Supertwisted Chiral Gyroid Mesophase in Chiral Rod-Like Compounds.

Among the intriguing bicontinuous self-assembled structures, the gyroid cubic is the most ubiquitous. It is found in block and star polymers, surfactants with or without solvent, in thermotropic liquid crystals with end- or side-chains, and in biosystems providing structural color and modelling cell mitosis. It contains two interpenetrating networks of opposite chirality and is thus achiral if, as usual, the content of the two nets is the same. However, we now find that this is not the case for strongly chiral compounds. While achiral molecules follow the opposite twists of nets 1 and 2, molecules with a chiral center in their rod-like core fail to follow the 70° twist between junctions in net 2 and instead wind against it by -110° to still match the junction orientation. The metastable chiral gyroid is a high-entropy high-heat-capacity mesophase. The homochirality of its nets makes the CD signal of the thienofluorenone compounds close to that in the stable I23 phase with 3 isochiral nets.

Enantiomers Self-Sort into Separate Counter-Twisted Ribbons of the Fddd Liquid Crystal─Antiferrochirality and Parachirality.

The recently discovered orthorhombic liquid crystal (LC) phase of symmetry Fddd is proving to be widespread. In this work, a chiral hydroxybutyrate linkage is inserted into the molecular core of hexacatenar rodlike compounds, containing a thienylfluorenone fluorophore. In addition to more usual tools, the methods used include grazing-incidence X-ray scattering, modulated differential scanning calorimetry (DSC), flash DSC with rates up to 6000 K/s, and chiro-optical spectroscopies using Mueller matrix method, plus conformational mapping. Although pure R and S enantiomers form only a strongly chiral hexagonal columnar LC phase (Colh*), the racemic mixture forms a highly ordered Fddd phase with 4 right- and 4 left-handed twisted ribbon-like columns traversing its large unit cell. In that structure, the two enantiomers locally deracemize and self-sort into the columns of their preferred chirality. The twisted ribbons in Fddd, with a 7.54 nm pitch, consist of stacked rafts, each containing ∼2 side-by-side molecules, the successive rafts rotated by 17°. In contrast, an analogous achiral compound forms only the columnar phase. The multiple methods used gave a comprehensive picture and helped in-depth understanding not only of the Fddd phase but also of the "parachiral" Colh* in pure enantiomers with irregular helicity, whose chirality is compared to the magnetization of a paramagnet in a field. Unusual short-range ordering effects are also described. An explanation of these phenomena is proposed based on conformational analysis. Surprisingly, the isotropic-columnar transition is extremely fast, completing within ∼20 ms. A clear effect of phase on UV-vis absorption and emission is observed.

Design of Nanostructured Hybrid Electrodes Based on a Liquid Crystalline Zn(II) Coordination Complex-Carbon Nanotubes Composition for the Specific Electrochemical Sensing of Uric Acid.

A metallomesogen based on an Zn(II) coordination complex was employed as precursor to obtain a complex matrix nanoplatform for the fabrication of a high-performance electrochemical hybrid sensor. Three representative paste electrodes, which differ by the weight ratio between Zn(II) metallomesogen and carbon nanotubes (CNT), i.e., PE_01, PE_02 and PE_03, were obtained by mixing the materials in different amounts. The composition with the largest amount of CNT with respect to Zn complex, i.e., PE_03, gives the best electrochemical signal for uric acid detection by cyclic voltammetry in an alkaline medium. The amphiphilic structure of the Zn(II) coordination complex likely induces a regular separation between the metal centers favoring the redox system through their reduction, followed by stripping, and is characterized by enhanced electrocatalytic activity towards uric acid oxidation. The comparative detection of uric acid between the PE_03 paste electrode and the commercial zinc electrode demonstrated the superiority of the former, and its great potential for the development of advanced electrochemical detection of uric acid. Advanced electrochemical techniques, such as differential-pulsed voltammetry (DPV) and square-wave voltammetry (SWV), allowed for the highly sensitive detection of uric acid in aqueous alkaline solutions. In addition, a good and fast amperometric signal for uric acid detection was achieved by multiple-pulsed amperometry, which was validated by urine analysis.

Bowls, vases and goblets-the microcrockery of polymer and nanocomposite morphology revealed by two-photon optical tomography.

On the >1 µm scale the morphology of semicrystalline plastics like polyethylene or Nylon features spherulites, "shish-kebabs", cylinddrites and other crystalline aggregates which strongly affect mechanical and other material properties. Current imaging techniques give only a 2D picture of these objects. Here we show how they can be visualized in 3D using fluorescent labels and confocal microscopy. As a result, we see spherulites in 3D, both in neat polymers and their nanocomposites, and observe how unevenly nanoparticles and other additives are distributed in the material. Images of i-polypropylene and biodegradable poly(lactic acid) reveal previously unsuspected morphologies such as "vases" and "goblets", nonspherical "spherulites" and, unexpectedly, "shish-kebabs" grown from quiescent melt. Also surprisingly, in nanocomposite sheets spherulite nucleation is seen to be copied from one surface to another, mediated by crystallization-induced pressure drop and local melt-flow. These first results reveal unfamiliar modes of self-assembly in familiar plastics and open fresh perspectives on polymer microstructure.

New Type of Columnar Liquid Crystal Superlattice in Double-Taper Ionic Minidendrons.

Wedge-shaped molecules, such as dendrons, are among the most important building blocks for directed supramolecular self-assembly. Here we present a new approach aimed at widening the range and complexity of potential mesophases by introducing double-tapered mesogens. Two series of compounds are presented, both alkali metal salts (Li, Na, Cs) of 3,4,5-tris-alkoxybenzoic acid with a second tapered tris-alkoxyaryl group attached at the end of an alkoxy chain. The double-tapered compounds all display an unusual hexagonal columnar phase consisting of one ionic and three non-ionic columns per unit cell. The cation size has an unexpectedly drastic effect on unit cell size. Unlike most columnar phases, the current phases show unusually high dimensional stability on heating, and high stiffness in spite of being 80-85 % aliphatic, attributed to their molecular topology. The described approach may lead to co-assemblies of multifunctional materials, for example, parallel p- and n-semiconducting nanowires or parallel ionic and electronic conductors.

Molecular ejection transition in liquid crystal columns self-assembled from wedge-shaped minidendrons.

Fan-shaped molecules with aromatic head-groups and two or more flexible pendant chains often self-assemble into columns that form columnar liquid crystals by packing on a 2d lattice. Such dendrons or minidendrons are essential building blocks in a large number of synthetic self-assembled systems and organic device materials. Here we report a new type of phase transition that occurs between two hexagonal columnar phases, Colh1 and Colh2, of Na-salt of 3,4,5-tris-dodecyloxy benzoic acid. Interestingly, the transition does not change the symmetry, which is p6mm in both phases, but on heating it involves a quantised drop in the number of molecules n in the cross-section of a column. The drop is from 4 to 3.5, with a further continuous decrease toward n = 3 as temperature increases further above Tc. The finding is based on evidence from X-ray diffraction. Using a transfer matrix formulation for the interactions within a column, with small additional mean field terms, we describe quantitatively the observed changes in terms of intermolecular forces responsible for the formation of supramolecular columns. The driving force behind temperature-induced molecular ejection from the columns is the increase in conformational disorder and the consequent lateral expansion of the alkyl chains. The asymmetry of the transition is due to the local order between 4-molecule discs giving extra stability to purely n = 4 columns.

Achieving Complexity at the Bottom. 2,6-Bis(arylidene)cyclohexanones and Anthocyanins: The Same General Multistate of Species.

As in supramolecular chemistry, complexity could also be achieved through a bottom-up approach. Anthocyanins and related compounds such as the compound (E)-6-(dimethylamino)-4-(4-(dimethylamino)-2-hydroxybenzylidene)-1,2,3,4-tetrahydroxanthylium chloride (1), here reported, exhibit this type of complexity. The thermodynamics and kinetics of the complex multistate of species of compound 1 were studied by conventional and stopped-flow UV-visible spectrophotometry as well as by NMR. The system follows the same multistate of species of anthocyanins, except for the presence at moderately basic pH values of a species possessing a spiro carbon. The introduction of two dimethylamino substituents in positions 4' and 7, modulates deeply the thermodynamic and kinetics of the system. A beautiful pH-dependent palette of colors is obtained, including a blue flavylium cation at unusually high pH values. The protonation of the dimethylamino substituents is the key aspect for explaining the details of the spiro opening kinetics. The system was fully characterized by representing the mole fraction distribution and the relative energy level diagram of all multistate species as a function of pH.

2,2'-Spirobis[chromene] Derivatives Chemistry and Their Relation with the Multistate System of Anthocyanins.

The chemistry of 2,2'-spirobis[chromene] derivatives is intimately related to the one of anthocyanins and similar compounds. The 2,2'-spirobis[chromene] species plays a central role in the network of chemical reactions connecting two different flavylium-based multistate systems. In the present work, a new asymmetric 2,2'-spirobis[chromene] intermediate possessing a constrained propylenic bridge between carbons 3 and 3' was isolated and its role as a pivot in the anthocyanins-type multistate of chemical reactions was investigated by the conjugation of absorption spectroscopy, stopped-flow, NMR, and X-ray crystallography. It was confirmed that the propylenic bridge is essential to stabilize the spirobis[chromene] species. Furthermore, under acidic conditions, two cis-trans styrylflavylium isomers were identified, which could be interconverted directly into one another with light. This is the first report of styrylflavylium cations with photoisomerization on the styryl moiety.

Body-centred cubic packing of spheres - the ultimate thermotropic assembly mode for highly divergent dendrons.

We have synthesized sodium tris(alkoxy)benzoates in which one of the three alkyl chains branches further into three C18H37 chains. These AB5 hyperbranched minidendrons melt directly into a body-centred cubic (BCC) mesophase formed by spherical "micelles". In contrast, their non-branched counterparts display various mesophases before they turn into BCC upon heating. This agrees with the predictions from a numerical geometric model that relates the shape of the molecular wedge to the type of mesophase they adopt. The spheres were found to shrink in volume upon heating and expand upon cooling, as molecules, in some cases nearly half of them, are ejected and reintegrated in the spheres. The ejection of dendrons is caused by their lateral thermal expansion. The BCC appears to be the ultimate mesophase for the extremely divergent wedges such as the hyperbranched minidendrons. In dendrons with chains of unequal length, the sphere size is fixed by the shorter chains, the longer ones back-folding or interdigitating to effectively widen the wedge. This new understanding of their assembly will help in designing new dendrons, e.g. for better encapsulation of guest molecules.

Added Alkane Allows Thermal Thinning of Supramolecular Columns by Forming Superlattice-An X-ray and Neutron Study.

We report a columnar superlattice formed by blends of dendron-like Li 3,4,5-tris(n-alkoxy)benzoates with n-alkanes. Without the alkane, the wedge-shaped molecules form liquid crystal columns with 3 dendrons in a supramolecular disk. The same structure exists in the blend, but on heating one dendron is expelled from the disks in every third column and is replaced by the alkane. This superlattice of unequal columns is confirmed by complementary X-ray and neutron diffraction studies. Lateral thermal expansion of dendrons normally leads to the expulsion of excess molecules from the column, reducing the column diameter. However, in the already narrow columns of pure Li salt, expulsion of one of only three dendrons in a disk is not viable. The added alkane facilitates the expulsion, as it replaces the missing dendron. Replacing the alkane with a functional compound can potentially lead to active nanoarrays with relatively large periodicity by using only small molecules.

Helically Twisted Chiral Arrays of Gold Nanoparticles Coated with a Cholesterol Mesogen.

Gold nanoparticles have been prepared and surface-functionalized with a 1:1 molar mixture of a hexylthiol ligand and a chiral mesogenic ligand consisting of a cholesterylbenzoate attached via an undecylthiol spacer. Grazing incidence X-ray diffraction showed that upon annealing a columnar liquid crystal (LC) structure develops with the nanoparticles forming strings on a regular oblique 2d lattice. Synchrotron radiation circular dichroism is substantially enhanced upon the isotropic-LC transition. In the proposed structural model, layers of Au columns rotate by a small angle relative to their neighbors, with the columns winding around a helical axis. The work demonstrates that it is possible to obtain chiral LC superstructures from nanoparticles coated with chiral mesogen without the addition of a separate LC or chiral dopants. The results provide direction in the development of plasmonic metamaterials interacting selectively with circularly polarized light.

Chemistry and photochemistry of 2,6-bis(2-hydroxybenzilidene)cyclohexanone. An example of a compound following the anthocyanins network of chemical reactions.

The kinetics and thermodynamics of the 2,6-bis(2-hydroxybenzilidene)cyclohexanone chemical reactions network was studied at different pH values using NMR, UV-vis, continuous irradiation, and flash photolysis. The chemical behavior of the system partially resembles anthocyanins and their analogue compounds. 2,6-Bis(2-hydroxybenzilidene)cyclohexanone exhibits a slow color change from yellow to red styrylflavylium under extreme acidic conditions. The rate constant for this process (5 × 10(-5) s(-1)) is pH independent and controlled by the cis-trans isomerization barrier. However, the interesting feature is the appearance of the colorless compound, 7,8-dihydro-6H-chromeno[3,2-d]xanthene, isolated from solutions of acid to neutral range, characterized by (1)H NMR and single crystal X-ray diffraction. Light absorption by 2,6-bis(2-hydroxybenzilidene)cyclohexanone solutions immediately after preparation exclusively results in cis-isomer as photoproduct, which via hemiketal formation yields (i) red styrylflavylium by dehydration under extremely acidic solutions (pH < 1) and (ii) colorless 7,8-dihydro-6H-chromeno[3,2-d]xanthene by cyclization in solutions of acid to neutral range.

Combinatorial synthesis and characterization of new asymmetric porphyrins as potential photosensitizers in photodynamic therapy.

Porphyrins play a major role as active photosensitizers in noninvasive optical photodynamic therapy (PDT). In a modular approach, this paper presents a short review of the recent developments of porphyrin structures and materials with improved photosensitizing properties and then presents the synthesis and characterization of a series of new second generation asymmetrical meso-tetraphenylporphyrins varied by substituent in the meta positions of the phenyl rings with either -OH or -OCH3 groups, whereas in the para positions only with -OCH3 groups. The new series of differentially functionalized porphyrins were obtained by a combinatorial multicomponent synthesis (Adler-Longo method) by simultaneously using two different aldehydes: 3,4-dimethoxybenzaldehyde and 3-hydroxybenzaldehyde. The porphyrins were isolated, purified and characterized by HPLC, TLC, UV-vis, fluorescence, MS, 1H-NMR, and 13C-NMR analysis, accompanied by DEPT 135 experiments. Because of the fact that the medium in cancerous tissues is often more acidic than in normal tissues, the capacity of these porphyrins to exist simultaneously in aggregated and protonated forms was also investigated, in tetrahydrofuran (THF) and acid THF-water systems, underlying the changes in the photophysical behaviour. The relative fluorescence quantum yields (Phif) were calculated in comparison with meso-tetraphenylporphyrin (TPP), and the values between 0.14-0.26 were found to be promising for further trials. The series of asymmetrically substituted tetraphenylporphyrins, as the new class of supramolecular materials, are suitable for further functionalization in order to improve their photophysical properties, and they could represent interesting potential PDT photosensitizers.

The design and investigation of room temperature thermotropic nematic gold nanoparticles.

Nematic gold nanoparticles, covered with a monolayer of calamitic mesogens and short hydrocarbon chains, have been synthesized and investigated. The materials are chemically stable and exhibit nematic phase behavior at room temperature.