Liquid Crystalline Phase Behavior Research Articles

Photoinduction of optical anisotropy in an azobenzene-containing ionic self-assembly liquid-crystalline material

Liquid-crystalline (LC) phase behavior and photoinduction of optical anisotropy in the ionic self-assembly complex 4-(4-diethylaminophenylazo) benzenesulfonate-dimethyldidodecylammonium (EO-C12D) has been investigated by polarized light microscopy, differential scanning calorimetry (DSC), x-ray scattering, null-ellipsometry, and UV-visible absorbance techniques. The complex exists in a bilayer smectic- A (smA{2}) LC phase at elevated temperatures (65-160 degrees C) and in a rectangular columnar (Col{r}) LC phase in the temperature range of -5-65 degrees C . Hysteresis in the transition from the smectic to the columnar LC phase was observed. Detailed experimental investigations of the phase behavior, film-forming properties, and induction of optical anisotropy were performed. High values of photoinduced anisotropy (dichroic ratio of approximately 50) were detected when thin films of the complex were irradiated with linearly polarized light ( Ar+ laser, lambda=488 nm ). It was shown that the azobenzene units align perpendicular to the polarization of the exciting light causing an alignment of the columns parallel to the light electric field vector. On the basis of all experimental results a model of the photoinduced alignment of the photochromic complex is proposed in which photoalignment of the material is connected to the reorientation of domains.

Biophysical characterization of synthetic rhamnolipids

Synthetic rhamnolipids, derived from a natural diacylated glycolipid, RL-2,2(14), produced by Burkholderia (Pseudomonas) plantarii, were analyzed biophysically. Changes in the chemical structures comprised variations in the length, the stereochemistry and numbers of the lipid chains, numbers of rhamnoses, and the occurrence of charged or neutral groups. As relevant biophysical parameters, the gel (beta) to liquid crystalline (alpha) phase behavior of the acyl chains of the rhamnoses, their three-dimensional supramolecular aggregate structure, and the ability of the compounds to intercalate into phospholipid liposomes in the absence and presence of lipopolysaccharide-binding protein were monitored. Their biological activities were examined as the ability to induce cytokines in human mononuclear cells and to induce chemiluminescence in monocytes. Depending on the particular chemical structures, the physicochemical parameters as well as the biological test systems show large variations. This relates to the acyl chain fluidity, aggregate structure, and intercalation ability, as well as the bioactivity. Most importantly, the data extend our conformational concept of endotoxicity, based on the intercalation of naturally originating amphiphilic virulence factors into membranes from immune cells. This 'endotoxin conformation', produced by amphiphilic molecules with a hydrophilic charged backbone and apolar hydrophobic moiety, and adopting inverted cubic aggregate structures, causes high mechanical stress in target immune cells on integral proteins, eventually leading to cell activation. Furthermore, biologically inactive rhamnolipids with lamellar aggregate structures antagonize the endotoxin-induced activity in a way similar to lipid A-derived antagonists.

Positional Isomers of Linear Sodium Dodecyl Benzene Sulfonate: Solubility, Self-Assembly, and Air/Water Interfacial Activity

Commercial linear alkyl benzene sulfonates (ABS) are a very important class of anionic surfactants that are employed in a wide variety of applications, especially those involving wetting and detergency. Linear ABS surfactants generally consist of a complex mixture of different chain lengths and positional isomers. This diversity and level of complexity makes it difficult to develop fundamental structure-property correlations for the commercial surfactants. In this work, six monodisperse headgroup positional isomers of sodium para-dodecyl benzene sulfonate (Na-x-DBS, x = 1-6) have been studied. The influence of headgroup position and added electrolyte (NaCl) on the solubility and self-assembly (micellar and vesicular aggregation and lyotropic liquid crystalline phase behavior) in the temperature range from 10 to 90 degrees C have been investigated. Additionally, the air-aqueous solution interfacial adsorption at 25 (no added NaCl) and 50 degrees C (from 0 to 1.0 M added NaCl) has been examined. The observed physicochemical behavior is interpreted in terms of local molecular packing constraints, and in the case of the lyotropic liquid crystalline behavior global aggregate packing constraints as well.

Monosaccharide composition, chain length and linkage type influence the interactions of oligosaccharides with dry phosphatidylcholine membranes

Sugars play an important role in the desiccation tolerance of most anhydrobiotic organisms and disaccharides have been extensively investigated for their ability to stabilize model membranes in the dry state. Much less is known about the ability of oligosaccharides to protect dry membranes. However, it has been shown that different structural families of oligosaccharides have different efficacies to interact with and protect membranes during drying. Here, we have compared three families of linear oligosaccharides (fructans, malto-oligosaccharides, manno-oligosaccharides) for their chain-length dependent lyoprotective effect on egg phosphatidylcholine liposomes. We found increased protection with chain length for the fructans, a moderate decrease in protection with chain length for malto-oligosaccharides, and a strong decrease for manno-oligosaccharides. Using Fourier-transform infrared spectroscopy and differential scanning calorimetry, we show that the degree of lyoprotection of the different sugars is closely related to their influence on the gel to liquid–crystalline phase behavior of the dry membranes and to the extent of H-bonding to different groups (C O, P O, choline) in the lipids. Possible structural characteristics of the different oligosaccharides that may determine the extent to which they are able to interact with and protect membranes are discussed.

Towards General Guidelines for Aligned, Nanoscale Assemblies of Hairy-Rod Polyfluorene

This account highlights recent progress towards understanding the complex hierarchical levels of solid-state structure in a prototypical helical hairy-rod polyfluorene, poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (or PF2/6). This branched-side-chain containing polyfluorene undergoes a systematic intermolecular self-assembly and liquid-crystalline phase behavior in combination with uniaxial and biaxial alignment. The latter processes yield full three-dimensional orientation of the crystallites and polymer chains. Also reviewed are the impact of the molecular structure and phase behavior on surface morphology, anisotropic film formation, and, ultimately, the overall impact of these physical attributes on optical constants. This particular polyfluorene also represents a model system for demonstrating the applicability of mean-field theory in detailing the self-organization of aligned hairy-rod block-copolymer systems. These results of PF2/6 are compared to those of other archetypical π-conjugated hairy-rod polymers. General guidelines of how molecular weight influences nanostructure, phase behavior, alignment, and surface morphology are given.

Entropy-Driven Formation of a Chiral Liquid-Crystalline Phase of Helical Filaments

We study the liquid-crystalline phase behavior of a concentrated suspension of helical flagella isolated from Salmonella typhimurium. Flagella are prepared with different polymorphic states, some of which have a pronounced helical character while others assume a rodlike shape. We show that the static phase behavior and dynamics of chiral helices are very different when compared to simpler achiral hard rods. With increasing concentration, helical flagella undergo an entropy-driven first order phase transition to a liquid-crystalline state having a novel chiral symmetry.

Divalent cations affect chain mobility and aggregate structure of lipopolysaccharide from Salmonella minnesota reflected in a decrease of its biological activity

The physicochemical properties and biological activities of rough mutant lipopolysaccharides Re (LPS Re) as preformed divalent cation (Mg 2+, Ca 2+, Ba 2+) salt form or as natural or triethylamine (Ten +)-salt form under the influence of externally added divalent cations were investigated using complementary methods: Differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopic (FT-IR) measurements for the β ↔ α gel to liquid crystalline phase behaviour of the acyl chains of LPS, synchrotron radiation X-ray diffraction studies for their aggregate structures, electron density calculations of the LPS bilayer systems, and LPS-induced cytokine (interleukin-6) production in human mononuclear cells. The divalent cation salt forms of LPS exhibit considerable changes in physicochemical parameters such as acyl chain mobility and aggregate structures as compared to the natural or monovalent cation salt forms. Concomitantly, the biological activity was much lower in particular for the Ca 2+- and Ba 2+-salt forms. This decrease in activity results mainly from the conversion of the unilamellar/cubic aggregate structure of LPS into a multilamellar one. The reduced activity also clearly correlates with the higher order – lower mobility – of the lipid A acyl chains. Both effects can be understood by an impediment of the interactions of LPS with binding proteins such as lipopolysaccharide-binding protein (LBP) and CD14 due to the action of the divalent cations.

The Internal Dynamic Modes of Charged Self-Assembled Peptide Fibrils

Photon correlation spectroscopy is used to study the internal dynamics of self-assembled charged peptide fibrils. Short neutral and charged polymeric aggregates have diffusive modes due to whole macromolecular motion. For long semiflexible fibrils the logarithm of the intermediate scattering function follows a q(2)t(3/4) scaling at long times consistent with a Kratky-Porod free energy and preaveraged Oseen hydrodynamics. Persistence lengths on the order of micrometers are calculated for the peptide fibrils consistent with estimates from the liquid-crystalline phase behavior. Fibril diameters (5-35 nm) calculated from the initial decay of the correlation functions are in agreement with transmission electron microscopy measurements.

Aqueous cholesteric liquid crystals using uncharged rodlike polypeptides.

The aqueous, lyotropic liquid-crystalline phase behavior of the alpha-helical polypeptide, poly(N(epsilon)-2-[2-(2-methoxyethoxy)ethoxy]acetyl-lysine) (1), has been studied using optical microscopy and X-ray scattering. Solutions of optically pure 1 were found to form cholesteric liquid crystals at volume fractions that decreased with increasing average chain length. At very high volume fractions, the formation of a hexagonal mesophase was observed. The pitch of the cholesteric phase could be varied by a mixture of enantiomeric samples L-1 and D-1, where the pitch increased as the mixture approached equimolar. The cholesteric phases could be untwisted, using either magnetic field or shear flow, into nematic phases, which relaxed into cholesterics upon removal of field or shear. We have found that the phase diagram of 1 in aqueous solution parallels that of poly(gamma-benzyl glutamate) in organic solvents, thus providing a useful system for liquid-crystal applications requiring water as solvent.

Side chain cholesteric liquid crystalline elastomers: synthesis and phase behaviour

A series of new side chain cholesteric liquid crystalline elastomers (P-2–P-6) containing the nematic crosslinking monomer 4-(10-undecen-1-yloyloxy)benzoyl-4′-allyloxybenzoyl-p-benzenediol bisate (M-1) and the cholesteric monomer 4-cholesteryl 4-(10-undecen-1-yloyloxy)benzoate (M-2) were synthesized. The chemical structures of the monomers and elastomers obtained were confirmed by FTIR and 1H NMR spectroscopy. Their liquid crystalline properties and phase behaviour were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The effect of the crosslinking units on phase behaviour is discussed. Elastomers containing less than 20 mol % of the crosslinking units showed elasticity, reversible phase transitions and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition and isotropization temperatures of P-2–P-6 increased with the increasing concentration of crosslinking unit M-1.

Copper-metallomesogen structures obtained by ionic self-assembly (ISA): molecular electromechanical switching driven by cooperativity.

In a stepwise noncovalent multiple-interaction strategy, copper(II) salts were complexed with the sodium salts of bathophenanthrolinedisulfonic acid (BPS) and bathocuproinedisulfonic acid (BCS), and organized into nanostructured materials by the addition of ammonium surfactants by means of the ionic self-assembly (ISA) route. In the case of the methyl-substituted BCS complexes, a slow color change from green to brick red was observed. UV and EPR investigations showed that the color change was due to a change in oxidation state, the resulting brick red color is typical for Cu(I) species. It is concluded that steric interactions and mechanical packing into a supramolecular structure drive this electronic transition at the metal center. When complexation is performed with double-tail ammonium surfactants, these metallomesogenic materials exhibit thermotropic liquid-crystalline phase behavior, as investigated by polarized light microscopy, differential scanning calorimetry (DSC), and temperature-dependent wide-angle and small-angle X-ray analyses. The complexity of the observed phases increased with increasing tail length of the surfactants. Complexation with double-tail C(18) surfactants yielded highly organized materials for both the BPS and BCS ligands.

Cross-linked normal hexagonal and bicontinuous cubic assemblies via polymerizable gemini amphiphiles.

The synthesis and lyotropic liquid-crystalline (LLC) phase behavior of a homologous series of intrinsically cross-linkable gemini surfactants are described. These novel bis(alkyl-1,3-diene)-based phosphonium gemini amphiphiles exhibit "normal" hexagonal (H(I)), Type I bicontinuous cubic (Q(I)), and lamellar (L(alpha)) phases in water, and can be photocross-linked with retention of phase architecture in each case. On the basis of their locations on the phase diagram, their powder X-ray diffraction profiles, and the physical properties of the cross-linked materials, the Q(I) phases formed by these gemini monomers are consistent with four possible bicontinuous cubic architectures with P or I space group symmetry that have been identified previously for small molecule amphiphiles. The extent of polymerization (i.e., the degree of diene conversion) achieved in the LLC phases was determined to be in the 23% to 71% range using UV-vis spectrometry, which is more than sufficient to extensively stabilize the systems. The resulting cross-linked H(I), L(alpha), and Q(I) phases are stable up to 300 degrees C in air. To our knowledge, these reactive amphiphiles constitute the first example of a polymerizable gemini surfactant, and the first example of a cross-linkable amphiphile system that can be polymerized in both the H(I) and a Q(I) mesophase with retention of phase microstructure.

Patterned Peptide Multilayer Thin Films with Nanoscale Order Through Engineered Liquid Crystallinity

Self‐assembling polypeptide molecules have been designed to explore aspects of hierarchical or multiscale long‐range order, and the interactions that occur at various length scales that drive order and patterning in complex materials. Three peptides were synthesized with repetitive motifs loosely based on insect silks. Thin films were obtained from each peptide using a solvent templating technique. Smectic layers were observed in the thin films and in bulk materials prepared for comparison. A variety of additional effects, including chirality and interfacial anchoring at the helix, compete in the thin films, resulting in complex but regular patterns within the materials over a hierarchy of length scales spanning several orders of magnitude. The relative magnitude of these effects can be tuned through sequence design, as can the smectic layer thickness. Thus, many parameters describing the patterns in the materials can be controlled at the molecular design level, allowing complex patterns of nanoscale to microscale topography and chemistry to be created through self‐assembly. In addition to the potential for designer materials, there may be some biological relevance to the observed phenomena. The silk‐like molecules used fold into “hairpins,” providing a rigid unit that can drive liquid crystalline phase behavior. A similar process could be at work in silks and other structurally similar proteins that form textured materials. Chiral smectic order and multiscale patterns that can often result are clearly rich areas, worthy of further exploration.

Ordering of quantum dots using genetically engineered viruses.

A liquid crystal system was used for the fabrication of a highly ordered composite material from genetically engineered M13 bacteriophage and zinc sulfide (ZnS) nanocrystals. The bacteriophage, which formed the basis of the self-ordering system, were selected to have a specific recognition moiety for ZnS crystal surfaces. The bacteriophage were coupled with ZnS solution precursors and spontaneously evolved a self-supporting hybrid film material that was ordered at the nanoscale and at the micrometer scale into approximately 72-micrometer domains, which were continuous over a centimeter length scale. In addition, suspensions were prepared in which the lyotropic liquid crystalline phase behavior of the hybrid material was controlled by solvent concentration and by the use of a magnetic field.

New challenges in quantum chemistry: Quests for accurate calculations for large molecular systems.

As quantum chemistry plays a more and more central role in many complicated chemical problems, it has become necessary to obtain accurate results for large molecular systems. Conventional quantum chemistry methods are either too expensive to apply to large systems or too approximate for the results to be reliable, and they fail to satisfy this requirement. A variety of different approaches is being developed with the aim of achieving this goal: local correlation methods; divide-and-conquer methods; linear-scaling density functional methods based on the fast multipole and other approximations; effective potential methods; and hybrid methods. ONIOM (our N-layered integrated molecular orbital plus molecular mechanics method), developed by the authors, is a hybrid method in which a large molecular system is divided into onion-skin-like layers, and different quantum chemistry/molecular mechanics methods are used for different parts of the system; the results are combined to extrapolatively estimate the results of high-level calculation for the real system. Several applications of ONIOM will be discussed.

Nonlinear Optical Properties and Liquid-Crystalline Behavior of New Polyesters with Dipole Moments Aligned Transverse to the Backbone

Side-chain azobenzene-based second-order nonlinear optical (NLO) polyesters (PNPAR, PNPAPAR, PPAR-TC, PPAPAR-TC) containing nitro or tricyanovinyl groups as acceptors with their dipole moments aligned transverse to the polymer backbone, were synthesized and characterized, and their thermal properties, liquid-crystalline (LC) phase behavior, and NLO properties studied. Polymers containing nitro groups as acceptors were found to exhibit LC properties, while those containing tricyanovinyl groups as acceptors did not. The LC polymers were of smectic nature. The polymer PNPAR with a nitro group, when poled at the LC phase-transition temperature range, was found to exhibit higher NLO activity than that with a tricyanovinyl group (PPAR-TC). When polymer PNPAPAR containing a nitro group, also exhibiting an LC phase, was poled at a temperature different from that of its transition temperature, the NLO activity observed was comparable to, and/or lower than, that of the similar polymer (PPAPAR-TC) having a tricyanobvinyl group in the place of the nitro group. All the polymers were found to possess moderate temporal stability of second harmonic generation activity and were found to have stability over the whole process.

The Synthesis and Investigation of the Liquid-Crystalline Phase Behaviour of a Series of N,N-bis(1-deoxy-D-mannitol-1-yl)-alkamides

The results of our synthetic efforts and the investigation of the liquid-crystalline phase behaviour of a series of bis-mannosyl-amides are presented. In this investigation the structure of the polar carbohydrate groups was kept constant and the length of the terminal alkyl chain was varied between seven and twelve methylene groups. The results obtained from the DSC measurements, optical polarising microscopy and X-ray diffraction experiments will be reported and a structural model will be discussed.

Liquid–crystal phase transitions in suspensions of plate–like particles

We study the liquid–crystalline phase behaviour of suspensions of hard colloidal platelets, in particular, the relationship between their size polydispersity and the stability of a nematic, columnar and smectic phase. A first–order nematic–to–columnar transition is observed for suspensions up to 25% polydispersity in platelet diameter (σD). However, at densities slightly above nematic–columnar coexistence, the columnar order appears to vanish in the case of σD = 25%, while columnar stability persists for σD = 17%. The observed tolerance of polydispersity in the columnar phase, being a two–dimensional crystal, seems remarkable in light of current predictions for the terminal polydispersity for hard–sphere crystallization and hard–rod smectic ordering.

Fluorescence Studies of Pyrene-Labeled Cholesterol-Bearing Polymer Films

Copolymers of cholesteryl 6-methacryloyloxy hexanoate and (1-pyrenernethyl) 6-methacryloyloxy hexanoate were prepared by free radical polymerization, with the amount of (1-pyrenemethyl) 6-methacryl oyloxy hexanoate this (f Py in the polymers ranging from 0-100 mol.%. The copolymers with f Py ≤ 20.0 mol.% show distinct liquid-crystalline (LC) phase behavior, According to the fluorescence of pyrene in these copolymer LC films, each pyrene raciety is individually incorporated into the LC domain, and the interaction between the pyrene moleties is restricted.

Novel Class of Ionic Metallomesogens without Organic Ligands: Thermotropic Liquid-Crystalline Behavior of Tetrachlorometalate Salts and Distinct Polymorphic Transition in Smectic A Phases

The thermotropic liquid-crystalline behavior of the metal complex salts [(C18H37)(CH3)3N]2[MCl4] and [(C18H37)2(CH3)2N]2[MCl4] (M = Co, Ni, Cu, Zn, Cd) was evaluated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffractometry, and electronic absorption as well as by Raman spectroscopy. Introduction of divalent metal ions into the cationic amphiphiles was found to result in enhancement of their thermal properties. Although the parent tetraalkylammonium chlorides showed no liquid-crystalline phase (or indistinct transition behavior), the derived tetrachlorometalate salts exhibited a stable liquid-crystalline phase and simple phase transition behavior. For instance, the tetrachlorocobaltate salt, [(C18H37)2(CH3)2N]2[CoCl4], showed a smectic A (SA) phase in an extremely wide temperature range from 45 to 155 °C on heating. In the SA phase, the complexes were confirmed to form a layered self-assembly that is built up from alternating layers of tetrahedral [MCl4]2- ions and flexible alkyl segments. Furthermore, distinct polymorphism in the SA phase was observed for several tetrachlorometalate salts, which differs essentially from the SA−SA transition described in the usual rodlike liquid crystals.