Synthesis and Characterisation of New Polymerisable Mesogens Containing Fluorene Moieties

Gebhardt, Philipp

Synthesis and Characterisation of New Polymerisable Mesogens Containing Fluorene Moieties

Sonogashira Cross Coupling

von Philipp Gebhardt (Autor:in)

Pharmazie

Zusammenfassung

Inhaltsangabe:Introduction:
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.
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, 1H and 13C 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. Inhaltsverzeichnis:Table of Contents:
Table of Abbreviations1
1.Introduction2
1.1Liquid Crystals: Structure and Properties2
1.2Classification of Liquid Crystals6
1.3Applications of Liquid Crystals8
1.4Liquid Crystalline Polymers and their Application11
1.5Luminescence13
1.6Aim and Scope of the present Thesis15
2.Results and Discussion17
2.1Synthetic pathways to the new LC fluorene derivatives17
2.2Structural Characterisation of the fluorene derivates20
2.3Differential Scanning Calorimetry27
2.4Polarisation […]

Leseprobe

Inhaltsverzeichnis

Philipp Gebhardt

Synthesis and Characterisation of New Polymerisable Mesogens Containing Fluorene

Moieties

Sonogashira Cross Coupling

ISBN: 978-3-8366-2730-6

Herstellung: Diplomica® Verlag GmbH, Hamburg, 2009

Zugl. Europa Fachhochschule Fresenius, Idstein, Idstein, Deutschland, Diplomarbeit,

2006

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http://www.diplomica.de, Hamburg 2009

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.

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,

H and

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.

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,

H y

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.

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,

H und

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 Contents

Table of Abbreviations...1

1. Introduction...2

1.1

Liquid Crystals: Structure and Properties...2

1.2

Classification of Liquid Crystals...6

1.3

Applications of Liquid Crystals...8

1.4

Liquid Crystalline Polymers and their Application...11

1.5

Luminescence...13

1.6

Aim and Scope of the present Thesis...15

2. Results and Discussion...17

2.1

Synthetic pathways to the new LC fluorene derivatives...17

2.2 Structural Characterisation of the fluorene derivates...20

2.3

Differential Scanning Calorimetry...27

2.4 Polarisation Microscopy...28

2.5 LC properties of the fluorene derivates...29

2.6 Conclusions and Outlook...31

3. Experimental Part...34

3.1 Solvents and Materials...34

3.2 Equipment...35

3.3 Syntheses...36

5. Bibliography...47

6. Appendix...50

6.1 IR-spectroscopy...50

6.2 NMR-spectroscopy...60

6.3 DSC...80

6.4 Photos obtained by polarisation microscopy...83

Table of Abbreviations

(1)

2,7-diiodofluorene

(2)

2,7-diiodo-9,9-dihexylfluorene

(3)

2,7-di(3-hydroxy-3-methylbutynyl)-9,9-dihexylfluorene

(4)

2,7-diethynyl-9,9-dihexylfluorene

(5a)

4-iodophenyl 4-hexyloxybenzoate

(5b)

4-iodophenyl 4-dodecyloxybenzoate

(6a)

2,7-bis[4-(4-hexyloxybenzoyl)phenylethynyl]-9,9-dihexylfluorene

(6b)

2,7-bis[4-(4-dodecyloxybenzoyl)phenylethynyl]-9,9-dihexylfluorene

(7)

4-(11-hydroxyundecyl)benzoic acid

(8)

4-[11-(acryloyloxy)undecyl]benzoic acid

(9)

4-Iodphenyl 4-[11-(acryloyloxy)undecyl]benzoate

(10)

2,7-bis[4-{4-(11-acrylundecyloxy)benzoyl}phenylethynyl]-9,9-dihexylfluorene

C-N Phase Transition Crystalline to Nematic

DCM Dichloromethane

DEPT Distortionless Enhancement by Polarisation Transfer

DSC Differential Scanning Calorimetry

FT-IR Fourier Transform Infrared Spectroscopy

KBr Potassium Bromide

KOH Potassium Hydroxide

LC Liquid Crystal / Liquid Crystalline

LCD Liquid Crystal Display

NaOH Sodium Hydroxide

N-I Phase Transition Nematic to Isotropic Liquid

NMR Nuclear Magnetic Resonance

SmA Smectic A

SmC Smectic C

STN Super Twisted Nematic

TEA Triethylamine

THF Tetrahydrofuran

TLC Thin Layer Chromatography

TMS Trimethylsilane

TN Twisted Nematic

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 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 14

, 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.

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.

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° each [4]. 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 n

and ordinary refractive index n

. The birefringence value

is given by n = n

- n

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 lost [5].

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 molecules [6]. 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) display [7]. Another application is in

organic photovoltaic cells [8].

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.

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 mesogen [9]. In the illustration, C-N means

phase transition temperature from crystalline to nematic, N-I from nematic to isotropic.

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 molecule [10].

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

formed [11]. If the lateral substituents are longer flexible alkylic chains, they may decrease

transition temperatures, favour nematic phases and faciliate the mesogens movements for

their orientation [12]. 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

substances [13].

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 core [14].

Many homologous series of compounds have been studied and reviewed [15]. 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 liquid-

crystalline nanostructures are abundant in living systems. Phospholipids, for example,

exhibit lyotrophic mesomorphism in the presence of water [16].

Thermotropic LCs were classified by Friedel [17] in three general types. They are called

nematic, cholesteric and smectic. They have a parallel alignment of the molecular long-

axis in common but differ in the lateral order.

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 band [18](fig. 6):

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 molecules [19] or nematic molecules that

are mixed up with a chiral substance [20]. In these systems, molecules align in layers.

Perpendicular to the director, layers are twisted relative to each other (fig. 7):

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 order [21]. The chiral pitch refers to the distance

(along the director) over which the mesogens undergo a full 360º twist.

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 transparent [24]. White and Taylor describe the guest-host

interaction using the phase transition from cholesteric to nematic [25].

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 light [26].

Heilmeier and other RCA enginieers designed and produced the first LCD, it was based

on the dynamic-scattering mode. On May 28

, 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

Japan [27].

Fig. 9: Seven-Segment LCD

In 1971, Martin Schadt and Wolfgang Helfrich found a new workable display mechanism

with twisted nematic LCs [28]. The technology takes advantage of nematic molecules that

redirect the direction of polarisation of light by 90° along their helix arrangement [29].

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 LCD [30]).

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 picture [31].

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 temperature [32].

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 range [33].

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 in [35].

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 fibers [36].

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 chains [37] (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 diacrylates [39] (fig. 11).

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 applications [40].

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 magnetic [41] fields or mechanical treatment [42] 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 formation [43]. Interesting applications

are for example materials that change colour when stretched, and artificial muscles [44].

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 temperature [45].

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.

Fig. 11: Jablonski Diagram, electronic state transitions [46]

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].

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 systems [48]. The benzene ring

with its six -electrons may act in conjunction with electron-donating groups

Details

Seiten
Erscheinungsform: Originalausgabe
Jahr: 2006
ISBN (eBook): 9783836627306
DOI: 10.3239/9783836627306
Dateigröße: 2.6 MB
Sprache: Englisch
Institution / Hochschule: Hochschule Fresenius Idstein – Chemie
Erscheinungsdatum: 2009 (März)
Note: 2,0
Schlagworte: sonogashira nematische flüssigkristalle photolumineszenz photopolymerisation

Autor

Philipp Gebhardt (Autor:in)