Birefringence Study in Hydrogen Bonded Complexes

A novel series of linear complementary hydrogen bonded complexes fashioned between p-n-alkylbenzoic acids (nBA) and m-fluorobenzoic acid (FBA) are isolated. The obtained homologous series comprising of seven mesogens are analyzed by polarizing optical microscope (POM), differential scanning calorimetry (DSC) and Fourier Transform Infra-Red (FTIR) spectroscopy. It is interesting to note the correlation between the alkyl chain length and the mesogenic phases. The order of the transitions is examined through Cox parameter, experimentally derived by thermal analysis (DSC). Optical tilt angle studies in smectic C and thermal stability factors for various phases are discussed. Birefringence study by various techniques enabled to measure the refractive index at various phase transitions. These results are compared and conferred with those obtained from conventional refractometer. Further orientational order parameter for various phases has been discussed.     

Phase transition studies in certain thermotropic liquid crystals thermal microscopy and DSC

Investigations on the phase transition studies in the thermotropic liquid crystals, (1) pcyanophenyl trans-4-propylcyclohexane carboxylate, (2) p-cyanophenyl trans-4-pentyl-cyclohexane carboxylate, (3) 4′-n-pentyloxy-4-cyanobiphenyl and (4) 4′-n-octyloxy-4-cyanobiphenyl are carried out by the thermal microscopy and the differential scanning calorimetric (DSC) studies. These studies reveal that the first three mesogens exhibit a single mesophase while the fourth one exhibits a smectic A phase in addition to the nematic phase. The textures exhibited by these mesogens in their mesophases and their transition temperatures, enthalpies and entropies are presented.     

Design,Synthesis and Analysis of Chlorohydroquinone Derivatives—Liquid Crystalline Complexes

A novel linear double hydrogen-bonded liquid crystalline material is designed and isolated. The ingredients of the mesogens synthesized are chloro hydroquinone (ClHQ) and p-n alkyloxy benzoic acid (nBAO) where n varied from pentyl to dodecyl alkyloxy carbon numbers. The series is abbreviated as ClHQ + nBAO, and seven of eight synthesized complexes are found to exhibit liquid crystalline phases. These liquid crystalline complexes are characterized by Fourier Transformation Infrared spectroscopy (FTIR) in order to confirm the formation of hydrogen bond. Mesophases and their corresponding transition temperatures are studied by polarizing optical microscopy (POM) and their respective enthalpy values along with the order of the transitions are determined by Differential Scanning Calorimetry (DSC). The experimental measurement of tilt angle is carried out by optical extinction method and fitted to the theoretically predicted mean field exponent which is found to be in good agreement. Dielectric relaxations at different temperatures in nematic phase for the complexes have been carried out and their corresponding activation energy values are calculated from Cole–Davidson plots. Light filtering action in the liquid crystalline complexes is done to validate the commercial applications of these mesogens.     

Growth and characterization of a novel NLO crystal bis-glycine hydrogen chloride (BGHC)

Single crystals of bis-glycine hydrogen chloride (BGHC), a semiorganic nonlinear optical (NLO) material, have been grown by slow solvent evaporation technique. Good optical quality single crystals with dimensions up to 33×5×5 mm³ are obtained. The crystals are characterized by optical transmission spectrum, FTIR and X-ray diffraction studies. The thermal stability of the crystal is studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The second harmonic generation (SHG) efficiency of BGHC was found to be 5.6 times that of KDP. The laser damage threshold values studied in single shot mode indicates that BGHC crystal possesses a fairly high value of 9.8 GW/cm² and thus the NLO studies confirmed the superiority of BGHC over KDP and urea crystals. The dielectric constant of the crystal was studied as a function of frequency and the results are discussed.     

Liquid Crystal Compounds Incorporating the Trans-1,4-Substituted Cyclohexane Ring System

A number of esters and other compounds containing a trans-1,4-substituted cyclohexane ring system-have been prepared, and the liquid crystal thermal stabilities of these new mesogens are discussed in relation to those of their analogues containing the 1,4-phenylene ring. The observed effects are discussed in terms of a possible thermal energy absorption by the cyclohexane ring in causing minor conformational changes.

Other physical properties of these new mesogens are also discussed with particular reference to their potential in applications. In relation to electro-optical display devices, such properties as birefringence, dielectric anistropy, viscosity etc., are important parameters, and the new materials have been assessed in this context as possible additives to the 4-n-alkyl- and 4-n-alkyloxy-4′-cyanobiphenyls for producing a favorable combination of physical parameters.

Also reported is a liquid crystal mixture which has a 100°C nematic range and which contains only materials which are aliphatic. This mixture is assessed for its potential application as an anistropic solvent in analytical techniques such as ultra-violet spectroscopy and nuclear magnetic resonance spectroscopy.     

Thermal and optical studies on induced smectic phases of inter molecular hydrogen bonded liquid crystals between decyloxy benzoic acid and citric acid

Hydrogen bonded liquid crystal complex (HBLC) is prepared from mesogenic 4-decyloxy benzoic acid (10OBA) and aliphatic nonmesogenic citric acid (CA). Liquid crystal (LC) phases are investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) studies. Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR) validate the intermolecular complementary, cyclic type of hydrogen bond (HB) and molecular environment in the designed HBLC complex. Mesomorphic phases like nematic, smectic X (Sm X) and smectic G (Sm G) are characterized by various textures and using different techniques such as POM, DSC and optical tilt angle measurements. Thermal span width and thermal stability factor for the observed phase is calculated. The complexes are prepared in different mole ratio and their corresponding influences on the phase transitions are discussed. Also it is observed that the HB units play a vital role in stabilizing the new Sm X phase. The variation in thermal stability of smectic phases due to the influence of aromatic cores and length of end chain in the different mole ratio of the present HBLC complexes are also discussed. The variation of tilt angle with respect to temperature in the smectic phase has been experimentally calculated and analyzed. The lowered melting and clearing transition temperatures and extended thermal span width in the Sm X phase are also reported.     

Induced Crystal‐G Phase through Intermolecular Hydrogen Bonding Part XIV: Influence of Alkyl Carbons of p‐Hydroxybenzoates on Phase Distribution of Induced Phase

Two novel series of intermolecular hydrogen‐bonded liquid crystal complexes resulting from equimolar combinations of p‐n‐alkoxybenzoic acids (nABA) (where n represents alkoxy carbon numbers 3 to10 and 12) and non‐mesogens viz. ethyl‐p‐hydroxybenzoate (EHB) and propyl‐p‐hydroxybenzoate (PHB) have been synthesized. The formation and stabilization of the hydrogen bonding is confirmed by infrared spectral study, which reveals the terminal functionalities, ‐COOH and ‐OH moieties are complementary to each other by acting as both proton donor and acceptor. The results of comparative thermal and phase analyses of both the series are discussed in conjunction with free‐ p‐n‐alkoxybenzoic acids. The induction of crystal‐G phase with wide thermal spans is observed in the present complexes.     

λ-Shaped mesogens based on chalconyl ester core having variable side chain: Synthesis and characterisation

In present study, we have synthesized two newly chalconyl-ester core LCs having variable side chain and fixed disubstituted butoxy group at another terminal end. All this compounds were well characterized by elemental analysis, FT-IR, ¹H NMR. The mesomorphic properties of these compounds were observed by optical polarized light microscopy (POM) and also confirmed by using differential scanning calorimetry (DSC) and X-ray diffraction pattern (XRD). Detailed XRD investigation endorses the presence of the nematic phase in higher angle region and SmC phase in lower angle region. In addition, it is found that all the prepared materials display enantiotropic LCs phases except first four members in series-1 and first member in series-2. Thermal stability of SmC and nematic phase of series-1 is higher as compare to series-2. To get more insights, the DFT based HOMO, LUMO studies are carried out which supports intramolecular charge transfer interactions and stability in this class of mesogens.     

Influence of MWCNT on the properties of hydrogen bonded liquid crystals

A hydrogen bonded complex molded between p-n-octylbenzoicacid (8BA) and m-fluorobenzoic acid (FBA) is isolated and doped with predetermined quantity of Multi Walled Carbon Nano Tube (MWCNT) in order to achieve better thermal and electrical performance. The obtained series of liquid crystal nano systems comprising of three mesogens, varying in the weight percentage of MWCNT are analyzed by Polarizing Optical Microscope (POM), Differential Scanning Calorimetry (DSC) and Fourier Transform Infra-Red (FTIR) spectroscopy. Different aspects of the MWCNT – liquid crystal combination, evaluation of MWCNT's effect on selected properties of liquid crystals which are relevant to display related applications are discussed. In addition, Frederik's transitions of MWCNT dispersed liquid crystals reveal the direct effect of bundles of MWCNT on liquid crystal reorientation.     

Structural characteristics and second harmonic generation in l-threonine crystals

l-threonine, an organic material with interesting non-linear optical properties, has been grown in solution and characterized by X-ray diffraction, FT infrared spectral, Raman spectral, UV VIS spectral and thermal techniques. The structural and spectral characteristics show that strong hydrogen bonds appearing in l-threonine have an influence on the stabilization of the crystal structure. The main contributions to the second harmonic generation in threonine results from the hydrogen bond and also from the vibrational part due to very intense IR bands of the hydrogen bond vibrations. The OH and NH involved in strong hydrogen bonds are responsible for the higher thermal stability of l-threonine compared to other organic non-linear optical materials. l-threonine is thermally stable up to 270 °C.     

Growth and characterization of semi organic nonlinear optical LHPCL crystals

The growth of single crystals of L‐histidinium perchlorate (LHPCL), a semi organic nonlinear optical material with dimension upto 5 × 6 × 2.5 mm³ is reported. Good optical quality single crystals were grown from aqueous solution by slow solvent evaporation technique. The grown crystals were characterized by single crystal X‐ray diffraction (XRD), FT‐IR and optical transmission studies. Thermal studies confirm that LHPCL has a fairly high thermal stability (272°C) when compared with other members of histidine family. The scanning electron microscopy (SEM) provides information on the quality of the samples and grain distribution over the surface of the sample. The dielectric constant and dielectric loss of the compound were measured at different frequencies, and also at varying temperatures and the results of these experiments are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of the Liquid Crystals Formed by the N-Oxides of Certain 4-(4′-pyridyl)phenyl 4″-n-Alkoxybenzoates: Comparison of the N-Oxide and Cyano-groups as Mesogenic Terminal Substituents

The liquid liquid crystal transition temperatures of the N-oxides of certain 4-(4′-pytidyl)phenyl 4″ -n-alkoxybenzoates are reported. The effects on smectic and nematice thermal stability of the N-oxide function and the cyano-group in terminal positions in mesogens are compared and discussed.     

Phase Transition and Optical Characterization of Mesophase Stability of Nematic and Columnar Biphasic Regions

We report the results of our studies on the optical and thermal properties of binary mixture of two compounds viz., abietic acid and alizarin dye. The mixture shows a very interesting co-existent biphasic regions of nematic (N + I) and columnar smectic (C + I) phases, sequentially when the specimen is cooled from its isotropic phase respectively at different concentrations of given molecule. The temperature variations of optical anisotropy, optical textures and electrical conductivity have also been discussed. Aggregated molecular size has been confirmed by X-ray studies.     

Synthesis of liquid crystals based on hydrogen-bonding of 4-(Octyloxy)benzoic acid with 4-alkylbenzoic acids

A molecular recognition process has been used to form new mesogenic molecular structures, where intermolecular hydrogen bonding occurs between 4-(octyloxy)benzoic acid (8BAO) and four 4-alkylbenzoic acids (nBAs, n = 2, 5, 6, 7). The synthesis of these complexes has been attained by resorting to mechanochemistry. The resulting materials have been characterized by polarizing optical thermal microscopy, differential scanning calorimetry, vibrational spectroscopy, X-ray powder diffraction, and ¹H NMR relaxometry. All the elements of the series show the formation of a mesophase. For one of the complexes, its electro-optical properties have also been assessed, resulting comparable to those of other widely used liquid crystals.     

Growth and characterization of tris allylthiourea mercuric chloride crystals

Single crystals of tris allylthiourea mercury chloride (ATMC), a semi‐organic nonlinear optical material, have been grown from a low‐temperature solution growth technique by slow cooling method. In the present work, we have grown ATMC crystal which is a new nonlinear optical material (NLO) having high optical quality and its second harmonic generation (SHG) efficiency is thrice that of Urea. The grown crystals have also been subjected to various characterization studies like X‐ray diffraction, UV‐VIS, FT‐IR and TGA‐DTA. X‐ray powder diffraction study confirmed the crystal structure of ATMC, UV‐VIS /FT‐IR spectral analysis showed the crystal has good optical transmission in the entire visible region and TGA‐DTA studies showed the thermal properties and complex degradation of ATMC crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of some novel liquid crystalline compounds of 1,3,4-oxadiazoline and their hydrazone derivatives

Three new series of mesogens have been synthesized by fixing 4-methoxy or 4-butoxy substituted phenyl ring in one side and 4-hydroxy, bromo or amino substituted phenyl group in the other side with different central linkage. The molecular structure of these series was confirmed by elemental analysis and FTIR and ¹HNMR spectroscopy. The mesomorphic properties were studied by using differential scanning calorimetry (DSC) and optical polarizing microscopy (OPM). All the compounds of series [I] exhibit enantiotropic smectic B (SmB) phase. All the compounds of series [II] and [III] display SmB and nematic (N) phases. The mesomorphic properties were found to be dependent on the two terminal 4-substituted phenyl groups and the central linkage group.     

Effects of barrier growth temperature on the properties of InGaN/GaN multi-quantum wells

The effects of the growth temperature and ambient of GaN quantum barriers on the characteristics of InGaN/GaN multi-quantum wells (MQWs) grown by a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated. The improvement of optical, structural properties and surface morphology in the MQWs with increasing the growth temperature of quantum barriers was found. Without a GaN capping layer, there were many pits and the thickness of quantum pair reduced by the thermal etching during the temperature-ramping process. Photoluminescence (PL) peaks showed a blue-shift and double peaks, but relative PL intensity abruptly increased due to the suppression of deep level related defects and smooth surface morphology caused by the increased surface mobility of adatom in the high temperature region. By using a GaN capping layer on the InGaN well layer, the thermal decomposition of the InGaN well layer was suppressed and pits on the surface abruptly reduced. A hydrogen carrier gas for the GaN barrier growth also improved the optical and structural properties of MQWs.     

Linear Double Hydrogen-bonded Thermotropic Liquid Crystals Formed Between Oxaloacetic Acid and p-n- Alkyloxy Benzoic Acids

Double hydrogen-bonded thermotropic liquid crystal complexes (DHBLC) have been isolated from molar ratios of Oxaloacetic acid (OAC) and eight alkyloxy benzoic acids (nBAO) whose carbon number varied from pentyloxy to dodecyloxy. The complexes are referred as OAC+nBAO where n varied from 5 to 12. In each of the synthesized complexes, complementary hydrogen bonds formed are confirmed by FTIR study and the complexes are further investigated by Polarizing Optical Microscopy (POM), Differential Scanning Calorimetry (DSC) for the construction of phase diagram. Characteristic phases like nematic, smectic C, smectic F, and smectic G are characterized by various textures. From DSC studies, odd–even effect is evinced in transition temperatures across isotropic to nematic phase transition and across smectic F to smectic G phase transition in enthalpy values. The order of all transitions observed in eight complexes is calculated by thermal analysis. The magnitude of optical tilt angle in smectic C is fitted to a power law and the scale of critical exponent is found to concur with the Mean Field theory predicted value.     

Optical Characterization of Lyotropic Chromonic Liquid Crystals

We report the results of our studies on the optical and thermal properties of binary mixture of compounds, viz., Berberine (BBE) and Poly-ethylene glycol (PEG). The mixture shows very interesting coexistent biphasic regions N + I and N + C phases respectively at different concentrations of BBE sequentially when the specimen is cooled from its isotropic liquid phase. The temperature variations of optical anisotropy and optical textures have also been discussed. Formation of above molecular orientations has been confirmed by X-ray studies.     

Inelastic neutron scattering studies of amorphous network solids

The inelastic neutron scattering technique provides information on the vibrational spectra of amorphous network glasses which extends that available from optical and resonance spectroscopy. A range of instrumentation is available to cover energies corresponding to relaxation phenomena (< 1 μeV) up to hydrogen stretching frequencies (500 meV). The resolution attainable is now sufficient to give data, the line width of which is dominated by the natural broadening caused by the structural variations in the networks. Additional information on atomic motions within a vibrational mode is given by the dependence of the scattered intensity on the magnitude of the scattering vector, Q, as revealed by the dynamical structure factor, S(Q, E). Low energy excitations (<10 meV) characteristic of the amorphous state have been related to anomalous low temperature thermal properties and also contain information on the range of topological order within the network. Experimental investigations using a variety of neutron inelastic scattering instrumentation to study vitreous silica, vitreous opal, vitreous boron trioxide, the amorphous germanium-selenium system and amorphous silicon containing hydrogen or deuterium are described. The results are used to discuss the possibilities and limitations of the method for testing structural models of network glasses.