Hybrid Nanoparticles of Conjugated Polymers with Multiple Incorporated Inorganic Semiconductor Particles

Hybrid Nanoparticles of Conjugated Polymers with Multiple Incorporated Inorganic Semiconductor Particles

Dissertation submitted for the degree of Doctor of Natural Sciences (Dr. rer. nat.)

Presented by Christoph Jung

At the

Universität Konstanz

Faculty of Sciences Department of Chemistry

Date of the oral examination: 15.04.2016

Die vorliegende Dissertation entstand in der Zeit von März 2010 bis Juni 2014 unter der Leitung von Herrn Prof. Dr. Stefan Mecking am Fachbereich Chemie der Universität Konstanz.

Acknowledgment / Danksagung

An erster Stelle möchte ich mich bei Prof. Dr. Stefan Mecking für die fordernde und spannende Themenstellung sowie Ideen und Hilfestellungen bedanken.

Herrn Prof. Dr. Alexander Wittemann danke ich herzlich für die Übernahme des Zweitgutachtens.

Mein Dank gilt auch Dr. Jonas Weickert aus der Gruppe von Prof. Dr. Lukas Schmidt-Mende für viele hilfreiche Diskussionen sowie für Messungen an den Solarzellen.

Großer Dank gilt ebenso meinen Bachelorstudenten und Mitarbeiterpraktikanten Arthur Groh und Ivan Zemskov, die sich mit großem Engagement an die Aufgabenstellungen herangewagt haben!

Ich danke Tjaard de Roo, Friederike Schütze und Christoph Fischer für das Korrekturlesen dieser Arbeit sowie viele hilfreiche Diskussionen auf dem Gebiet der konjugierten Polymere. Danke auch an Dr. Inaqui Göttker-Schnetmann, der auf so ziemlich jede Frage eine Antwort hatte.

An alle Serviceabteilungen des Fachbereichs Chemie der Universität Konstanz wie das Proteomics Center geht mein Dank für ihre gute Arbeit. Ein besonderes Dankeschön geht dabei an Ulrich Haunz und Anke Friemel von der NMR-Core-Facility für Hilfe und ein immer offenes Ohr was alle Belange der NMR Spektroskopie anging. Ich danke Lars Bolk für das Messen meiner DSC und GPC Proben sowie Hilfestellungen in sämtlichen IT Problemen. Vielen Dank auch an Dr. Maren Dill für Time Correlated Single Photon Counting Messungen, Yvonne Binder für TGA Messungen und Dr. Michael Krumm für Hilfestellungen beim Bau von Solarzellen.

Ein ausgesprochen großes Dankeschön geht an dieser Stelle an Dr. Marina Krumova für das Einlernen in die Transmissionselektronenmikroskopie und AFM, ihr großes Interesse und Engagement sowie viele lehrreiche Diskussionen! Robin Kirsten und Dr. Werner Röll danke ich für ein offenes Ohr bei vielen chemischen und praktischen Fragestellungen, sowie das Versorgen mit allem, was man im Labor brauchen kann.

Susan Kyncl gilt mein Dank dafür, dass sie mir den meisten Papierkram ferngehalten hat und immer ein offener Ansprechpartner für organisatorische und bürokratische Fragen war.

II

An dieser Stelle danke ich auch allen aktuellen und ehemaligen Mitgliedern der AG Mecking – Anna, Moritz, Stefan, Benjamin, Tjaard, Thomas R., Thomas W., Hannes, Philipp, Wuchi, Doro, Carla, Johannes P., Johannes H., Marius, Jussy, Alex, Patrick, Veronica, Zhongbao, Uli, Samir, Phil, Franz, Inaqui, Timo, Christoph, Robin, Susan, Marina, Lars, Werner, Nici, Hanna, Flo, Frieda und Arthur – für die angenehme Stimmung und gute Atmosphäre sowie die schöne Zeit hier in Konstanz.

Last but not least: Danke an meine Familie, meine Freunde und meine Freundin, ohne Euch wäre das nicht möglich gewesen!

Publications and Communications

Parts of this thesis have been published

Manuscripts

 Huber, J.; Jung, C.; Mecking, S. Nanoparticles of Low Optical Band Gap Conjugated Polymers, Macromolecules 2012, 45, 7799.

 Jung, C.; Krumova, M.; Mecking, S. Hybrid Nanoparticles by Step-Growth Sonogashira Coupling in Disperse Systems, Langmuir 2014, 30, 9905.

 Jung, C.; de Roo, T.; Mecking, S. Conjugated Polymer Composite Nanoparticles by Rapid Mixing, Macromol. Rapid Commun. 2014, 35, 2038.

Posters

Jung, C.; Mecking. S.: Zsigmondy-Colloquium, Konstanz/2014: “Hybrid Nanoparticles of Conjugated Polymers with Multiple Incorporated Inorganic Semiconductor Particles.”

Abstract / Zusammenfassung

Die vorliegende Arbeit beschäftigt sich mit der Entwicklung von Hybridnanopartikeln bestehend aus konjugierten Polymeren und anorganischen Halbleiternanopartikeln. Solche Materialien und Materialkombinationen sind von großem Interesse im Bereich der Optoelektronik, Labeling und Trackingverfahren sowie im Gebiet der organischen Photovoltaik.

Eine grundsätzliche Schwierigkeit bei der Herstellung von organisch- anorganischen Hybridmaterialien liegt in der Kompatibilisierung beider Materialien, da Entmischung und Phasentrennung in der Regel thermodynamisch begünstigt sind. Eine Möglichkeit diese makroskopische Entmischung verhindern zu können liegt in der Vorgabe der Morphologie der Hybridmaterialien in Form nanopartikulärer Komposite.

In diesem Zusammenhang wurden als Testsystem des Konzepts Hybridnanopartikel bestehend aus Polydiethynylfluorenen und mit Ölsäure funktionalisierten Titandioxid-Stäbchen mittel Glaser Kupplung in Miniemulsion synthetisiert. Um eine größere Vielfalt an Monomeren und damit Polymer Mikrostrukturen nutzen zu können, wurde das Konzept auf die Sonogashira Kupplungspolymerisation in wässriger Miniemulsion erweitert. Poly(arylen ethynylen)e mit verschiedenen Polymer-Mikrostrukturen und Seitenketten wurden in Gegenwart von mit Ölsäure hybrophobisierten TiO2 Partikeln sowie in Gegenwart von CdSe Quantenpunkten polymerisiert. Dabei zeigte sich dass die Mikrostruktur der Polymere großen Einfluss auf den Erfolg der Einbettung der anorganischen Partikel sowie auf die Homogenität deren Verteilung in der organischen Matrix hat. Poly(arylen ethynylen)e bestehend nur aus Fluoren basierten Monomeren zeigten eine homogene Verteilung der anorganischen Partikel im Polymer. Die Einführung von Phenylen-basierten Monomeren führte dabei zu einer inhomogeneren Verteilung der anorganischen Gastpartikel bzw. zu einem Nichteinbau im Falle von Titandioxid. Dies ist zum einen auf verstärkte Wechselwirkungen der Polymerketten untereinander durch π-π stacking der planareren Phenylringe zurückzuführen, was zu einer Verdrängung der größeren TiO2 Partikel führen kann. Ein anderer Grund wurde in der Reaktivität der Monomere gefunden, welche per NMR Spektroskopie untersucht wurde. So führt eine niedrige Polymerisationsgeschwindigkeit zu begünstigter Phasentrennung. Dies wird durch

VI

Aminen, Phosphinoxiden und Carbonsäuren konnte im Falle von Phosphinoxiden keinen positiven Effekt auf Einbau und Homogenität der Hybridpartikel erzielen. Aminogruppen sorgen für eine homogenere Verteilung von CdSe in den Polymerpartikeln, haben aber keinen positiven Einfluss auf Titandioxid. Carbonsäure-funktionalisierte Polymere konnten keinen Einbau von Titandioxid verzeichnen, was der niedrigen Reaktivität dieser Monomere geschuldet ist. Dieses Carbonsäure funktionalisierte Monomer konnte in der Synthese von Titandioxid Partikeln als polymerisierbarer Ligand eingesetzt werden, womit eine kovalente Anbindung der Polymerketten an die Oberfläche und damit eine Verbesserung der Wechselwirkungen zwischen organischer und anorganischer Phase gewährleistet werden konnte.

Da sich die Reaktivität der Monomere und damit die Geschwindigkeit der Bildung der Polymerpartikel als kritische Größe herausstellte, wurde ein Sekundärdispersionsansatz untersucht. Dabei wurden Lösungen aus Polymer und anorganischen Partikeln in wässrigen Tensidlösungen emulgiert, was eine schnellere Partikelbildung zur Folge hat. Poly(arlyen ethynylen)e sind für dieses Verfahren aufgrund ihrer niedrigen Löslichkeit wenig geeignet, sodass eine Reihe an kommerziellen Polymeren wie MEH-PPV oder mittels Suzuki Kupplung synthetisierter Polymere wie Polyfluoren und Derivate davon (PFTBT) eingesetzt wurden. Kompositpartikel konnten für die untersuchten Verbindungen sowohl für TiO2 als auch CdSe mit diesem Ansatz erhalten werden. Energieübertragungsprozesse vom Polymer auf CdSe Quantenpunkte konnten anhand der Quantenausbeuten und Fluoreszenzlebensdauern festgestellt werden.

Die Dispersionen ließen sich zu geschlossenen Filmen mit einer homogenen Verteilung der anorganischen Partikel in der organischen Matrix verarbeiten. Im Falle von MEH- PPV konnten sowohl für TiO2 als auch für CdSe Prototypen von Hybridsolarzellen aus Dispersion realisiert werden.

Um einen größeren Einfluss auf die Kinetik der Partikelbildung wurden Hybridpartikel mittels schnellem Mischen von THF Lösungen aus Polyfluoren bzw.

F8BT und anorganischen Partikeln wie TiO2, CdSe und CdSe/CdS mit wässrigen SDS Lösungen in einer eigens angefertigten Mischapparatur hergestellt. Dabei hängt die Mischeffizienz und damit die Geschwindigkeit der Partikelbildung von den Flussgeschwindigkeiten der organischen und wässrigen Ströme ab. Im Bereich turbulenter Strömung werden kleine (< 50 nm) und homogene Hybridnanopartikel erhalten wogegen im Bereich laminarer Strömung größere und inhomogenere bis phasengetrennte Partikel erhalten werden. Für diese kleinen Hybridpartikel kann eine

starke Verminderung der Polymerfluoreszenz bei nahezu ausschließlicher Emission seitens der Quantenpunkte für Komposite aus F8BT/CdSe/CdS und Polyfluoren/CdSe/CdS und damit ein Energieübertrag vom Polymer auf die Quantenpunkte beobachtet werden.

Fluoreszente Nanopartikel die im roten bis nahinfraroten Bereich des Spektrums emittieren sind interessant für Markierungs- und Detektionsanwendungen beispielsweise in biologischen Systemen. Vor diesem Hintergrund konnten rot emittierende Polymerpartikel durch Einbau eines polymerisierbaren Perylenmonomers mittels Miniemulsionspolymerisation in konjugierte Polymerpartikel erreicht werden. Ein vollständiger Energieübertrag vom Polymerrückgrad auf die Perylen Fluorophore konnte bereits mit Peryleneinbauraten von 1 mol% erreicht werden. Selbst-stabilisierte rot emittierende Polymerpartikel wurden in Form eines Triblockcopolymers bestehend aus einem konjugierten Benzothiadiazol-Fluoren Block mit PEG Endfunktionalisierung synthetisiert. Der konjugierte Block wurde dabei durch Wahl einer geeigneten Stöchiometrie mit Alkin Endgruppen aufgebaut und durch eine Azid-Alkin „Klick“

Reaktion mit PEG-Aziden verknüpft. 55 nm große selbststabilisierte Polymerpartikel mit einem Fluoreszenzmaximum bei 672 nm und einer Fluoreszenzquantenausbeute von 10%

konnten auf diese Weise erhalten werden.

I. Table of Contents

1 Introduction ... 9

1.1 Organic-Inorganic Hybrid Materials ... 9

1.2 Methods to Prepare Hybrid Nanoparticles in Dispersion ... 14

1.2.1 Miniemulsion Polymerization ... 14

1.2.2 Post Polymerization Dispersion ... 17

1.2.3 Nanoprecipitation ... 18

1.3 Inorganic Semiconductors ... 20

1.3.1 Metal Oxide Semiconductors ... 22

1.3.2 Quantum Dots ... 23

1.4 Conjugated Polymers ... 25

1.5 Synthetic Protocols to Conjugated Polymers ... 31

1.5.1 Sonogashira Coupling ... 31

1.5.2 Glaser Coupling ... 34

1.5.3 Suzuki-Miyaura Coupling ... 35

2 Scope of the Thesis ... 39

3 Results and Discussion ... 41

3.1 Nanocomposites of Rigid, Insoluble Polymers by Miniemulsion Polymerization ... 41

3.1.1 Titanium Dioxide Nanoparticles with Oleic Acid as Ligand ... 41

3.1.2 Titanium Dioxide Nanoparticles with a Polymerizable Ligand ... 43

3.1.3 Monomers with Functional Groups ... 44

3.1.4 Composites with Titanium Dioxide by Glaser Coupling in Miniemulsion ... 49

3.1.5 Poly(Arylene Ethynylene) Composites with Titanium Dioxide by Sonogashira Coupling in Miniemulsion ... 54

3.1.6 Poly(Arylene Ethynylene) Composites with Cadmium Selenide ... 58

3.1.7 Kinetic Investigations of Monomer Reactivity ... 61

2

3.1.9 Summary ... 70

3.2 Nanocomposites by Post Polymerization Dispersion from Emulsions . 71 3.2.1 Composites with Titanium Dioxide ... 71

3.2.2 Composites with Cadmium Selenide ... 77

3.2.3 Summary ... 82

3.3 Composites of Polyfluorene and Benzothiadiazole Derived Polymers with Inorganic Nanoparticles by Nanoprecipitation ... 83

3.3.1 Manual Precipitation via Syringe ... 83

3.3.2 Nanoprecipitation in a Multi-Inlet Vortex Mixer ... 88

3.3.3 Size Control and Influence of the Surfactant ... 89

3.3.4 Composites of Poly(9,9-dioctylfluorene-alt-benzothiadiazole), Titanium Dioxide and Cadmium Selenide ... 92

3.3.5 Composites of Poly(9,9-di-n-hexylfluorenyl-2,7-diyl) and Poly(9,9- dioctylfluorene-alt-benzothiadiazole) with Cadmium Selenide/Cadmium Sulfide and Optical Properties ... 96

3.4 Luminescent Polymer Nanoparticles with Small Band-gaps ... 98

3.4.1 Perylene Based Polymers ... 99

3.4.2 Self-Stabilized Low Band-gap Polymer Nanoparticles ... 102

3.5 Polymer Antenna with Perylene Linker ... 106

4 Summary ... 111

5 Experimental Section ... 115

5.1 Materials and General Considerations ... 115

5.1.1 General Synthetic Procedures ... 115

5.1.2 Solvents and Reagents ... 115

5.1.3 NMR Spectroscopy ... 115

5.1.4 IR Spectroscopy ... 115

5.1.5 UV-Vis Spectroscopy ... 116

5.1.6 Fluorescence Spectroscopy and Quantum Yields ... 116

5.1.7 Time-Correlated Single Photon Counting ... 116

5.1.8 Transmission Electron Microscopy ... 116

5.1.9 Dynamic Light Scattering ... 116

5.1.10 Mass Spectrometry ... 116

5.1.11 Differential Scanning Calorimetry ... 117

5.1.12 Gel Permeation Chromatography ... 117

5.1.13 Atomic Force Microscopy ... 117

5.1.14 Scanning Electron Microscopy ... 117

5.1.15 Photovoltaic Device Characterization ... 117

5.2 Monomer Synthesis ... 118

5.3 Miniemulsion Polymerizations ... 144

5.3.1 Glaser Coupling Polymerizations ... 144

5.3.2 Sonogashira Coupling Polymerization ... 145

5.4 Polymerizations in Solution ... 145

5.4.1 Sonogashira Coupling ... 145

5.4.2 Suzuki Coupling ... 145

5.5 Post Polymerization Dispersion by Solvent Evaporation ... 146

5.6 Nanoprecipitation ... 146

5.6.1 Manual Nanoprecipitation via Syringe ... 146

5.6.2 Nanoprecipitation in a Multi-Inlet Vortex Mixer ... 147

5.7 Blockcopolymer Synthesis ... 147

5.8 Antenna Synthesis ... 148

5.9 Synthesis of Titanium Dioxide Nanoparticles ... 153

5.9.1 Titanium Dioxide Particles with Oleic Acid as a Ligand ... 153

5.9.2 Titanium Dioxide Particles with a Polymerizable Ligand ... 153

5.10 Synthesis of Cadmium Selenide Quantum Dots ... 153

5.11 Photovoltaic Device Fabrication ... 154

6 References ... 155

7 Appendix ... 167

4

II. Annotations

Abbreviations

AFM atomic force microscopy

Ar aryl

BHJ bulk heterojunction

BuLi n-butyllithium

δ chemical shift in ppm

DLS dynamic light scattering

DMF dimethylformamide dnbpy 4,4′-dinonyl-2,2′-dipyridyl

DPn degree of polymerization

DSC differential scanning calorimetry

DSSC dye sensitized solar cell

Et ethyl EtHex 2-ethylhexyl eq. equivalent(s) F8BT poly(9,9-dioctylfluorene-alt-

benzothiadiazole)

GPC gel permeation chromatography

HOMO highest occupied molecular orbital

iPr iso-propyl

IR infrared

ITO indium tin oxide

LUMO lowest unoccupied molecular orbital

MALDI-TOF matrix-assisted laser

desorption/ionization – time of flight

MIVM Multi-inlet vortex mixer

MEH-PPV poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene]

Mn number average molecular weight

∑

∑

Mw weight average molecular weight

∑

∑

MWD molecular weight distribution Mw/Mn

n.d. not determined

NIR near infrared

NMP N-methyl-2-pyrrolidone

NMR nuclear magnetic resonance

NP nanoparticle Ph phenyl

OPV organic photovoltaic

P3HT poly(3-hexylthiophene-2,5-diyl)

PAE poly(arylene ethynylene)

PCE power conversion efficiency

PDI polydispersity index

PEDOT:PSS poly(3,4-ethylenedioxythiophene):

polystyrene sulfonate

PEG polyethylene glycol

PF polyfluorene PFTBT poly(9,9-dioctylfluorene)-co-(4,7-di-2-

thienyl-2,1,3-benzothiadiazole)

PPE poly(phenylene ethynylene)

ppm parts per million

PPP poly(p-phenylene)

PS polystyrene PPV poly(p-phenylenevinylene)

QD quantum dot

QY (fluorescence) quantum yield

Re Reynolds number

rpm revolutions per minute

RT room temperature

6

tBu tert-butyl

TEM transmission electron microscopy

THF tetrahydrofuran

TLC thin layer chromatography

TMAO trimethylamine N-oxide

TMSA trimethylsilylacetylene

TOPO trioctylphosphine oxide

TTAB tetradecyltrimethyl ammonium bromide

UV-Vis ultra violet - visible

wt% weight percent

Overview of Monomers

Introduction: Organic-Inorganic Hybrid Materials

1 Introduction

1.1 Organic-Inorganic Hybrid Materials

Organic-inorganic hybrid materials are a fascinating field of research due to the vast possibilities to combine and control properties and compositions. This promises the potential to generate hybrid materials with properties which exceed the sum of the individual material contributions.¹ Inspiration for many chemists, physicists and biologists comes from nature where many types of hybrid materials have been applied for millions of years to achieve optimal balance between mechanical properties, durability or other features such as hydrophobicity or permeability. ²

Hybrid materials can be broadly described as (nano)composites with intimately mixed organic and inorganic components. Even though man-made hybrid materials date back thousands of years – Maya blue for example is an organic-inorganic hybrid material that consists of blue indigo pigments and inorganic clay palygorskite³ - industrial development and production started as late as in the middle of the nineteenth century.

Examples are natural polymers such as casein, hydroxyethylated starch or gum arabicum as binders mixed with inorganic components as kaolin or clay. These mixtures were used in various domains such as paints or paper coatings.⁴ One of the oldest and most commonly available product made from hybrid materials is a rubber tire. Performance characteristics such as abrasion resistance are enhanced by reinforcing natural and synthetic polymers like natural rubber and styrene-butadiene rubber with inorganic particle such as carbon black and silica.⁵

During the last sixty years, the field of hybrid materials has boomed (Figure 1.1) and great progress has been made in controlling texture and composition of composite materials leading to first industrial applications. Coatings are the most widespread application of hybrid materials to date due to the low cost and ease of processing in a well implemented and understood technology. Common household articles like non-stick coatings based on hybrid materials for cookware or ironing have entered the market,^6,7 where longevity and stability over conventional polytetrafluoroethylene (PTFE) coatings were improved by the SEB company by the use of fluorinated silanes and micro-

8,9

Introduction: Organic-Inorganic Hybrid Materials

10

in LED devices by Sony.¹² Hybrid materials have also found their way into human care and health care industries.

Figure 1.1 Overview of hybrid materials in research and industry. Design taken from reference 13.

Hybrid organic-silica based polymers developed by Fraunhofer ISC are used in the Admira® series as dental fillers and other medicinal applications.¹⁴ These materials highlight the range of properties accessible by tuning the composition of hybrid compounds as these products can be used for protective coatings, anticorrosion,

Introduction: Organic-Inorganic Hybrid Materials

polymeric ion conductors, optics or barrier coatings. Other commercial fields are hair cosmetics (Intra-Cylane™ by L`Oréal) or UV absorbers in sunscreens and skincare.

Strong research efforts are put into the new and growing field of nanomedicine.

Among the most promising applications are materials for a controlled release of biologically active compounds. Examples are mesoporous silica and organosilica materials suitable for tissue engineering or bone regeneration.^15,16 The incorporation of bisphosphonates into mesoporous silica may slow down or inhibit bone resorption in osteoporotic bones and could be employed in transplantable devices and materials.¹⁷ Some of these smart hybrid nanomaterials have already entered the market in recent years.¹⁸ Bio-based hybrids are being actively developed for biosensing applications or bioreactors, taking advantage of the extraordinary effectiveness and activity of enzymes.

Active biocompounds are immobilized on solids to grant them recyclability and reduce leaching of the bio-molecules such as lipases that can be used for esterification and transesterification reactions in the refining of fats and oils.¹⁹

General synthetic strategies to obtain hybrid materials can be grouped into three main chemical paths.²⁰ One route is heavily based on classical sol-gel and soft chemistry.

The synthesis of such materials revolves around molecular precursors such as metal salts, multifunctional ligands, organosilanes or organically modified metal alkoxides.

Hydrolysis or hydrothermal synthesis forms hybrid networks with incorporated organic polymers which are present during the condensation process. This method is simple, versatile and cheap, yielding usually amorphous nanocomposites suitable for example for functional coatings. Self-assembly and template synthesis enables greater control over micro- and nanostructures.

A second common strategy employs the hybridization of nanoscale building blocks.

This allows a well defined and characterized inorganic structure (in contrast to amorphous materials synthesized by sol-gel chemistry) due to the availability of pre-synthesized nanoscale objects such as functionalized nanoparticles (metals, metal oxides, chalcogenides), clusters or clays.^21-25 These compounds are then dispersed in organic network-modifiers or -formers.

The third major procedure relies on the self-assembly of amphiphilic polymers or molecules in combination with sol-gel polymerization. Prime examples for this strategy

Introduction: Organic-Inorganic Hybrid Materials

12

combination of different approaches such as the template assisted self-assembly of nanoscale building blocks.^27,28

Hybrid Particles

In principle, a hybrid material or hybrid particle is simply a material that includes two moieties blended on a nanoscale level with distinct phases. In the context of this work, hybrid materials and particles will imply that these moieties consist of an organic and an inorganic part, so that organic-inorganic hybrid particles can be defined as colloidal particles which contain organic and inorganic domains.²⁹ The preparation of colloidal dispersions of hybrid particles is particularly useful to resolve issues of reproducibility and phase segregation behavior, which is more commonly observed for melt or solution processed hybrid materials. Generally, hybrid particles can be grouped by the method of preparation. Three different synthetic routes can be distinguished: 1) Hybrid colloids constructed by the assembly of preformed organic and inorganic building blocks; 2) Colloids produced by in situ polymerization of organic/inorganic precursors in the presence of organic/inorganic particles; 3) Simultaneous reaction of organic and inorganic precursors.

Most common examples of composite particles are based on approach number two, the polymerization in the presence of the pre-formed inorganic part. Different types of particle morphologies are possible, depending on the synthetic process and the materials involved (Figure 1.2). Raspberry-like or “currant bun” structures can be obtained, where the inorganic material is located on the surface or dispersed throughout the organic phase, respectively, as well as core-shell particles with either an organic or inorganic core are accessible.

Figure 1.2 Schematic illustration of possible hybrid particle morphologies.

Introduction: Organic-Inorganic Hybrid Materials

Composites of this type were obtained early on by electrochemical polymerization of aniline, pyrrole³⁰ or 3,4-ethylenedioxythiophene³¹ in the presence of silica in water to obtain conducting silica particles. The concept was then quickly expanded onto vinylic monomers, which are today the dominant group of monomers employed in colloidal hybrid particle synthesis. Surfactant free colloidal hybrids were presented by free-radical polymerization of 4-vinylpyridine and silica due to its water solubility and acid-base interaction with the silica surface.³² 4-Vinylpyridine was gradually replaced by commodity monomers like styrene and acrylates³³ where additional strategies like grafting from the inorganic particle surface could be employed. Depending on the types of monomers, the glass transition temperature can be adjusted, so that film forming silica hybrids can be obtained. A commercial example for an acrylic/silica composite particle dispersion is Col.9® by BASF, a hybrid coating that improves soiling resistance, color retention and surface hardness.³⁴

An important procedure in the preparation of hybrid colloids is the utilization of the emulsion polymerization and miniemulsion polymerization techniques (vide infra). In this context, various types of inorganic fillers have been embedded into polymeric particles. Hydrophobic or hydrophobized solids can be encapsulated by miniemulsion polymerization (hybridization of nanoscale inorganic particles within an organic matrix).

Silver particles,³⁵ titanium dioxide pigments,³⁶ silica particles³⁷ or CdSe quantum dots³⁸ could be encapsulated by emulsion polymerization of vinylic monomers. The possibilities to combine materials with different properties are also desirable for biomedical applications. Incorporating a fluorescent dye and magnetite particles into a polystyrene/polyacrylic acid particle allows for the dual detection by MRI and fluorescence microscopy in cells.³⁹ Functional particles are interesting candidates for drug delivery, cell imaging or hyperthermic treatment.

In recent years, coating quantum dots with polymer shells has attracted considerable interest. These coatings improve the stability and biocompatibility of the quantum dots and can enable energy transfer processes when conjugated polymers are employed, thus enlarging the field of potential applications. Coating strategies include emulsion polymerization as mentioned above, miniemulsion polymerization,⁴⁰ attachment of polymerizable groups and subsequent polymerization from the surface,⁴¹ ligand exchange

Introduction: Methods to Prepare Hybrid Nanoparticles in Dispersion

14

chemical modification for example to introduce biological recognition markers for cellular imaging.⁴⁴

The vast majority of procedures yielding hybrid colloids via in situ polymerization rely on free-radical polymerization of vinylic monomers. This excludes a wide range of polymers typically synthesized by catalytic polymerization techniques such as many polyolefins or conjugated polymers. There are only a few examples for successful syntheses of composites by non-radical in situ polymerization, including polyethylene/silica composites prepared by emulsion polymerization⁴⁵ and the embedding of single CdSe QDs into polyfluorene particles by miniemulsion polymerization.⁴⁶

1.2 Methods to Prepare Hybrid Nanoparticles in Dispersion

There are many different techniques to form colloidal hybrid particles, starting from seeded polymerizations in aqueous media, as briefly mentioned above, over emulsion polymerizations, in situ polymerizations of inorganic and organic precursor molecules to blending techniques. In the following sections though, three methods which are relevant to the content of this work will be discussed: miniemulsion polymerizations, a solvent evaporation technique and a controlled precipitation protocol.

1.2.1 Miniemulsion Polymerization

Emulsions are dispersed systems with liquid droplets dispersed in another immiscible liquid. Traditional emulsions (‘macroemulsions’) contain monomer droplets

> 1 µm in the initial reaction mixture. These droplets are not stable on the time scale of a polymerization experiment, they continuously coalesce and at the same time new droplets are generated by stirring. By contrast, in miniemulsions the droplets ideally preserve their identity. They are metastable over prolonged periods of time. Different to a

‘macroemulsion’, miniemulsions contain small droplets generated by an initial high shear and Ostwald ripening is suppressed by an additional hydrophobe (vide infra).

The miniemulsion polymerization is a special case of the emulsion polymerization and in some respect similar to a suspension polymerization. In contrast to classical emulsion polymerization, the initiation of the polymerization does not occur in the continuous phase (e.g. water) but in the monomer droplets. Under ideal conditions, the final polymer particles reflect the initial droplet distribution and composition.

Introduction: Methods to Prepare Hybrid Nanoparticles in Dispersion

Miniemulsification of the oil phase leads to droplets in the size range between 50 and several hundred nanometers, depending on the oil to water ratio, type of oil and the amount of surfactant. Since this process is associated with a large increase of interphase surface, a high energy emulsification technique is required. In the laboratory scale, ultrasonication is commonly used while high-pressure homogenizers or rotor-stator dispersers are employed to manufacture larger quantities of miniemulsions.⁴⁷ The increase in specific surface energy has to be accommodated for by the use of a surfactant, stabilizing the droplets against the thermodynamically favored coalescence.

The stabilization of the emulsion by the surfactant can be achieved either by electrostatic stabilization or by steric stabilization. The latter is typically performed by the adsorption of amphiphilic block-copolymers. Colloidal stability is based on the entropic repulsion of the surface-attached chains when monomer droplets or polymer particles come into close proximity. Due to the more favored mixing enthalpy and entropy of the block-copolymer and the solvent (water), colloidal stability is provided by the de- mixing of the “coating shells”. The process is temperature sensitive, leading to a destabilization at the θ-temperature (the temperature where the excess chemical potential of mixing is zero, i.e. the free energy of mixing Δ 0).

Electrostatic stabilization is based on the adsorption of charged molecules onto the surface of the droplets, which can be either cationic or anionic surfactants. The surface charges of the droplets lead to an electrostatic repulsion which prevents coalescence of the droplets or particles. The stability of a dispersion stabilized in such a manner can be described by the DLVO theory which accounts for the attractive van der Waals forces which destabilize the system and the repulsive electrostatic forces which contribute to the stability of the dispersion. The total interaction potential between two particles is given by the sum of the attractive and repulsive potentials:

The attractive potential between two spherical particles of a radius R is described by

∙ 12

where A is the Hamaker constant for the particle/medium pair and x is the distance between the two particles. The repulsive forces are then described by the repulsion between two electrical double layers of the particles. Surface charges introduced by the

Introduction: Methods to Prepare Hybrid Nanoparticles in Dispersion

16

that an electrical double layer is formed. The variation of the electrical potential within the double layer between two charged spheres can then be expressed as

∙32 ∙

∙ ∙ with

1 1 and

where ε and ε0 are the relative permittivity and the permittivity of the vacuum, respectively; z is the ion charge, is the elementary charge; x is the distance between the particles; R the radius of the particles; ψ0 is the potential at the surface and is the inverse of the Debye screening length. Combining both expressions gives

∙32 ∙

∙ ∙ ∙

12

which describes the interaction of two charged particles through a liquid medium. A graphical representation is given in Figure 1.3.

Figure 1.3 Schematic potential diagram of the interaction between two equally charged particles.

The sum of both potentials leads to a secondary minimum, a point of reversible aggregation called flocculation. The higher the energy barrier before the primary minimum is reached, the more stable is the dispersion. Once the two particles reach a close, critical distance, the attractive van der Waals forces dominate the total potential

Introduction: Methods to Prepare Hybrid Nanoparticles in Dispersion

and the colloids are coagulated irreversibly. The strength of the stabilizing electrical potential decreases exponentially and is further dependent on the ion concentration in the dispersant. Increasing the salt concentration decreases the Debye length, caused by the increased amount of ions shielding the surface charges, effectively decreasing the stabilizing effect of the electrostatic repulsion.

Miniemulsions typically not only need to be stabilized against coagulation and flocculation. Especially in the early stages of the polymerization, the diffusion of monomer between droplets through the aqueous phase leads to a change in droplet or particle size and number (Ostwald ripening). The driving force behind this process is the decrease in surface energy which is achieved by the growth of larger droplets at the expense of smaller ones. This undesired diffusion can be suppressed by the addition of a superhydrophobe, a long chain alkane such as hexadecane. An osmotic pressure is created, counteracting the diffusion of the monomers.

In order to incorporate hydrophilic inorganic particles like calcium carbonate, titanium dioxide, magnetite or silica in a miniemulsion polymerization process utilizing hydrophobic polymers, a hydrophobization of the particle surfaces is required.⁴⁸ They are then mixed with the monomers, initiator and if required additional stabilizers. This hydrophobic phase is then well dispersed in an aqueous medium and the polymerization occurs within the droplets (Figure 1.4).

Figure 1.4 Schematic miniemulsion polymerization process in the presence of inorganic nanoparticles.

1.2.2 Post Polymerization Dispersion

A common technique to prepare polymeric nanoparticles is based on the dispersion

Introduction: Methods to Prepare Hybrid Nanoparticles in Dispersion

18

emulsion droplets. In contrast to emulsion polymerization or miniemulsion polymerization, the solvent evaporation process offers several advantages on a laboratory scale: absence of potentially interfering monomer residues, catalysts, transfer agents or initiators. Additionally, substances or nanoparticles co-dissolved in the hydrophobic phase can be incorporated into the polymer particles through this approach. The purification and characterization of the materials employed before emulsification also allows for a better defined material composition. However, the size distribution of particles obtained by solvent evaporation is typically broader than for heterophase polymerizations. This is very likely due to the preparation process itself and not attributed to coalescence of the droplets and particles.⁴⁹ Particle sizes and the encapsulation process depend on a number of factors, e.g. nature of the substance to encapsulate, type of polymer, its concentration and molecular weights, temperature, emulsification efficiency and viscosities.

The versatility of the technique can be demonstrated by a variety of particle systems presented. Using pre-formed polymers gives access to nanoparticles which cannot be prepared by other means or only with greater synthetic effort. Biodegradable particles for drug delivery,⁵⁰ anisotropic magnetic Janus particles,⁵¹ magnetic nanosensors⁵² or fluorescent superparamagnetic polylactide nanoparticles⁵³ are just a small excerpt of the materials that can be obtained.

1.2.3 Nanoprecipitation

Sub-micron sized particles and composite particles can be obtained through a rapid precipitation process. The compound to be precipitated is dissolved in a good solvent and a second miscible non-solvent is added. The most common pair is THF and water. The mixing of both solvents leads to a decrease in overall solvent quality for the dissolved compounds, causing them to precipitate. Small particles are formed when the mixing occurs fast and the resulting particles are self-stabilized or stabilized by a surfactant or amphiphilic polymers.

The most basic method to perform a nanoprecipitation is to add a THF solution of the compound(s) to rapidly stirred water via a syringe. At low concentrations, stable nanoparticles can be obtained without the need of an additional stabilizer as claimed for conjugated polymer nanoparticles.⁵⁴ Colloidal stabilization is likely due to impurities in the water or surface charges on the nanoparticles originating from chemical defects.⁵⁵ Upon employing suitably functionalized polymers, magnetic organic-inorganic

Introduction: Methods to Prepare Hybrid Nanoparticles in Dispersion

composite particles for nanotherapeutics can be obtained by this emulsifier free process.⁵⁶ Molecular weights and polymer concentrations in the solvent greatly influence the resulting particle size of the composite materials. Stabilization provided by block- copolymers in conjunction with functionalized polymers allowed for the encapsulation of CdSe QDs for bioimaging and labeling purposes.⁴³

A larger degree of control over the mixing process can be gained by customized mixing devices. Controlled mixing in a microfluidic device was presented to encapsulate fluorescent dye molecules into amphiphilic conjugated block-copolymer particles.⁵⁷ Flow focusing a THF solution stream of the polymer with two aqueous streams leads to a controlled mixing process at the interphase of both liquids. Uniform particle sizes can be obtained by this method. In this procedure, solvent mixing between the two streams – which are in the laminar flow regime – is relatively slow. Likely, this step determines the rate of particle formation.

Since organic-inorganic composite formation is generally thermodynamically disfavored and often results in aggregation of the inorganic compounds or phase separation, especially when the material compatibility is poor, a fast mixing process is required to achieve kinetic control. A “flash” nanoprecipitation technique presented by the group of Prud’homme is able to conduct mixing within milliseconds.⁵⁸ Kinetically arresting particles is possible when the mixing speed is higher than particle growth and nucleation.^59,60 Rapid mixing is achieved in a multi-inlet vortex mixer (MIVM). The device consists of four inlet streams arranged tangentially to a circular mixing chamber with a centered outlet (Figure 1.5).

Figure 1.5 Schematic drawing of the mixing chamber of a MIVM.

Introduction: Inorganic Semiconductors

20

flow velocities, a vortex is formed in the mixing chamber with turbulent flow characteristics. Supersaturation is achieved by a change of solvent quality from good (THF) to poor (water) within milliseconds. Reynolds numbers over 6000 can be reached with this setup, promoting very fast and efficient mixing of the two solutions.

1.3 Inorganic Semiconductors

The perhaps most important and most thoroughly investigated semiconductor is doped silicon, finding application in e.g. computer chip manufacturing and photovoltaic panels. In this chapter though, two classes of inorganic semiconductors shall be discussed which are relevant to the contents of this work: semiconductors based on metal oxides such as tin oxide, zinc oxide or titanium oxide and semiconductors used for the preparation of quantum dots, mainly based on II – VI group elements such as GaAs or CdSe.

In extended inorganic solids, atomic orbitals overlap and form continuous electronic energy levels known as bands.⁶¹ Depending on the structure of the bands, materials can be classified as conductors, semiconductors or insulators (Figure 1.6).

Metals are conductors being characterized by a partially or completely filled band;

semiconductors possess a filled valence band and a usually empty conduction band. In principle, the latter also applies to insulators, the main difference being the energetic gap between valence band and conduction band. Values for the band-gaps of semiconductors are usually between 0.5 and 3.5 eV while band-gaps for insulators are greater than 4 eV.

The conductive properties depend on the band-gap and can be strongly influenced by doping processes. Here, impurities are intentionally introduced to modify the electronic properties of the pure semiconductor by forming additional energetic states between valence band and conduction band in the energy band model.

Introduction: Inorganic Semiconductors

Figure 1.6 Simplified energy diagram of conductors, semiconductors and insulators. Filled boxes represent filled valence bands and the empty boxes the empty conduction bands. The length of the arrows represent the size of the band-gap.

Two types of doping agents are distinguished. Introducing electron donating impurities is called n-type doping, where atoms containing an additional valence electron are introduced. The surplus electrons are only loosely bound and highly mobile upon applying a voltage. A located, immobile positive charge is retained at the doping atom, rendering the charge balance neutral. An example would be the doping of silicon with phosphorous atoms. The reverse case is p-type doping with electron deficient acceptor atoms such as aluminum or boron in silicon. When a voltage is applied, a silicon-bound electron can jump to these acceptor atoms, effectively generating a moving “hole”, acting as a mobile positive charge carrier. A negative, immobile charge is effectively generated at the doping atom. The degrees of doping are typically in the parts per million range but influence the electronic properties strongly. N-type doping introduces energy states located slightly below the conduction band, reducing the amount of energy required to raise an electron into the conduction band. On the other hand, p-type doping introduces energy levels slightly above the valence band. The energy gap between these states and the nearest band is very small, so that the thermal energy at room temperature is sufficient to ionize the dopant atoms and free charge carriers are generated in the respective bands.

Doping not only leads to higher charge carrier numbers in semiconductors, but it also changes the position of the bands relative to the Fermi level EF. The Fermi level is

Introduction: Inorganic Semiconductors

22

state at the Fermi level (ϵ = EF), then this state will have a chance of being occupied 50%

of the time. In an insulator, the Fermi level lies in the band-gap, away from any states that could potentially carry charges, thus rendering the material non-conducting. In a conductor, EF lies within a band so that a large number of states are near that energy level and able to carry charges. For a semiconductor, EF lies close to a band edge so that a number of carriers that are thermally excited are found close to that band edge. The exact position can be influenced by the amount and type of doping.

1.3.1 Metal Oxide Semiconductors

The most common metal oxide (MO) semiconductors are transition metal oxides such as ZnO, SnO2, In2O3 or TiO2. In contrast to semiconductors like Si or Ge, their chemical bonding shows a high degree of ionic character. The band-gaps for those materials are typically between 3.0 and 3.5 eV, leading to a rather low conductivity at room temperature but a high transparency on the other hand. As discussed in the preceding section, doping changes the energetic level of conduction and valence bands in respect to the Fermi level. An illustrative example is tin-doped indium oxide (ITO) which is extensively used as a transparent electrode in displays and organic photovoltaic devices. Its large band-gap of >3 eV gives the material a high transparency in the full range of the visible spectrum while the high doping with tin leads to a Fermi level that is positioned well within the conduction band, giving rise to a high amount of charge carriers even at room temperature.⁶²

With the exception of indium, most metal oxides listed above are cheap and readily available. This makes the materials interesting for a range of applications. Especially TiO2

is used in a variety of different areas, ranging from pigments over water photoelectrolysis, photocatalysis and photovoltaic devices. An exemplary use is the catalytic reduction of carbon dioxide to methane through irradiation on a TiO2 surface.⁶³ The photocatalytic properties of TiO2 are additionally exploited for the oxygen-mediated oxidation of organic substances, an important topic in water purification and degradation of toxic pollutants.⁶⁴ The oxidative nature of TiO2 under UV illumination is attributed to the strong oxidizing power of the hole generated in the valence band (3.0 V vs. NHE).⁶⁵

For photovoltaic applications, a prominent much studied device based on metal oxide semiconductors is the dye-sensitized solar cell (DSSC).^66,67 The operation principle of a DSSC features a metal oxide semiconductor (e.g. TiO2 in the anatase modification due to a higher charge carrier mobility) which is in contact with a redox electrolyte or an

Introduction: Inorganic Semiconductors

organic hole conductor. A monolayer of a sensitizer is attached to the surface of the semiconductor and the structure is sandwiched between two transparent electrodes such as ITO or fluorine-doped tin oxide (FTO). Photoexcitation of the sensitizer dye leads to the injection of a charge into the conduction band of the metal oxide. The dye is then regenerated by the redox couple electrolyte which is in turn regenerated at the counter electrode, leading to no permanent chemical change in the system. Power conversion efficiencies (PCE) have exceeded 11 %⁶⁸ and first modules are close to commercialization.⁶⁹ Applications in BHJ solar cells are yet less successful in terms of application by comparison. Blends from sintered mesoporous titania particles and P3HT could achieve PCEs of 2.5 %⁷⁰ while ZnO nanoparticles blended with P3HT⁷¹ and poly(2- methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV)⁷² afforded efficiencies of 0.9% and 1.5%, respectively. Several concepts aimed at improving PCEs have been reported, using optimized dye-polymer couples to sensitize the MO semiconductors⁷³ or to control the structure of the MO layer to improve charge transport properties, e.g. by growing nanowires via a hydrothermal growth process.⁷⁴

1.3.2 Quantum Dots

Quantum dots are semiconductors or certain metal nanoparticles with all three dimensions in the range of 1-10 nm. In this size domain, the particles show quantum mechanical effects not present in the bulk material. One can imagine the quantum dots having a band structure (Figure 1.7) intermediate between bands (as in bulk semiconductors) and bonds (as in molecules). As a consequence, the band-gap Eg is correlated with the particle size and is no longer a material constant. This phenomenon is commonly referred to as the quantum size effect. The band-gap can be mathematically related to the particle size by an effective mass model:⁷⁵

8

1 1

1.8 ²/4 ₀

Eg is the band-gap of the material, R the particle radius, me the effective mass of the electron in the solid, mh the effective mass of the hole in the solid and ε the dielectric constant of the material.

The quantum size effect is not strictly limited to semiconductors but also occurs in noble metal nanoparticles, predominately gold and silver particles. This behavior is based

Introduction: Inorganic Semiconductors

24

Figure 1.7 Schematic energy level diagram and band structure of bulk semiconductors, molecules and quantum dots placed in between.

II-VI semiconductors like CdSe QDs represent the most used and investigated type of quantum dots. An early example of colloidal QDs was presented in 1982 when they were prepared by the reaction of Cd(ClO4)2 and Na2S on the surface of silica particles.⁷⁷ A breakthrough was achieved in 1993 in the form of nearly monodisperse CdX (X = S, Se, Te) nanocrystals via a hot injection method.⁷⁸ In this approach, organometallic reagents like dimethyl cadmium and trioctylphosphine selenide (TOPSe) undergo reaction in a coordinating solvent at high temperatures of up to 300 °C. This permits a controlled growth of the nanocrystals by a discrete nucleation period. This basic principle is still widely used in today’s synthesis of CdX QDs, though the precursors have changed to less hazardous compounds. Typically, the cadmium precursor CdO (or Cd(AcO)2, CdCO3) - which is less volatile and pyrophoric than CdMe2 – is dissolved in a coordinating solvent like trioctylphosphine oxide (TOPO) or fatty acids in the presence of phosphonic acids and long chain amines.⁷⁹ TOPSe is then injected under an argon atmosphere at temperatures over 260 °C. After nucleation, the temperature is decreased to ca. 200-240 °C to separate nucleation and particle growth.

The choice of the coordinating solvent or solvent mixture has large effects on the size, shape⁸⁰ and properties of the nanocrystals as the solvent also acts as surfactant.

According to the Gibbs-Curie-Wulff theorem, the shape of a crystal is determined by the surface free energy of the respective crystallographic faces. The final crystal will adopt a shape that minimizes the total surface free energy of the system. The surfactant mixture

Introduction: Conjugated Polymers

will selectively adsorb onto different crystallographic faces, thus modifying its surface free energy and promoting crystal growth along different crystallographic faces.⁸¹ Using this technique, dots, rods or tetrapods can be synthesized. The relative growth rates of the faces can be controlled by a variation of the ligands used (for an overview of additional models to control semiconductor particle shapes, see reference 80).

The “pure” single compound quantum dots suffer from surface defects, physical and photostability issues as well as moderate fluorescence quantum yields. This is due to the possibility of the formed charges to escape from the core or become trapped. A solution for this shortcoming was found in the coating of the quantum dots with a protecting shell. In most cases a material with a larger band-gap like CdS or ZnS is employed to cap the quantum dot core, preventing the photogenerated exciton from spreading over the entire particle. This is usually accompanied by an increase in robustness and fluorescence quantum yield as well as a red shift of the emission maximum. Related to this approach, alloyed ternary CdZnSe/ZnSe quantum dots have been reported which show further improved optical properties like non-blinking behavior and significantly higher continuous output of photons.⁸² The shell also prevents the leaking of the toxic Cd from the cores, thus enabling the use of these types of QDs in biological systems.⁸³ Quantum dots of this type are already commercially available and are produced in a kilogram scale in continuous flow reactors.⁸⁴

1.4 Conjugated Polymers

Conjugated polymers are of key interest in today’s materials research due to their actual and potential applications in low cost flexible displays, photovoltaic devices, sensing⁸⁵ and labeling. Ever since the discovery of the conductive properties of polyacetylene,⁸⁶ conjugated polymers have been studied intensely due to their unique properties such as fluorescence, electroluminescence and conductivity. Most commonly, conjugated polymers with functional groups for organic light emitting diodes are prepared by catalytic carbon-carbon coupling reactions. The most widely used ones are the Suzuki, Sonogashira, Heck, acyclic diene metathesis polymerization (ADMET)⁸⁷ and the Glaser⁸⁸ reaction. Most of these reactions are step-growth reactions, which require high yielding reaction conditions in order to obtain high molecular weights. The resulting polymers

89

Introduction: Conjugated Polymers

26

by Suzuki or Yamamoto coupling). Other routes to obtain conjugated polymers include chemical oxidation polymerization through common oxidants such as ammonium peroxydisulfate e.g. to synthesize polythiophenes⁹⁰ or polyanilines⁹¹ and electrochemical polymerization, a method to obtain polypyrroles and polyanilines.⁹² This technique is especially interesting for the electrochemical coating of surfaces and complex structures with these polymers. Progress has been made in recent years in developing controlled polymerizations of conjugated polymers.⁹³ A main drawback of the classical step-growth C-C cross coupling polymerizations is the lack of control over molecular weight distribution (MWD), regioregularity and the strict stoichiometry requirements.

Figure 1.8 Chemical structures of common conjugated polymers.

In this context, PPVs were prepared in a living fashion via ring opening metathesis polymerization (ROMP) resulting in a polymer with very few defects and a narrow MWD close to 1.2.⁹⁴ High molecular weight poly(3-alkylthiophene)s with a narrow polydispersity and high regioregularity could be prepared^95,96 by a nickel catalyzed Grignard metathesis chain-growth mechanism, a procedure that is also used in industry by the Merck company to produce regioregular polythiophenes.⁹⁷ These poly(3- alkylthiophenes) are among the most studied types of conjugated polymers, due to their application as active components in organic electronic devices. Regioregular poly(3- hexylthiophene) (P3HT) is a key material in organic field effect transistors (OFETs) and organic photovoltaics (OPV) where the control of the regioregularity is of fundamental importance for the electronic properties.⁹⁸ For example, regioirregular P3HT shows only very low charge mobilities <10^-5 cm²/V s while the regular polymer exhibits mobilities as high as 0.2 cm²/V s.

Suzuki polycondensations are widely used in the synthesis of polyphenylenes or polyfluorenes and, as discussed above, usually follow a step-growth mechanism.

Introduction: Conjugated Polymers

Recently, Yokozawa reported first examples of Suzuki polycondensations that are able to produce polymers with a very narrow MWD.⁹⁹ The catalyst of choice is a T-shaped three- coordinate and therefore coordinatively unsaturated Pd(Ar)P(^tBu)3Br complex. The aromatic ligand is found as an end group in every chain, giving rise to the possibility of end functionalization of the resulting polymers and the synthesis of conjugated block copolymers.¹⁰⁰ The reaction seems to follow a chain-growth mechanism where one can conceive that a ligand stabilized Pd(0) fragment rests on the π-system of the growing polymer chain, “slides” across the newly added terminal monomer unit and then adds oxidatively into the C-Br bond. The catalyst does not leave the growing chain during this process and can therefore not start new chains, keeping the MWD narrow.

Especially in the field of biosensing, chemosensing¹⁰¹ and optical labeling and imaging as well as in optoelectronic devices¹⁰², PPEs (or more general, poly(arylene ethynylene)s, PAEs) are a prominent class of materials.⁸⁹ They often feature high fluorescence quantum yields in solution (with reduced fluorescence quantum yields in the solid state due to strong π–π interactions creating non-radiative deactivation pathways) as well as excellent photostability. They can be produced by two different synthetic approaches: molybdenum hexacarbonyl catalyzed alkyne metathesis of propynylated dialkylarenes or via palladium catalyzed Sonogashira polycondensation. The metathesis reactions work best for alkyl substituted dipropynyl benzenes. Yields and molecular weights are lower for dialkoxy substituted dipropynyl benzenes, limiting the types of monomers which are compatible with the catalyst system. Sonogashira coupling polymerization has the advantage of a significantly higher tolerance for functional groups and heteroatoms. This allows the introduction of ester, phosphonate, sulfonate, distyrylbenzene, peptide, polylactide, polyester, sugars and oligoethyleneglycol groups and expands the potential field of applications tremendously.⁸⁹ It has to be noted though that the palladium catalyzed method has its own drawbacks, most notably lower yields and molecular weights than its metathesis counterpart. Defect structures such as oxidative ethynyl homocoupling are a common issue. These polymers are often no longer soluble once precipitated from methanol. One possible explanation could be crosslinking by Pd nanoparticles which are formed as reductive elimination and decomposition products.¹⁰³

Typically, conjugated polymers suffer from a low solubility and high viscosity of

Introduction: Conjugated Polymers

28

common organic solvents and are therefore not suited for post polymerization processes.

Another concept to improve the processability of these materials is to synthesize them in aqueous dispersion, which allows for the facile handling of the compounds even at high solid contents. This is especially attractive for polymers which suffer from low solubility or insolubility after precipitation - regardless of side chain functionalization - such as poly(arylene ethynylene)s.¹⁰⁴ Conjugated polymer nanoparticles¹⁰⁵ were obtained by miniemulsion polymerization for poly(arylene diethynylene)s¹⁰⁶ and poly(arylene ethynylenes).¹⁰⁷

Low Band-gap Conjugated Polymers

Classical conjugated polymers like polyphenylenes, polyfluorenes or polythiophenes feature an absorption and emission spectrum which is usually in the UV to green regime. In recent years, considerable effort has been put into the development of polymers with a strong absorption in the red to NIR region, i.e. which have a low band- gap between HOMO and LUMO. This is of interest for numerous reasons. In the case of biological applications like fluorescence labeling or bio imaging, higher excitation wavelength corresponds to less irradiation energy and therefore less tissue damage and background fluorescence originating from the numerous aromatic compounds - such as phenylalanine - found in cellular material. Electronic devices also benefit from a broader absorption profile of the active component. The most efficient organic photovoltaic devices are based on a bulk heterojunction approach (BHJ),¹⁰⁸ where the charges are generated by the dissociation of an exciton at the interface of two semiconducting materials. The exciton is generated through excitation of an electron donating material, typically a conjugated polymer. The electron generated after the exciton dissociation is collected by the electron accepting compound, typically a fullerene derivative or inorganic semiconductors like CdSe, TiO2 or ZnO2 and transported to the cathode.

Looking at organic photovoltaic devices, low band-gap conjugated polymers are well suited because the highest photon flux in the incident solar spectrum is at 700 nm.¹⁰⁹ To harvest the maximum number of photons, the polymers should feature a broad absorption preferentially in this spectral region, a requirement not fulfilled by popular OPV materials like P3HT. P3HT for example only collects a maximum of ~20% of the solar photons, which is one of the reasons for the rather low power conversion efficiencies (PCE) of common OPVs. Increasing the amount of collected photons leads to larger short circuit current densities (JSC) and an improved photovoltaic charge generation.

Introduction: Conjugated Polymers

Conjugated polymers with low band-gaps are most commonly achieved via the donor – acceptor (D-A) approach. These donor-acceptor polymers consist alternatingly of electrondonating and electronaccepting units. The orbital mixing of these units (Figure 1.9) leads to smaller band-gaps of the resulting polymers.

Figure 1.9 Schematic orbital mixing diagram of donor-acceptor conjugated polymers.¹¹⁰

It is possible to fine-tune the energy levels of the donor and acceptor HOMOs and LUMOs by attaching electron donating groups to the donor unit and electron- withdrawing groups at the acceptor unit. In these systems, the HOMO of the polymer is mainly located at the donor groups and the LUMO at the acceptor groups and they are therefore susceptible to modifications on the respective donors and acceptors.¹¹¹ The fine tuning of these energy levels has a strong influence on the device properties. A deeper lying HOMO is correlated with higher open circuit voltages (VOC)¹¹² as well as the offset between the HOMO of the donor polymer and the LUMO of the acceptor where a minimum of difference 0.3 is estimated to be required for a facilitated exciton splitting and charge dissociation. The electron rich donor units in D-A polymers are mainly derived from thiophene and benzene motifs as depicted in Figure 1.10.

Introduction: Conjugated Polymers

30

Figure 1.10 Chemical structures of common electron donating motifs in D-A polymers.

The electron donating ability of thiophene is stronger than that of benzene, therefore moieties built mainly from thiophenes or fused or bridged thiophenes are considered stronger donors than fluorenes or terphenyl-based moieties. Most of the donors are planar and possess a high degree of symmetry which is beneficial for intra- and intermolecular ordering of the polymer chains in solid state applications as in films.

The acceptor units possess at least one or more strong electron-withdrawing groups like imine nitrogens or carboxyl groups. The most common structures consist mainly of thiazole, thiadiazole, pyrazine or annulated amides. Strong acceptor monomers usually contain one or two thiadiazole rings such as benzothiadiazole and its derivative as depicted in Figure 1.11. Most D-A polymers utilized in organic electronics were developed from a combination of these donors and acceptors out of the pool briefly described here.

Figure 1.11 Chemical structures of common, strong acceptor units in D-A polymers.

This concept has been further expanded onto the inclusion of different main-group elements such as Group 14 elements such as silicon and germanium, Group 15 element

Introduction: Synthetic Protocols to Conjugated Polymers

phosphorous or Group 16 elements selenium and tellurium.¹¹³ Energetic modification of the LUMO levels and an influence on the width of the band gap can be achieved with this strategy.^114-116

Silicon-containing conjugated polymers have quite different electronic and optical properties compared to their carbon counterparts. HOMO and LUMO energies e.g. in the silicon analogue to polyfluorene, tend to be lower along with good thermal and environmental stability.¹¹⁷ Besides electronic properties, introducing silicon into key positions of the polymers also has an impact on the solid state packing of the chains. The longer C-Si bond is believed to reduce steric hindrance from the bulky alky substituents, enabling a denser chain packing and improved charge transport.¹¹⁸

1.5 Synthetic Protocols to Conjugated Polymers

As briefly outlined above, the majority of conjugated polymers are synthesized by transition metal catalyzed reaction. The exceptions are polymers obtained by electrochemical means (vide supra). The most common protocols to obtain conjugated polymers are based on palladium or nickel chemistry, most notably Stille, Suzuki- Miyaura and Sonogashira couplings. The latter two methods are highly relevant to this work and will be discussed – alongside the Glaser-Hay coupling – in greater detail in the following sections.

1.5.1 Sonogashira Coupling

The Sonogashira reaction, as it was discovered by Kenkichi Sonogashira and Nobue Hagihara, is a coupling reaction between terminal alkynes and aryl or vinyl halides.¹¹⁹ It utilizes palladium(0) complexes as a catalyst and a copper(I) salt as a co-catalyst. The base (usually amines) traps the produced halide acid. The mechanism¹²⁰ is still not fully understood, a proposed mechanism of coupled copper and palladium cycles is displayed in Figure 1.12.

Introduction: Synthetic Protocols to Conjugated Polymers

32

Figure 1.12 Proposed catalytic cycle of the Sonogashira coupling reaction.

The palladium cycle is based on the oxidative addition of the aryl or vinyl halide and reductive elimination of the coupled alkyne. It is suggested that the initial aryl halides are strongly involved in the turnover-determining step of the catalytic cycle.¹²¹ In this context, substrates with electron donating groups are more stable and decrease the reaction rates while electron withdrawing groups lower the EHOMO and would facilitate the oxidative addition. The cycle connects with the copper cycle by a transmetalation process, though the true mechanisms of the copper cycle are poorly understood. An open issue in this mechanistic proposal is the deprotonation of the terminal alkyne: an amine is not basic enough to abstract the proton. It is assumed that the copper halides increase the acidity of the acetylene proton through coordination to the triple bond. The Sonogashira reaction can be employed in polymerization reactions even in aqueous systems.¹²² Water stable catalysts are available, e.g. by the use of specific phosphine ligands.

A common side reaction of the copper co-catalyzed protocols of the Sonogashira reaction is the copper mediated Glaser-Hay reaction,¹²³ which makes the exclusion of molecular oxygen a necessity. This reaction will be discussed in detail in the next section.