Synthesis and Characterisation of New Polymerisable Mesogens Containing Fluorene Moieties

Diploma Thesis , 2006 , 91 Pages , Grade: 2

Chemistry - Bio-chemistry

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

Table of Abbreviations

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

5. Bibliography

6. Appendix
6.1 IR-spectroscopy
6.2 NMR-spectroscopy
6.3 DSC
6.4 Photos obtained by polarisation microscopy

Danksagung

An erster Stelle möchte Ich mich bei Professor Dr. Carlos Aguilera Jorquera bedanken für die Möglichkeit, an dieser Diplomarbeit zu arbeiten, die freundliche Aufnahme in seiner Arbeitsgruppe und seine Gastfreundschaft während meiner Zeit an der Universidad de Concepción

Ich danke Scarlette Heggie, Paola Linda, Karen Bustamante und Mauricio Morel für die Hilfe und ihren Rat im Labor und für die schöne Zeit in Chile

Mein besonderer Dank gilt Silvia Fernandes für ihre Gastfreundschaft und ihre Hilfe bei der Kernresonanzspektroskopie, Doña Rosita für ihre Hilfe bei der Infrarotspektroskopie, Leonardo Bernal und Don Guillermo für nette Gespräche und ihren Rat im Labor

Ich danke meinen Professoren und Kommilitonen an der Fachhochschule Fresenius und meinen Eltern für die finanzielle Unterstützung und weil sie mir das Studium ermöglichten

Summary

Liquid crystals present an intermediate state of matter. In a liquid the molecules are in contact but are able to move past each other. In a crystal the molecules are not able to move past one another, they are incorporated in the cystalline lattice, giving the system a long-range order. In nematic liquid crystals, the molecules are arranged in such a way that their longitudinal axes are mutually parallel but they are easily able to move in the direction of their longitudinal axes. Thus, liquid properties like fluidity and viscosity as well as optic properties that are shown by crystals like the reflection of different colours depending on the viewing angle are observed simultaneously

The incorporation of photopolymerisable groups provides monomers for temperature independent polymerisation. After polymerisation in the LC phase and subsequent cooling, the molecular orientation within the system can be frozen in, thus, materials with special qualities can be obtained. These materials have direction-depending optical and mechanical properties consequently they represent an area of scientific interest and technological potential

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In the present work three new mesogens, molecules with liquid crystalline behaviour in a determined temperature range were synthesized. They have the above illustrated structure. One of them is a direactive monomer for the creation of a threedimensional network. Due to their structure, the compounds show fluorescence and are suitable for new materials with application in electro-optical devices like LCDs

The present thesis describes the synthesis of the new mesogens and their characterisation with FT-IR,¹ H and¹³ C NMR. The influence of the molecular structure on the thermotropic properties is discussed and the liquid crystalline properties are examined by polarisation microscopy and DSC

Moreover, ways for obtaining and characterising orientated thin films are bescribed

Resumen

Los cristales líquidos presentan un estado intermedio de la materia. En un líquido las moléculas están en contacto pero pueden moverse unas contra otras. En un cristal las moléculas están incorporadas en la red cristalina restringiendo su movilidad y dando el sistema un orden de gama larga

Los cristales líquidos nemáticos exhiben orden en la orientación de sus moléculas y al mismo tiempo desorden en la posición de sus centros de masa. Las moléculas pueden moverse lateralmente, girar alrededor del eje común o deslizarse paralelamente a él. Así mantienen las características de los líquidos como: fluidez y viscosidad y las características ópticas que presentan los cristales como reflexión de distintos coloures dependiendo del ángulo bajo el cual se les observe

La incorporación de grupos fotopolimerizables en moléculas de este tipo proporciona monómeros para realizar una polimerización de moléculas con estructuras mesógenas independiente de la temperatura. Mediante polimerización en el estado líquido cristalino se puede mantener la orientación molecular del sistema al enfriar. Así se pueden obtener materiales con calidades especiales. Estos materiales cuyas propiedades ópticas y mecánicas dependen de la dirección representan un área de gran interés científico y un excelente potencial tecnológico

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En este trabajo se han sintetizado tres nuevos mesógenos. Las moléculas tienen la estructura ilustrada arriba y muestran un comportamiento líquido cristalino en una gama de temperaturas. Una de ellas es un monómero direactivo para la formación de una red tridimensional. Debido a su estructura, los compuestos demuestran fluorescencia y por lo tanto son convenientes para obtener nuevos materiales en el uso de dispositivos electroópticos como LCDs

La actual tesis describe la síntesis de nuevos mesógenos y su caracterización con FT-IR,¹ H y¹³ C RMN. Las características líquidas cristalinas son obtenidas a través de microscópia de luz polarizada, DSC y la influencia de la estructura molecular en las características termotrópicas es analizada y discutida. Además se describe la preparación de películas orientadas y su caracterización

Zusammenfassung

Der Zustand „flüssigkristallin“ liegt zwischen den Aggregatzuständen flüssig und fest. In einer Flüssigkeit stehen die Moleküle in Kontakt zueinander, aber sie sind in der Lage, sich aneinander vorbei zu bewegen. In einem Kristall sind die Moleküle nicht in der Lage sich aneinander vorbeizubewegen, sie sind in das Kristallgitter fest eingebaut, was dem System über weite Bereiche eine hohe Ordnung gibt. In nematischen Flüssigkristallen sind die Moleküle mit ihren Längsachsen zueinander parallel angeordnet, aber sie sind auch in der Lage, sich in Richtung ihrer Längsachsen gegeneinander zu bewegen. Dadurch zeigen sich in diesen Systemen Eigenschaften von Flüssigkeiten wie Fluidität und Viskosität sowie gleichzeitig optische Eigenschaften von Kristallen wie die Reflektion verschiedener Farben abhängig vom Betrachtungswinkel

Der Einbau von fotopolymerisierbaren Gruppen in flüssigkristalline Moleküle liefert Monomere für die temperaturunabhängige Polymerisation. Polymerisiert man im flüssigkristallinen Aggregatzustand und kühlt dann ab, wird die Orientierung der Moleküle eingefroren, wodurch man Materialien mit speziellen Eigenschaften erhält. Diese Materialien haben richtungsabhängige optische und mechanische Eigenschaften und präsentieren dadurch vielfältige technische Anwendungsmöglichkeiten und ein Gebiet von hohem wissenschaftlichem Interesse

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In der vorliegenden Arbeit werden drei neue Mesogene hergestellt. Das sind Moleküle mit flüssigkristallinem Verhalten in einem bestimmten Temperaturbereich. Sie haben die oben dargestellte Struktur. 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,¹ H und¹³ C 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 Abbreviations

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1. Introduction

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 derivative¹. 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.

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Fig. 1: DSC - Diagram of Mesogen (6b)

Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. Using DSC, it is possible to observe energy changes that occur as matter transitions from solid to mesophase and from mesophase to isotropic liquid (fig. 1).

LCs present an intermediate state of matter. Such compounds posess properties of liquids such as fluidity and viscosity and, on the other hand, optical properties that appear in crystals such as birefringence. This is the resolution or splitting of a light wave into two different waves by an optically anisotropic medium such as calcite.

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Fig. 2: Splitting of light by a calcite crystal

A birefringent (or double refractive) material posesses two indices of refraction. Splitting a calcite crystal can easily be performed. Thus, a rhombus is formed with corners possessing angles of 78° and 102°. The crystal’s optical axis goes through two corners that are formed by three angles of 102° each⁴. Along this axis, the material’s two refractive indices are equal. A light ray entering a calcite crystal perpendicular to its optical axis is split into two rays of polarised light. In contrast to “normal” light, in which the electric field vectors vibrate in all perpendicular planes with respect to the direction of propagation, in polarised light all waves vibrate in the same plane.

Fig. 2 illustrates light splitting by a calcite crystal. The resulting rays are called extraordinary ray and ordinary ray. Their waves are polarised and vibrate in a perpendicular way to each other. The two corresponding refractive indices are called extraordinary refractive index ne and ordinary refractive index no. The birefringence value is given by n = ne - no.

The order of LC phases depends on molecular geometry and polarisability. The magnitude of dipole-dipole forces and forces that support LC phase are critical. If these forces are very weak, or at the other hand very strong, the LC character is lost⁵. LC phases can be formed by molecules having several different general molecular shapes. Calamitic LCs are formed by rod-like molecules, the more recently discovered discotic LCs are formed by disc-like molecules⁶. In the late 1990s the discotic molecules found application in electronic displays. They are used to make a sheet of film that expands the viewing angle of a twisted-nematic (TN) display⁷. Another application is in organic photovoltaic cells⁸.

In rod-shaped molecules, anisotropic shape and resulting anisotropic forces result in the formation of LC phases. A typical calamitic LC forming molecule has the idealised molecular structure shown in fig. 3.

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Fig. 3: Idealised structure of a typical calamitic LC forming molecule

A is a linking group between the two (or more) ring systems B and B’. It increases the molecules length to breadth ratio and can also influence the polarisability and flexibility of the molecule. Fig. 4 shows the effect of different linking groups on liquid crystal stability (position and range of LC phase) of a given mesogen⁹. In the illustration, C-N means phase transition temperature from crystalline to nematic, N-I from nematic to isotropic.

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Fig. 4: Comparison of the effect of different linking groups on liquid crystal stability

The terminal groups X and X’ (fig. 3) may extend the molecular long axis and have significant effect on the LC properties. Maier and Saupe suggest that the temperature of nematic to isotropic phase transition of a compound is related to the molecular polarisability which is itself related to the terminal group and its influence on the conjugation of the molecule¹⁰.

The lateral substituents Y and Y’ broaden the molecule thus reducing lateral attractions and lowering LC phase stability. It can be useful to lower attractive forces because ring systems may favour crystalline phases so much that very high melting point materials are formed¹¹. If the lateral substituents are longer flexible alkylic chains, they may decrease transition temperatures, favour nematic phases and faciliate the mesogens movements for their orientation¹². Decreased transition temperature is generally desirable as it means decreased processing temperature, for example for the manufacture of LC polymer materials. Especially for the photopolymerisation of such materials, it is necessary to lower the working temperature to avoid premature, termally induced polymerisation. LCDs, on the other hand, require materials that are nematic over a range of 0 - 60 °C. Eutectic mixtures can be used to cover this temperature range for higher melting substances¹³.

In conclusion, terminal as well as lateral substitution affect the type and temperature stability of the mesophase. To reveal the LC behaviour, it is generally necessary to reduce the crystal melting point which is achieved by attaching flexible substituents like alkylic chains to the mesogenic core¹⁴.

Many homologous series of compounds have been studied and reviewed¹⁵. In each case the melting points show no coherent structure dependence and therefore are still unpredictable.

1.2 Classification of Liquid Crystals

LCs are generally classified as thermotropic or lyotropic based on whether the temperature or the presence of a solvent stabilises the mesophase. Lyotropic liquidcrystalline nanostructures are abundant in living systems. Phospholipids, for example, exhibit lyotrophic mesomorphism in the presence of water¹⁶.

Thermotropic LCs were classified by Friedel¹⁷ in three general types. They are called nematic, cholesteric and smectic. They have a parallel alignment of the molecular longaxis in common but differ in the lateral order.

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Fig. 5: Molecular alignment in the nematic phase

In nematic LCs, the molecular long axes are preferably oriented in one direction, defined as the director n (fig. 5). The nematic phase posesses a relatively low viscosity, even small external forces cause deformations. Deformations or distorsions can be described in terms of three basic types: splay, twist and band¹⁸ (fig. 6):

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Fig. 6: Deformations occuring in LCs

Neighboring molecules may be forced to be angled with respect to one another, rather than aligned (twist). Bending may occur parallel (bend) or perpendicular to the director (splay).

The nematic phase is the least ordered, the molecular axes point in the same direction, but molecules may move relative to each other because of the lack of lateral attractive forces. The cholesteric phase is formed of chiral molecules¹⁹ or nematic molecules that are mixed up with a chiral substance²⁰. In these systems, molecules align in layers. Perpendicular to the director, layers are twisted relative to each other (fig. 7):

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Fig. 7: Molecular alignment in the cholesteric phase

The finite twist angle between adjacent molecules is due to their asymmetric packing, which results in longer-range chiral order²¹. The chiral pitch refers to the distance (along the director) over which the mesogens undergo a full 360º twist.

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Fig. 8: Molecular alignment in a smectic phase

The smectic phase shows the highest order within the LCs. In smectic A (SmA) phases, on average, the molecules are parallel to one another and are arranged in layers, with the long axes perpendicular to the layer plane (fig. 8). Further smectic phases exist. The structure of the smectic C (SmC) liquid crystals is closely related to the structure of the SmA. The molecules are arranged in layers, but the long axes of the molecules are tilted relative to the layers’ planes. The different smectic and the other above mentioned LC phases can be identified by polarisation microscopy or by means of x-ray diffraction [22, 23].

1.3 Application of Liquid Crystals

Todays major application of LCs is their use in Liquid Crystal Displays (LCD). These developments began in 1964, when George Heilmeier of RCA Laboratories discovered the guest-host mode and the dynamic-scattering mode.

The guest-host mode evokes a colour switching effect when an electric field is applied to the mixture of a dye and liquid crystals. The dye is called the guest and the liquid crystals are called host. With no voltage applied, the guest and host molecules align at right angles to the direction of incident light and therefore absorb light and are coloured. When an electric field is applied, the guest dye molecules reorientate along with the host nematic liquid-crystal molecules. They are now aligned parallel to the direction of incident light and the mixture becomes transparent²⁴. White and Taylor describe the guest-host interaction using the phase transition from cholesteric to nematic²⁵.

The guest-host mode is not stable over a long period of time in applied fields and shows further disadvantages. When trying to solve these problems, Heilmeier discovered another efficient way to electronically influence the reflection of light.

In the dynamic-scattering mode, an electrical field causes nematic LCs to tumble. With no current, the molecules are aligned to electrode plates in a perpendicular way. When applying an electrical field perpendicular to the electrodes, the LC’s molecules align parallel to the plates. In nematic liquid crystals, the electrical conductivity in the direction along the long axis is larger than in the short-axis directions, which causes charge buildup. The induced field and external field generate a shear torque on the molecules, which causes a circular motion so that they scatter light²⁶.

Heilmeier and other RCA enginieers designed and produced the first LCD, it was based on the dynamic-scattering mode. On May 28th, 1968, RCA held a press conference at its headquarters at Rockefeller Plaza, New York. They proudly announced the discovery of a totally new type of electronic display device. It was lightweight, consumed little electrical power, and was very thin. The press conference aroused the attention of scientific and industrial communities all over the world. This announcement initiated the development of digital watches in the U.S., Japan, and Germany and the work on pocket calculators in Japan²⁷.

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Fig. 9: Seven-Segment LCD

In 1971, Martin Schadt and Wolfgang Helfrich found a new workable display mechanism with twisted nematic LCs²⁸. The technology takes advantage of nematic molecules that redirect the direction of polarisation of light by 90° along their helix arrangement²⁹. Fig. 9 illustrates the function of a common 7-segment TN-LCD as it is used in small electronic devices. The LC is sandwiched between two glass plates with seven electrodes each. Molecular alignment of the LC is pre-orientated by the use of rubbed surfaces. Light passing through polariser 1 is polarised in the vertical direction. When no voltage is applied to the electrodes, the liquid crystalline phase induces a 90 degree "twist" of the light. It can therefore pass through polariser 2, which is oriented perpendicular to polariser 1. When voltage is applied to the electrodes, the LC molecules rearrange to a linear order perpendicular to the planes of the polarisers and passing light is no longer twisted. Thus, it cannot pass polariser 2 and the respective area appears dark. Light can either be provided by a lightsource in the back of the display (backlight), or a reflector is used to illuminate the device - that itself consumes very little power - with external light.

In colour-LCDs, each pixel is devided into three subpixels which are coloured red, green and blue. These subpixels can be controlled independly, to yield thousands or millions of colours for each pixel. To adress each subpixel separately, larger high-resolution devices depend on integrated transistor-circuits (Active-Matrix LCD³⁰ ).

Conventional colour LCDs use a broadband white backlight to illuminate an LCD incorporating a transparent microdot colour filter. The contrast, brightness and colour of LCDs vary with viewing angle due to the inherent optical anisotropy of the liquid crystal material. The colour filters absorb more than 75% of the light from the backlight by generating a colour picture³¹.

Photoluminescent Liquid Crystal Displays (PLLCDs) are illuminated by a near UV backlight. A phosphor screen in front of the display emits visible light generated by photoluminescence. This novel architecture provides higher efficiency and combines the flat panel format of an LCD with the viewing angle and brightness of a cathode ray tube like it is used in ordinary television screens.

Another application for LCs is for example the use as temperature indicators due to the selective reflection of light as a function of temperature in cholesteric phases. The chiral pitch depends on temperature and is generally of the same order of magnitude as the wavelengths of visible light. This causes these systems to change their visible absorption spectrum as a function of temperature³².

High resolution temperature-indication is possible, applications are for example suitable in medical thermography. Cholesteryl-phenyltetradecaethionate provides light reflection changing from 500 to 600 nm in a 0.02 K range³³.

1.4 Liquid Crystalline Polymers and their Application

In recent years the photoinitiated polymerisation of LCs has been of high interest in the technological and scientific research area, because these monomers are suitable for the preparation of thin, orientated solid films for electronic components, especially for applications such as solid state polarisers, interference filters, etc.³⁴. The advantage of photoinitiated polymerisation compared to thermal polymerisation is its independence of temperature which enables the selection of the mesophase formed to be frozen in³⁵.

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Fig. 10: Polymer with mesogenic side groups

Two classes of liquid crystalline polymers (LCPs) are distinguished. In main chain LCPs, the mesogens are part of the polymer main chain. A well known application for this class of compounds is “Kevlar”, the trade name for high-strength aramid fibers³⁶. In side group LCPs, the mesogens are attached to the polymer backbone. If the mesogenic groups are covalently fixed side chains of a given main chain, their ability to move and orient is changed drastically. In the liquid state of the polymer, the tendency of statistical chain conformation hinders an orientation of the side chains. If the anisotropic interactions of side chains are strong enough to form the mesophase, a liquid crystalline structure can neverthless be formed, but only in accordance with the limited motions of the main chain. In the liquid state the motions of the polymer main chain have to be decoupled from those of the anisotropically oriented mesogenic side chains. The decoupling should be possible, if flexible spacer groups are inserted between the main chain and the rigid mesogenic side chains³⁷ (fig. 10).

In 1987, D. J. Broer and co-workers obtained a highly ordered polymer sample by “in-situ photopolymerisation of an oriented liquid-crystalline acrylate”³⁸. In the absence of solvents, the monomer is heated to the mesomorphic phase, macroscopically oriented and irradiated with ultraviolet (UV) light. The photoinitiator, which is dissolved in the mesomorphic monomer, is fragmented into free radicals which initiate the chain polymerisation. Homogenous orientation can be obtained by applying the monomer onto a substrate which has been coated with a thin polymer layer and rubbed unidirectionally with tissue. Subsequently, the monomer orients itself according to the rubbing direction. The ordering of the mesogens is frozen-in, yielding uniaxially oriented networks by carrying out photo-initiated chain crosslinking of liquid crystalline diacrylates³⁹ (fig. 11).

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Fig. 11: Photo-crosslinking of an oriented liquid crystalline diacrylate

Liquid crystalline gels can be obtained by polymerisation of liquid crystal diacrylates in mixtures of nonreactive mesogens. Although these host molecules are not chemically incorporated in the crosslinked network, they support orientation due to anisotropic interactions. Some applications were found. The transparent material shows light scattering behavior as a response to an electric field and can be used for switchable polarisers and further display applications⁴⁰.

Liquid crystalline elastomers are materials with remarkable properties. They can be obtained by incorporating mesogenic monomers into poymer networks with a determined density of crosslinking. Nematic elastomers with a single, global director of orientation can be produced through the use of magnetic⁴¹ fields or mechanical treatment⁴² during the cross-linking procedure which results in a globally anisotropic chain trajectory. Such samples retain a memory of their chain shape and, through coupling, a memory of the single global director present at the time of network formation⁴³. Interesting applications are for example materials that change colour when stretched, and artificial muscles⁴⁴.

1.5 Luminescence

Luminescence is the emisson of photons in different wavelength regions (ultraviolet, visible or infrared) occuring when electrons in excited orbitals decay to their ground states. The phenomenon was first described by Eilhard Wiedemann. In his original paper of 1888 he proposed that a ‘luminescent substance’ was one which ‘becomes luminous by the action of an external agent which does not involve an appropriate rise in temperature⁴⁵. Fluorescence does, in comparision to phosphorescense, not involve the spin change of the exited electrons and is short-lived. It disappears when the source of excitation is removed.

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Fig. 11: Jablonski Diagram, electronic state transitions⁴⁶

The Jablonski Diagram, named after the Polish physicist Aleksander Jablonski (1898 - 1980), illustrates the electronic states and the transitions between them (fig. 11). Depending on the way of excitation, different types of luminescence are distinguished ⁴⁷.

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Fig. 12: Examples for organic compounds that show luminescence

Photoluminescence is shown by different organic compounds (fig. 12). They tend to have rigid, conjugated structures and are often aromatic ring systems⁴⁸. The benzene ring with its six -electrons may act in conjunction with electron-donating groups (auxochromes like -NH2-, RHN-, R1R2N-, -O-, -OH, -OR, etc.) and electron accepting groups (like -CN, -C=C-, -CO-, etc.) to produce strong absorption in the UV or visible regions which may give rise to fluorescence⁴⁹.

The fluorene derivates synthesised in this thesis show strong photoluminescence; the excitation is caused by absorption of photons. In thin layer chromatography, they appear as bright blue spots on the plates when irradiated with UV-light.

In electroluminescence, excitation is established through an electric current or a strong electric field. It is the radiative recombination of electrons and holes in a material. Limited conjugation leads to lower charge carrier mobility. Thus, the ideal electroluminescent material for organic light emitting devices has uninterrupted - conjugation and shows a high yield of fluorescence. It is suggested that due to a more rigid conjugated backbone, electroluminescent polymers with cabon-carbon triple bond linkages exhibit higher fluorescence quantum yields, compared with other families of conjugated polymers⁵⁰. Material research in this area has intensified since the first demonstrations of organic light-emitting diodes (OLEDs) and polymer light-emitting diodes (PLED) have taken place, because they show advantages to common technologies.

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Fig. 13: Schematic drawing of a single layer OLED

A single layer OLED consists of a thin film of light emitting material which is sandwiched between two electrodes (fig. 13). One of these electrodes has to be semi-transparent to allow the light to leave the device. As an anode material indium-tin-oxide (ITO) is mainly used. It ensures conductivity and is transparent.

Fluorescence microscopy is used to view luminescent species, mainly in the sphere of biology. This technique works with UV-irradiation and filters to make only the emitted part of light visible for the eye or a camara.

In a spectrophotometer, monochromators allow to record wavelength/intensity curves for absorbed and emitted light. Polarisers are used to illuminate the sample with polarised light and to analise emitted light to determine any anisotropic properties.

1.6 Aim and Scope of the present thesis

In recent years a wide range of emissive materials has been reported for the use in electroluminescent devices; these vary from low molecular mass molecules to processable polymers. In addition, highly conjugated liquid crystals are desirable. The self-organising properties of these materials can be exploited to improve device performance and to achieve linearly polarised electroluminescence. Polarised electroluminescence from nematic networks has been reported, offering a possible substitute for one of the polarisers and the back light of TN-LCDs and STN-LCDs, with a lower power consumption and/or a higher brightness.

For this application, highly -polarisable and conjugated systems with aromatic moieties, linked to a mesogenic core through triple C C bonds may be used. Diarylacetylenes with different mesophases and strong blue fluorescence from 390 to 460 nm with good quantum yields from 50 to 85% have been reported⁵¹.

Fluorene containing materials are often used for organic light-emitting devices. The synthesis and characterisation of oligo(9,9-dihexyl-2,7-fluorene ethynylene)s has been reported for the application as blue OLED⁵². Recent research led to the use of mesogens containing fluorene and thiophene units with acrylate and diene photopolymerisable end groups for full-colour OLEDs which emit linearly polarised light [53, 54].

The preparation of self-organising polymers is a field of investigation with growing interest. There is a need for further research on new materials for the improvement of organic light- emitting devices and display technologies. For this purpose, new mesogens containing fluorene moieties are synthesised and studied in the course of this thesis (fig. 14).

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Fig. 14: Target molecule

In mesogens, an incorporated multiple bond can conjugate with the phenylene rings, enhancing the anisotropic polarisability. This function increases the molecular length, restricts freedom of rotation and maintains rigidity. The ester linkage is also effective since it has a planar trans conformation and since resonance interactions impart double-bond character on its -C(=O)O- bond restricting rotational motions⁵⁵. The terminal alkoxy groups may give rise to higher liquid crystal thermal stability compared to the alkyl analogues, this being due to increased conjugation and rigidness, also. Due to its rodlike character, the molecule is expected to display nematic LC behaviour.

In the framework of this thesis, three new mesogens are synthesised, starting from fluorene. A Sonogashira cross-coupling reaction is used as the final step, followed by column chromatography. At first, two compounds are studied. They differ with respect to the number of carbons in their terminal chains. Lenghtening of these chains may diminish the melting point of the substance but usually leads to a decreased mesophase termal stability. After the examination of LC properties, a terminal chain with appropiate length and terminal acrylic group is choosen for the synthesis of a reactive, polymerisable mesogen.

The compounds are checked for purity with TLC. They are identified and characterised by FT-IR- and NMR-Spectroscopy. The LC properties are measured with DSC (phase behaviour) and polarised light microscopy (identification of mesophase type).

2. Results and Discussion

2.1 Synthetic pathways to the new LC fluorene derivatives

Fluorene can be seen as a biphenyl molecule that is additionally connected by a methylene bridge. This connection forces the rings to stay in plane. Due to its structure, it shows acid properties, a fact which faciliates the substitution of the hydrogens in position 9. At the other hand, positions 2 and 7 are activated for electrophilic aromatic substitution such as Friedel-Crafts Alkylation.

After the iodation of position 2 and 7 of the fluorene core, position 9 and 9’ are alkylated. Subsequently the ethynyl function is introduced in a first Sonogashira cross-coupling reaction [56, 57], substituting iodine. This is the coupling of terminal alkynes with aryl or vinyl halides, using a palladium catalyst, a copper(I) cocatalyst and an amine base under anhydrous and anaerobic conditions. Acetylene gas can be used for the synthesis of disubstituted acetylenes rather than for terminal ethynyls⁵⁷. To avoid undesired coupling reactions, one end of the acetylene can reversibly be attatched at position 2 of a 2-hydroxypropyl group (fig. 15). The protective group is removed in the next step by treatment with potassium hydroxide in 2-propanol, leading to the terminal ethynyl compound under the formation of acetone. The syntheses are carried out in a modified way as described by S. H. Lee and coworkers for the synthesis of 2-ethynyl-9,9- dihexylfluorene⁵². The obtained 2,7-diethynyl-9,9-dihexylfluorene (4) is the core structure for the attatchment of 3 different ester moieties. The synthetic pathways are illustrated in the following two pages (fig. 16 and 17).

illustration not visible in this excerpt

Fig. 16: Synthetic pathway for compounds (1) - (6a), (6b)

After (4) was obtained, 4-alkoxybenzoic acids were transformed to the esters ((5a), (5b)) by reaction with 4-iodophenol using dicyclohexylcarbodiimide and 4- dimethylaminopyridine as condensation catalyst. This esterification was first reported by Bernhard Neises and Wolfgang Steglich ⁵⁸. The obtained compounds can be purified by simple recrystallisation from ethanol. In a second Sonogashira-reaction, the esters are coupled to the ethynylfluorene (4), leading to (6a), (6b).

illustration not visible in this excerpt

Fig. 17: Synthetic pathway for compounds (7) - (10)

4-(11-hydroxyundecyl)benzoic acid (7) is obtained in the way described by D.J. Broer and coworkers ³⁹. Methyl 4-hydroxybenzoate reacts with 11-bromo-1-undecanol and the reaction product is opened for further esterification by KOH. The intermediate is purified and used in a subsequent step for the reaction with acryloyl chloride, the following acidification yields (8). Esterification with 4-iodophenol leads to (9), which is coupled to (4) in the following Sonogashira reaction.

2.2 Structural Characterisation of the fluorene derivates

2.2.1 Fourier transform infrared spectroscopy

In the following text some characteristic absorption bands will be assigned to functional groups. Compound (3) for example shows an intense, broad band with its maximum at 3339 cm-¹ that is caused by O-H stretching vibration from hydroxyl groups at both sides of the molecule. The sharper band at 1158 cm-¹ results from the C-O-stretching vibration in tertiary alcohols.

illustration not visible in this excerpt

Fig. 18: Compound (3) and its FT-IR Spectrum

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^[1] F. Reinitzer; Mh. Chem.; 1888; 9; 421

^[2] O. Lehmann; Annalen der Physik; 1908; 27; 1099

^[3] O. Lehmann; Z. Phys. Chem.; 1989; 4, 462

^[4] E. Hering, R. Martin, M. Stohrer; Physik für Ingenieure; Springer Verlag Berlin Heidelberg New York; 2002; 6; 471

^[5] W.Maier, A.Saupe; Z. Naturforsch.; 1958; 13a, 564

^[6] G. W. Gray, K. J. Harrison; Symp. Faraday Soc.; 1971; 5; 54

^[7] H. Kawamoto; Proc. IEEE; 2002; 90; 4; 462

^[8] T. Hassheider, S. A. Benning, M. W. Lauhof, H. S. Kitzerow, H. Bock, M. D.

Watson, K. Müller; Mol. Cryst. Liq. Cryst.; 2004; 413; 2597-2608

^[9] D. Demus, H. Demus, H. Zaschke; Flüssige Kristalle in Tabellen; VEB Deutscher

Verlag für Grundstoffindustrie, Leipzig; 1976

^[10] W. Maier, A. Saupe; Z. Naturforsch.; 1960; 15a; 287

^[11] B. Bahadur; Liquid Crystals, Applications and Uses; World Scientific; 1990; 1; 119

^[12] A. Kelker, R. Hatz, Handbook of Liquid Crystals; Verlag Chemie, Deerfield Beach, Florida; 1980; 2

^[13] D. Hume, P. Rynes, K. J. Harrison; J. Chem. Soc. Commun.; 1974; 98

^[14] P. Ksazynski, A. G. Douglass; J. Organometal. Chem.; 1999; 581; 29

^[15] G. W. Gray, P. W. Winsor; Liquid Crystals and Plastic Crystals; Pub. E. Horwood, Chichester; 1974; 1

^[16] D. Chapman; Symp. Faraday Soc.; 1971; 5; 163 – 174

^[17] G. Friedel; Ann. Phys.; 1922; 18; 273

^[18] T. E. Faber; Proc. R. Soc. Lond. A.; 1977; 353; 247 – 259

^[19] H. Stegemeyer; Ber. Bunsenges. Phys. Chem.; 1974; 78; 860

^[20] H. Finkelmann, H. Stegemeyer; Ber. Bunsenges. Phys. Chem.; 1974; 78; 869

^[21] J.L. Furgason; Mol. Cryst.; 1966; 1; 293

^[22] H. Sackmann, D. Demus; Fortsch. Chem. Forsch.; 1969; 12; 349

^[23] A. De Vries; D. L. Fishel; Mol. Cryst. Liq. Cryst.; 1972; 16; 311

^[24] G. H. Heilmeier, L. A. Zanoni; Appl. Phys. Lett.; 1968; 13; 3; 91–92

^[25] G.N. White, D.L. Taylor; J. Appl. Phys.; 1974; 45; 4718

^[26] G. H. Heilmeier, L. A. Zanoni, and L. A. Barton; Proc. IEEE; 1968; 56; 1162–1171

^[27] H. Kawamoto; Proc. IEEE; 2002; 90; 4; 466

^[28] M. Schadt, W. Helfrich; Appl. Phys. Lett.; 1971; 18; 4; 127

^[29] J. F. Dryer; Proc. 3rd Int. Liquid Crystal Conf., Berlin, Germany; 1970; 25

^[30] A. I. Snell, K. D. Mackenzie,W. E. Spear, P. G. Le Comber, A. J. Hughes; Appl. Phys.; 1981; 24; 4; 357–362

^[31] I. Springle, P. Bayley, W. Crossland, B. Needham, A. Davey; Macromol. Symp.; 2000; 154; 37-44

^[32] H. Koswig; Flüssige Kristalle; Aulis Verlag Berlin; 1985; 119

^[33] W. Elser, R. D. Ennulat; Adv. Liq. Cryst.; 1976; 2; 337

^[34] P. Saez, M. Rodriguez-Baeza, C. Aguilera; European Polymer Journal; 2005; 41; 1053-1060

^[35] Jahromi S, J Lub; Mol GN. Polymer; 1994; 35; 622

^[36] S. L. Kwolek, P. W. Morgan, J. R. Schaefen, L. W. Gulrich; Macromolecules; 1977; 10; 1390

^[37] H. Finkelmann, H. Ringsdorf, J. H. Wendorff; Makromol. Chem.; 1978; 179; 273

^[38] D. J. Broer, H. Finkelmann, K. Kondo; Makromol. Chem.; 1988; 189; 185-194

^[39] D. J. Broer, J. Boven, G. N. Mol; Makromol. Chem.; 1989; 190; 2255-2268

^[40] H. Ren, S.-T. Wu; Appl. Phys. Lett.; 2002; 81; 8; 1432 – 1434

^[41] C. H. Legge, F. J. Davis, G. R. Mitchell; J. Phys. II France; 1991; 1; 1253

^[42] J. Kupfer, H. Finkelmann; Makromol. Chem. Rapid Commun.; 1991; 12; 717

^[43] P. M. S. Roberts, G. R. Mitchell, F. J. Davis; J. Phys. II France; 1997; 7; 1337

^[44] P. Xie, R. Zhang; J. Mater. Chem.; 2005; 15; 2529-2550

^[45] E. Wiedemann; Ann. Phys.; 1888; 34; 446

^[46] P. Atkins; The Elements of Physical Chemistry; W. H. Freeman and Company; 2000; 448

^[47] T. Jüstel, H. Nikol, C. Ronda; Angew. Chem. Int. Ed.; 1998; 37; 3084

^[48] T. H. Richardson; Functional Organic and Polymeric Materials; Wiley & Sons Ltd.; 2000; 365

^[49] R. A. Vora, A. K. Prajapati; Chem. Sci.; 2001; 113; 2; 95-102

^[50] A. P. Davey, S. Elliot, O. O’Connor, W. Blau; J. Chem. Soc., Chem. Commun.; 1995; 1433-1434

^[51] R. Christiano, A. A. Vieira, F. Ely, H. Gallardo; Liqu. Cryst.; 2006; 33; 4; 381-390

^[52] S. H. Lee, T. Nakamura, T. Tsutsui; Org. Lett.; 2001; 3; 13; 2005-2007

^[53] M. P. Alred, A. J. Eastwood, S. M. Kelly, P. Valchos, A. E. A. Contoret, S. R. Farrar, B. Mansoor, M. O’Neill, W. C. Tsoi; 2004; Chem. Matter.; 16; 4928

^[54] M. P. Alred, A. E. A. Contoret, S. R. Farrar, S. M. Kelly, D. Mathieson, M. O’Neill, W. C. Tsoi, P. Valchos; Adv. Mater; 2005; 17; 1368

^[55] V. Percec, R. Yourd; Macromolecules; 1988; 21; 3379-3386

^[56] K. Sonogashira, Y. Tohda, N. Hagihara; Tetrahedron Lett.; 1975; 50; 4467-4470

^[57] S. Takahashi, Y. Kuroyama, K. Sonogashira, N. Hagihara; Synthesis; 1980; 627- 630

^[58] B. Neises, W. Steglich; Angew. Chem.; 1978; 90; 556-557

^[59] L. G. Wade; Organic Chemistry; Prentice Hall; 2003; 1; 640

• College: Fresenius University of Applied Sciences Idstein
• Grade: 2
• Author: Dipl.-Ing (FH) Philipp Gebhardt (Author)
• Publication Year: 2006
• Pages: 91
• Catalog Number: V186607
• ISBN (eBook): 9783656996859
• ISBN (Book): 9783869433073
• Language: English
• Tags: synthesis, characterisation, polymerisable, mesogens, containing, fluorene, moieties