In der vorliegenden Arbeit werden drei neue Mesogene hergestellt. Das sind Moleküle mit
flüssigkristallinem Verhalten in einem bestimmten Temperaturbereich. Über die Sonogashira Kreuzkupplungsreaktion wurden Dreifachbindungen eingbracht. Dies führt zu hoher Elektronendichte und guter Polarisierbarkeit. Eines der Moleküle ist ein direaktives Monomer zur Herstellung
eines dreidimensionalen Netzwerkes. Außerdem zeigen die Verbindungen Fluoreszenz
und eröffnen Möglichkeiten für die Verbesserung elektro-optischer Geräte wie z.B. LCDs.
Diese Diplomarbeit beschreibt die Synthese der neuen Mesogene und die Aufklärung
ihrer Struktur mit FT-IR, 1H und 13C NMR. Der Einfluss der Struktur auf den
flüssigkristallinen Temperaturbereich wird diskutiert, und die Flüssigkristalleigenschaften
werden polarisationsmikroskopisch und kalorimetrisch (DSC) untersucht.
Außerdem wird diskutiert, wie mit Fotopolymerisation orientierte Dünnfilme hergestellt
werden können und wie man ihre Orientierung bestimmt.
Table of Contents
1. Introduction
1.1 Liquid Crystals: Structure and Properties
1.2 Classification of Liquid Crystals
1.3 Applications of Liquid Crystals
1.4 Liquid Crystalline Polymers and their Application
1.5 Luminescence
1.6 Aim and Scope of the present Thesis
2. Results and Discussion
2.1 Synthetic pathways to the new LC fluorene derivatives
2.2 Structural Characterisation of the fluorene derivates
2.3 Differential Scanning Calorimetry
2.4 Polarisation Microscopy
2.5 LC properties of the fluorene derivates
2.6 Conclusions and Outlook
3. Experimental Part
3.1 Solvents and Materials
3.2 Equipment
3.3 Syntheses
Research Objectives and Topics
The primary objective of this thesis is the synthesis and characterization of new, polymerizable liquid crystalline mesogens containing fluorene moieties, specifically designed to investigate their potential for use in advanced opto-electronic devices such as light-emitting diodes.
- Synthesis of fluorene-based mesogens via Sonogashira cross-coupling
- Characterization of structural properties using FT-IR and NMR spectroscopy
- Analysis of thermotropic and liquid crystalline behavior using DSC and polarization microscopy
- Investigation of photoluminescence and potential applications in LCD or OLED technologies
Excerpt from the Book
1.1 Liquid Crystals: Structure and Properties
Liquid Crystals (LC) were discovered in 1888 when the Austrian botanist Friedrich Reinitzer observed a “double melting” behaviour of a cholesterol derivative [1]. At 145.5 °C, the substance melts to form a turbid, milky liquid. The melt suddenly becomes clear and transparent at temperatures above 178.5 °C. On March 14th, 1888, he wrote a letter to the German physicist Professor Otto Lehmann, describing these substances as “apparently living crystals” [2].
Lehmann carried out investigations on crystals and had developed a microscope that works with polarised light. He performed a lot of work on this phenomenon and published an article with the title “Über fliessende Krystalle” (“On flowing Crystals”) [3] in 1889.
Anisotropic properties (properties that depend on the direction under which they are observed) occur exclusively in a system with a regular, ordered structure. While crystals have highly ordered structures with ions or molecules at defined places in a lattice, typical liquids are isotropic with randomly distributed mobile molecules. The liquid crystalline phase, in contrast, is a fluid system in which the molecules still show some form of regularly ordered structure, a so called mesophase. In case of the cholesteryl benzoate, the substance melts at 145.5 °C under loss of the crystalline lattice structure. The turbidity of the melt above this temperature and below 178.5 °C is caused by light scattering as a result of a still existing order in small domains. Above 178.5 °C, this spatial arrangement of the molecules is lost and the melt becomes transparent. This point is called clearing point. Fig. 1 shows the corresponding DSC diagram of compound (6b) of this thesis with a melting point of 138.9 °C and a clearing point of 205.2 °C.
Summary of Chapters
1. Introduction: This chapter provides the theoretical background on liquid crystals, their classification, applications, and the relevance of liquid crystalline polymers, concluding with the specific research scope.
2. Results and Discussion: This section details the synthesis pathways for fluorene derivatives and provides a comprehensive structural and thermal analysis of the synthesized compounds.
3. Experimental Part: This chapter documents the technical procedures, including equipment utilized and detailed synthetic protocols for each chemical compound prepared in the study.
Keywords
Liquid Crystals, Fluorene, Synthesis, Mesogens, Sonogashira Coupling, Polymerisation, DSC, Polarisation Microscopy, Photoluminescence, LCD, OLED, FT-IR, NMR, Anisotropy, Thermotropic
Frequently Asked Questions
What is the core focus of this thesis?
The work focuses on the synthesis and structural characterization of novel, polymerizable liquid crystalline mesogens that contain fluorene moieties.
What are the central thematic fields?
The research bridges organic synthesis and materials science, specifically exploring liquid crystalline behavior, luminescence, and polymer chemistry for display applications.
What is the primary goal of the study?
The goal is to develop and analyze new materials that show stable liquid crystalline phases and luminescence, which could be utilized in future electronic display devices.
Which scientific methods were applied?
The research employed chemical synthesis techniques (Sonogashira cross-coupling), structural analysis (FT-IR, 1H and 13C NMR, DEPT-135), and physical characterization methods (Differential Scanning Calorimetry and polarization microscopy).
What does the main body cover?
The main body details the synthetic pathways of several fluorene derivatives, their spectroscopic verification, thermal behavior analysis via DSC, and the examination of their mesomorphic properties under a polarization microscope.
Which keywords characterize this research?
Key terms include liquid crystals, fluorene derivatives, photoluminescence, polymerizable mesogens, and electro-optical applications.
Why are fluorene moieties chosen for these mesogens?
Fluorene units provide a rigid, conjugated structure that is highly desirable for creating electroluminescent materials with high fluorescence quantum yields.
What was the outcome of the photopolymerization attempt?
A first attempt to create a stable, oriented thin film via UV-irradiation was performed with compound (10), but no stable film was obtained, suggesting further optimization of experimental conditions is required.
How is the phase transition determined?
Thermal transitions were determined using Differential Scanning Calorimetry (DSC), which measures the heat flow and identifies endothermic or exothermic changes as the material moves between phases.
- Arbeit zitieren
- Dipl.-Ing (FH) Philipp Gebhardt (Autor:in), 2006, Synthesis and Characterisation of New Polymerisable Mesogens Containing Fluorene Moieties, München, GRIN Verlag, https://www.grin.com/document/186607
