystals (HBLC) 4-hexylbenzoic acid (6BA), 4-(octyloxy)benzoic acid (8OBA), and 4-(decyloxy)benzoic acid (10OBA) have been investigated by means of differential scanning calorimetry (DSC) and polarize optic microscope (POM). The DSC and POM results clearly indicate the existence of smectic and nematic phase transitions in binary mixtures. The phase transition temperature values of 6BA/10OBA mixtures have clearly increased with increasing heating rate. The activation energies were calculated for the phase transitions of 6BA/10OBA liquid crystal (LC) mixture. The optical transmittance of these mixed hydrogen-bonded nematic liquid crystals was investigated in terms of temperature variations through electrooptic methods. The electrooptic experiments indicate that, while low in the nematic phase, the optical transmittance is very high at the nematic-isotropic phase transition. The transmitted light intensity values of 6BA/8OBA mixture are somewhat higher than those of other binary mixtures, 6BA/10OBA and 8OBA/10OBA, a result associated with the different alkyloxy chain lengths.     

Dielectric investigation on some binary mixtures of hockey-stick-shaped liquid crystal and octyloxy-cyanobiphenyl

We report the measurement of temperature-dependent dielectric parameters in some binary liquid crystal mixtures comprising of a hockey-stick-shaped mesogen 4-(3-decyloxyphenyliminomethyl) phenyl-4-decyloxycinnamate (SF7) and calamitic compound 4′-octyloxy-4-cyanobiphenyl (8OCB). All the investigated mixtures possess a large positive dielectric anisotropy (Δε), although a noticeable reduction has been found by increasing the diverse-shaped dopant concentration. Investigation on the pretransitional behavior in the vicinity of isotropic to nematic (I–N) phase transition suggesting a tricritical character for all the studied mixtures. Parameterization of dielectric permittivity close to the nematic to smectic-A (N–Sm-A) phase transition exhibits non-universal values of the critical exponents describing a second-order nature of the transition. Systematic variation of critical exponents against dopant concentration and McMillan ratio reveals a well consistency with those obtained from the high-resolution optical birefringence measurements.     

An EPR study of the orientational transition in smectic liquid crystals in a magnetic field

Spin probe EPR spectroscopy is used to detect an orientational transition in the smectic mesophase in 4-n-octyl-4'-cyanobiphenyl (8CB) liquid crystal under the action of an intense magnetic field (5.87 T).     

Experimental and theoretical study of the IR spectrum of 4-amyloxy-4′-cyanobiphenyl

The IR absorption spectrum of polycrystalline 4-amyloxy-4′-cyanobiphenyl (5OCB) is measured in a KBr pellet over the range 400–4000 cm^?1. The structure of the molecule and the frequencies and intensities of the bands in the spectrum are calculated in the approximation of the B3LYP hybrid density functional with the 6-31G(d) and 6-31+G(d) basis sets. With the method of linear scaling of frequencies, 39 bands of the experimental IR spectrum are assigned. On the basis of calculations for related compounds, the vibrations belonging to the substituents and the biphenyl fragment are ascertained. It is shown that the IR absorption spectra of polycrystalline 4-hydroxypropyl-4′-cyanobiphenyl and 5OCB are almost identical and differ by the occurrence of three bands associated with vibrations localized on the oxyamyl radical.     

Effect of surface and molecular structure on the dielectric behaviour in thick layers of cyanobiphenyl liquid crystals

The static dielectric studies of hexoxy heptoxy and octoxy cyanobiphenyl liquid crystals (6OCB, 7OCB and 8OCB) and the euteric nematic mixture of 6OCB/7OCB indicate that molecular alignment can be induced by the steel electrod surface of a dielectric thick cell with an electrode gap of about 1 mm. Whereas the permittivity values of alkyl cyanobiphenyls (6CB, 7CB and 8CB) in the nematic phase indicate imperfect alignment of the liquid crystal molecules. The good parallel molecular alignment of alkoxy cyanobiphenyls was to existence of oxygen atom in their molecular structures. These observations demonstrated a sensitive relationship between the molecular alignment, molecular structure and surface effect. Analysis of the dielectric data re-confirmed that the dipole components of the alkoxy cyanobiphenyls along the long molecular axes have a strong tendency for anti-parallel alignment which increases with increasing nematic order, whereas the dipole components perpendicular to the long molecular axes indicate a tendency for parallel alignment.     

Photorefractive properties in homogeneous and heterogeneous polymer/liquid crystal composites

The photorefractivity of a photorefractive composite consisting of poly(N-vinylcarbazole) (PVK) doped with a non-room-temperature liquid crystal, 4-butyloxy-4’-cyanobiphenyl (4OCB), and C₆₀ was studied by means of measuring the two-beam coupling coefficients. The results show that the photorefractivity is enhanced by phase separation. Received: 6 December 2000 / Revised version: 5 March 2001 / Published online: 9 May 2001     

Self-aggregates formation of tetrachloroperylene acid esters in Langmuir and Langmuir-Blodgett films

Using Langmuir and Langmuir-Blodgett techniques molecular films of chlorinated perylene derivatives, namely tetra-n-butyl-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxylate (PCn for n = 1, 5, 9) have been studied. The Langmuir films of pure compounds and mixed with liquid crystalline 4-octyl-4′-cyanobiphenyl (8CB) were characterized by surface pressure-mean molecular area isotherms. An additive rule reveals miscibility of all the dyes with 8CB but shows different types of intermolecular interaction forces. The pure and mixed Langmuir films were transferred onto quartz plates and characterized spectroscopically. Absorption and fluorescence spectra were recorded for the samples in form of diluted chloroform solution, the dye with 8CB mixtures in monomolecular Langmuir-Blodgett films and in liquid crystal cells. Different tendency to aggregation of the dye with short and long alkyl chains was observed. It is shown that the dye molecule stacking and aggregation of the chlorinated perylene dyes depend on the dye concentration and are related to the torsion of the perylene core.     

Orientation of pentyl-cyanobiphenyl layers and variations in their luminescence spectra under UV irradiation

Polarized fluorescence spectra of layers of a nematic liquid crystal (LC) of n-pentyl-n′-cyanobiphenyl (5CB), oriented by two different methods, namely, by rubbing the polyimide-coated substrates holding the LC layer and by irradiating the LC layer placed between isotropic fused quartz substrates by polarized UV radiation, are comparatively analyzed. The photoluminescence spectra in the two cases show anisotropy with parameters virtually independent of the orientation method. Contrary to the case of rubbing orientation, the photoluminescence spectrum of the LC layer, oriented by irradiation, substantially differs from the initial one. Possible photochemical processes and mechanisms of photoorientation are discussed.     

High-frequency dielectric spectra from liquid crystals of series nCB and nOCB

The design of a resonant frequency-tunable high-sensitivity microstrip sensor is suggested. The permittivity dispersion of liquid crystals of two homologic series, alkylcyanobiphenyls (7CB and 8CB) and alkyloxycyanobiphenyls (7OCB and 8OCB), is studied at frequencies of 100–900 MHz. The dielectric spectra are shown to be the sum of the Debye relaxation and dielectric resonances observed at f≈160, 280, 360, 450, 550, and 650 MHz. The dielectric resonances are present in the spectra of all the samples in both the nematic and isotropic phase. The substitution of an oxygen atom (series nOCB) for a carbon atom (series nCB) in liquid crystal molecules has a minor effect on the dielectric resonance frequencies but changes the resonance intensities and splits some of the resonance lines.     

Low-temperature photoluminescence of 5CB liquid crystal

The photoluminescence spectra of the 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal have been investigated in detail at low temperatures 4.2-200 K, for the first time. The spectral data obtained are compared to the results of the luminescence study for the 5CB nematic phase at Т=300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth non-monotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T=160 K. The 5CB crystal-crystal transition at Т=80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T=140 K, a new spectral band at 424 nm appears; it corresponds to the significantly overlapping 5CB dimers, its intensity grows under further heating. The present investigation gives a tool for the further characterization of the molecular alignments and changes in the 5CB molecular conformations, using the monomer and excimer fluorescence emissions as a probe. The conclusions made are confirmed by the IR-spectroscopy data, measured and analyzed for the 5CB in the same temperature region.     

Fluorescence of the nematic liquid crystal 5CB in nanoporous glasses

The fluorescence spectra of the nematic liquid crystal n-pentyl-n′-cyanobiphenyl (5CB) in porous glasses with pores from 1 to 44 nm in diameter are investigated. A decrease in the pore diameter leads to suppression of some long-wavelength spectral components corresponding to H-type predimer and dimer pairs (the molecular sieve effect). The spectrum of 5CB in small pores (smaller than 4 nm in diameter) can be explained by the superposition of the monomer fluorescence and the fluorescence of J-type dimer pairs of 5CB molecules, as well as associates of 5CB molecules and surface groups on pore walls. Exposure of samples to UV light enhances the molecular interaction in associates, possibly, due to the formation of strong chemical bonds.     

Dynamics of 8CB confined into porous silicon probed by incoherent neutron backscattering experiments

Confinement in the nanochannels of porous silicon strongly affects the phase behavior of the archetype liquid-crystal 4-n-octyl-4-cyanobiphenyl (8CB). A very striking phenomenon is the development of a short-range smectic order, which occurs on a very broad temperature range. It suggests in this case that quenched disorder effects add to usual finite size and surface interaction effects. We have monitored the temperature variation of the molecular dynamics of the confined fluid by incoherent quasielastic neutron scattering. A strongly reduced mobility is observed at the highest temperatures in the liquid phase, which suggests that the interfacial molecular dynamics is strongly hindered. A continuously increasing slowdown appears on cooling together with a progressive growth of the static correlation length.     

Study of the structural and morphological changes during the phase transition of ZnS to ZnO

Nanocrystalline zinc sulfide (ZnS) (nanorods) was synthesized by chemical precipitation method, and they were annealed in air at different temperatures in the range 200–700 °C for the phase transition of ZnS to ZnO. The characterization of the system was done by different techniques such as X-ray diffraction (XRD), high-resolution transmission electron microscopy, UV–Vis spectroscopy, photoluminescence spectroscopy and differential scanning calorimetry (DSC). From the XRD and TEM analysis, the crystal structure is found to convert from cubic ZnS phase to the hexagonal ZnO phase and its morphology from nanorods to nanoparticles with the increasing annealing temperatures. The UV–Visible absorption and photoluminescence measurements revealed that the relative changes in the phases alter the band gap and introduce new kinds of defects in the system. The percentage of the ZnS and ZnO phase has been found to be proportional to the annealing temperature for a fixed time interval, and the DSC measurement has also found similar results as in the XRD pattern during the phase transitions.     

Distribution function of relaxation times for a 4-<Emphasis Type="Italic">n</Emphasis>-pentyl-4′-cyanobiphenyl liquid crystal

The distribution function of relaxation times is reconstructed from the dielectric loss spectrum measured over a wide range of frequencies for a 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal. It is demonstrated that the distribution function for the isotropic and nematic phases is asymmetric in shape. Comparison shows that the reconstructed distribution function is in qualitative agreement with similar functions that are analytically derived from the Cole-Davidson and Havriliak-Negami empirical equations. The specific features observed in the behavior of the distribution function with a variation in the angle between the direction of polarization of a microwave electric field and the director of the liquid-crystal molecules are analyzed. A complex dependence of the permittivity for the liquid crystal in the range of ultrahigh frequencies is explained in terms of additional relaxation mechanisms associated with different motions of molecular fragments of alkyl chains.