Liquid crystalline materials forming columnar mesophases are of importance for both the fundamental research and technological applications due to their supramolecular architecture allowing for one-dimensional charge transport. The potential applications of these materials include light emitting diodes, solar cells, field effect transistors and photovoltaic cells. However, to design a LC material suitable for a particular application, a fundamental understanding of the structure-property relationships is needed.<p>In the present thesis, a variety of systems forming columnar mesophases have been explored. They include small molecular weight compounds (triphenylene, phthalocyanine derivatives and star-shaped mesogens) and polymer materials. The research was focused on the study of the influence of the molecular architecture and specific interactions such as hydrogen bonding on the supramolecular organization in the mesophase, as well as on the influence of columnar mesophase on crystal growth. The main results of the thesis are summarized below.<p>The influence of hydrogen bonding on the structure and charge carrier mobility was investigated for a triphenylene derivative, hexaazatriphenylene, having lateral alkyl chains linked to the core via amide groups. These linking groups provide the possibility to form inter- and intra-molecular hydrogen bonds. Acting as “clamps”, the inter-molecular hydrogen bonds are found to enforce the attractive interactions between the molecules in the column. Thus, the columnar mesophase formed by this system is characterized by the smallest inter-disk distance ever found in columnar mesophases (3.18 Å). The improved intra-columnar order brings about a higher charge carrier mobility (0.02 cm2/Vs) as compared to other triphenylene derivatives without hydrogen bonds. <p>Phthalocyanine derivatives, which are liquid crystalline at ambient temperature, could be suitable for opto-electronic applications due to their improved processibility and self-healing of structural defects. Our interest in these systems was inspired by the fact that, in spite of numerous studies performed to date, only very a few phthalocyanine derivatives were found to exhibit columnar mesophases at ambient temperature. We observed that by introducing branches in alkyl chains close to the core, we were able to render the material LC at ambient temperature. Analysis of X-ray diffraction patterns measured on oriented samples showed that these systems form hexagonal and rectangular ordered columnar mesophases. This finding is in contradiction with the general view stating that non-hexagonal mesophases can be only disordered. Since the absolute majority of applications require fabrication of films, it was very important to achieve the visualization of the organization of the phthalocyanine derivatives at the nanometer scale. AFM images on thick spin-coated films with columnar resolution are presented for the first time. They allowed the examination of columnar curvatures and breaks at the boundaries between different single crystal-like domains. <p>The possibility of templating columnar crystal growth was studied for a star-shaped mesogen using a combination of direct- and reciprocal-space techniques. AFM images with columnar resolution showed that the crystal growth initiated in the monotropic columnar mesophase occurs almost in register with the mesomorphic template. In the final crystalline structure, the placement of the crystalline columns is controlled by the mesomorphic tracks at the scale of an individual column, i.e. at the scale of approximately 3.5 nm. <p>The mesophase-assisted crystallization was also studied for the case of a polymer material forming columnar mesophase, poly(di-n-propylsiloxane). X-ray diffraction on oriented fibers allowed us to correct the previous indexation and solve the structure of the unit cell. The crystallization process was studied on samples crystallized in different conditions. It was found that, depending on crystallization conditions, both folded-chain and extended-chain crystals can be obtained. Thus, crystallization of the material from the mesophase results in the formation of 100-150nm thick crystals, which corresponds to a nearly extended-chain conformation. By contrast, when crystallized from a dilute solution, folded-chain crystals result. The mechanisms of chain unfolding was studied by variable temperature atomic force microscopy on PDPS single crystals. It was found that crystals rapidly thicken above the initial melting point, up to 80 nm. / Doctorat en sciences, Spécialisation physique / info:eu-repo/semantics/nonPublished

Les mésophases hexagonales sont des systèmes quaternaires formés de tensioactifs et co-tensioactifs, eau salée et huile. Ces mésophases sont utilisées comme moules « mous » pour la synthèse de différents nanomatériaux tels que des nanostructures métalliques poreuses, des nanostructures de polymères conjugués et des nanocomposites métalliques-polymères. Contrairement aux matrices (ou moules) durs, qui nécessitent des réactifs chimiques corrosifs pour extraire les nanomatériaux synthétisés in situ, le processus d'extraction des nanomatériaux synthétisés dans les mésophases hexagonales est simple : les nanomatériaux peuvent être extraits simplement par lavage avec de l'éthanol ou du 2-propanol. Un autre intérêt à utiliser ces mésophases comme matrice de synthèse est qu’elles peuvent être gonflées en contrôlant le rapport huile / eau. Cette thèse est divisée en trois parties: 1) La synthèse de nanostructures métalliques poreuses dans la phase aqueuse des mésophases hexagonales et leur application dans les piles à combustible (oxydation de l'éthanol), 2) La synthèse de nanostructures de polymères conjugués dans la phase huile des mésophases hexagonales pour des applications en photocatalyse et en particulier pour la dégradation de polluants, 3) La synthèse combinée dans les phases huile et eau des mésophases hexagonales de nanocomposites métal-polymère. Plusieurs nanostructures métalliques telles que des nanoballes PdPt de porosité et composition contrôlées, des nanostructures poreuses cœur-coquille AuPd et AuPt, bimetalliques PtNi et trimétalliques AuPdPt ont été synthétisées par radiolyse dans la phase aqueuse des mésophases hexagonales. Les nanoballes PdPt de porosité et composition contrôlées ont été utilisées comme électro-catalyseurs pour l'oxydation de l'éthanol. L'effet de la taille des pores sur la surface électro-active des nanostructures métalliques et leur activité électrocatalytique pour l'oxydation de l'éthanol a été étudié. Les nanostructures poreuses cœur-coquille bimétalliques AuPd et AuPt, et trimétalliques AuPdPt ont été utilisées pour l'oxydation de l'éthanol et du glucose. Les nanoballes poreuses PtNi ont été utilisées pour l'évolution de H2 et la réaction de réduction de l’oxygène. Des nanostructures de polymères conjugués (poly(3-hexylthiophène), P3HT) ont été synthétisées dans la phase huile des mésophases hexagonales. Ces nanostructures de polymères ont une activité photocatalytique élevée sous UV et lumière visible. Le phénol et la rhodamine B ont été utilisés comme polluants modèles. Ces photocatalyseurs sont très stables même après plusieurs cycles photocatalytiques. L'ajout de molécules capteurs et l’étude du mécanisme montrent que les radicaux O2.− sont les principaux radicaux responsables de la dégradation du phénol. De manière très intéressante, l'activité photocatalytique de ces nanostructures de P3HT est fortement augmentée lorsqu'elles sont supportées sur une surface solide. Ce résultat ouvre de nouvelles perspectives pour des applications dans des réacteurs photocatalytiques et des surfaces autonettoyantes. Des résultats préliminaires sur la synthèse des nanocomposites Pt-PDPB (polydiphenylbutadiyne) sont également présentés dans cette thèse. / Soft hexagonal mesophases, which consist of quaternary systems (surfactants, brine, oil, and co-surfactant) are used as templates for the synthesis of different nanomaterials such as metal nanostructures, conjugated polymer nanostructures, and metal-polymer nanocomposites. Unlike hard templates, which need a harsh chemical reagent to extract nanomaterials after the synthesis, in soft template hexagonal mesophases, the extraction process of nanomaterials is simple, only by washing with ethanol or 2-propanol. Another interesting property of this class of template lies on its ability to be swollen by controlling the ratio of oil to water.This thesis is divided into three parts: 1) Radiolytic synthesis of metal nanostructures in the aqueous phase of hexagonal mesophases and their application in fuel cells (ethanol oxidation), 2) Synthesis of conjugated polymer nanostructures in the oil phase of hexagonal mesophases for photocatalytic degradation of pollutants, 3) Combined synthesis in the oil and water phases of hexagonal mesophases of metal-polymer nanocomposites.Several metal nanostructures such as PdPt nanoballs with controlled composition and porosity, AuPd and AuPt core shell, bimetallic PtNi and trimetallic AuPdPt porous nanoballs were synthesized by radiolysis in the aqueous phase of hexagonal mesophases. PdPt nanoballs with controlled porosity and composition were used as electrocatalysts for ethanol oxidation. The effect of the pore size on their electro active surface and their electrocatalytic activity towards ethanol oxidation were studied. AuPd and AuPt core-shell, and trimetallic AuPdPt porous nanoballs were used for ethanol and glucose oxidation. PtNi porous nanoballs were used for H2 evolution and oxygen reduction reaction. Conjugated polymer nanostructures namely P3HT (poly(3-hexylthiophene)) were synthesized in the oil phase of hexagonal mesophases. These polymer nanostructures are highly active for photocatalysis under UV and visible light. Phenol and rhodamine B were used as model pollutants. These photocatalysts are very stable even after repeated cycling. Addition of scavengers and mechanistic studies show that O2.− is the main radical responsible for degradation of phenol. Most interestingly, the photocatalytic activity of these P3HT nanostructures is highly enhanced when they are supported on a solid surface opening new perspectives in photocatalytic reactors and self-cleaning surfaces. Premiminary results on the synthesis of Pt-PDPB (polydiphenylbutadiyne) nanocomposites are also presented in this thesis.