Raquel Giménez

Widely tunable ultraviolet-visible liquid crystal laser

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Forster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

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.

Metallomesogens: a promise or a fact?

The discipline of metallomesogens has been actively investigated in the past decade. Recent research has focused mainly on building up basic knowledge by exploring new shapes. The interest comprises both the molecular and supramolecular level. Important results are the generation of helical superstructures or the discovery of a McMillan phase. Different approaches include lanthanide complexes, specially designed ligands and polymeric compounds. Applications have been proposed on the basis of the optical, mechanical and electronic properties of these materials. Up to now, the only true application is in solid state chemistry where some lyotropic metallomesogens are used as templates for mesoporous materials.

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.

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.

Self-assembly in helical columnar mesophases and luminescence of chiral 1H-pyrazoles.

Chiral polycatenar 1H-pyrazoles self-assemble to form columnar mesophases that are stable at room temperature. X-ray diffraction and CD studies in the mesophase indicate a supramolecular helical organization consisting of stacked H-bonded dimers. The liquid-crystalline compounds reported are 3,5-bis(dialkoxyphenyl)-1H-pyrazoles that incorporate two or four dihydrocitronellyl chiral tails. It can be observed that the grafting of these branched chiral substituents onto the 3,5-diphenyl-1H-pyrazole core has a beneficial role in inducing mesomorphism, because isomeric linear-chain compounds are not liquid crystalline; this is not the usual scheme of behavior. Furthermore, the molecular chirality is transferred to the columnar mesophase, because preferential helical arrangements are observed. Films of the compounds are luminescent at room temperature and constitute an example of the self-organization of nondiscoid units into columnar liquid-crystalline assemblies in which the functional molecular unit transfers its properties to a hierarchically built superstructure.

Mesomorphism of a tetrahedral zinc complex

Thermotropic smectic phases have been observed for the first time in a zinc coordination complex with tetrahedral geometry; this complex, which contains the pyrazole dimer bis[3,5-bis(p-decyloxyphenyl)pyrazolyl]ethane as the ligand, exhibits fluorescence.

BETA -DIKETONE, PYRAZOLE AND ISOXAZOLE DERIVATIVES WITH POLAR GROUPS : LIQUID CRYSTALLINE AND NON-LINEAR OPTICAL PROPERTIES

The liquid crystalline behaviour of 1-(4-n-decyloxyphenyl)-3-(4-X-phenyl)propan-1,3-diones and their pyrazole and isoxazole derivatives has been studied by optical microscopy, DSC and X-ray techniques. The 4-substituents (X) were chosen to include a range of different polar and non-polar substituents: H, OCH3, Cl, Br and CN. A monotropic SA phase is observed for the beta-diketone derivative in which X CN and this is the first example of this phase found in a 1,3-diphenylpropan-1,3-dione derivative. The majority of the pyrazole and isoxazole compounds show SA phases. As regards the cyano-substituted compounds, X-ray diffraction studies on the mesophase show that the layer spacing is consistent with a partial bilayer SA mesophase. The first hyperpolarizabilities of the cyano-derivatives due to their push-pull structure have been measured by the EFISH method. Values for these compounds were found to be comparable to those for other conjugated CH3O-pi-CN systems.

Cyanostilbene bent-core molecules: a route to functional materials

The synthesis and characterization of novel compounds, that incorporate the high current interest functional cyanostilbene unit, are reported. Four examples, derived from 3,4′-biphenylene, have a 2D molecular-shape that promotes a variety of bent-core liquid crystalline phases (columnar, polar smectic C, and dark-conglomerate phases). The mesomorphism has been characterized by optical microscopy, differential scanning calorimetry, X-ray diffraction, and by detailed electrooptic and dielectric studies. The liquid crystalline properties can be modulated depending on the position of the cyano group on the CC bond (α- or β-isomers) and the number of cyanostilbene units in the molecule (one or two). The polar smectic C mesophases have an antiferroelectric ground state. Furthermore, the multiresponsive nature of the cyanostilbene structure has led to very attractive and diverse photoactivity, including luminescence and second harmonic generation. The results obtained broaden the possibilities for the use of bent-core liquid crystals to design novel multiresponsive soft materials.

Silver Pyrazolates as Coordination-Polymer Luminescent Metallomesogens

Silver pyrazolates with columnar liquid-crystal phases that are stable at room temperature have been prepared by reaction of silver nitrate with 3,5-diarylpyrazolates. The complexes consist of open-chain oligomers, despite the fact that the most common structural type for homoleptic coinage metal pyrazolates is the trimeric metallacycle [M(μ-pz)](3). The special characteristics of silver in forming reversible metal-ligand bonds in solution, evidenced experimentally, leads to supramolecular organizations in which the silver cations promote self-organization of the nonmesomorphic pyrazolates into helical 1D polymers that exhibit columnar mesophases. The materials are readily soluble in common organic solvents and are liquid-crystalline over a broader temperature range than their gold counterparts, which are known to form discrete cyclic trinuclear species. Thin films of the silver complexes show luminescence at room temperature. The compounds described here are the first examples of luminescent metallomesogens formed by a main-chain coordination polymer.