Nélida Gimeno

Liquid crystalline and nonlinear optical properties of bent-shaped compounds derived from 3,4'-biphenylene

The synthesis of different bent-core molecules derived from 3,4′-biphenylene bearing azo, azoxy, imine or ester linkages in their lateral structures is reported. Structure–activity relationships for their liquid crystalline behaviour are discussed. SmCP, USmCP and Colob mesophases are found depending on the type and number of these connecting units. The sequence ester ≈ azoxy > imine > azo can be proposed for the mesophase range, with significant differences observed in terms of mesopahase stabilization. SHG studies on these compounds give nonlinear coefficients in the range of 1–8 pm V−1. The molecular origin for these values is analyzed semi-quantitatively. It was concluded that the SHG performance of bent-core mesogens in general can still be increased substantially. An approach to improve the properties of these materials is briefly outlined.

Interface structure of the dark conglomerate liquid crystal phase

The dark conglomerate (DC) phase is the spontaneously chiral, isotropic fluid formed by achiral bent-core liquid crystal molecules characterized by well-defined local smectic liquid crystal layering deformed into a disordered “plumbers nightmare” array of 100 nm-scale, focal conic-like domains. Here we report the interface structure of this phase, showing that surface confinement orders the focal conics into a hexagonal plumbers nightmare lattice, directly confirming the topology proposed for the DC phase. The observations suggest that a bulk phase of saddle–splay dominated focal conics may be produced in DC materials under suitable conditions, which opens the way for the development of two-dimensional or even three-dimensional periodic dielectric media for photonic crystal applications.

“Click chemistry” as a versatile route to synthesize and modulate bent-core liquid crystalline materials

Three series of bent-shaped compounds containing the 1,2,3-triazole ring in the central core of the molecule have been prepared by the most extended “click chemistry” reaction, the copper-catalyzed azide-alkyne cycloaddition (CuAAC). This research demonstrates the versatility of this synthetic approach with the aim of achieving innovative compact supramolecular organizations. The appropriate combination of the 1,2,3-triazole synthon linked either to a methylene unit (series M) or to a methylenoxycarbonyl block (series MC) has allowed the induction of a variety of non-classical bent-core liquid crystal phases versus the classic mesophases promoted by 1,4-diphenyl-1,2,3-triazole derivatives (series T). Through a suitable selection of common lateral structures connected by “click chemistry” both the transition temperatures and mesomorphism, ranging from lamellar to columnar or B4-like supramolecular liquid crystalline organizations, can be tuned.

On the supramolecular packing of bent-shaped molecules: the influence of the central bent-core on the liquid crystalline behaviour

Two series of bent-shaped molecules have been synthesized and their supramolecular self-organization at variable temperature has been studied. The majority of these bent-compounds form liquid crystalline phases and the mesomorphic behaviour of them has been fully characterized by polarised optical microscopy, calorimetry, X-ray diffraction and dielectric techniques. A series of bis-4-n-tetradecyloxybenzoyloxybenzoates derived from naphthalene, resulting in a change of the bending angle, have proved that when these cores bend around 120°, a polar lamellar packing (SmCaPA) can be induced, whereas when the angle was around 60°, an orthogonal lamellar arrangement (SmA) that responded to an electric field, are found to occur. Liquid-crystal order can also be achieved over a significant range of temperatures by using appropriate bis-4,4′-disubstituted phenylene derivatives (–Ar–X–Ar–). Groups such as –CO–, SO– and –CH2– connecting the aromatic rings induce the SmCaPA phase while a sulfide group (–S–) leads to columnar order. Further, the formation of isotropic textures as chiral conglomerates or a field-induced phase transition have also been observed as unusual properties of the self-organizations of these compounds.

Bent-core liquid crystal phases promoted by azo-containing molecules: from monomers to side-chain polymers

New bent-core monomethacrylates and their side-chain polymers containing the 3,4′-biphenylene moiety as a central core and both all-ester and azo-ester lateral structures have been prepared and characterized. Attractive results concerning the scarcely studied side-chain homo- and copolymers, synthesized either by in situ polymerization in the mesophase or ATRP in solution, are reported. The research is focused on the synthesis of these materials and the effect that structural changes, such as the number of aromatic rings (5 or 6), the presence of –NN– versus –COO– links on the lateral structures, and the relative positions of the reactive groups, have on the liquid crystal properties of these new bent-core based-materials. The mesomorphic behavior of these low and high-molecular compounds, and also of the synthetic intermediates with bulky terminal bromo-substituents, have been characterized by POM, XRD, DSC and electrooptic studies, showing that the majority of these new molecules have the ability to organize forming non classical lamellar or columnar mesophases. Significant stabilization of the lamellar SmCP mesophase ranges is achieved upon polymerization in comparison to the monomers and interestingly the vitrification of these supramolecular organizations occurs at room temperature.

The SmA phase of a bent-core V-shaped compound: structure and electric-field response

The smectic A (SmA) phase of a V-shaped mesogen with a molecular bending angle of 60° was studied. According to X-ray measurements, the molecules are arranged with the ‘bow-string’ direction parallel to the smectic layer. This point was confirmed by birefringence analysis and the result is compatible with a non-polar structure. A detailed study of the (001) smectic layer reflection versus temperature was also carried out to determine the smectic translational order parameter. The results indicate that the material presents a high degree of translational order. In addition, the electric field response of the material was studied by means of second harmonic generation. This technique turned out to be highly sensitive to small structural distortions due to the V-shape of the constituent molecules.

Bent-rod liquid crystal dimers: synthesis and mesomorphic properties

The synthesis and characterization of three bent-rod dimers are reported. Very long flexible spacers that include methylene units and either a phenyl ring or a triazole moiety as connecting structures join the bent- and rod-promesogenic cores. Polarizing microscopy, modulated differential scanning calorimetry, X-ray diffraction at variable temperature and dielectric measurements have been performed to establish the mesophase behaviour of the dimers. The results show a complex supramolecular organization for this kind of flexible dimer in the mesophase. Interestingly, the length and the chemical nature of the flexible linking spacers condition the liquid crystalline properties of these novel compounds. Additionally, it has been found that the chemical structure of the rod-like core plays a key role in determining the kind of mesophase, either nematic or lamellar, formed by these materials.

Supramolecular Architectures from Bent‐Core Dendritic Molecules

Control of the self-assembly of small molecules to generate architectures with diverse shapes and dimensions is a challenging research field. We report unprecedented results on the ability of ionic, bent dendritic molecules to aggregate in water. A range of analytical techniques (TEM, SEM, SAED, and XRD) provide evidence of the formation of rods, spheres, fibers, helical ribbons, or tubules from achiral molecules. The compact packing of the bent-core structures, which promotes the bent-core mesophases, also occurs in the presence of a poor solvent to provide products ranging from single objects to supramolecular gels. The subtle balance of molecule/solvent interactions and appropriate molecular designs also allows the transfer of molecular conformational chirality to morphological chirality in the overall superstructure. Functional motifs and controlled morphologies can be combined, thereby opening up new prospects for the generation of nanostructured materials through a bottom-up strategy.

Topography of bent-core liquid crystals at the air/liquid crystal interface

Bent-core liquid crystals show a variety of novel structures involving the interplay of molecular bend, tilt and polarisation. Here we investigate the microstructures of the B4, B7 and the dark conglomerate (DC) phases at the air/liquid crystal interface. In these phases, bent-core molecules undergo complex self-assembly, forming helical nanofilaments (the B4 phase), layer undulations (the B7 phase) and disordered focal conics (the DC phase) in the bulk. However, due to the fluidity of the phases and the homeotropic alignment at the interface, several different topographies are observed at the air/liquid crystal interface. We will discuss the surface structures discovered so far in the B4, B7 and DC phases and show how they help us to understand the microstructure and the self-assembly of the liquid crystal phases of bent-core molecules.

Study of an ethylene oxide-terminated bent-core compound: synthesis and Langmuir-Blodgett film structure.

Bent-core compounds have attracted interest due to their unusual supramolecular structures, uncommon physical properties such as ferro- and antiferroelectricity and potential applications in fields such as nonlinear optics. Their incorporation into nanostructured materials, however, needs to be improved in terms of accurate control of the packing and orientation of the molecules in practical structures. Here, we have synthesized a novel bent-core compound bearing a tetraethylene glycol (TEG) chain as the hydrophilic head group and studied its capacity to obtain monomolecular films by means of the Langmuir-Blodgett technique. We developed a synthetic route to the material and studied its behavior in Langmuir and Langmuir-Blodgett films by means of a variety of characterization techniques, including a new model for the interpretation of UV-Vis reflection spectra of bent-core compounds. We found that the new head group, while destroying the formation of bulk mesophases, stabilizes the formation of the monolayer at the air-water interface and allows core-core interactions to dominate film dynamics, thus providing a promising alternative to carboxylic acid head groups.