Amphiphilic Mesogens Research Articles

The effect of fluorination on the liquid crystal and optical behaviors of amphiphilic cyanostilbene-based mesogens

Three series of amphiphilic mesogens containing a cyanostilbene core with zero, one or two lateral fluorine atoms, which attach one polar glycerol group at one end and three lipophilic alkyl chains at the other end were synthesized by Suzuki coupling and Knoevenagel reactions. The thermodynamics, liquid crystalline and photophysical properties of these amphiphilic mesogens were evaluated by polarized optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction, density functional theory calculation, absorption and emission spectra. The non-fluoro and mono-fluoro amphiphilic mesogens display a mesophase transition from hexagonal columnar phase to three-dimensional micellar cubic phase with Im3¯m lattice by the increase of temperature or the elongation of terminal lipophilic alkyl chains. The bi-fluoro amphiphilic mesogens can only self-organize into hexagonal columnar mesophase. Similar spectral properties and weak solvatochromism in these amphiphilic mesogens are observed attributed to the weak intramolecular charge transfer and weak influence of fluorine atom on the electronic properties in both ground and excited states. In addition, aggregation-induced emission behavior could be achieved in these amphiphilic mesogens due to the twisted molecular configuration. Therefore, complex self-assembly structures and interesting photophysical properties may be altered by varying the degree of fluorination of amphiphilic mesogens, which could promote the application of liquid crystalline materials.

Amphiphilic benzothiadiazole derivatives: synthesis, self-assembly and applications as light-emitting liquid crystal display and switchable anisotropic scattering device

Series of benzothiadiazole based amphiphilic mesogens consisting of a rigid 4,7-diphenyl-2,1,3-benzothiadiazole core with three lipophilic flexible alkoxy chains at one end and a polar glyceryl group at the other end were synthesized. The influence of the type of the linking groups in the rigid core and the length of terminal alkoxy chain on their self-assembly behavior and photophysical properties was investigated. These compounds can self-assemble into luminescent liquid crystals (LCs) with columnar/p6mm and CubI/Im3m lattices as well as luminescent organo gels with different morphologies. Furthermore, the applications of these compounds as light-emitting liquid crystal display (LE-LCD) as well as electrically switchable anisotropic scattering device were realized.

Air/Water Interfacial Monolayer Assembly of Peptide-Conjugated Liquid-Crystalline Molecules

Abstract Potential applications of functional liquid crystals such as biosensors strongly rely on control of the molecular orientation at interfaces. However, little knowledge regarding detailed molecular arrangements at such interfaces is available. In this work, two-dimensional self-assembling behavior at air/water interfaces of two types of amphiphilic mesogens with different peptide chains, arginine-glycine-aspartic acid and glycine-glycine-aspartic acid is investigated. Surface pressure–mean molecular area isotherms indicate that both bioconjugated mesogens are in the liquid expanded state to high surface compression conditions. The monolayer thickness of 16–19 Å derived by atomic force microscopic images is much smaller than the molecular length of ∼50 Å of a completely stretched motif. This implies that the bioconjugated molecules align in an inflected manner where tetraethylene glycol connecting hydrophobic rigid-rod and hydrophilic peptides is the inflection point. Contact angles of water for substrate surface with monolayers remarkably change depending on the surface pressure at the substrate transfer. This can be explained by the varied molecular arrangements with surface compression at the air/water interfaces. Understanding of molecular orientation at air/water interfaces is of fundamental importance for study of the ordering of liquid crystals at various other interfaces, leading to the design and further development of functional liquid-crystalline molecules for attractive sensor platforms.

Bicontinuous Gyroid Phase of a Water-Swollen Wedge-Shaped Amphiphile: Studies with In-Situ Grazing-Incidence X-ray Scattering and Atomic Force Microscopy.

We report on formation of a bicontinuous double gyroid phase by a wedge-shaped amphiphilic mesogen, pyridinium 4′-[3″,4″,5″-tris-(octyloxy)benzoyloxy]azobenzene-4-sulfonate. It is found that this compound can self-organize in zeolite-like structures adaptive to environmental conditions (e.g., temperature, humidity, solvent vapors). Depending on the type of the phase, the structure contains 1D, 2D, or 3D networks of nanometer-sized ion channels. Of particular interest are bicontinuous phases, such as the double gyroid phase, as they hold promise for applications in separation and energy. Specially designed environmental cells compatible with grazing-incidence X-ray scattering and atomic force microscopy enable simultaneous measurements of structural parameters/morphology during vapor-annealing treatment at different temperatures. Such in-situ approach allows finding the environmental conditions at which the double gyroid phase can be formed and provide insights on the supramolecular structure of thin films at different spatial levels.

Nuclear Magnetic Resonance and Calorimetric Investigations of Extraction Mode on Flaxseed Gum Composition.

We discussed about the influence of extraction mode on the flaxseed gums composition and their thermal stabilities. In order to do so, flaxseed gum was extracted by both classical magnetic stirring method and ultrasonic-assisted extraction (UAE). As a function of time, protein content, gum yield, pH values were evaluated and samples were characterized by 1H and 13C nuclear magnetic resonance (NMR) experiments as well as scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The flaxseed gum extracted in aqueous solution correspond to a mixture of different components, including polysaccharides, proteins and sometimes lignan derivatives. It is found that the protein and gum contents increase with the extraction duration for both the ultrasonic assisted and the traditional extraction while the pH decreases at the same time. As expected, compared to traditional magnetic stirring method, ultrasonic assisted extraction method can significantly enhance the yield of polysaccharides, lignans and proteins. The variation of pH is correlated to the increase of lignan molecules in the extracted samples. For thermic methods, SEM experiments showed that lignan derivatives which ester-bonded to polysaccharides associated to proteins are responsible to the formation of globular aggregates. Supplementary rod-like molecular organization were obtained from UAE and questions on the nature of the amphiphilic mesogen carbohydrate structures.

Bottom-Up Fabrication of Nanostructured Bicontinuous and Hexagonal Ion-Conducting Polymer Membranes

We report a simple photo-cross-linking process to chemically arrest the different liquid-crystalline structures formed by self-assembly of wedge-shaped amphiphilic mesogens. Using this route, we obtained free-standing polymer membranes with columnar or bicontinuous cubic structures, depending primarily on the relative humidity conditions during UV-induced cross-linking. These cross-linked mesostructures show much higher thermal stability in comparison with that of the liquid-crystalline structures of the initial monomers. The ionic conductivity of the membranes strongly depends on the water uptake preceding the polymerization reaction. According to NMR diffusometry, which can quantify water transport in one or two environments in these materials, the water diffusion rate in the membrane with bicontinuous cubic structures can approach values of commercial ion conducting membranes. These studies show promise for use of this fabrication route in practical applications for selective ion and water transport.

Humidity-induced formation of water channels in supramolecular assemblies of wedge-shaped amphiphiles: the effect of the molecular architecture on the channel topology.

In supramolecular assemblies, absorption of water can assist the channel formation, similarly to biological systems and Nafion-like commercial ion-selective membranes. In this work, we investigate humidity-induced formation of water channels in wedge-shaped amphiphilic molecules, namely sodium 4'-[3'',4'',5''-tris(alkyloxy)benzoyloxy]azobenzene-4-sulfonates. The studied molecules contain a polar sulfonate group at the tip and a hydrophobic periphery composed of alkyl chains of two different lengths. Upon increasing the relative humidity (RH) the amount of absorbed water significantly increases for the mesogen with dodecyl chains as compared to the one with octyl groups. In the former case, water sorption is accompanied by a considerable enhancement of ionic conductivity and a phase transition. In particular, an increase of RH induces a transition from a lamellar to a columnar phase resulting in the formation of 1D water channels running along the axis of the supramolecular columns. For the compound with shorter alkyl chains the lamellar phase exists in the entire RH-range exhibiting pronounced swelling at high RH-values and thereby forming a 2D water channel structure. NMR diffusometry was used to address the different molecular motions in the lyotropic mesophases of the studied amphiphiles.

Polymerizable wedge-shaped ionic liquid crystals for fabrication of ion-conducting membranes: Impact of the counterion on the phase structure and conductivity

In this manuscript, we report on the phase behavior of a novel family of ionic liquid crystals based on wedge-shaped amphiphilic mesogens of benzene sulfonates with different counterions (Li, Na, K and Cs) and polymerizable acrylic groups in the alkyl chain periphery. The phase diagrams of these compounds were explored with differential scanning calorimetry, polarized optical microscopy and temperature-dependent small-angle X-ray scattering. Lamellar and columnar liquid-crystalline phases were identified. Ionic conductivity was measured by impedance spectroscopy at different relative humidities. The influence of the counterion on the mesostructure and electrical conductivity is further discussed. Finally, it is demonstrated how photopolymerization can be employed to arrest the different supramolecular structures and transform the low-molecular-weight nanostructured liquid into self-standing ion conducting polymer membranes.

Self-Assembly of Bioconjugated Amphiphilic Mesogens Having Specific Binding Moieties at Aqueous–Liquid Crystal Interfaces

New bioconjugated amphiphilic mesogens with recognition groups have been synthesized, and their self-assembly behavior has been characterized at aqueous–liquid crystal (LC) interfaces. Specifically, the rod-shaped 2,3-difluoro-4′-(4-trans-pentylcyclohexyl)biphenyl-based mesogen was conjugated with either biotin or the arginine–glycine–aspartic acid (RGD) peptide sequence through a tetraethylene glycol chain. Langmuir film measurements revealed that the two biorecognition moieties lead to very different surface pressure–area isotherms, indicating that biotin and RGD have distinct effects on self-assembled monolayers formed by these bioconjugated mesogens at aqueous surfaces. Measurements of the surface-induced orientations of LCs exhibited by biotin-conjugated mesogens mixed with the room-temperature nematic LC 4-cyano-4′-pentylbiphenyl (5CB) revealed formation of cholesteric phases (consistent with the chiral nature of biotin) and evidence of the presence of the conjugated mesogen at aqueous interfaces. P...

Thermotropic liquid crystalline amphiphilic materials with hydrophilic groups at both terminals

We synthesized thermotropic amphiphilic mesogens with hydrophilic groups at both terminals through the reaction of 2-hydroxyethylamine with ω-bromo-1-(4-(4-(2-hydroxyethyl)phenylazo)phenoxy)alkylamine. We examined the thermal and orientational properties of these mesogens. The mesogens formed fluid smectic phases due to interactions between the hydrophilic units. On heating a thermotropic amphiphilic mesogen (HE-6-OH) with a hexamethylene spacer chain, smectic A and C phases were formed. On cooling, HE-6-OH exhibited a smectic F phase and a G state in addition to smectic A and C phases. Further, the thermotropic amphiphilic mesogen with a dodecamethylene spacer chain showed smectic A and F phases on heating and cooling.

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.

Structure of Mesogenic 4′-Hexadecyloxy-3′-Nitrobiphenyl-4-Carboxylic Acid (ANBC-16) at the Air--Water Interface

The Langmuir film behavior of an amphiphilic thermotropic mesogen at the air-water interface has been studied from surface pressure-molecular area isotherms. The isotherms exhibited a transition from an expanded to a condensed phase and on further compression also indicated the formation of an unstable monolayer, evidenced through a pronounced hysterisis observed during the decompression cycle. This phenomenon is explained on the basis of stronger hydrophobic forces dominating the orientation in the condensed phase. Increase in the limiting molecular area at lower temperatures provides supporting evidence for the observed loss of a stable condensed phase, which is attributed to the dissolution of the molecules into the aqueous subphase.

A New Family of Polymerizable Lyotropic Liquid Crystals: Control of Feature Size in Cross-Linked Inverted Hexagonal Assemblies via Monomer Structure

An efficient and versatile synthesis of a series of polymerizable amphiphilic mesogens that affords control over tail length and position of the polymerizable group is described. The synthesis employs a novel and facile method of preparing styrene ethers. The monomers are sodium salts of styrene ether-modified fatty acids that can be used to form cross-linkable inverted hexagonal (HII) lyotropic liquid crystal (LLC) phases at ambient temperature with controllable nanometer-scale dimensions. Examination of a series of regioisomers with the same alkyl chain length but with the styrene ether group at different locations along the chain revealed that the position of the styrene ether has a profound effect on the dimensions of the resulting HII phase at a fixed temperature and composition. Increasing overall monomer tail length also has a significant, although smaller, effect on the unit cell dimensions of the LLC phase. By controlling the structure of the LLC monomer in this manner, cross-linked HII phases wi...

Amphipilic and mesogenic carbohydrates XII. New thermotropic mesogens based on perfluoroalkyl-substituted carbohydrates

In continuing studies of amphiphilic and mesogenic carbohydrates, the mesomorphic properties of a new group of perfluoroalkylated amphiphilic mesogens are discussed. Smectic A phases dominate the mesomorphic behaviour of the new materials, but one compound exhibits a columnar mesophase and three others have SmA-SmC* transitions. The synthesis of these materials has already been reported and involved dithionite-mediated O- and S-perfluoroalkylations with 1-iodoperfluoroalkanes.

Heterogeneous Catalysis with Cross-Linked Lyotropic Liquid Crystal Assemblies: Organic Analogues to Zeolites and Mesoporous Sieves.

Organic, nanoporous heterogeneous catalysts based on a carboxylate-containing, amphiphilic mesogen catalyze the Knoevenagel condensation (see schematic representation). These networks maintain their order in solution and can be recycled. Enhanced basicity, excellent site accessibility, and substrate size exclusion are features of these nanostructured systems.

Dithionite-Catalysed Addition of Perfluoroalkyl Iodides to Unsaturated Carbohydrates

The perfluoroalkyl chains (−C4F9, −C6F13, −C8F17) were terminally connected to the 5-enopyranose 1, and the 5-enofuranoses 8 and 23 by addition, in the presence of sodium dithionite, of the corresponding homologous perfluoroalkyl iodides to the double bond. Up to six products were separated from each reaction mixture and then fully characterised. The 5-iodo derivatives 9, 10 (prepared from 8), 24, 25, and 26 (prepared from 23) were diastereomeric mixtures (5R-5S). Compounds 9, 10, and 25 were hydrodeiodinated to the methylene derivatives 17, 18, and 27, respectively. Complete deprotection of the perfluoroalkyl-substituted pyranose 3 (prepared from 1), and of the furanoses 17 and 18 led to the carbohydrate-based amphiphilic mesogens 29/30, 21, and 22, respectively, with a perfluoroalkyl tail. These formed mesophases of the smectic A type.

Formation of thermotropic and lyotropic smectic and columnar liquid crystalline phases by a novel type of rigid rod-like amphiphilic molecule

4-Benzyloxy-4′-(2,3-dihydroxypropyloxy)biphenyls with lateral methyl substituents are a novel class of amphiphilic mesogens displaying thermotropic and lyotropic smectic and columnar mesophases due to microsegregation of hydrophilic regions from all-aromatic segments in the absence of a flexible alkyl chain.

Novel Amphiphilic Phthalocyanine Mesogens

Abstract A number of novel amphiphilic phthalocyanines (Pcs) have been prepared containing long alkyl (-C16H33) and oligo(ethyleneoxy) chains (-O(C2H4O)3CH3) in the peripheral positions. All the Pcs prepared showed liquid crystalline behaviour, with most compounds giving two mesophases. Optical microscopy suggests that the lower temperature mesophase (M1) is a discotic rectangular mesophase, and that the higher temperature mesophase (M2) is a discotic hexagonal phase.

Thermotropic Liquid Crystals Derived from Amphiphilic Mesogens

Abstract It is well-established that the presence in amphiphilic molecules of two distinct and incompatible segments leads, in general, to the formation of molecular organizates certain of which exhibit ther-motropic liquid crystalline phases. In the present review the decisive role of amphiphilicity in inducing the formation of thermotropic smectic and columnar mesophases is discussed. Following a brief discussion of the main classes of ionic and nonionic amphiphiles that exhibit thermotropic liquid crystalline character, the emphasis is being placed to amphiphiles bearing multiple hydroxy groups and primarily to those with molecularly recognizable moieties able to form a system of hydrogen bonds. The discussion of this topic, primarily seen from the organic chemistry scope, is hoped to trigger further interest towards the synthesis of novel and unconventional amphiphiles with liquid crystalline properties.

Investigation of bulk properties and monolayer behaviour of amphiphilic mesogens: structural variations of the head group

Non-ionic amphiphilic compounds—butane-1.2,3-triol derivatives, 2-hydroxycarboxylic acids and carboxylic acids—incorporating rigid aromatic structural units like 1,4-disubstituted benzene, 4,4′-disubstituted biphenyl or the 2-phenyl-1,3,4-thiadiazole have been prepared. In most cases these compounds exhibit smectic liquid crystalline phases which can be stabilized by the addition of water. Their mesomorphic properties are compared with those of the structurally related 1,2-diol derivatives. Surprisingly, also some ω-(4-alkylphenoxy)alkanoic acids which have no thermotropic mesophases were found to aggregate to lyotropic mesophases in the presence of water. Most of the new compounds form stable monolayers at the air–water interface. The force–area isotherms of the thiadiazole derivatives have been recorded and it was found that they strongly depend on the chemical structure of the head group.