Chromonic liquid crystal phases

Multiscale dynamics of pretransitional fluctuations in the isotropic phase of a lyotropic liquid crystal

Using an improved static and dynamic light-scattering technique, we have observed multiscale relaxation of the pretransitional fluctuations in the isotropic phase of a cromolyn aqueous solution, a lyotropic liquid crystal where rods are formed by aggregates of disklike molecules. We have detected the onset of cromolyn aggregation about 12 degrees C above the transition temperature. The onset is manifested by the emergence of strong scattering due to the fluctuations of local anisotropy and by the split of the diffusion dynamics into two distinctly different modes, one associated with the relatively fast diffusion of monomer-size particles and the other one with the much slower diffusion of the cromolyn aggregates. A third observed dynamic mode is associated with the pretransitional slowing down of fluctuations of the local anisotropy. This mode behaves differently in polarized and depolarized light scattering, due to a coupling between fluctuations of the local-anisotropy and velocity fluctuations.

Controllable side-by-side and end-to-end assembly of Au nanorods by lyotropic chromonic materials

We present a simple and universal technique for assembling gold nanorods (NRs) using self-assembled stacks of lyotropic chromonic materials, without covalent bonding between NRs and the linking agent. The anisotropic electrostatic interaction between the chromonic stacks and NRs allows one to achieve either side-by-side or end-to-end assembly, depending on the surface charge of NRs. The assembled superstructures are stable within an extended temperature range; the degree of NR aggregation can be controlled by a number of factors influencing the self-assembly of chromonic materials, such as the concentration and pH of the solution.

A study of the structural effects on the liquid-crystalline properties of ionic perylenebis(dicarboximide)s using UV-Vis spectroscopy, polarized light microscopy, and NMR spectroscopy

Ionic perylenebis(dicarboximide)s 1-5 were synthesized. The aggregation and liquid-crystalline properties of these compounds in aqueous solutions were investigated. In the concentration range of approximately 5 x 10-7-5 x 10-4 M, the structures of the ammonium side chains and counterions did not have a significant effect on the electronic transition properties and H-aggregate formation of these compounds. However, the liquid-crystalline phase properties varied with the structure of the side chains and the counterions. Ionic perylenebis(dicarboximide)s 1, 3, and 5 with chloride ions formed nematic (N) phases from the isotropic (I) phase, while 2 and 4 with p-methylbenzenesulfonate ions formed chromonic ribbons from the I phase. Studies by polarized light microscopy and 2H NMR spectroscopy indicated that the N phase of 5 (with gem dimethyl groups) formed at higher concentrations than those observed for 1 and 3 at the same temperature. Furthermore, the N phase of 5 was less ordered compared to those of 1 and 3 at a similar concentration and temperature, presumably due to the bulkiness of the side chains of 5 that hindered the stacking and pi-interactions of the aromatic rings.

X-ray microscopy study of chromonic liquid crystal dry film texture

Soft x-ray spectromicroscopy has been used to investigate the degree of the molecular alignment of sulfonated benzo[de]benzo[4.5]imidazo[2,1-a]isoquinoline[7,1], a lyotropic chromonic liquid crystal (LCLC). LCLC thin films cast from concentrated aqua solution (20%wt.) , aligned by shear flow and dried, show strong linear dichroism in their C-, N-, O-, S- K edge near edge x-ray spectra (NEXAFS). The carbon K edge has been used for quantitative evaluation of the orientational texture of the films at a submicron spatial scale. This has verified there is predominantly in-plane alignment of the LC director. To highlight the role of hydrophobic-hydrophilic interactions, two stereoisomers of the same dye has been synthesized with different positioning of terminal sulfonate groups, in the form of a mixture of isomers with sulfonate groups in 2,10 and 2,11 positions (Y104 compound) and in a 5,10-disulfo arrangement (Y105). Both compounds develop characteristic herringbone-type texture with similar domain sizes. Polarized optical microscopy and higher resolution x-ray microscopy show sinusoidal-like undulations of the molecular director, with occasional crisscross appearance. Such behavior is found to be consistent with earlier observation of striations, characteristic of the columnar phase. The drastic difference in the degree of undulation ( +/-15 degrees in Y104 and +/-7 degrees in Y105 films) and long period of undulation (approaching the film thickness) requires further analysis. It was also found that the degree of in-plane order within domains changes from 0.8 for Y104 to >0.9 in Y105 films.

Supra-molecular association and polymorphic behaviour in systems containing bile acid salts

A wide number of supra-molecular association modes are observed in mixtures containing water and bile salts, BS, (with, eventually, other components). Molecular or micellar solutions transform into hydrated solids, fibres, lyotropic liquid crystals and/or gels by raising the concentration, the temperature, adding electrolytes, surfactants, lipids and proteins. Amorphous or ordered phases may be formed accordingly. The forces responsible for this very rich polymorphism presumably arise from the unusual combination of electrostatic, hydrophobic and hydrogen-bond contributions to the system stability, with subsequent control of the supra-molecular organisation modes. The stabilising effect due to hydrogen bonds does not occur in almost all surfactants or lipids and is peculiar to bile acids and salts. Some supra-molecular organisation modes, supposed to be related to malfunctions and dis-metabolic diseases in vivo, are briefly reported and discussed.

Lyotropic liquid crystal as a real-time detector of microbial immune complexes

AIMS

To design a simple method for the detection of microbe-immune complexes exploiting the optical and elastic properties of a biocompatible liquid crystalline material.

METHODS AND RESULTS

Aqueous solution of disodium cromoglycate (DSCG), a lyotropic chromonic liquid crystal (LCLC), was aligned in a glass cell so as to be optically dark in polarized light. Immune complexes of at least three to four organisms altered the DSCG alignment such that polarized light was subsequently transmitted to reveal the presence of pathogens as optically bright regions around the immune complexes.

CONCLUSIONS

This work describes the first method to detect viable micro-organisms in real time using LCLC.

SIGNIFICANCE AND IMPACT OF THE STUDY

This technique provides a powerful tool for the detection of microbes in minutes, exploiting the optical and elastic properties of LC.

Template-guided organization of chromonic liquid crystals into micropatterned anisotropic organic solids

We have developed an approach to generate micropatterns of anisotropic organic materials which exploits the self-organization, driven by pi-stacking and hydrophobic interactions, of an ionic perylenebis(dicarboximide) in aqueous solutions.

Aggregation behavior and chromonic liquid crystal properties of an anionic monoazo dye

X-ray scattering and various optical techniques are utilized to study the aggregation process and chromonic liquid crystal phase of the anionic monoazo dye Sunset Yellow FCF. The x-ray results demonstrate that aggregation involves pi-pi stacking of the molecules into columns, with the columns undergoing a phase transition to an orientationally ordered chromonic liquid crystal phase at high dye concentration. Optical absorption measurements on dilute solutions reveal that the aggregation takes place at all concentrations, with the average aggregation number increasing with concentration. A simple theory based on the law of mass action and an isodesmic aggregation process is in excellent agreement with the experimental data and yields a value for the "bond" energy between molecules in an aggregate. Measurements of the birefringence and order parameter are also performed as a function of temperature in the chromonic liquid crystal phase. The agreement between these results and a more complicated theory of aggregation is quite reasonable. Overall, these results both confirm that the aggregation process for some dyes is isodesmic and provide a second example of a well-characterized chromonic system.

Optical characterization of the nematic lyotropic chromonic liquid crystals: light absorption, birefringence, and scalar order parameter

We report on the optical properties of the nematic (N) phase formed by lyotropic chromonic liquid crystals (LCLCs) in well aligned planar samples. LCLCs belong to a broad class of materials formed by one-dimensional molecular self-assembly and are similar to other systems such as "living polymers" and "wormlike micelles." We study three water soluble LCLC forming materials: disodium chromoglycate, a derivative of indanthrone called Blue 27, and a derivative of perylene called Violet 20. The individual molecules have a planklike shape and assemble into rodlike aggregates that form the phase once the concentration exceeds about 0.1 M. The uniform surface alignment of the N phase is achieved by buffed polyimide layers. According to the light absorption anisotropy data, the molecular planes are on average perpendicular to the aggregate axes and thus to the nematic director. We determined the birefringence of these materials in the N and biphasic N-isotropic (I) regions and found it to be negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic low-molecular-weight nematic materials. In the absorbing materials Blue 27 and Violet 20, the wavelength dependence of birefringence is nonmonotonic because of the effect of anomalous dispersion near the absorption bands. We describe positive and negative tactoids formed as the nuclei of the new phase in the biphasic N-I region (which is wide in all three materials studied). Finally, we determined the scalar order parameter of the phase of Blue 27 and found it to be relatively high, in the range 0.72-0.79, which puts the finding into the domain of general validity of the Onsager model. However, the observed temperature dependence of the scalar order parameter points to the importance of factors not accounted for in the athermal Onsager model, such as interaggregate interactions and the temperature dependence of the aggregate length.

Salt Effects on the Phase Behavior, Structure, and Rheology of Chromonic Liquid Crystals

Chromonic liquid crystals are formed by the addition of aromatic molecules such as disodium chromoglycate (cromolyn) to water. In this study, we investigate the addition of salts to the lyotropic nematic phase of cromolyn aqueous solutions. The addition of sodium and potassium salts shifts the isotropic-nematic phase boundary upward by more than 10 degrees C, so that samples that were isotropic at room temperature are transformed into nematic phases. Salt effects are predominantly dictated by the cation, not the anion, and appear to differ based on cation size. In contrast to small, hydrated cations like sodium, large, weakly hydrated cations such as tetraethylammonium and tetrabutylammonium shift the phase boundary downward, thus stabilizing the isotropic phase at the expense of the nematic one. The phase behavior results are highly correlated with viscosity measurements, with an upward shift in the phase boundary correlating with an increase in solution viscosity and vice versa. We also probe the microstructure in cromolyn-salt solutions, both indirectly by small-angle neutron scattering (SANS) and directly by cryo-transmission electron microscopy (cryo-TEM). The cryo-TEM images show the presence of rodlike aggregates that possibly undergo a higher order aggregation into bundles in the presence of salt.

“Majority-Rules” Operative in Chiral Columnar Stacks of C3-Symmetrical Molecules

C(3)-symmetrical disks 1, preorganized by acylated 2,2'-bipyridine-3,3'-diamine moieties and decorated with nine identical chiral, lipophilic tails, aggregate into a dynamic helix in apolar solvents. The aggregates, previously shown to be governed by the "sergeants-and-soldiers" principle when mixed with achiral analogues, are now also revealed to obey the "majority-rules" effect, a phenomenon not earlier observed in nonpolymers. Our experimental circular dichroism data can be accurately described with a recently developed theory. A fit of the theory to the experimental results shows that the mismatch penalty, i.e., the free energy of a monomer present in a helix of its nonpreferred screw sense (0.94 kJ/mol), is about 8 times lower than the penalty for a helix reversal (7.8 kJ/mol). This corresponds well to our vision of the supramolecular architecture of the disks.

Real-time microbe detection based on director distortions around growing immune complexes in lyotropic chromonic liquid crystals

We describe director distortions in the nematic liquid crystal (LC) caused by a spherical particle with tangential surface orientation of the director and show that light transmittance through the distorted region is a steep function of the particle's size. The effect allows us to propose a real-time microbial sensor based on a nontoxic lyotropic chromonic LC (LCLC) that detects and amplifies the presence of immune complexes. A cassette is filled with LCLC, antibody, and antigen-bearing particles. Small and isolated particles cause no macroscopic distortions of the LCLC. Upon antibody-antigen binding, the growing immune complexes distort the director and cause detectable optical transmittance between crossed polarizers.

Pretransitional fluctuations in the isotropic phase of a lyotropic chromonic liquid crystal

We have studied isotropic-to-nematic pretransitional fluctuations in an aqueous solution of disodium cromoglycate (cromolyn) by static and dynamic light scattering. Cromolyn is a representative of lyotropic chromonic liquid crystals with building units being elongated rods formed by aggregates of disk-like molecules. By combining light-scattering and viscosity measurements we have determined the correlation length and relaxation time of the orientational order-parameter fluctuations and estimated the size of the cromolyn aggregates. The pretransitional behavior of light scattering does not completely follow the classic Landau-de Gennes model. This feature is most probably associated with the variable length of cromolyn aggregates. We have observed a dramatic increase of the shear viscosity near the transition to the nematic phase, the fact which correlates with the idea of growing supramolecular aggregates. The steep temperature dependence of the viscosity is accompanied by a practically temperature-independent translational diffusion coefficient.

Cholesteryl-grafted functional amphiphilic poly(N-isopropylacrylamide-co-N-hydroxylmethylacrylamide): synthesis, temperature-sensitivity, self-assembly and encapsulation of a hydrophobic agent

A thermally responsive amphiphilic grafted copolymer comprised of hydroxyl-containing random poly(N-isopropylacrylamide-co-N-hydroxylmethylacrylamide) as the hydrophilic chain and cholesteryl groups as hydrophobic side arms was developed for the controlled release of hydrophobic drugs. The polymer was temperature-sensitive with a lower critical solution temperature of 40.5 degrees C and a critical micelle concentration of 4 mg/l. Dynamic light-scattering studies showed that the amphiphilic polymer self-assembled into micelles in aqueous media with their mean sizes in the range of 25-34 nm. Transmission electron microscope studies showed that the nanoparticles prepared from the micelle solutions exhibited multiple morphologies including unusual cubic and cuboids-like shapes, and normal spherical shapes, which could be controlled by the formation conditions. Wide-angle X-ray scattering studies showed that these nanoparticles were amorphous in nature but a small crystalline phase existed and the crystallinity of particles increased with the decrease of initial formation concentration. Pyrene was employed as a model hydrophobic agent to examine the encapsulation ability of the polymer with respect to hydrophobic agents in aqueous media. The loading level of the polymer with respect to pyrene was 4.4 mg/g, indicating that the thermally responsive amphiphilic polymer would be able to be used for the encapsulation of hydrophobic drugs.

Study of the chromonic liquid-crystalline phases of bis-(N,N-diethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide dihydrochloride by polarized optical microscopy and 2H NMR spectroscopy

The chromonic liquid-crystalline properties of bis-(N,N-diethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide dihydrochloride in an aqueous solution were investigated by polarized light microscopy and 2H NMR spectroscopy. Both techniques indicate a narrow I + N biphasic region and a broad N phase region at concentrations ranging from approximately 6.9 to approximately 30 wt % at room temperature. Optical microscopy indicates that a hexagonal M phase exists at higher concentrations. The variation of the I --> N + I and N + I --> N transition temperatures with concentration was studied by 2H NMR spectroscopy. Finally, the effects of temperature and concentration on the order parameter of the N phase were investigated by 2H NMR using a tetra-deuterated derivative. A value of 0.97 was obtained for the N phase at its upper concentration limit.

Hierarchical formation of helical supramolecular polymers via stacking of hydrogen-bonded pairs in water

Bifunctional ureido-s-triazines provided with penta(ethylene oxide) side chains are able to self assemble in water, leading to helical columns via cooperative stacking of the hydrogen-bonded pairs (DADA array). Monofunctional ureido-s-triazines do not form such helical architectures. The presence of a linker, covalently connecting the two ureido-s-triazine units, is essential as it generates a high local concentration of aromatic units, favorable for stacking interactions. This hydrophobic stacking of the aromatic units occurs at concentrations as low as 5 x 10(-6) M and can be visualized by using fluorescence spectroscopy. The stacking generates a hydrophobic microenvironment that allows intermolecular hydrogen bonding to occur at higher concentrations because the hydrogen bonds are shielded from competitive hydrogen bonding with water. This hierarchical process results in the formation of a helical self-assembled polymer in water at concentrations above 10(-4) M. Chiral side chains attached to the ureido-s-triazine units bias the helicity of these columns as concluded from CD spectroscopy and "Sergeants and Soldiers" experiments.

Supramolecular organization of alpha,alpha'-disubstituted sexithiophenes

The self-assembly of alpha,alpha'-linked sexithiophenes with chiral and achiral penta(ethylene glycol) chains attached at the alpha-positions of the terminal rings, that is, 2,2':5',2'':5'',2''':5''',2'''':5'''',2'''''-sexithiophene-5,5'''''-dicarboxylic acid-2S)-2-methyl-3,6,9,12,15-pentaoxahexadecyl ester (1) and 2,2':5',2'':5'',2''':5'''',2''''':5''''',2'''''-sexithiophene-5,5'''''-dicarboxylic acid-3,6,9,12,15-pentaoxahexadecyl ester (2), respectively is described. Analysis of the UV/vis, fluorescence, circular dichroism, and circular polarization of luminescence spectroscopic data shows that these compounds form chiral aggregates in polar solvents and in the solid state. In n-butanol aggregation occurs at temperatures below 30 degrees C, while above this threshold temperature the aggregates break up without an intermediate disordered state of aggregation, and the compounds are molecularly dissolved. The "melting temperature" of the aggregates depends on the concentration of sexithiophene, indicating that the optical changes observed are a result of intermolecular processes. Mass spectrometric measurements reveal that 1 and 2 can form mixed aggregates. Analysis of the optical spectra reveals that in these mixed aggregates, chiral 1 molecules act as "sergeants" to direct the packing of the "soldiers" 2, illustrating cooperativity within the columns. In water, the same type of chiral aggregates are formed as in n-butanol below 30 degrees C; however, these aggregates are still present, but the chirality is lost above 30 degrees C. In spin-coated films of 1 chiral aggregates are present. AFM studies show that 1 self-organizes into chiral fiberlike structures in the solid state. Furthermore both 1 and 2 display thermotropic liquid crystalline behavior between 180 and 200 degrees C.

Helical self-assembled polymers from cooperative stacking of hydrogen-bonded pairs

The double helix of DNA epitomizes this molecule's ability to self-assemble in aqueous solutions into a complex chiral structure using hydrogen bonding and hydrophobic interactions. Non-covalently interacting molecules in organic solvents are used to design systems that similarly form controlled architectures. Peripheral chiral centres in assemblies and chiral side chains attached to a polymer backbone, have been shown to induce chirality at the supramolecular level, and highly ordered structures stable in water are also known. However, it remains difficult to rationally exploit non-covalent interactions for the formation of chiral assemblies that are stable in water, where solvent molecules can compete effectively for hydrogen bonds. Here we describe a general strategy for the design of functionalized monomer units and their association in either water or alkanes into non-covalently linked polymeric structures with controlled helicity and chain length. The monomers consist of bifunctionalized ureidotriazine units connected by a spacer and carrying solubilizing chains at the periphery. This design allows for dimerization through self-complementary quadruple hydrogen bonding between the units and solvophobically induced stacking of the dimers into columnar polymeric architectures, whose structure and helicity can be adjusted by tuning the nature of the solubilizing side chains.