Ignacio Sánchez

Luminescent liquid crystal materials based on unsymmetrical boron difluoride β-diketonate adducts

A strategic design based on the use of asymmetrically 1,3-alkyloxyphenyl substituted β-diketonate ligands towards the BF2 group allowed us to find unsymmetrical compounds that, in addition to a high photoluminescent quantum yield, possess a liquid crystal behaviour, which is related to the presence of the banana-shaped liquid crystal mesophases B1.

Polycatenar unsymmetrical β-diketonate ligands as a useful tool to induce columnar mesomorphism on highly luminescent boron difluoride complexes

Three families of polycatenar 1,3-alkyloxyphenyl β-diketonate boron difluoride complexes with three (I) four (II) or five (III) peripheral chains unsymmetrically distributed, have been prepared and characterised. The thermal behaviour was analysed in light of their molecular characteristics, allowing us to establish that the complexes of III are liquid crystal materials in contrast to the absence of mesomorphism for the related derivatives I and II. Compounds III were found to be discotic liquid crystals exhibiting hexagonal columnar (Colh) mesophases. In addition, when the complexes presented a higher asymmetry degree related to the presence of flexible chains with different lengths at both substituents, the mesophases were raised at room temperature. The photophysical characterisation of the boron derivatives was also carried out, and in all cases it showed a high fluorescent emission in the green region for solution and for the solid state. All these features make these bifunctional luminescent mesogens good candidates to be potentially applied in technological fields, which require soft emissive compounds.

Pyrazolium salts as a new class of ionic liquid crystals

New alkyloxyphenyl substituted pyrazolium salts [H2pzR(n)][A] (R(n) = C6H4OCnH2n+1; n = 8, 10, 12, 14, 16, 18; [A] = Cl−, BF4−, ReO4−, SbF6−, CF3SO3−, CH3-p-C6H4SO3−) have been synthesised and characterised. Those containing Cl−, BF4−, ReO4− and SbF6− anions behave as liquid crystal compounds exhibiting smectic A (SmA) mesophases. Because the precursor neutral pyrazoles [HpzR(n)] were not liquid crystals, their protonation and strategic choice of the counter-anion was found useful to achieve mesomorphism and to modulate the transition temperatures, so giving rise to a new class of ionic liquid crystals. The X-ray crystal structures of selected derivatives as representative examples of each type of compound have been resolved. In most of the cases, for X-ray purposes, the salts were designed to contain one methoxyphenyl-substituted pyrazolium cation. Different structural features have been found depending on the nature of the anion. Dimeric units held through hydrogen bonds are arranged in chains which pack in a layer-like array for compounds [H2pzR(10)][BF4], [H2pzR(1)][OTf] and [H2pzR(1)][PTS], but 2D structures were found for [H2pzR(1)][Cl] and [H2pzR(1)][ReO4]. Relationships between the solid and the mesophase structures are proposed. The absence of mesomorphism for CF3SO3− and CH3-p-C6H4SO3− derivatives is also explained on the basis of the main structural features.

New Pyrazolium Salts as a Support for Ionic Liquid Crystals and Ionic Conductors

Ionic liquid crystals (ILCs) are a class of materials that combine the properties of liquid crystals (LCs) and ionic liquids (ILs). This type of materials is directed towards properties such as conductivity in ordered systems at different temperatures. In this work, we synthesize five new families of ILCs containing symmetrical and unsymmetrical substituted pyrazolium cations, with different alkyl long-chains, and anions such as Cl−, BF4−, ReO4−, p-CH3-6H4SO3− (PTS) and CF3SO3− (OTf). We study their thermal behavior by polarized light optical microscopy (POM) and differential scanning calorimetry (DSC). All of them, except those with OTf as counteranion, show thermotropic mesomorphism. The observations by POM reveal textures of lamellar mesophases. Those agree with the arrangement observed in the X-ray crystal structure of [H2pzR(4),R(4)][ReO4]. The nature of the mesophases is also confirmed by variable temperature powder X-ray diffraction. On the other hand, the study of the dielectric properties at variable temperature in mesomorphic (Cl− and BF4−) and non-mesomorphic (OTf) salts indicates that the supramolecular arrangement of the mesophase favors a greater ionic mobility and therefore ionic conductivity.