Emma Cavero

H-bonded complexes containing 1,3,4-Oxadiazole derivatives. Mesomorphic behaviour, Photophysical Properties and Chiral Photoinduction.

Two series of new V-shaped acids derived from 1,3,4-oxadiazoles are described. These acids were used to prepare supramolecular complexes through hydrogen bonding with 2,4-diamino-6-dodecylamino-1,3,5-triazine in a 3 : 1 ratio, respectively. The formation of the complexes was studied by infrared and NMR techniques. The thermal behaviour and mesomorphic properties of all the complexes were investigated by polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction. Hexagonal and rectangular columnar mesophases were observed for all complexes at room temperature, without evidence of crystallization. The results of circular dichroism studies allowed us to propose that in the liquid crystalline state these materials adopt a helical columnar organization and this chirality can be controlled by irradiation with CPL. Furthermore, the complexes display strong blue luminescence in solution and in the mesophase.

Polar Switching in a Lyotropic Columnar Nematic Liquid Crystal Made of Bowl‐Shaped Molecules

A polar response in a lyotropic columnar nematic material is reported. The material accommodates bowl-shaped molecules with strong axial dipole moments in column segments without head-to-tail invariance. Optical second-harmonic-generation methods confirm that the nematic columns align unidirectionally under an applied electric field and the material develops remnant macroscopic polarization observable for hours. The switching takes place by a flip of the columns.

Liquid crystalline dendrimers based on cinnamates and coumarins

ABSTRACT New poly(propylene imine) (PPI) dendrimers modified at the periphery with mesomorphic 4-cyanobiphenyl units and cinnamate- or coumarin-based photoactive moieties were synthesised and structurally characterised. The photoactive moieties were structurally designed to be liquid crystalline compatible and to reduce the tendency towards crystallisation of the final material. Thermal and liquid crystal properties were studied by POM, DSC, TGA and XRD. The results show that all dendrimers displayed a smectic A mesophase. Dendrimers with high transparency in the visible region but with different absorption spectra were obtained, rendering materials that are easy to monitor in light-irradiation experiments. GRAPHICAL ABSTRACT

Influence of the liquid crystal behaviour on the Langmuir and Langmuir–Blodgett film supramolecular architecture of an ionic liquid crystal

A new luminescent ionic liquid crystal, called Ipz-2, has been synthesised and its mesophase behaviour and also at the air-liquid interface has been studied and compared with Ipz, another ionic pyrazole derivative, with a similar molecular structure, previously studied. The X-ray diffraction pattern shows that Ipz-2 exhibits hexagonal columnar mesomorphism, while Ipz adopts lamellar mesophases. Langmuir films of both compounds are flat and homogeneous at large areas per molecule, but create different supramolecular structures under further compression. Ipz-2 Langmuir films have been transferred onto solid substrates, and Atomic Force Microscopy (AFM) images of the Langmuir-Blodgett films have shown that large columnar structures hundreds of nm in diameter are formed on top of the initial monolayer, in contrast with well-defined trilayer LB films obtained for Ipz. Our results show that Ipz-2 has a tendency to stack in columnar arrangements both in liquid crystalline bulk and in Langmuir and Langmuir-Blodgett films.