Structural analysis of the lyotropic polymorphism of four-stranded aggregates of 2'-deoxyguanosine 3'-monophosphate derivatives

Guanosine derivatives, dissolved in water, can form chromonic cholesteric and hexagonal phases. The common structural unit is a chiral stack of Hoogsteen-bonded guanosine tetramers. Using optical microscopy, circular dichroism and X-ray diffraction techniques, we have analysed the complex columnar polymorphism of two derivatives of 2'-deoxyguanosine—the 3'-monophosphate d(Gp) and the 3'-monophosphate monoisobutyl ester d(Gp)iBu. As a function of water concentration, both compounds exhibit four different columnar phases: in addition to the well-known cholesteric (Ch) and hexagonal (H) phaseg, a 2-dimensional square (Sq) lattice and a new hexagonal packing have been observed. In such a structure (Hb), two columns are packed in the 3m special positions of co-ordinates 1/3,2/3 and 2/3, 1/3 of the two-dimensional hexagonal unit cell. The unusual phase sequence appears to be related to the presence of strong short range intercolumnar interactions. Moreover, the easing of stacking constraints on increasing dilution seems to induce a tilting of the guanosine residues which form the tetramers.     

Study of the liquid crystalline phases of benzopurpurin in water

We report the NMR study of the liquid crystalline phases of aqueous solutions of the dye benzopurpurin (BP-4B). Upon changing dye concentration, the system exhibits a phase transition between two ordered phases at about 3·5 wt %. The structure of these phases was not determined but the evidence suggests that they consist of columns or helically twisted columns of stacked dye molecules which are randomly oriented, similar to nematic phases.     

Proton NMR relaxation study of molecular dynamics of chromonic liquid crystal Edicol Sunset Yellow

Proton nuclear magnetic resonance (¹H NMR) relaxometry, over about five decades in Larmor frequency, and pulsed field gradient NMR were used to study the molecular dynamics in the chromonic nematic and isotropic phases of stacked molecules of the binary mixture composed by Edicol Sunset Yellow (ESY) and deuterated water. Our results evidence that in both phases collective motions are responsible for the spin-lattice relaxation dispersion in the Larmor frequency range below 1 MHz. In the nematic phase, the collective motion are attributed to columnar undulations within the stacked molecules, while, in the isotropic phase, the results are explained by local order fluctuations due to the formation of the stacks. The high frequency dispersion was explained by individual molecular motions like rotations around and perpendicular to the stack axis, and also self-diffusion.     

NMR and X-ray studies of the chromonic lyomesophases formed by some xanthone derivatives

Optical microscopy, NMR and X-ray measurements are presented for four chromonic lyomesogens derived from 9-xanthone. The measurements provide details about the mesogen-water binary phase diagrams of the four compounds as well as quantitative information about the ordering and structural parameters of the mesophases. All four systems exhibit peritectic phase diagrams with a nematic (N) phase at low mesogen concentration and a hexagonal (H) phase at high concentration. The results are consistent with previously suggested models for chromonic lyomesophases in which columnar aggregates are formed by stacked mesogenic molecules. In the N phase these columns are parallel to the director but are otherwise randomly distributed in the bulk solvent, while in the H phase they form a two dimensional hexagonal array.     

Chromonic mesophases

Over the last 10 years, there has been a growing acceptance of the concept of chromonic phases and a wider recognition that they form a well-defined family of lyotropic liquid crystalline phases, with a package of properties distinct in almost every aspect, from those of conventional amphiphiles. New chromonogenic compounds have appeared and new technological uses for chromonic systems are being actively explored. Recent promising investigations include the synthesis of a chromonic dye, C.I. Direct Blue 67, which has an N phase of high order parameter and which can be dried down to give well-oriented films of solid. When dried down on a ‘command surface’ of photoaligned substrate this can produce a highly patterned film. The use of chromonic phases in the construction of compensating plates for improving the viewing characteristics of twisted nematic displays has been explored. Although this technology may not be suitable for commercially exploitation in its present form, the success of the devices is significant. It is suggested that current studies of the way in which the temperature range of thermotropic discotic mesophases is enhanced in 1:1 CPI mixtures may well lead to improved formulations for chromonic dyes. It is predicted that the marriage of chromonic phase technology with current biochemical analytical techniques will give rise to a new generation of medical diagnostic tests.     

Chromonic liquid crystal formation by Edicol Sunset Yellow

The solution and liquid crystalline phases formed by dissolution of the dye Edicol Sunset Yellow (ESY) in water have been examined using optical microscopy, multinuclear NMR (1H, 2H, 13C, 23Na), and X-ray diffraction. From the solution 1H and 13C spectra (particularly 13C), it is clear that the tautomeric form present in all these phases is the hydrazone, NH, structure, not the usually given azo, OH, form. Two chromonic mesophases occur: a nematic (N) phase at approximately 30-40 wt % and a hexagonal (M) phase at approximately 40-45 wt %. X-ray diffraction data show that the aggregates in the mesophases are single molecule stacks, with a typical spacing of approximately 3.5 angstroms, as expected for these systems. The NMR quadrupole splittings (2H2O, 23Na) are similar to those observed for surfactant lyotropic mesophases, suggesting that there are no water molecules or counter ions that are tightly bound to the ESY aggregates. An unusual feature of the X-ray diffraction pattern of the mesophases is the occurrence of diffuse off-axis reflections at approximately 6.8 angstroms. It is proposed that these arise from a head-to-tail packing of the molecules within the stacks.     

Optical and surface morphological properties of polarizing films fabricated from a chromonic dye by the photoalignment technique

The inherent chromonic lyotropic liquid crystalline properties of a dye have been manipulated to fabricate multi-axial micropolarizing thin films by means of the photoalignment technique. The dye aqueous solution is deposited on a photopatterned polymer film to result in the macroscopic alignment of the dye molecules, followed by drying at ambient temperature. The solid polarizing dye layers thus produced exhibit very a high contrast ratio and degree of polarization in the region of visible light. Addition of a small amount of surfactant to the dye solution is a prerequisite for the generation of a nematic chromonic phase and for the formation of homogeneous thin dye layers on the polymer film. The correlation between the optical and surface morphological properties of the dye layers is discussed.     

Isodesmic self-assembly in lyotropic chromonic systems

We have developed simple models of chromonic molecules and by carrying out Monte Carlo simulation in a binary mixture of model chromonic and water molecules, have studied the effect of concentration and molecular shape on the pattern of molecular aggregation. We have also computed the free energy change associated with the formation of chromonic columnar aggregates by umbrella sampling. This helps us to verify the isodesmic behaviour which is characteristic of chromonic systems.     

Isodesmic self-assembly in lyotropic chromonic systems

We have developed simple models of chromonic molecules and by carrying out Monte Carlo simulation in a binary mixture of model chromonic and water molecules, have studied the effect of concentration and molecular shape on the pattern of molecular aggregation. We have also computed the free energy change associated with the formation of chromonic columnar aggregates by umbrella sampling. This helps us to verify the isodesmic behaviour which is characteristic of chromonic systems.     

Aggregation behavior and chromonic liquid crystal phase of a dye derived from naphthalenecarboxylic acid

Polarizing microscopy, X-ray scattering, and absorption spectroscopy are used to investigate the aggregation process and chromonic liquid crystal of the anionic compound Bordeaux dye, a product of the sulfonation of the dibenzimidazole derivative of naphthalenetetracarboxylic acid. Polarizing microscopy reveals that the liquid crystal phase forms at room temperature when the concentration is only about 6 wt%, a value lower than what is found in many aggregating systems. The X-ray results indicate that the aggregation is via columns, with a cross-sectional area about 2.5 times larger than the individual molecule. Absorption spectroscopy shows a significant change in the absorption spectrum due to aggregation, which is nicely explained by a simple theory of isodesmic aggregation and excitonic coupling between the molecules in an aggregate. The "stacking free energy change" for a molecule in an aggregate relative to a molecule in solution is estimated to be about 9 kBT, a larger value than that found in the one other system where it has been estimated.     

Oriented monolayers prepared from lyotropic chromonic liquid crystal

We use a layer-by layer electrostatic self-assembly technique to obtain in-plane oriented aggregates of mesogenic dye molecules cast from lyotropic chromonic liquid crystals (LCLCs) on mica substrates. The aqueous solutions of dye used for deposition are in the nematic phase. Atomic force microscopy and X-ray photoelectron spectroscopy of the dried film reveal that the LCLC molecules adsorb at the charged substrate preserving ordered aggregates of elongated shape characteristic of the nematic phase in the aqueous solution. These elongated aggregates of LCLC molecules form films with in-plane orientational order and are compositionally distinct from the substrate.     

Hydrogen-bond-assisted excited-state deactivation at liquid/water interfaces

The excited-state dynamics of eosin B (EB) at dodecane/water and decanol/water interfaces has been investigated with polarization-dependent and time-resolved surface second harmonic generation. The results of the polarization-dependent measurements vary substantially with (1) the EB concentration, (2) the age of the sample, and (3) the nature of the organic phase. All of these effects are ascribed to the formation of EB aggregates at the interface. Aggregation also manifests itself in the time-resolved measurements as a substantial shortening of the excited-state lifetime of EB. However, independently of the dye concentration used, the excited-state lifetime of EB at both dodecane/water and decanol/water interfaces is much longer than in bulk water, where the excited-state population undergoes hydrogen-bond-assisted non-radiative deactivation in a few picoseconds. These results indicate that hydrogen bonding between EB and water molecules at liquid/water interfaces is either much less efficient than in bulk water or does not enhance non-radiative deactivation. This strong increase of the excited-state lifetime of EB at liquid/water interfaces opens promising avenues of applying this molecule as a fluorescent interfacial probe.     

A sharp J-band-like electronic absorption peak observed for the chromonic liquid crystal of bisazo dye Ponceau SS in water

Abstract  

The bisazo dye Ponceau SS forms an acute oblique (V-type) dimer in dilute aqueous solution. In concentrated aqueous solution, these V-type dimers are considered to stack to form V-type blocks of chromonic liquid-crystalline (LC) columns. The weak interactions among these LC columnar V-type blocks have been spectroscopically investigated. On increasing the concentration of Ponceau SS at 25 °C, the V-type LC columnar blocks form a rhombus-type LC columnar block at 2 wt% concentration. These rhombus-type LC columnar blocks are further aligned at an acute angle at 5 and 10 wt% concentrations. The rapidly cooled 10 wt% LC sample shows an unusually sharp J-band-like peak in the electronic absorption spectrum. The emergence of this J-band-like peak has been analyzed from the viewpoint of an exciton model, suggesting that many nearest neighbor unique molecules belonging to many different rhombus-type LC columnar blocks are aligned in a head-to-tail manner to give a giant quasi-linear head-to-tail-type exciton.     

Chromonic/Silica nanohybrids: synthesis and macroscopic alignment

Some dye molecules self-aggregate to exhibit a lyotropic columnar liquid crystal state (chromonic liquid crystal) via pi stacking in relatively highly concentrated aqueous solutions. In this work, the chromonic liquid crystal structure was immobilized, for the first time, with silica networks by way of the sol-gel condensation process. The immobilization of the columnar structure was successfully attained in the presence of 2-(2-aminoethoxy)ethanol, which favorably mediates the interface between the anionic charge of the dye aggregates and the silica network. Without this molecule, the sol-gel process gave rise to a transformation from columnar to lamellar structure. Both spin-coating and dip-coating methods gave essentially the same results. In the dip-coated films, the dye molecules were aligned over a large area with orientation orthogonal to the lifting direction.     

Surfactant encapsulated DNA: structure characterization and interaction with dye molecules in organic media

The recognition of electrostatically-bound DNA-didodecyldimethylammonium (DNA-DDDA) complex by three dye molecules, acridine orange (AO), ethidium bromide (EB) and 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin tetra(p-toluenesulfonate) (TMPyP) in organic media was investigated through 1H NMR, UV-vis, and circular dichroism (CD) spectroscopies. When the organic solvent in which DNA-DDDA complex dissolves is changed from ethanol to chloroform, the adsorbed AO undergoes a reversible transformation from a monomer to a highly aggregated state at the interface between DNA and DDDA. EB also adsorbs at the interface between DNA and DDDA when EB interacts with the DNA-DDDA complex in organic media, but its existing state is independent of the used solvents. The third dye, TMPyP cation can intercalate into the G-C region while its anionic p-tosylate counterion remains unbound when it mixes with DNA complex in organic media. The complexes of DDDA with previously recognized DNA by the three dye molecules (DNA-dye), respectively, are also investigated. AO seems having changed its location from the grooves of DNA to the interface between DNA and DDDA after DNA-AO complex was electrostatically encapsulated with DDDA. The aggregation behavior of AO also shows a dependence on the polarity of the organic solvent. EB molecules are believed to intercalate into the base pairs of DNA in aqueous solution. The intercalation mode is still maintained after the encapsulation for DNA-EB in organic solvents, which is different from the situation between DNA-DDDA complex and EB. But in both cases, the existing states of EB are independent of the polarity of the organic solvents. Finally, TMPyP in the complex of DNA-TMPyP and DDDA is also judged to intercalate into the G-C region of DNA while its anionic p-tosylate counterion remains separated from DNA complex, which is similar to its interaction with DNA-DDDA complex in organic media. These data also strongly suggest that the intercalation state of TMPyP is more stable than its adsorption state in grooves when it is recognized with DNA. The present results are significant for the designs of both laser dye and conductive materials.     

The distinction between chromonic and amphiphilic lyotropic mesophases

Optical microscopy, X-ray diffraction and NMR spectroscopy have been used to examine whether the hexagonal phases of representative chromonic and amphiphilic mesogens are miscible. The systems studied were octaoxyethylene-glycol dodecylether with either disodium cromoglycate or 5-n-hexyl-7-(5-methyl-sulphonimidoyl) xanthone-2-carboxylic acid. The results clearly demonstrate that the hexagonal phases of these two systems are not miscible, although miscibility does occur in the isotropic solution. These observations suggest that chromonic mesophases are a new breed of lyotropic liquid crystals.     

Self-assembly of thiacyanine dyes in water for the synthesis of active hybrid nanofibres

Water-soluble self-assembled nanostructures were synthesised by simple counter ion exchange of thiacyanine dyes which helps in the formation of nematic (N) and hexagonal (M) chromonic liquid-crystalline (CLCs) phases. Conjugated double bonds as central spacers connected between two benzothiazole segments affect water solubility and liquid crystal formation. Aggregation-dependent properties characterised by ultraviolet–visible, fluorescence and ¹H nuclear magnetic resonance spectroscopy. Sol–gel reaction of dye aggregates with silica species furnishes entangled nanotubular fibres with constant diameter and their length in excess of micrometres, having templates of pore sizes below the mesoporous range. The π–π stacked chromonic aggregate dyes are also of importance in shape selective catalysis, adsorption, desorption micro-patterned materials, and provide a significant step towards biosensor medical applications because of their water-soluble nature.     

Vesicle systems for mimicking the effects of 2,4-dichlorophenol on cell membranes

The effect of 2,4-dichlorophenol (DCP) on the phosphatidylethanolamine ( --dipalmitoyl-phosphatidylethanolamine (DPPE))/water liposomes was studied in the temperature domains of the gel and liquid crystalline phases at the DCP/DPPE molar ratios of 10⁻¹ and 10⁻³ by using differential scanning calorimetry (DSC) as well as small and wide angle X-ray scattering (SAXS and WAXS). Different character of the transitions between the gel and the liquid crystalline phases was observed in the lipid/water and by DCP-doped systems. The different DCP concentrations caused similar effects in the change of the layer arrangements of the gel phase, while the perturbation of the subcells of this phase was different. In the liquid crystalline phase, the DCP molecules did not affect the layer structure significantly. The calorimetrical behaviour of the systems were rather correlated to the changes of the subcells than to the layer arrangements.     

Thermal and colour properties of leuco dye-based thermochromic composite with dodecanol solvent

Leuco dye–developer–solvent composites with thermochromic properties were studied. The applied 1-dodecanol solvent shows β crystalline, α solid (rotator) and liquid phases. The inclusion of the crystal violet lactone dye increases transition temperatures of all phases and strengthens the α phase. The benzyl 4-hydroxybenzoate developer causes opposite effects and prevents the appearance of the α phase. These consequences combine in ternary composites, where lower transition temperatures were obtained as in the solvent, but the α phase is formed more likely as expected due to the presence of the developer. The hydration of the composite strengthens its crystalline network, but makes α phase more unstable. The thermal properties were related to the temperature-dependent colour. Composites with well-resolved α and β phases at cooling show a typical colour hysteresis. If only weak α phase or none at all was found in DSC curves, the colour hysteresis has an unusual shape with heating and cooling curves crossing each other. The hydration of the composite changes the colorimetric properties in accordance with the relation found for anhydrous composite. More conclusions and interrelations between the two properties require application of other analytical methods.