De Novo Design of secondary structures: Synthesis and conformational studies

Chemists have long sought to extrapolate the power of biological catalysis and recognition to synthetic systems. These efforts have focused largely on low molecular weight catalysts and receptors; however, biological systems themselves rely almost exclusively on polymers, proteins and RNA, to perform complex chemical functions. Proteins and RNA are unique in their ability to adopt compact, well-ordered conformations, and specific folding provides precise spatial orientation of the functional groups that comprise the “active site”. These features suggest that identification of new polymer backbones with discrete and predictable folding propensities (“foldamers”) will provide a basis for design of molecular machines with unique capabilities. The foldamer approach complements current efforts to design unnatural properties into polypeptides and polynucleotides. The aim of this thesis is the synthesis and conformational studies of new classes of foldamers, using a peptidomimetic approach. Moreover their attitude to be utilized as ionophores, catalysts, and nanobiomaterials were analyzed in solution and in the solid state. This thesis is divided in thematically chapters that are reported below. It begins with a very general introduction (page 4) which is useful, but not strictly necessary, to the expert reader. It is worth mentioning that paragraph I.3 (page 22) is the starting point of this work and paragraph I.5 (page 32) isrequired to better understand the results of chapters 4 and 5. In chapter 1 (page 39) is reported the synthesis and conformational analysis of a novel class of foldamers containing (S)-β3-homophenylglycine [(S)-β3-hPhg] and D- 4-carboxy-oxazolidin-2-one (D-Oxd) residues in alternate order is reported. The experimental conformational analysis performed in solution by IR, 1HNMR, and CD spectroscopy unambiguously proved that these oligomers fold into ordered structures with increasing sequence length. Theoretical calculations employing ab initio MO theory suggest a helix with 11-membered hydrogenbonded rings as the preferred secondary structure type. The novel structures enrich the field of peptidic foldamers and might be useful in the mimicry of native peptides. In chapter 2 cyclo-(L-Ala-D-Oxd)3 and cyclo-(L-Ala-DOxd) 4 were prepared in the liquid phase with good overall yields and were utilized for bivalent ions chelation (Ca2+, Mg2+, Cu2+, Zn2+ and Hg2+); their chelation skill was analyzed with ESI-MS, CD and 1HNMR techniques and the best results were obtained with cyclo-(L-Ala-D-Oxd)3 and Mg2+ or Ca2+. Chapter 3 describes an application of oligopeptides as catalysts for aldol reactions. Paragraph 3.1 concerns the use of prolinamides as catalysts of the cross aldol addition of hydroxyacetone to aromatic aldeydes, whereas paragraphs 3.2 and 3.3 are about the catalyzed aldol addition of acetone to isatins. By means of DFT and AIM calculations, the steric and stereoelectronic effects that control the enantioselectivity in the cross-aldol addition of acetone to isatin catalysed by L-proline have been studied, also in the presence of small quantities of water. In chapter 4 is reported the synthesis and the analysis of a new fiber-like material, obtained from the selfaggregation of the dipeptide Boc-L-Phe-D-Oxd-OBn, which spontaneously forms uniform fibers consisting of parallel infinite linear chains arising from singleintermolecular N-H···O=C hydrogen bonds. This is the absolute borderline case of a parallel β-sheet structure. Longer oligomers of the same series with general formula Boc-(L-Phe-D-Oxd)n-OBn (where n = 2-5), are described in chapter 5. Their properties in solution and in the solid state were analyzed, in correlation with their attitude to form intramolecular hydrogen bond. In chapter 6 is reported the synthesis of imidazolidin-2- one-4-carboxylate and (tetrahydro)-pyrimidin-2-one-5- carboxylate, via an efficient modification of the Hofmann rearrangement. The reaction affords the desired compounds from protected asparagine or glutamine in good to high yield, using PhI(OAc)2 as source of iodine(III).

Cocristalli di acidi dicarbossilici: sintesi, caratterizzazione e stabilità allo stato solido

It is well-known that crystalline materials obtain their fundamental physical properties from the molecular arrangement within the solid, and altering the placement and or interactions between these molecules can impact the properties of the particular solid. Solid state chemistry looks at an attempt to alter the chemical and physical solid-state properties of APIs through many different strategies as the formation of salts, polymorphs, hydrates, solvates, and cocrystals. The final aim of this work is to study the chemical and physical propriety of new crystal structures. The work consists of three parts. The first is the cocrystallization of α,ω-alkanedicarboxylics acids with pirimidine. Single-crystal X-ray diffraction analysis of this adduct have been carried out at RT, 150 and 200 K. The cocrystals show an alteration of their melting point similar to pure acids. The two significant deviations are for the cocrystals with succinico and glutarico acids. The second object of work is the structure determination of β polymorph undecandioic acids. In literature is known the other polymorph α. We observed that the thermodynamic relation for this dimorphics system is monotropic. In the third part we synthesized and analyzed the stability of four new salts of serine and oxalic acid. This project highlights the advantage of the solid state synthesis.

Materiali Politiofenici Otticamente Attivi

Much effort has been devoted in the recent years to the investigation of optically active polythiophenes characterized by the presence of a chiral moiety linked to the 3-position of the aromatic ring. In addition to their potential technological applications as materials for enantioselective electrodes and membranes, chiral poly(thiophene)s offer the possibility of studying the structural changes accompanying the transition from the disordered state by following the variation of their chiroptical properties by circular dichroism (CD). In solution of a good solvent, that kind of polythiophenes doesn’t display any optical activity arising from the presence of dissymmetric conformation of the backbone, as shown by circular dichroism (CD) spectra. When the macromolecules begin to aggregate, as it occurs e.g. by addition of a poor solvent, or lowering the solution temperature, or when the macromolecules are assembled in the solid state as thin films obtained by solution casting or spin coating, significant CD bands are observed in the spectral region related to the electronic absorptions of the aromatic polythiophene chromophore. These CD bands are indicative of a chiral macromolecule arrangement of one prevailing chirality. The synthesis of -substituted polythiophenes can be carried out starting from the corresponding -substituted mono- or oligomeric thiophenic monomers under regioselective or regiospecific conditions in order to minimize or avoid the formation of head-to-head dyads unfavourably affecting the presence of coplanar conformations of thiophene rings as a consequence of steric interactions between the side-chain substituents, both in solution and in the solid state. To this regard, non-symmetrically substituted monomers require therefore to perform the polymerization in the presence of highly demanding catalysts and reaction condition, whereas with symmetrically substituted oligothiophenic monomers containing the -substituents located far apart from the reacting sites, it is instead possible to obtain regioregular macromolecules by adopting more simple and economic polymerization methods, such as, e. g., the chemical oxidative polymerization with iron (III) trichloride. In order to verify how the polymer structure affects its optical activity, further poly-3-alkylthiophenes, substituted by an enantiomerically pure chiral alkyl group, namely poli[3,3”-di[2((S)-(+)-2-methylbutoxy)ethyl]-2,2’:5’,2”-terthiophene] (PDMBOETT), poli[3,3’di[2((S)-(+)-2-methylbutoxy)ethyl]-2,2’-bitiofene] (PDMBOEBT), poli[3,3””-didodecyl-4’,3”’-di(S)-(+)-2-methylbutyl-2,2’:5’,2”:5”,2”’:5”’,2””-quinquethiophene (PDDDMBQT) have been synthesized and characterized by instrumental techniques. The spectroscopic behaviour of thin films of poly(DDDMBQT) has been investigated in the solid state under different sample preparation procedures. It was also compared with the behaviour of polymers previously made. The experimental results are interpreted in terms of influence of the side-chain substituents on the extent of planarity of the polymeric chains and the formation of optically active chiral aggregates. In recent years conjugated block copolymers have received considerable attention. It is well known that conjugated block copolymers composed of two electronically different blocks can have morphologic and optical properties, that differ from those of their homopolymers. A recent study has also shown that the electronic properties and the supramolecular organization of one conjugated block can also be influenced by the other block. In order to study better this behavior, a new conjugated block copolymers, composed of a regioregular hydrophylic block and a regioregular hydrophobic block namely poli[3[2-(2-metossietossi)etossi]metiltiofene]-co- poli[3(1-octilossi)tiofene], has been synthesized and characterized.

Charakterisierung und Funktionalisierung von Polyphenylen-Dendrimeren

Die vorliegende Arbeit beschäftigt sich mit der Synthese, Funktionalisierung und Charakterisierung hochverzweigter Polyphenylen-Dendrimere. Dendrimere sind sternförmig aufgebaute Makromoleküle mit regelmäßig verzweigten Armen. Trotz der Herstellung einer Vielfalt von unterschiedlichen Dendrimertypen bleibt die Herstellung steifer formpersistenter nanometergroßer Dendrimere eine Herausforderung. Ein Ansatz zur Herstellung von form- und größenstabilen 'Nanoobjekten' wird in dieser Arbeit vorgestellt. Grundlage der Synthese der in dieser Arbeit hergestellten Polyphenylen-Dendrimere ist die Diels-Alder-Cycloaddition zwischen Tetraphenylcyclopentadienonen und Ethinylderivaten. Auf diese Weise können monodisperse Makromoleküle mit Molekularmassen größer 20 kDa und Durchmessern von 6 nm erhalten werden. Funktionalisiert werden die Dendrimere mit Funktionen wie z. B. Alkyl, Hydroxy oder Carboxy. Die Charakterisierung erfolgt u. a. mit Hilfe der NMR, GPC, Lichtstreuung oder MALDI-TOF Massenspektrometrie, aber auch mit abbildenden Methoden, wie z. B. der AFM und TEM und der Kristallstrukturanalyse. Die Dynamik der vorgestellten Dendrimere wird zum einen mit molekulardynamischen Berechnungen, zum anderen mit der Festkörper-NMR untersucht.Die Ergebnisse der Untersuchungen beweisen, dass Polyphenylen-Dendrimere nanometergroße steife formstabile Moleküle sind. Sie besitzen in erster Näherung eine globuläre Form, die große Hohlräume enthalten, in die Gastmoleküle eindringen können. Weiterhin erlauben sie eine bezüglich der Anzahl und Position definierte Funktionalisierung.

Coil-ring-coil block copolymers as building blocks of supramolecular hollow cylindrical brushes

This work describes the synthesis of a new class of rod-coil block copolymers, oligosubstituted shape persistent macrocycles, (coil-ring-coil block copolymers), and their behavior in solution and in the solid state.The coil-ring-coil block copolymers are formed by nanometer sized shape persistent macrocycles based on the phenyl-ethynyl backbone as rigid block and oligomers of polystyrene or polydimethylsiloxane as flexible blocks. The strategy that has been followed is to synthesize the macrocycles with an alcoholic functionality and the polymer carboxylic acids independently, and then bind them together by esterification. The ester bond is stable and relatively easy to form.The synthesis of the shape persistent macrocycles is based on two separate steps. In the first step the building blocks of the macrocycles are connected by Hagiara-Sogonaschira coupling to form an 'half-ring' as precursor, that contains two free acetylenes. In the second step the half-ring is cyclized by forming two sp-sp bonds via a copper-catalyzed Glaser coupling under pseudo-high-dilution conditions. The polystyrene carboxylic acid was prepared directly by siphoning the living anionic polymer chain into a THF solution, saturated with CO2, while the polydimethylsiloxane carboxylic acid was obtained by hydrosilylating an unsaturated benzylester with an Si-H terminated polydimethylsiloxane, and cleavage of the ester. The carbodiimide coupling was found to be the best way to connect macrocycles and polymers in high yield and high purity.The polystyrene-ring-polystyrene block copolymers are, depending on the molecular weight of the polystyrene, lyotropic liquid crystals in cyclohexane. The aggregation behavior of the copolymers in solution was investigated in more detail using several technique. As a result it can be concluded that the polystyrene-ring-polystyrene block copolymers can aggregate into hollow cylinder-like objects with an average length of 700 nm by a combination of shape complementary and demixing of rigid and flexible polymer parts. The resulting structure can be described as supramolecular hollow cylindrical brush.If the lyotropic solution of the polystyrene-ring-polystyrene block copolymers are dried, they remain birefringent indicating that the solid state has an ordered structure. The polydimethylsiloxane-ring-polydimethylsiloxane block copolymers are more or less fluid at room temperature, and are all birefringent (termotropic liquid crystals) as well. This is a prove that the copolymers are ordered in the fluid state. By a careful investigation using electron diffraction and wide-angle X-ray scattering, it has been possible to derive a model for the 3D-order of the copolymers. The data indicate a lamella structure for both type of copolymers. The macrocycles are arranged in a layer of columns. These crystalline layers are separated by amorphous layers which contain the polymers substituents.

Festkörper-NMR-Spektroskopie

Die innerhalb dieser Arbeit mittels moderner Festkörper-NMR-Methoden untersuchte molekulare Dynamik in Poly(methacrylat)-Schmelzen und Polyphenylen-Dendrimeren ist durch eine bemerkenswerte Anisotropie gekennzeichnet.Die Anisotropie der molekularen Dynamik zeigt sich in geschmolzenen, ataktischen und isotaktischen Poly(ethylmethacrylaten) (PEMA) durch die Zeitskalenseparation der segmentellen alpha-Relaxation von einem etwa zwei Größenordnungen langsameren Relaxationsprozeß, welcher die Isotropisierung der Polymerhauptkette wiedergibt. Die Isotropisierungsdynamik der Polymerhauptkette wird - mit Ausnahme von PMMA - durch eine universelle, nicht-korrelationszeitenverteilte Relaxationsmode der Poly(methacrylate) quantifiziert, deren Temperaturabhängigkeit durch einen einheitlichen WLF-Parametersatz beschrieben werden kann. Geometrisch läßt sich die Isotropisierung der Hauptkette durch Sprungprozesse beliebiger Amplitude von Kettenstücken mit gestreckter all-trans-Konformation interpretieren. Die Kette zeigt eine außergewöhnliche konformative Stabilität. WAXS-Messungen deuten für PEMA und seine höheren Homologen die Existenz einer Schichtstruktur an, in der sich die steifen, polaren Hauptketten lokal in Monolagen anordnen, welche durch Bereiche zusammengelagerter Seitengruppen getrennt sind. Die Festkörper-NMR-Untersuchungen an Polyphenylen-Dendrimeren bringen zwei zentrale Aspekte in der wechselseitigen Beziehung von Struktur und Dynamik hervor. Zum einen ist die beobachtete molekulare Dynamik auf lokale Reorientierungen einzelner, terminaler Phenylringe um definierte Achsen beschränkt. Polyphenylen-Dendrimermoleküle sind unter diesen Bewegungen formstabil. Zum anderen können sowohl schnelle, als auch langsame Phenylreorientierungen nachgewiesen werden, wobei jeweils die intramolekulare Packungsdichte der Phenylringe das dynamische Verhalten der Polyphenylen-Dendrimere kontrolliert.

Polymer controlled mineralization of zinc phosphate hydrates and applications in corrosion protection, catalysis and biomedicine

ZUSAMMENFASSUNG Die Tauglichkeit von Hybridmaterialien auf der Basis von Zinkphosphathydrat-Zementen zum Einsatz als korrosionshemmende anorganische Pigmente oder zur prothetischen und konservierenden Knochen- und Zahntherapie wird weltweit empirisch seit den neunziger Jahren intensiv erforscht. In der vorliegenden Arbeit wurden zuerst Referenzproben, d.h. alpha-und beta-Hopeite (Abk. a-,b-ZPT) dank eines hydrothermalen Kristallisationsverfahrens in wässerigem Milieu bei 20°C und 90°C hergestellt. Die Kristallstruktur beider Polymorphe des Zinkphosphattetrahydrats Zn3(PO4)2  4 H2O wurde komplett bestimmt. Einkristall-strukturanalyse zeigt, daß der Hauptunterschied zwischen der alpha-und beta-Form des Zinkphosphattetrahydrats in zwei verschiedenen Anordnungen der Wasserstoffbrücken liegt. Die entsprechenden drei- und zweidimensionalen Anordnungen der Wasserstoffbrücken der a-und b-ZPT induzieren jeweils unterschiedliches thermisches Verhalten beim Aufwärmen. Während die alpha-Form ihr Kristallwasser in zwei definierten Stufen verliert, erzeugt die beta-Form instabile Dehydratationsprodukt. Dieses entspricht zwei unabhängigen, aber nebeneinander ablaufenden Dehydratationsmechanismen: (i) bei niedrigen Heizraten einen zweidimensionalen Johnson-Mehl-Avrami (JMA) Mechanismus auf der (011) Ebene, der einerseits bevorzugt an Kristallkanten stattfindet und anderseits von existierenden Kristalldefekten auf Oberflächen gesteuert wird; (ii) bei hohen Heizraten einem zweidimensionalen Diffusionsmechanismus (D2), der zuerst auf der (101) Ebene und dann auf der (110) Ebene erfolgt. Durch die Betrachtung der ZPT Dehydratation als irreversibele heterogene Festkörperstufenreaktion wurde dank eines „ähnlichen Endprodukt“-Protokolls das Dehydratationsphasendiagramm aufgestellt. Es beschreibt die möglichen Zusammenhänge zwischen den verschiedenen Hydratationszuständen und weist auf die Existenz eines Übergangszustandes um 170°C (d.h. Reaktion b-ZPT  a-ZPT) hin. Daneben wurde auch ein gezieltes chemisches Ätzverfahren mit verdünnten H3PO4- und NH3 Lösungen angewendet, um die ersten Stufe des Herauslösens von Zinkphosphat genau zu untersuchen. Allerdings zeigen alpha- und beta-Hopeite charakteristische hexagonale und kubische Ätzgruben, die sich unter kristallographischer Kontrolle verbreitern. Eine zuverlässige Beschreibung der Oberfächenchemie und Topologie konnte nur durch AFM und FFM Experimente erfolgen. Gleichzeitig konnte in dieser Weise die Oberflächendefektdichte und-verteilung und die Volumenauflösungsrate von a-ZPT und b-ZPT bestimmt werden. Auf einem zweiten Weg wurde eine innovative Strategie zur Herstellung von basischen Zinkphosphatpigmenten erster und zweiter Generation (d.h. NaZnPO4  1H2O und Na2ZnPO4(OH)  2H2O) mit dem Einsatz von einerseits oberflächenmodifizierten Polystyrolatices (z.B. produziert durch ein Miniemulsionspolymerisationsverfahren) und anderseits von Dendrimeren auf der Basis von Polyamidoamid (PAMAM) beschritten. Die erhaltene Zeolithstruktur (ZPO) hat in Abhängigkeit von steigendem Natrium und Wassergehalt unterschiedliche kontrollierte Morphologie: hexagonal, würfelförmig, herzförmig, sechsarmige Sterne, lanzettenförmige Dendrite, usw. Zur quantitativen Evaluierung des Polymereinbaus in der Kristallstruktur wurden carboxylierte fluoreszenzmarkierte Latices eingesetzt. Es zeigt sich, daß Polymeradditive nicht nur das Wachstum bis zu 8 µm.min-1 reduzierten. Trotzdem scheint es auch als starker Nukleationsbeschleuniger zu wirken. Dank der Koordinationschemie (d.h. Bildung eines sechszentrigen Komplexes L-COO-Zn-PO4*H2O mit Ligandenaustausch) konnten zwei einfache Mechanismen zur Wirkung von Latexpartikeln bei der ZPO Kristallisation aufgezeigt werden: (i) ein Intrakorona- und (ii) ein Extrakorona-Keimbildungsmechanismus. Weiterhin wurde die Effizienz eines Kurzzeit- und Langzeitkorrosionschutzes durch maßgeschneiderte ZPO/ZPT Pigmente und kontrollierte Freisetzung von Phosphationen in zwei Näherungen des Auslösungsgleichgewichts abgeschätzt: (i) durch eine Auswaschungs-methode (thermodynamischer Prozess) und (ii) durch eine pH-Impulsmethode (kinetischer Prozess. Besonders deutlich wird der Ausflösungs-Fällungsmechanismus (d.h. der Metamorphismus). Die wesentliche Rolle den Natriumionen bei der Korrosionshemmung wird durch ein passendes zusammensetzungsabhängiges Auflösungsmodell (ZAAM) beschrieben, das mit dem Befund des Salzsprühteste und der Feuchtigkeitskammertests konsistent ist. Schließlich zeigt diese Arbeit das herausragende Potential funktionalisierter Latices (Polymer) bei der kontrollierten Mineralisation zur Herstellung maßgeschneiderter Zinkphosphat Materialien. Solche Hybridmaterialien werden dringend in der Entwicklung umweltfreundlicher Korrosionsschutzpigmente sowie in der Dentalmedizin benötigt.

Electronic energy transfer processes in pi-conjugated polymers

Currently pi-conjugated polymers are considered as technologically interesting materials to be used as functional building elements for the development of the new generation of optoelectronic devices. More specifically during the last few years, poly-p-phenylene materials have attracted considerable attention for their blue photoluminescence properties. This Thesis deals with the optical properties of the most representative blue light poly-p-phenylene emitters such as poly(fluorene), oligo(fluorene), poly(indenofluorene) and ladder-type penta(phenylene) derivatives. In the present work, laser induced photoluminescence spectroscopy is used as a major tool for the study of the interdependence between the dynamics of the probed photoluminescence, the molecular structures of the prepared polymeric films and the presence of chemical defects. Complementary results obtained by two-dimensional wide-angle X-ray diffraction are reported. These findings show that the different optical properties observed are influenced by the intermolecular solid-state interactions that in turn are controlled by the pendant groups of the polymer backbone. A significant feedback is delivered regarding the positive impact of a new synthetic route for the preparation of a poly(indenofluorene) derivative on the spectral purity of the compound. The energy transfer mechanisms that operate in the studied systems are addressed by doping experiments. After the evaluation of the structure/property interdependence, a new optical excitation pathway is presented. An efficient photon low-energy up-conversion that sensitises the blue emission of poly(fluorene) is demonstrated. The observed phenomenon takes place in poly(fluorene) derivatives hosts doped with metallated octaethyl porphyrins, after quasi-CW photoexcitation of intensities in the order of kW/cm2. The up-conversion process is parameterised in terms of temperature, wavelength excitation and central metal cation in the porphyrin ring. Additionally the observation of the up-conversion is extended in a broad range of poly-p-phenylene blue light emitting hosts. The dependence of the detected up-conversion intensity on the excitation intensity and doping concentration is reported. Furthermore the dynamics of the up-conversion intensity are monitored as a function of the doping concentration. These experimental results strongly suggest the existence of triplet-triplet annihilation events into the porphyrin molecules that are subsequently followed by energy transfer to the host. After confirming the occurrence of the up-conversion in solutions, cyclic voltammetry is used in order to show that the up-conversion efficiency is partially determined from the energetic alignment between the HOMO levels of the host and the dopant.

Polymorphs solvates and co-crystals of molecular materials

The scope of my research project is to produce and characterize new crystalline forms of organic compounds, focusing the attention on co-crystals and then transferring these notions on APIs to produce co-crystals of potential interest in the pharmaceutical field. In the first part of this work co-crystallization experiments were performed using as building blocks the family of aliphatic dicarboxylic acids HOOC-(CH2)n-COOH, with n= 2-8. This class of compounds has always been an object of study because it is characterized by an interesting phenomenon of alternation of melting points: the acids with an even number of carbon atoms show a melting point higher than those with an odd one. The acids were co-crystallized with four dipyridyl molecules (formed by two pyridine rings with a different number of bridging carbon atoms) through the formation of intermolecular interactions N•••(H)O. The bases used were: 4,4’-bipyridine (BPY), 1,2-bis(4-pyridyl)ethane (BPA), 1,2-(di-4-pyridyl)ethylene (BPE) and 1,2-bis(4-pyridyl)propane (BPP). The co-crystals obtained by solution synthesis were characterized by different solid-state techniques to determine the structure and to see how the melting points in co-crystals change. In the second part of this study we tried to obtain new crystal forms of compounds of pharmaceutical interest. The APIs studied are: O-desmethylvenlafaxine, Lidocaine, Nalidixic Acid and Sulfadiazine. Each API was subjected to Polymorph Screening and Salt/Co-crystal Screening experiments to identify new crystal forms characterized by different properties. In a typical Salt/Co-crystal Screening the sample was made to react with a co-former (solid or liquid) through different methods: crystallization by solution, grinding, kneading and solid-gas reactions. The new crystal forms obtained were characterized by different solid state techniques (X-ray single crystal diffraction, X-ray powder diffraction, Differential Scanning Calorimetry, Thermogravimetric Analysis, Evolved gas analysis, FT-IR – ATR, Solid State N.M.R).

Struktur- und Funktionsuntersuchungen des C 4-Dicarboxylat-Sensors DcuS von Escherichia coli

Das Zweikomponentensystem DcuSR reguliert die Expression der Gene der anaeroben Fumaratatmung in E. coli in Abhängigkeit von externen C4-Dicarbonsäuren. Die membranständige Histidinkinase DcuS detektiert den Reiz und leitet ihn über die Membran an den Responseregulaor DcuR weiter, der die Aktivität der Zielgene reguliert. Das Substratspektrum von DcuS wurde näher untersucht und strukturelle Eigenschaften der Substrate sowie ihre Affinität zu DcuS bestimmt. Es wird vermutet, dass Histidinkinasen im aktiven Zustand als Dimere oder höhere Oligomere vorliegen. Der Oligomerisierungszustand von DcuS in der Membran wurde mittels EPR-Spektroskopie untersucht. Es wurden funktionelle Cysteinmutanten von DcuS hergestellt, die nur an bestimmten Positionen der periplasmatischen Domäne Cysteinreste, aber sonst keine weiteren Cysteinreste, enthielten. Die Proteine wurden isoliert, über die Cysteinreste mit Nitroxiden markiert und in Liposomen rekonstituiert. Erste EPR-Messungen zeigten, dass rekonstituiertes DcuS in einem geordneten Zustand in der Membran vorliegt, der diskrete Abstände zwischen den Monomeren aufweist. Die Struktur von rekonstituiertem DcuS in der Membran soll durch Festkörper-NMR aufgeklärt werden. Ein geeignetes C-terminal verkürztes Konstrukt, DcuS-PD/PAS wurde zu diesem Zweck hergestellt. Das Protein ließ sich in hoher Reinheit isolieren und konnte wieder in Liposomen rekonstituiert werden. Vorbereitende NMR-Messungen zeigten, dass eine Strukturaufklärung an diesem Protein möglich ist. Weitere Strukturuntersuchungen werden zur Zeit durchgeführt.

Electronic energy transfer processes and charge carrier transport in pi-conjugated polymers

Conjugated polymers have attracted tremendous academical and industrial research interest over the past decades due to the appealing advantages that organic / polymeric materials offer for electronic applications and devices such as organic light emitting diodes (OLED), organic field effect transistors (OFET), organic solar cells (OSC), photodiodes and plastic lasers. The optimization of organic materials for applications in optoelectronic devices requires detailed knowledge of their photophysical properties, for instance energy levels of excited singlet and triplet states, excited state decay mechanisms and charge carrier mobilities. In the present work a variety of different conjugated (co)polymers, mainly polyspirobifluorene- and polyfluorene-type materials, was investigated using time-resolved photoluminescence spectroscopy in the picosecond to second time domain to study their elementary photophysical properties and to get a deeper insight into structure-property relationships. The experiments cover fluorescence spectroscopy using Streak Camera techniques as well as time-delayed gated detection techniques for the investigation of delayed fluorescence and phosphorescence. All measurements were performed on the solid state, i.e. thin polymer films and on diluted solutions. Starting from the elementary photophysical properties of conjugated polymers the experiments were extended to studies of singlet and triplet energy transfer processes in polymer blends, polymer-triplet emitter blends and copolymers. The phenomenon of photonenergy upconversion was investigated in blue light-emitting polymer matrices doped with metallated porphyrin derivatives supposing an bimolecular annihilation upconversion mechanism which could be experimentally verified on a series of copolymers. This mechanism allows for more efficient photonenergy upconversion than previously reported for polyfluorene derivatives. In addition to the above described spectroscopical experiments, amplified spontaneous emission (ASE) in thin film polymer waveguides was studied employing a fully-arylated poly(indenofluorene) as the gain medium. It was found that the material exhibits a very low threshold value for amplification of blue light combined with an excellent oxidative stability, which makes it interesting as active material for organic solid state lasers. Apart from spectroscopical experiments, transient photocurrent measurements on conjugated polymers were performed as well to elucidate the charge carrier mobility in the solid state, which is an important material parameter for device applications. A modified time-of-flight (TOF) technique using a charge carrier generation layer allowed to study hole transport in a series of spirobifluorene copolymers to unravel the structure-mobility relationship by comparison with the homopolymer. Not only the charge carrier mobility could be determined for the series of polymers but also field- and temperature-dependent measurements analyzed in the framework of the Gaussian disorder model showed that results coincide very well with the predictions of the model. Thus, the validity of the disorder concept for charge carrier transport in amorphous glassy materials could be verified for the investigated series of copolymers.

Struktur- und Reaktionsaufklärung durch Kombination von Theorie und Experiment

Die vorliegende Dissertation beinhaltet Anwendungen der Quantenchemie und methodische Entwicklungen im Bereich der "Coupled-Cluster"-Theorie zu den folgenden Themen: 1.) Die Bestimmung von Geometrieparametern in wasserstoffverbrückten Komplexen mit Pikometer-Genauigkeit durch Kopplung von NMR-Experimenten und quantenchemischen Rechnungen wird an zwei Beispielen dargelegt. 2.) Die hierin auftretenden Unterschiede in Theorie und Experiment werden diskutiert. Hierzu wurde die Schwingungsmittelung des Dipolkopplungstensors implementiert, um Nullpunkt-Effekte betrachten zu können. 3.) Ein weiterer Aspekt der Arbeit behandelt die Strukturaufklärung an diskotischen Flüssigkristallen. Die quantenchemische Modellbildung und das Zusammenspiel mit experimentellen Methoden, vor allem der Festkörper-NMR, wird vorgestellt. 4.) Innerhalb dieser Arbeit wurde mit der Parallelisierung des Quantenchemiepaketes ACESII begonnen. Die grundlegende Strategie und erste Ergebnisse werden vorgestellt. 5.) Zur Skalenreduktion des CCCSD(T)-Verfahrens durch Faktorisierung wurden verschiedene Zerlegungen des Energienenners getestet. Ein sich hieraus ergebendes Verfahren zur Berechnung der CCSD(T)-Energie wurde implementiert. 6.) Die Reaktionsaufklärung der Bildung von HSOH aus di-tert-Butyl-Sulfoxid wird vorgestellt. Dazu wurde die Thermodynamik der Reaktionsschritte mit Methoden der Quantenchemie berechnet.

Synthetical engineering of supramolecular properties of large polycyclic aromatic hydrocarbons

An important property for devices is the charge-carrier mobility values for discotic organic materials like hexa-peri-hexabenzocoronenes. A close relation exists between the degree of their columnar self-arrangement of the molecules and their mobilities. Within this first step an induction of a higher order via hydrogen-bonding was considered, which mainly pointed towards the improvement of the intracolumnar stacking of the materials. For the analytics a broad range of methods was used including differential scanning calorimetry (DSC), wide-angle X-ray diffractometry (WAXS), solid-state NMR spectroscopy and scanning tunneling microscopy (STM). Indeed, a specific influence of the hydrogen-bonds could be identified, although in several cases by the cost of a severe reduction of solubility and processability. This effect was dampened by the addition of a long alkyl chain next to the hydrogen-bond exerting functional group, which resulted in an improved columnar arrangement by retention of processability. In contrast to the before mentioned example of inducing a higher intracolumnar order by hydrogen-bonding, the focus was also be set upon larger aromatic systems. The charge-carrier mobility is also in close relation to the size of the aromatic core and larger π-areas are expected to lead to improved mobilities. For photovoltaic applications a high extinction coefficient over a broad range of the spectrum is favorable, which can also be achieved by enlarging the aromatic core component. In addition the stronger π-interactions between the aromatic core components should yield an improved columnar stability and order. However the strengthening of the π-interactions between the aromatic core components led to a reduction of the solubility and the processability due to the stronger aggregation of the molecules. This required the introduction of efficiently solubilizing features in terms of long alkyl chains in the corona of the aromatic entity, in combination of a distortion of the aromatic core moiety by bulky tert-butyl groups. By this approach not only the processing and cleaning of the materials with standard laboratory techniques became possible, but moreover the first structure-rich UV/vis and a resolved 1H-NMR spectra for an aromatic system two times larger than hexa-peri-hexabenzocoronene were recorded. The bulk properties in an extruded fiber as well as on the surface showed a columnar self-assembly including a phase in which a homeotropic alignment on a substrate was observed, which turns the material into an interesting candidate for future applications in electronic devices.

Silicon-containing block copolymers with well-defined, branched architectures

In the past decade, block copolymers (BCPs) have attracted increasing scientific and technological interest because of their inherent capability to spontaneously self-assemble into ordered arrays of nanostructures. The importance of nanostructures in a number of applications has fostered the need for well-defined, complex macromolecular architectures. In this thesis, the influence of macromolecular architecture on the bulk morphologies of novel linear-hyperbranched and linear brush-like diblock copolymer structure is investigated. An innovative, generally applicable strategy for the preparation of these defined diblock copolymers, consisting of linear polystyrene and branched polycarbosilane blocks, is demonstrated. Furthermore, complete characterization and solid-state morphological studies are provided. Finally, the concept is extended to linear-hyperbrached and linear brush-like polyalkoxysilanes. A shift of the classical phase boundaries to higher PS weight fractions as well as the appearance of new morphologies confirms the dramatic effect that polymer topology has on the morphology of BCPs.

Synthesis and surface modification of semiconductor nanocrystals

The last decade has witnessed an exponential growth of activities in the field of nanoscience and nanotechnology worldwide, driven both by the excitement of understanding new science and by the potential hope for applications and economic impacts. The largest activity in this field up to date has been in the synthesis and characterization of new materials consisting of particles with dimensions in the order of a few nanometers, so-called nanocrystalline materials. [1-8] Semiconductor nanomaterials such as III/V or II/VI compound semiconductors exhibit strong quantum confinement behavior in the size range from 1 to 10 nm. Therefore, preparation of high quality semiconductor nanocrystals has been a challenge for synthetic chemists, leading to the recent rapid progress in delivering a wide variety of semiconducting nanomaterials. Semiconductor nanocrystals, also called quantum dots, possess physical properties distinctly different from those of the bulk material. Typically, in the size range from 1 to 10 nm, when the particle size is changed, the band gap between the valence and the conduction band will change, too. In a simple approximation a particle in a box model has been used to describe the phenomenon[9]: at nanoscale dimensions the degenerate energy states of a semiconductor separate into discrete states and the system behaves like one big molecule. The size-dependent transformation of the energy levels of the particles is called “quantum size-effect”. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective bandgap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of semiconductor nanaocrystals shift to the blue (higher energies) as the size of the particles gets smaller. This color tuning is well documented for CdSe nanocrystals whose absorption and emission covers almost the whole visible spectral range. As particle sizes become smaller the ratio of surface atoms to those in the interior increases, which has a strong impact on particle properties, too. Prominent examples are the low melting point [8] and size/shape dependent pressure resistance [10] of semiconductor nanocrystals. Given the size dependence of particle properties, chemists and material scientists now have the unique opportunity to change the electronic and chemical properties of a material by simply controlling the particle size. In particular, CdSe nanocrystals have been widely investigated. Mainly due to their size-dependent optoelectronic properties [11, 12] and flexible chemical processibility [13], they have played a distinguished role for a number of seminal studies [11, 12, 14, 15]. Potential technical applications have been discussed, too. [8, 16-27] Improvement of the optoelectronic properties of semiconductor nanocrystals is still a prominent research topic. One of the most important approaches is fabricating composite type-I core-shell structures which exhibit improved properties, making them attractive from both a fundamental and a practical point of view. Overcoating of nanocrystallites with higher band gap inorganic materials has been shown to increase the photoluminescence quantum yields by eliminating surface nonradiative recombination sites. [28] Particles passivated with inorganic shells are more robust than nanocrystals covered by organic ligands only and have greater tolerance to processing conditions necessary for incorporation into solid state structures or for other applications. Some examples of core-shell nanocrystals reported earlier include CdS on CdSe [29], CdSe on CdS, [30], ZnS on CdS, [31] ZnS on CdSe[28, 32], ZnSe on CdSe [33] and CdS/HgS/CdS [34]. The characterization and preparation of a new core-shell structure, CdSe nanocrystals overcoated by different shells (CdS, ZnS), is presented in chapter 4. Type-I core-shell structures as mentioned above greatly improve the photoluminescence quantum yield and chemical and photochemical stability of nanocrystals. The emission wavelengths of type-I core/shell nanocrystals typically only shows a small red-shift when compared to the plain core nanocrystals. [30, 31, 35] In contrast to type-I core-shell nanocrystals, only few studies have been conducted on colloidal type-II core/shell structures [36-38] which are characterized by a staggered alignment of conduction and valence bands giving rise to a broad tunability of absorption and emission wavelengths, as was shown for CdTe/CdSe core-shell nanocrystals. [36] The emission of type-II core/shell nanocrystals mainly originates from the radiative recombination of electron-hole pairs across the core-shell interface leading to a long photoluminescence lifetime. Type-II core/shell nanocrystals are promising with respect to photoconduction or photovoltaic applications as has been discussed in the literature.[39] Novel type-II core-shell structures with ZnTe cores are reported in chapter 5. The recent progress in the shape control of semiconductor nanocrystals opens new fields of applications. For instance, rod shaped CdSe nanocrystals can enhance the photo-electro conversion efficiency of photovoltaic cells, [40, 41] and also allow for polarized emission in light emitting diodes. [42, 43] Shape control of anisotropic nanocrystals can be achieved by the use of surfactants, [44, 45] regular or inverse micelles as regulating agents, [46, 47] electrochemical processes, [48] template-assisted [49, 50] and solution-liquid-solution (SLS) growth mechnism. [51-53] Recently, formation of various CdSe nanocrystal shapes has been reported by the groups of Alivisatos [54] and Peng, [55] respectively. Furthermore, it has been reported by the group of Prasad [56] that noble metal nanoparticles can induce anisotropic growth of CdSe nanocrystals at lower temperatures than typically used in other methods for preparing anisotropic CdSe structures. Although several approaches for anisotropic crystal growth have been reported by now, developing new synthetic methods for the shape control of colloidal semiconductor nanocrystals remains an important goal. Accordingly, we have attempted to utilize a crystal phase control approach for the controllable synthesis of colloidal ZnE/CdSe (E = S, Se, Te) heterostructures in a variety of morphologies. The complex heterostructures obtained are presented in chapter 6. The unique optical properties of nanocrystals make them appealing as in vivo and in vitro fluorophores in a variety of biological and chemical investigations, in which traditional fluorescence labels based on organic molecules fall short of providing long-term stability and simultaneous detection of multiple emission colours [References]. The ability to prepare water soluble nanocrystals with high stability and quantum yield has led to promising applications in cellular labeling, [57, 58] deep-tissue imaging, [59, 60] and assay labeling [61, 62]. Furthermore, appropriately solubilized nanocrystals have been used as donors in fluorescence resonance energy transfer (FRET) couples. [63-65] Despite recent progress, much work still needs to be done to achieve reproducible and robust surface functionalization and develop flexible (bio-) conjugation techniques. Based on multi-shell CdSe nanocrystals, several new solubilization and ligand exchange protocols have been developed which are presented in chapter 7. The organization of this thesis is as follows: A short overview describing synthesis and properties of CdSe nanocrystals is given in chapter 2. Chapter 3 is the experimental part providing some background information about the optical and analytical methods used in this thesis. The following chapters report the results of this work: synthesis and characterization of type-I multi-shell and type-II core/shell nanocrystals are described in chapter 4 and chapter 5, respectively. In chapter 6, a high–yield synthesis of various CdSe architectures by crystal phase control is reported. Experiments about surface modification of nanocrystals are described in chapter 7. At last, a short summary of the results is given in chapter 8.