Le domaine des senseurs chimiques et biochimiques est en pleine expansion, tant au niveau de la recherche fondamentale que du design et de la fabrication de nouveaux « nez artificiels ». Les recherches actuelles visent à la mise au point de dispositifs de taille réduite permettant l’identification et la quantification d’espèces multiples, avec une réponse rapide et réversible, une détection sensible et sélective, le tout intégré dans une technologie simple et peu coûteuse. Les composantes actives des nez artificiels dont il est question ici reposent sur l’utilisation en tant que composante active de polymères et molécules conjugués. Les polymères conjugués sont attractifs pour leur grande sensibilité (tributaire du processus de diffusion efficace des excitations électroniques le long des chaînes) tandis que les molécules conjuguées répondent généralement favorablement au cahier des charges imposé à un senseur, dont les interactions fortes et spécifiques entre le senseur et la molécule à détecter ou encore une grande stabilité chimique et photochimique. Dans ce contexte, le but de cette thèse est d’associer polymères et molécules conjugués pour mettre au point un dispositif hybride combinant les avantages des deux composantes. Afin de caractériser ces dispositifs et de les optimiser, nous nous sommes focalisés dans un premier temps sur l’étude des propriétés géométriques, électroniques et optiques des entités séparées, que nous avons étudiées par le biais de méthodes issues de la chimie quantique. D’une part, nous avons considéré les améliorations pouvant être apportées aux détecteurs moléculaires tant au niveau des limites de détection que de ses propriétés spectroscopiques. D’autre part, nous avons mené une étude mécanistique des processus de transfert d’énergie le long de chaînes de polymères conjugués rigides, en apportant une attention particulière sur les différentes approches envisageables pour en améliorer l’efficacité (effets conformationnels, introduction d’unités à longues durées de vie, écart énergétique donneur-accepteur). Enfin, sur base des résultats obtenus pour les constituants séparés, nous avons proposé des structures chimiques de systèmes ‘hybrides’, constitués de polymères conjugués substitués en bouts de chaîne par des groupements ionophores. Ces études ont été menées en étroite collaboration avec le groupe du Prof. Noël Boens à la KULeuven, où les molécules et polymères conjuguées étudiés théoriquement dans le cadre de ce travail ont été synthétisés et caractérisés d’un point de vue spectroscopique.

In Leuchtdioden wird Licht durch die Rekombination von injizierten Ladungsträgern erzeugt. Das kann einerseits in anorganischen Materialien geschehen. In diesem Fall ist es notwendig, hochgeordnete Kristallstrukturen herzustellen, die die Eigenschaften der Leuchtdioden bestimmen. Ein anderer Ansatz ist die Verwendung von organischen Molekülen und Polymeren. Auf Grund der Vielseitigkeit der organischen Chemie können die Eigenschaften der verwendeten halbleitenden Polymere schon während der Synthese beeinflusst werden. Außerdem weisen auch diese Polymere die bekannte mechanische Flexibilität auf. Die Herstellung von flexiblen, großflächigen Beleuchtungsquellen und Anzeigelementen ist so möglich.<br> Die erste Leuchtdiode mit einem halbleitenden Polymer als Emitter wurde 1990 hergestellt. Seither hat das Forschungsgebiet eine rasante Entwicklung genommen. Auch erste kommerzielle Produkte sind erhältlich. Im Zuge dieser Entwicklung wurde deutlich, dass die Eigenschaften von polymeren Leuchtdioden – beispielsweise Farbe und Effizienz – durch die Verwendung mehrerer Komponenten in der aktiven Schicht deutlich verbessert werden können. Gleichzeitig ergeben sich neue Herausforderungen durch die Wechselwirkungen der verschiedenen Filmbestandteile. Während die Komponenten oft entweder zur Verbesserung des Ladungstransportes oder zur Beeinflussung der Emission zugegeben werden, muss darauf geachtet werden, dass die anderen Prozesse nicht negativ beeinflusst werden. In dieser Arbeit werden einige dieser Wechselwirkungen untersucht und mit einfachen physikalischen Modellen erklärt.<br> So werden zunächst blau emittierende Leuchtdioden auf der Basis von Polyfluoren untersucht. Dieses Material ist zwar ein sehr effizienter blauer Emitter, jedoch ist es anfällig für chemische Defekte, diese sich nicht vollständig verhindern lassen. Die Defekte bilden Fallenzustände für Elektronen, ihr Einfluss lässt sich durch die Zugabe von Lochfallen unterdrücken. Der zugrunde liegende Prozess, die Beeinflussung der Ladungsträgerbalance, wird erklärt. Im Folgenden werden Mischsystemen mit dendronisierten Emittern, die gleichzeitig eine Falle für Elektronen bilden, untersucht. Hier wird die unterschiedliche Wirkung der isolierenden Hülle auf die Ladungs- und Energieübertragung zwischen Matrix und Farbstoffkern der Dendrimere untersucht. In Mischsystemen haben die Natur der angeregten Zustände sowie die Art und Weise des Ladungsträgertransportes einen großen Einfluss auf diese Transferprozesse. Außerden hat auch hier die Ladungsträgerbalance Auswirkungen auf die Emission. Um den Ladungsträgereinfang in Fallenzuständen zu charakterisieren, wird eine Methode auf Grundlage der Messung des zeitaufgelösten Photostroms in organischen Mischfilmen weiterentwickelt. Die erzielten Ergebnisse zeigen, dass die Übertragung der für geordnete Systeme entwickelten Modelle des Ladungsträgertransportes nicht ohne weiteres auf Polymersysteme mit hoher Unordnung übertragen werden können. Abschließend werden zeitaufgelöste Messungen der Phosphoreszenz in entsprechenden Mischungen aus Polymeren und organometallischen Verbindungen vorgestellt. Auch diese Systeme enthalten üblicherweise weitere Komponenten, die den Ladungstransport verbessern. In diesen Filmen kann es zu einer Übertragung der Tripletts vom Emitter auf die weiteren Filmbestandteile kommen. Bei Kenntnis der in Frage kommenden Wechselwirkungen können die unerwünschten Prozesse vermieden werden. / Light-emitting diodes generate light from the recombination of injected charge carriers. This can be obtained in inorganic materials. Here, it is necessary to produce highly ordered crystalline structures that determine the properties of the device. Another possibility is the utilization of organic molecules and polymers. Based on the versatile organic chemistry, it is possible to tune the properties of the semiconducting polymers already during synthesis. In addition, semiconducting polymers are mechanically flexible. Thus, it is possible to construct flexible, large-area light sources and displays.<br> The first light-emitting diode using a polymer emitter was presented in 1990. Since then, this field of research has grown rapidly up to the point where first products are commercially available. It has become clear that the properties of polymer light-emitting diodes such as color and efficiency can be improved by incorporating multiple components inside the active layer. At the same time, this gives rise to new interactions between these components. While components are often added either to improve the charge transport or to change the emission, it has to made sure that other processes are not influenced in a negative manner. This work investigates some of these interactions and describes them with simple physical models.<br> First, blue light-emitting diodes based on polyfluorene are analyzed. This polymer is an efficient emitter, but it is susceptible to the formation of chemical defects that can not be suppressed completely. These defects form electron traps, but their effect can be compensated by the addition of hole traps. The underlying process, namely the changed charge carrier balance, is explained. In the following, blend systems with dendronized emitters that form electron traps are investigated. The different influence of the insulating shell on the charge and energy transfer between polymer host and the emissive core of the dendrimers is examined. In the blend, the nature of the excited states as well as the method of the charge transport through the layer are of great importance to the transfer. Again, the charge carrier balance influences the emission. To characterize the trapping of charges in trap states, a method based on the measurement of transient photocurrents is enhanced. The results show that models developed for ordered systems can not simply be transferred to polymer systems with a high degree of disorder. Finally, time-resolved measurements of the phosphorescence decay in blends of polymers with organo-metallic compounds are shown. Usually, these systems contain more components that facilitate charge transport. Thus, triplets may be transferred from the phosphorescent dye other components of the film. Knowing the underlying interactions, unwanted processes can be suppressed.

Charge transport below the mobility edge, where the charge carriers are hopping between localized electronic states, is the dominant charge transport mechanism in a wide range of disordered materials. This type of incoherent charge transport is fundamentally different from the coherent charge transport in ordered crystalline materials. With the advent of organic electronics, where small organic molecules or polymers replace traditional inorganic semiconductors, the interest for this type of hopping charge transport has increased greatly. The work documented in this thesis has been dedicated to the understanding of this charge transport below the mobility edge. While analytical solutions exist for the transport coefficients in several simplified models of hopping charge transport, no analytical solutions yet exist that can describe these coefficients in most real systems. Due to this, Monte Carlo simulations, sometimes described as ideal experiments performed by computers, have been extensively used in this work. A particularly interesting organic system is deoxyribonucleic acid (DNA). Besides its overwhelming biological importance, DNA’s recognition and self-assembly properties have made it an interesting candidate as a molecular wire in the field of molecular electronics. In this work, it is shown that incoherent hopping and the Nobel prize-awarded Marcus theory can be used to describe the results of experimental studies on DNA. Furthermore, using this experimentally verified model, predictions of the bottlenecks in DNA conduction are made. The second part of this work concerns charge transport in conjugated polymers, the flagship of organic materials with respect to processability. It is shown that polaronic effects, accounted for by Marcus theory but not by the more commonly used Miller-Abrahams theory, can be very important for the charge transport process. A significant step is also taken in the modeling of the off-diagonal disorder in organic systems. By taking the geometry of the system from large-scale molecular dynamics simulations and calculating the electronic transfer integrals using Mulliken theory, the off-diagonal disorder is for the first time modeled directly from theory without the need for an assumed parametric random distribution.

In this diploma work, organic electrochemical transistors based on PEDOT:PSS have been studied, focusing on the influence of the geometry and physical dimensions on the transistor characteristics. The geometrical parameters studied are the area ratio between the gate and channel, the channel width and the channel length. Each parameter has been varied in five steps with each step containing three identical transistors. Results concerning the geometrical influence of the linear region resistance, the saturation current (density) and the on/off ratio are presented and discussed. Also, empirical curve-fits of the geometrical influence on the linear region resistance and the saturation current have been performed. In addition, tentative results of the locus of the saturation current versus saturation voltage for specific transistors have been obtained.

Oligo(ethylene glycol) groups have been used as substituents in poly(p-phenyleneethynylene)s (PPEs) to provide solubility, and to boost quantum yield. Properties such as water-solubility and increased quantum yield in aqueous solution make these conjugated systems promising for biosensory applications. In this thesis, a PPE containing a branched ethylene glycol side chain is synthesized as part of a polymer array for glycan biosensing. I also report that the same side chain can be put to use in a red-emissive polymer to lend water solubility. Another monomeric unit, containing ethylene glycol chains, is incorporated into a PPE to create an ampiphilic polymer. The versatility of these polymers allows them to be used for a variety of purposes, some of which will be described herein.

Dithienopyrrole-based conjugated materials, including oligomers and polymers, for potential organic electronic applications, were designed, synthesized and characterized. The optical and electrochemical properties of these materials were investigated, and their structure-property relationships were studied. Some of the materials can be oxidized (or reduced) chemically or electrochemically. Furthermore, the utility of these materials in organic electronic devices, such as OFETs and OPVs, were assessed. In OFETs, they can function as hole-transport materials with mobilities up to 4.8 × 10-2 cm2/(Vs), and one example serves as an ambipolar material with comparable hole and electron mobilities of 1.2 × 10-3 and 5.8 ×10-4 cm2/(Vs), respectively. Some of the materials can also be used as electron donors in OPVs in conjunction with PCBM, and exhibited power conversion efficiencies up to 1.4% after optimizations. They may also be used in other applications such as electrochromic devices, photodetectors, and optical limiting.

The field of molecular electronics includes the study of conjugated oligomers and polymers that have significant potential for use in devices such as light emitting diodes (LEDS), field effect transistors (FETS), and photovoltaic solar cells. These materials may replace inorganic semiconductors in these devices, Achieving better device performance through lowering the band-gap and achieving higher field effect mobilities will benefit from a greater fundamental understanding of charge transfer through the aromatic subunits. π-stacking of segments of conjugated polymers has been identified as a key feature that influences the charge transfer through semiconducting organic materials. Optimizing the molecular architecture of conjugated polymers has the potential to provide materials with better charge mobility. While devices might benefit from materials that take advantage of π-stacking, access to π-stacked structures presents a synthetic challenge. 1,8-Disubstituted naphthalenes may serve as simple covalent bridging scaffolds which might hold conjugated oligomers in a π-stacked arrangement. The research described in this thesis focuses on the synthesis of well-defined phenylene ethynylene oligomers coupled to naphthalene to serve as experimental models of closely π-stacked aromatic units in conjugated polymers. The π-stacked molecules reported in this dissertation are characterized by NMR, IR, and mass spectrometry. The effects of π- stacking on the structure and behavior of conjugated oligomers are determined by X-ray crystallography, spectroscopy, and electrochemistry.

<p>In this diploma work, organic electrochemical transistors based on PEDOT:PSS have been studied, focusing on the influence of the geometry and physical dimensions on the transistor characteristics. The geometrical parameters studied are the area ratio between the gate and channel, the channel width and the channel length. Each parameter has been varied in five steps with each step containing three identical transistors. Results concerning the geometrical influence of the linear region resistance, the saturation current (density) and the on/off ratio are presented and discussed. Also, empirical curve-fits of the geometrical influence on the linear region resistance and the saturation current have been performed. In addition, tentative results of the locus of the saturation current versus saturation voltage for specific transistors have been obtained.</p>

Morphology is the origin of life and function. Defining and designing morphology, understanding the relationship between morphology and function, is an essential theme in a number of research areas. In conjugated polymer research, the major obstacles to achieving these goals are the heterogeneity and complexity of conjugated polymer films. In the study presented in this dissertation, various single molecule spectroscopy techniques were used as an approach to minimize the complexity of these problems. By using excitation polarization spectroscopy, it was discovered that single chains of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) assume a highly ordered rod conformation despite the fact that the morphology of bulk films is known to be amorphous. The comparison of results from experiments and a coarse grained bead-on-a-chain simulation suggested that single chains have the ability to use a thermally induced defect to maximize [pi]-[pi] stacking and adopt a rod conformation as a stable conformation. Bias-induced centroid spectroscopy (BIC) on highly ordered single chains demonstrated that the energy transfer scale could be an order of magnitude larger than the value typically measured for bulk films. It was further demonstrated that such an extraordinary long energy transfer was not a unique property of single chains but was also observed in aggregates as long as the morphology was ordered. These studies were extended to another model compound poly(3-hexylthiophene) (P3HT) to generalize the mechanism of morphology formation and the structure-property relationship. For P3HT, it was shown that side-chains were a very important factor in determining single chain conformation, while the conformation of MEH-PPV was not affected by side-chains. By controlling the side-chains, both ordered and disordered P3HT chains were obtained. The comparison of results from experiments and an energy transfer model simulation quantified that energy transfer was at least twice as efficient in ordered chains as in disordered chains. In aggregates, the difference between the energy transfer efficiency of ordered and disordered morphology was even larger than that in the case of single chains. These results could suggest that there is a very fast energy transfer mechanism that occurs through interchain interactions when chains are packed in ordered fashion. / text

A series of low band-gap conjugated polymers with intramolecular charge transfer properties were synthesized and bulk heterojunction devices based on these polymers were fabricated. The electrochemical and photophysical properties of the polymers were tuned by using different electron withdrawing molecules or ruthenium complexes as the comonomer. Preliminary results suggested that the electronic structures of the polymer were significantly altered by the incorporation of different acceptor units. The polymers also demonstrated intense absorption bands in the visible region, indicating that they are suitable photoactive materials in bulk heterojunction devices. The synthesis and characterization of a series of organic donor-acceptor copolymers were studied. All of the polymers contained alternating cyclopenta[2,1- b:3,4-b’]dithiophene (CPDT) units. The effects of the different acceptor monomers were evaluated. The electron-withdrawing carboxylate and amide functional groups of the acceptors were found to effectively stabilize the HOMO levels of the polymer, and the optical band-gaps were significantly reduced. Bulk heterojunction devices were fabricated using the polymers and 6,6-phenyl C61 butyric acid methyl ester (PCBM) as donors and acceptors respectively. These devices exhibited high open circuit voltage (Voc) up to 0.86 V. The extended photosensitizing range was confirmed by the external quantum efficiency (EQE) spectra. The device performance was further improved by optimizing the active layer thickness and applying 1,8-diiodooctane in the blend solution. A new synthetic route to novel ruthenium containing polymers was also reported. [Ru(L)(L’)Cl2] complexes (L and L’ = bidentate N^N ligands) with a dibromo-substituted ligand were found to be polymerizable by Stille cross-coupling reaction. The subsequent displacement of the chloride ligands by thiocyanate was highly effective and the structures of the target polymers were fully characterized. The main chain absorption showed a significant red-shift upon metal coordination and the metal-to-ligand charge transfer (MLCT) of the complex strengthens the photon harvesting ability of the polymer. The dual function of these Ru(II) complexes demonstrated a new avenue to develop new classes of optoelectronic materials. The extent of _-delocalization of the ancillary ligands also showed interesting effects on the electronic properties of the polymers. Bulk heterojunction devices were fabricated. Photovoltaic response was observed in these devices, and the device performance can be improved by further modifying the surface morphology of the blend films. / published_or_final_version / Chemistry / Master / Master of Philosophy