José Luis Serrano

Helical supramolecular organizations from metal-organic liquid crystals

Abstract The present paper is aimed at providing a survey of investigations undertaken regarding the preparation and characterization of helical supramolecular architectures using a strategy based on two concepts: coordination chemistry and mesomorphic behavior. Metal-coordination procedures have been used to design and synthesize molecular units capable of organizing into helical superstructures. In order to ensure this capability, the molecular building blocks have been complemented with a pro-mesogenic character, most commonly columnar, so that this type of mesophase is used as a template to build up helical supramolecular architectures with long-range orientational order and long-range positional order. For example, metal-containing liquid crystals have been described that organize into helical columnar mesophases whose structure has been fully investigated. To date, molecular units that display this organizational behavior have been found to be very different in shape, and hence the formation of helical structures has a different origin in each case. In this paper, three molecular structures are discussed (i.e. ‘disk-like’ molecules, ‘propeller-like’ molecules and polycatenar elongated molecules) that have been shown to give rise to helical organizations. Models have been proposed for the three systems described and in the present overview these have been classified as azimuthal rotation of planar disk-like complexes, interdigitated stacking of octahedral complexes and staggered stacking of polycatenar elongated planar complexes, respectively. Throughout this paper, emphasis is placed on studies carried out in the mesophase that have allowed authors to propose the corresponding models.

Dendromesogens: liquid crystal organizations of poly(amidoamine) dendrimers versus starburst structures.

A new series of liquid crystalline poly(amidoamine) (PAMAM) dendrimers is described. These dendrimers are made by attaching to the 0-, 1-, 2-, 3-, and 4-generation of PAMAM-terminal promesogenic units that carry two decyloxy chains in the 3- and 4-positions of their peripheral aromatic ring. X-ray diffraction studies show that all the compounds display a hexagonal columnar mesophase. A high density of aliphatic chains imposes a curved interface with the promesogenic units that forces the molecules to adopt a radial conformation, and therefore, the columnar structure. A model for the supramolecular organization of the different generations within the columnar mesophase is proposed based on the variation of some of the structural parameters.

Dendromesogens: Liquid Crystal Organizations versus Starburst Structures

Poly(amidoamine) starburst dendrimers which contain peripheral mesogenic ester units were found to exhibit a smectic A mesophase. A structural model is proposed that is valid for several generations of dendrimers. The mesogenic units form parallel aggregations that give rise to the smectic structure (shown schematically).

Functional star-shaped tris(triazolyl)triazines: columnar liquid crystal, fluorescent, solvatofluorochromic and electrochemical properties

Columnar liquid crystals with a C3 star-shaped heteroaromatic nitrogen-rich core, i.e. 2,4,6-tris(1′,2′,3′-triazol-4′-yl)-1,3,5-triazine (TTT), that show luminescence and solvatofluorochromism, and are also electron-acceptors are described. The core has been extended with polyalkoxybenzoyloxyphenyl groups situated at the 1 position of the 1,2,3-triazole rings to obtain molecules with three, six or nine n-decyloxy chains (series TB) or three, six or nine (S)-3,7-dimethyloctyloxy chains (series TB*). The preparation of the target compounds involved a copper-catalysed alkyne–azide cycloaddition (CuAAC) “click chemistry” procedure with a 1,3,5-triazine precursor and aromatic azides and this proved to be a versatile way to functionalise the periphery of the heteroaromatic core. Comparison of these compounds and those bearing polyalkoxyphenyl substituents (series T) has led to a deeper understanding of the self-assembly of the compounds with the TTT core. It was found that they self-organise into columnar mesophases, in which two molecules with a low-symmetry conformation occupy on average a columnar stratum. These molecules tend to adopt a polar conformation instead of the apolar C3 conformation and arrange in an antiparallel assembly. For some compounds the molecules in this arrangement have enough flexibility to tilt upon lowering temperature or by applying an electric field. Interestingly, the hexagonal columnar arrangement is preserved in a glassy state at room temperature. Furthermore, the mesophases show luminescence in the blue region of the visible spectrum depending on the peripheral substitution and molecular structure. We also report a novel property of TTT compounds (series T, TB and TB*) as they are able to act as solvatofluorochromic probes, a property that allowed us to estimate the overall polarity of the liquid crystalline medium. An aromatic azide precursor was also found to be mesomorphic and exhibited a monolayer SmA mesophase.

Bent-core liquid crystals in a route to efficient organic nonlinear optical materials

The design, synthesis and nonlinear optical (NLO) response of some bent compounds, at both the molecular and macroscopic level, are reported. Based on a bent-core NLO-phore with a μβ0 in the range of 390 × 10−48 esu (1907 nm), the second order susceptibility tensor for second harmonic generation is evaluated at excitation wavelengths of 1064 and 1600 nm in a mesogenic material with a SmCP mesophase. The NLO coefficients are in the range 10–100 pm V−1. Thus it is deduced that a molecular design leading to bent-core liquid crystals with large β values is a correct strategy to achieve NLO-materials for bulk second order effects.

Versatile optical materials: fluorescence, non-linear optical and mesogenic properties of selected 2-pyrazoline derivatives

A study of the structure-property relationships in a series of 3-(4-n-decyloxyphenyl)-1-(p-X-phenyl)-2-pyrazolines has been performed. By simply changing the substituent in the 1-phenyl ring we were able to tune the physical properties of the compounds. If this ring is non-substituted or substituted with a 4-methoxy, 4-chloro or 4-carboxy group, the pyrazoline compounds are fluorescent. If the ring is 4-nitro- or 2,4-dinitro-substituted, the compounds have interesting second-order non-linear optical properties. The first hyperpolarizability has been measured using the Hyper-Rayleigh Scattering technique in solution. The 4-nitro derivative displays liquid crystalline behaviour, showing a monotropic smectic A phase with a partial bilayer structure due to an antiparallel arrangement of molecules as confirmed by X-ray studies in the mesophase.

Halide and pseudohalide effects in Pd-catalysed cross-coupling reactions

The presence of halide and pseudohalide anions dramatically affects the outcome of transition-metal catalysed reactions, both in terms of selectivity and activity. In this perspective, the roles and effects of these anionic ligands in topical and important Pd-catalysed cross-coupling reactions involving the formation of C–C bonds, e.g. Sonogashira, Stille and Suzuki–Miyaura cross-coupling, will be described. The use of imidate anions (derived from succinimide, maleimide and phthalimide) in several different classes of neutral and anionic Pd catalysts/precatalysts will be highlighted.

Synthesis and Characterization of Maltose-Based Amphiphiles as Supramolecular Hydrogelators

Low molecular mass amphiphilic glycolipids have been prepared by linking a maltose polar head and a hydrophobic linear chain either by amidation or copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition. The liquid crystalline properties of these amphiphilic materials have been characterized. The influence of the chemical structure of these glycolipids on the gelation properties in water has also been studied. Glycolipids obtained by the click coupling of the two components give rise to stable hydrogels at room temperature. The fibrillar structure of supramolecular hydrogels obtained by the self-assembly of these gelators have been characterized by electron microscopy. Fibers showed some torsion, which could be related with a chiral supramolecular arrangement of amphiphiles, as confirmed by circular dichroism (CD). The sol-gel transition temperature was also determined by differential scanning calorimetry (DSC) and NMR.

Control of self-assembly of a 3-hexen-1,5-diyne derivative: toward soft materials with an aggregation-induced enhancement in emission.

The supramolecular architectures of a fluorophore are controlled through the design of a conjugated polycatenar molecule, the self-assembly of which can be addressed toward a columnar liquid-crystalline phase and organogels. Thus, depending on the environmental conditions for self-assembly, compound CA9 organizes into an unprecedented hexagonal columnar mesophase in the condensed state, in which half a molecule constitutes the slice of the column, or into a rectangular mesomorphic-like organization in the presence of apolar solvents such as cyclohexane and dodecane, at a concentration in which fibers form and gelling conditions are fulfilled. In this Col(r)-type arrangement, the organization within the columns depends on the solvent. All of the materials prepared show luminescence, and moreover, a remarkable 3-fold increase in fluorescence intensity was observed in going from the solution to the gel state.

Sugar-Coated Discotic Liquid Crystals

The constitutional marriage between a classical thermotropic discotic molecule and carbohydrate-based amphiphiles (e.g., see Figure) is shown to result in hybrid compounds, where, for example, a triphenylene unit serves as the central core from which oligomethyleneoxy chains terminated with sugar residues radiate, which have interesting liquid crystal properties