Mesogenic Behavior Research Articles

Study of the Effect of Lateral Substitution on Mesogenic Behavior with Reference to -CH=CH- Unit

ABSTRACTA novel cinnamate ester homologous series has been synthesized and studied with a view to understanding and establishing the effects of molecular structure on liquid crystal (LC) properties with a focus on the highly polar methoxy group as a lateral substituent. The series consists of twelve homologs; of which C1–C5 are nonmesogenic, and the rest of the homologs are enantiotropically smectogenic or and nematogenic. The texture of nematic phases is threaded or Schlieren and that of the smectic is either smectic A or C, as recognized and determined through an optical polarizing microscope equipped with a heating stage. The Sm-N I and N-I transition curves exhibit odd-even effects and behave in normal manner. The Cr-I M behaves in normal manner. Analytical and spectral data confirm the molecular structures of homologs. The average thermal stabilities for smectic and nematic are 125.3 C and 129.8 C, respectively, whose total mesophase length varies from 13 C to 51 C. Some LC properties of present series are compared with the structurally similar known series.

Mesomorphism dependence on molecular rigidity and flexibility

ABSTRACTA novel homologous series of mesogens was synthesized and studied with a view to understanding and establishing the relations between molecular structure and liquid crystal properties. Eleven members of the series were synthesized. Mesogenic behavior commences from the C6 member and continues up to the C14 member. The rest of the members, C1 to C5 and C16, are non-mesogenic. Mesogenic homologues (C6 to C14) are enantiotropically nematogenic. Textures of nematic phase are threaded or Schlieren. Transition temperatures were determined by an optical polarizing microscope equipped with a heating stage. Analytical and structural data confirm the molecular structures of homologues. Thermal stability for nematic phase is 198.0°C. Mesomorphic phase length ranges from 11.0°C to 50.0°C. Cr-I/N and N-I transition curves of a phase diagram behave in normal manner from C1 to C12 members, and higher members C14 and C16 deviated from normal behavior. Mesogenic properties of present novel ester series are compared with the structurally similar series. Odd-even effect is missing for the N-I transition curve. Thus, the present series is partly nematogenic without exhibition of smectic property whose transition temperatures vary between 153°C and 210°C.

Influence of the ester chain length on the mesogenic behavior and optical anisotropy of 4-[[4-(butoxy)phenyl]diazenyl]phenyl alkanoates

In this manuscript, we report synthesis and physico-chemical characterization of 4-[[4-(butoxy)phenyl]diazenyl]phenyl alkanoates homologous series. For the first time, nineteen derivatives are described here. The enantiotropic nematic phase is typically observed among all members of this series. However, in the case of 4-[[4-(butoxy)phenyl]diazenyl]phenyl stearate, the nematic phase shows a monotropic character. In addition to liquid-crystalline polymorphism, a second crystalline form was observed in some homologs. Furthermore, using a photoelastic modulator, the optical anisotropy in the nematic phase was determined in the first nine compounds of this series. Temperature dependence of optical anisotropy at significantly lower values of reduced temperature is relatively weak. In contrast, optical anisotropy shows a strong temperature effect near isotropization. Moreover, the influence of the ester chain elongation on liquid crystalline and optical properties was established.

Synthesis and Characterization of Novel Thermotropic Liquid Crystals with a Lateral Chloro Group

Twelve novel homologues having azolinkage of the series 4-Carbethoxy-[3′chloro-4′ (4″-n-alkoxy benzoyloxy)] azobenzenes has been synthesized. The methyl to n-hexyl homologues exhibit only a nematic mesophase, while the n-heptyl to n-dodecyl homologues exhibit both smectic and nematic mesophases. The n-tetradecyl and n-hexadecyl homologues exhibit only smectic mesophases. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-isotropic transition temperatures. The mesogenic behavior of present series is explained by comparing its homologue with other homologue of related mesogenic series. The synthesized compounds were characterized by combination of elemental analysis and standard spectroscopic methods. For the exhibition of mesomorphic property the role of ester and azo linkages has been discussed. The impact of lateral chloro group on mesomorphism is also discussed.

A study on polymorphism of hydrogen-bonded thermotropic liquid crystals

A novel mesogenic homologous series comprising of eight hydrogen-bonded liquid crystalline complexes are isolated and analyzed by forming a hydrogen bond between p-n alkyloxy benzoic acids (where n represent alkyloxy carbon number which varies from 5 to 12) and mesaconic acid, respectively. Eight synthesized complexes are subjected to Fourier transform infra-red spectroscopy, polarizing optical microscopy and differential scanning calorimetry studies to meet the basic characterization. The variation of optical tilt angle with respect to temperature in various conventional and smectic X phases are investigated and analyzed. A phase diagram is constructed to elucidate the mesogenic behavior of novel liquid crystalline series. The Cox ratio that reveals the order of various mesophases and the stability factor, to invoke the thermal stability of mesophases, is studied under the results of DSC thermogram.

Effect of bent-shape and calamitic-shape of hydrogen-bonded mesogens on the liquid crystalline properties

Six series of hydrogen-bonded (H-bonded) liquid crystals (LCs) were synthesised to study the effect of H-bonded mesogens with calamitic-shape or bent-shape structure on mesogenic behaviours by using isonicotinate and nicotinate derivatives as proton acceptors. The lateral fluoro-substituent and the length of the terminal chains were also studied in detail. When the structure of H-bonded LCs changed from calamitic-shape to bent-shape, the length to breadth ratio reduced greatly, which led to the temperature range of mesophases narrowed and a decrease in melting point and clear point. Although the introduction of fluorine substituent could reduce the length to breadth ratio and lead to the mesophase range narrowed, even disappeared, it could effectively inhibit the emergence of the smectic A phase and obviously enhance the strength of H-bond self-assembly. Additionally, it could help reduce the phase transition points to increase the length of terminal chains.

Controlling the Spatial Organization of Liquid Crystalline Nanoparticles by Composition of the Organic Grafting Layer.

Understanding how the spatial ordering of liquid crystalline nanoparticles can be controlled by different factors is of great importance in the further development of their photonic applications. In this paper, we report a new key parameter to control the mesogenic behavior of gold nanoparticles modified by rodlike thiols. An efficient method to control the spatial arrangement of hybrid nanoparticles in a condensed state is developed by changing the composition of the mesogenic grafting layer on the surface of the nanoparticles. The composition can be tuned by different conditions of the ligand exchange reaction. The thermal and optical behavior of the mesogenic and promesogenic ligands were investigated by using differential scanning calorimetry (DSC) and hot-stage polarized optical microscopy. The chemical structure of the synthesized hybrid nanoparticles was characterized by (1) H NMR spectroscopy, thermogravimetric analysis (TGA), XPS, and elemental analysis, whereas the superstructures were examined by small-angle X-ray diffraction (SAXSRD) analysis. Structural studies showed that the organic sublayer made of mesogenic ligands is denser with an increasing the average ligand number, thereby separating the nanoparticles in the liquid crystalline phases, which changes the parameters of these phases.

Differential response of cholesterol based pyrimidine systems with oxyethylene type spacers to gelation and mesogen formation in the presence of alkali metal ions.

A new series of lipophilic cholesteryl derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde has been synthesized. Oxyethylene spacers of variable lengths were inserted between the hydrogen bonding promoting pyrimidine core and the cholesteryl tail in order to understand their effect on the self-assembly of these compounds. Only compound 1a with the shortest spacer formed a gel in organic solvents such as n-butanol and n-dodecane. While other members (1b and c) having longer spacers led to sol formation and precipitation in n-butanol and n-dodecane respectively. The self-assembly phenomena associated with the gelation process were investigated using temperature-dependent UV-Vis and CD-spectroscopy. The morphological features of the freeze-dried gels obtained from different organic solvents were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The solid phase behaviours of these molecules and their associated alkali metal ion complexes were explored using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The molecular arrangements in the xerogel and in the solid state were further probed using a wide-angle X-ray diffraction (WAXD) technique. Analysis of the wide-angle X-ray diffraction data reveals that this class of molecules adopts a hexagonal columnar organization in the gel and in the solid state. Each slice of these hexagonal columnar structures is composed of a dimeric molecular-assembly as a building block. Significant changes in the conformation of the oxyethylene chains could be triggered via the coordination of selected alkali metal ions. This led to the production of interesting metal ion promoted mesogenic behaviour.

Emissive bis-salicylaldiminato Schiff base ligands and their zinc(II) complexes: Synthesis, photophysical properties, mesomorphism and DFT studies

Bis-salicylaldiminato Schiff base ligands and their Zn(II) complexes derived from 2,3-Diaminomaleonitrile (DAMN) were synthesized. Their molecular structures, photophysical properties and mesogenic behaviors were investigated. The ligands and their Zn(II) complexes were characterized by using elemental analysis, FT-IR, 1H NMR and molar conductivity measurements. Photophysical properties of ligands and their Zn(II) complexes were investigated in different polar solvents by using UV–visible and fluorescence spectroscopic studies. Ligands emit green light whereas complexes emit orange light upon irradiation with UV–visible light. The liquid crystalline phases of ligands and their Zn(II) complexes were characterized by polarizing optical microscopy and differential scanning calorimetry. The ligand having longer 4-n-octadecyloxy chain (n=18) displays columnar phase whereas the lower homologues (n=16, 12) did not show mesophase. The Zn(II) complexes having 4-n-octadecyloxy end chain display smectic B like phase whereas other lower homologues are non mesogenic in nature. The thermal stability of the compounds were studied by using thermo gravimetric analysis. The density functional theory was carried out to obtain the stable molecular conformation, dipole moment, molecular orbitals and polarizability of the ligands and their Zn(II) complexes.

Effect of counter-ion on the thermotropic liquid crystal behaviour of bis(alkyl)-tris(imidazolium salt) compounds

Recently, new thermotropic ionic liquid crystals (LCs) with a hexyl-linked tris(imidazolium bromide) core and two terminal alkyl chains were synthesised and characterised. To explore the effect of different counter-ions on the LC behaviour of this system, derivatives with BF4− and Tf2N− counter-ions were prepared and analysed. Five of the BF4− derivatives were found to exhibit thermotropic LC behaviour. The 12-, 14- and 16-carbon tail BF4− compounds form SmA phases. The 18- and 20-carbon tail homologues form what appears to be a smectic phase but are weakly mesogenic and harder to characterise. Only two of the Tf2N− derivatives exhibited mesogenic behaviour. The 18-carbon tail Tf2N− compound forms an as-yet unidentified, highly periodic smectic phase with positional order while the 20-carbon tail homologue forms a periodic SmA phase. The Tf2N− mesogens have much lower clearing points even though their LC phases have more order than the Br− and BF4− mesogens. X-ray diffraction showed that these mesogens have different amounts of tail interdigitation between the smectic layers depending on the counter-ion present. Atomistic molecular dynamics simulations indicated that counter-ion size plays an important role in defining the density of the ionic region, which in turn affects the amount of interdigitation in the smectic phases.

The Mesogenic Behavior of Isomeric and Nonisomeric Series of Novel Azoesters. 4-(4′-n-Alkoxy Benzoyloxy) Phenylazo-2″-Bromobenzenes

A novel homologous series: 4-(4′-n-alkoxy benzoyloxy) phenylazo-2″-bromobenzenes is synthesized and studied with a view to understanding and establishing the effects of molecular structure on mesogenic behavior in a series. The mesogenic property commences from fifth homologue to the last homologue. The transition temperatures of the series are relatively low ranging between 68°C and 143°C. The mesogenic range varies between 6°C and 22°C. The novel azoester series is nematogenic without exhibition of any smectogenic property and an average thermal stability of 89.4°C. The mesogenic behavior of the novel series is compared with structurally similar isomeric/nonisomeric other known series.

Novel Azoester Compounds with a Lateral Methyl Substituent

Twelve novel homologues with an azo linkage of the series 4-carbethoxy-[3′methyl-4′ (4″-n-alkoxy benzoyloxy)] azobenzenes have been synthesized. The methyl to n-hexyl homologues exhibit only nematic mesophases, while the n-heptyl to n-tetradecyl homologues exhibit both smectic and nematic mesophases. The n-hexadecyl homologue exhibits only a smectic mesophase. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-isotropic transitions. The mesogenic behavior of present series is explained by comparing each homolog of the related mesogenic series. The synthesized compounds were characterized by a combination of elemental analysis and standard spectroscopic methods. For the exhibition of mesomorphic property the role of ester and azo linkages has been discussed. The impact of the lateral methyl group on mesomorphism is also discussed.

The Phase Morphology of 2,5 DI-Substituted Pyridine Derivatives: A Quantum Mechanical Analysis

A comparative analysis of some mesogenic and nonmesogenic molecules of 2, 5 di-substituted pyridine derivatives molecules has been carried out. The multicenter multipole expansion method is used to obtain the various interaction energy terms. The phase morphology of the mesogenic molecular system has been further investigated with the aid of various energy terms of configurations. The multicenter multipole expansion method is satisfactory and predicts the mesogenic behavior of molecular systems.

Azo-functionalised achiral bent-core liquid crystalline materials: effect of presence of –N=N– linkage at different locations in the molecular architecture

Here, we report four new homologous series of azo-functionalised achiral bent-core compounds. The paper deals with the design, synthesis and liquid crystalline properties of four new homologous series of photochromic materials. In this study, we have investigated the effect of the presence of –N=N– linkage at different locations of the molecular architecture, on the mesomorphic properties. The molecular structures of all the newly synthesised compounds are established using the organic spectroscopic methods. The liquid crystalline properties are investigated using polarising optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction and electro-optical studies. They exhibit B1 (colr) and B2 (SmCAPA) mesophases. We find that the presence of the –N=N– linkage at different locations in the molecular architecture does not seem to have much effect on the mesogenic behaviour of such compounds. However, we clearly see a profound effect of the location of the –N=N– linkage on the photo-induced electro-optical properties of these compounds.

Mesogenic behaviour of p-nitrophenyl group linked with biphenyl mesogens via flexible spacers

Three series of nematogens with a terminal p-nitrophenyl group linked with biphenyl groups via flexible spacers were synthesised and their transitional properties were studied by means of differential scanning calorimetry (DSC) and polarising microscopy (POM). The nematic–isotropic transition temperature and the associated entropy change showed an odd–even effect as the length of the spacers was varied, in which the even members exhibited higher values. Such an odd–even effect was in consistency with the feature of liquid crystal dimers and trimers. On the basis of these data, it seemed reasonable to assume that the terminal p-nitrophenyl group played a role of mesogen in the nematic liquid crystal. The p-nitrophenyl group may work as a mesogen as a result of the interaction with the biphenyl mesogens. The mesogenic behaviour of the terminal p-nitrophenyl group is explained on the basis of neighbouring group effect.

Dissipative Particle Dynamics Simulation of the Phase Behavior of T-Shaped Ternary Amphiphiles Possessing Rodlike Mesogens

We employed dissipative particle dynamics simulations to explore the phase behavior of T-shaped ternary amphiphiles composed of rodlike cores connected by two incompatible end chains and side grafted segments. By fine-tuning the number of terminal and lateral beads, three phase diagrams for the model systems with different terminal chain lengths are constructed in terms of temperature and lateral chain length, which have some common features and mostly compare favorably with experimental studies with the exception a couple of new phases. It is worthwhile to highlight that the mixed cylindrical phase and the perforated layer phase, as the experimentally observed mesophases exclusive for facial amphiphilies, are found in simulations for the first time. Also, a novel gyroid structure is observed in series of T-shaped ternary amphiphiles for the first time. Furthermore, by evaluating the effective volume fraction of lateral chains, the phase sequence spanning from conventional smectic layer phase via perforated layer structures and polygonal cylindrical arrays to novel lamellar mesophase is established, which is not just qualitatively consistent with the related experimental findings but even the stability windows of some mesophases quantitatively correspond well to experimental results. The success of reproducing the in-plane ordering of rods in the lamellar phase as well as the generic phase diagram of such T-shaped ternary amphiphiles in great detail implies that our genetic model qualitatively captures many of the characteristics of the phase behavior of real T-shaped molecules and could serve as a satisfactory basis for further exploration of self-organization in other related soft matter systems.

Theoretical study on liquid crystal cyanobiphenyls: Phase stability and phase behavior

This article presents a theoretical study on liquid crystalline materials in homologous series of 4'-n-alkyl-4-cyanobiphenyl (nCB) with propyl (3CB), pentyl (5CB), and heptyl (7CB) groups. The atomic net charge and dipole moment components at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh–Schrodinger perturbation theory along with the multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for short-range interactions. Further, these interaction energy values have been used as input to calculate the translational entropy, and free energy of nCB (n=3, 5, and 7) molecules during the stacking, and in-plane interactions. The observed results have been correlated with the mesogenic behavior and phase stability based on the thermodynamic parameters introduced in this article. Further, an attempt has been made to elucidate the flexibility of a configuration at a particular temperature, which has a direct relation with phase transition property of the molecules.

Closo-1-CB11H11-1-Ph]− as a structural element for ionic liquid crystals

Ion pairs 2[Pyr] and 3[Pyr] containing [closo-1-CB11H11-1-Ph]− as the structural element were synthesized and their liquid crystalline properties were investigated by thermal and optical methods. Their mesogenic behavior was compared to that of the analogous series 1[Pyr] having a COO linking group between the benzene ring and the cluster, and the observed structure–property relationships are discussed in the context of general trends in related non-ionic liquid crystals.

Synthesis and Mesogenic Behavior of Novel Liquid Crystals With a –CH˭CH–CO– Central Bridge

A novel homologous series α-3-[-4′-n-alkoxy benzoyloxy] phenyl-β-4″-methoxy benzoyl ethylene, which consists of 11 members, has been synthesized and the mesomorphic behavior studied in relation to the molecular structure. The pentyl to hexadecyl homologues are mesomorphic and the remaining homologues are non-mesomorphic. The pentyl, tetradecyl, and hexadecyl derivatives are only enantiotropic nematic, while the hexyl, octyl, decyl, and dodecyl homologue derivatives exhibit smectic mesomorphism in addition to the enantiotropic nematic phase. The transition and melting temperatures were determined by an optical polarizing microscope with a heating stage. Analytical and spectral data confirm the molecular structures. The texture of the nematic phase is of the threaded or Schlieren type and that of the smectic mesophase is a typical smectic A phase. The transition curves of the phase diagram behave in normal manner without showing an odd–even effect. The smectic and nematic thermal stabilities are 129.5°C and 150.7°C, respectively. The novel series is partly smectogenic and predominantly nematogenic with a middle-ordered melting type whose mesomorphic phase length varies from 7°C to 46°C. The mesomorphic properties are compared with other structurally similar homologous series.

Mesogenic Behavior of a Novel Homologous Series: α-4-[4′-n-Alkoxy benzoyloxy] benzoyl-β-4″-chloro Phenyl Ethylenes

A novel homologous series entitled α-4-[4′-n-Alkoxy benzoyloxy] benzoyl β-4″-chloro phenyl ethylenes consists of 12 homologues. The pentyl to tetradecyl homologues are enatiotropically nematogenic and all of the other homologues (methyl to butyl and hexadecyl) are nonmesogenic. The textures of the nematogenic mesophase are threaded or Schlieren in nature. The transition temperatures were determined by an optical polarizing microscope with a heating stage. The phase diagram of the novel series shows a normal behavior with negligible abnormality in trend of the nematic-isotropic transition curve and exhibits an odd–even effect as expected. The nematic thermal stability is 149.4°C on average. The series is nematogenic without any smectogenic character. The mesogenic properties of the novel series are compared with known series that are structurally similar.