Universität Stuttgart
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Item Open Access Spectroscopic investigations of the magnetic anisotropy of lanthanide- and cobalt-based molecular nanomagnets(2016) Rechkemmer, Yvonne; Slageren, Joris van (Prof. Dr.)Single-molecule magnets are metal complexes exhibiting an energy barrier for spin reversal, leading to magnetic bistability and slow relaxation of the magnetization. Their potential for practical applications such as high-density magnetic data storage was recognized early on and with the goal of achieving high energy barriers, different kinds of single-molecule magnets have been synthesized. The quadratic dependence of the barrier height on the spin motivated chemists to synthesize metal complexes with very high total spins; however, with limited success. It was shown that high spins come along with low anisotropies and increased interest thus focused on the synthesis and investigation of (mononuclear) complexes of highly anisotropic metal centers, e.g. lanthanide or cobalt complexes. Although rather high energy barriers can be achieved in such systems, practical application remains problematic and has not been realized yet. Reasons are for example the lack of rational design criteria and the complex interplay of different magnetic relaxation pathways. The aim of this work was therefore the comprehensive magnetic and spectroscopic investigation of selected molecular lanthanide and cobalt compounds in order to obtain a deeper insight into the correlation of molecular and electronic structures as well as the corresponding magnetic properties. The applied spectroscopic methods included electron paramagnetic resonance spectroscopy, far-infrared spectroscopy and optical methods. Special emphasis was placed on magnetic circular dichroism (MCD) spectroscopy, which served as a main tool for electronic structure determination. However, since the MCD-spectrometer was not part of the available experimental equipment at the University of Stuttgart, its design, setup and characterization were the first part of this work. In the further course of this work MCD-spectroscopy was employed for the electronic structure determination of selected lanthanide and cobalt compounds. The studied lanthanide compounds were literature-known molecular tetra-carbonates of erbium (1-Er) and dysprosium (1-Dy). Detailed magnetometric studies showed that both 1-Er and 1-Dy are field-induced single-molecule magnets; however, 1-Er and 1-Dy show significant differences in their magnetic relaxation behavior. The magnetic studies were complemented by detailed spectroscopic investigations.The combination of far-infrared-, luminescence- and MCD-spectroscopy allowed for the experimental determination of 48 energy levels for 1-Er and 55 levels for 1-Dy, which built the foundation for the subsequent crystal field analysis and electronic structure determination. In addition, the results of EPR-spectroscopic studies were used for fine-tuning and verifying the respectively determined crystal field parameters. Calculating the magnetic dipole strengths for transitions between the relevant states led to a quantitative understanding of the magnetic relaxation pathways. Besides the investigation of lanthanide compounds, this thesis deals with two classes of cobalt complexes. The first class comprises mononuclear complexes in which one Co(II) ion is ligated by the nitrogen donors of two doubly deprotonated 1,2-bis(methanesulfonamido)-benzene-ligands. Rather acute N-Co-N bite angles indicate strong deviations from ideal tetrahedral symmetry. The static magnetic properties hint at very high energy barriers for spin reversal and with the help of far-infrared spectroscopy, largely negative axial zero-field splitting parameters were determined. The corresponding energy barriers belong to the highest ever reported for 3d-transition metal complexes and investigating the dynamic magnetic properties confirmed single-molecule magnet behavior. The unique magnetic properties were fully explained by analyzing spectroscopic results. The MCD-spectra showed intense signals that were assigned to spin-allowed d-d-transitions. Subsequent crystal field analysis revealed that the strong axial crystal field generated by the ligands leads to a large splitting of the electronic terms and thus in turn to a relatively small energy gap between the electronic ground state and the first excited state. The resulting increase in second-order spin-orbit coupling explains the high energy barriers observed in the studied complexes. The second class of cobalt compounds studied in this work included dimers of distorted octahedrally coordinated Co(II) ions bridged by symmetrical or asymmetrical quinone based bridging ligands. The main focus of investigation lay on the impact of the bridging ligand on the magnetic coupling between the cobalt centers. Thus, the magnetic properties of the complexes were studied with the help of static susceptibility and magnetization measurements and analyzed by means of different models. Depending on the bridging ligand, different signs for the exchange coupling constants were found. The varying signs can be explained by different relative contributions of possible exchange paths, influenced by the different substituents at the bridging ligands or slight geometry differences. The observations indicate that electron withdrawing substituents favor ferromagnetic couplings, which are preferred in the context of molecular magnetism. All in all, it can be concluded that this work provides a contribution to the deeper understanding of the features relevant for single-molecule magnets. The electronic structure determination for selected lanthanide and cobalt complexes applying advanced magnetometric and spectroscopic techniques not only led to an understanding of the static and dynamic magnetic properties but also allowed for the development of design criteria and new approaches for improved single-molecule magnets in the future.Item Open Access Monodisperse highly ordered and polydisperse biobased solid foams(2018) Andrieux, Sébastien; Stubenrauch, Cosima (Prof. Dr.)The aim of this work was the synthesis of monodisperse highly ordered biobased polymer foams and a comparison with their polydisperse counterparts. We used the biobased and biodegradable polymer chitosan, which we cross-linked with genipin. The polymer foams were synthesised via foam templating, i.e. via a liquid foam whose continuous phase contains a polymer and can be solidified. In order to obtain monodisperse highly ordered polymer foams, one first has to generate monodisperse highly ordered liquid foam templates. We did so by using microfluidics, which allows to produce monodisperse liquid foams with bubble sizes from 200 µm to 800 µm and polydispersities below 5%. The monodisperse foams were collected outside of the microfluidic channels and left to self-order under the influence of gravity and confinement. We studied the kinetics of the cross-linking reaction to find the optimal storage conditions during cross-linking. Once cross-linked we freeze-dried the gelled foams to obtain solid chitosan foams. We compared the morphological properties of the solid foams with those of the liquid templates in order to test the efficiency of the developed templating route. We observed how modifying the cross-linking and drying conditions can strongly affect the morphology of the solid foams. The main issue was to maintain the key properties of the liquid foam template throughout the solidification process, namely the bubble size distribution, the structural order and the density. We then compared the synthesised monodisperse polymer foams with their polydisperse counterparts. Although easy foaming methods exist for the generation of polydisperse foams, they do not allow the control over the polydispersity. We thus used microfluidics to generate liquid chitosan foams with tunable polydispersities from below 5% up to 26%. Microfluidics allows to match the average bubble size and density of the polydisperse liquid chitosan foam with those of the monodisperse counterpart. After solidifying the liquid templates we obtained solid foams with controlled polydispersities and studied the in uence of the polydispersity on the mechanical properties. However, we observed that not the polydispersity but the foam density was the main parameter at play. Moreover, the solid chitosan foams had weak mechanical properties with elastic moduli below 100 kPa. To overcome this issue, we incorporated cellulose nanofibres to the original chitosan solution and followed the developed route for foam templating. We had to adapt the microfluidic parameters to account for the viscosity changes brought about by the nanofibres. However, we managed to produce monodisperse liquid foams having the same bubble size, i.e. ~300 µm, but different amounts of cellulose nanofibres. The cellulose content had a strong influence on the solid foam morphology in general and on the pore connectivity in particular.Item Open Access Soft materials for acoustic applications(2022) Choi, Eunjin; Fischer, Peer (Prof. Dr.)Ultrasound finds wide application in imaging and testing because ultrasound can penetrate tissue and is benign. Gaseous microbubbles strongly scatter ultrasound and are therefore used as contrast agents. Ultrasound responsive materials can be used for many industrial and biomedical applications. Ultrasound can also be used to exert forces and manipulate particles solution and biological cells. In this thesis, material systems are developed for three application areas: 1) models of human organs for the quantitative evaluation of surgical procedures with ultrasound; 2) the fabrication of soft objects by assembling polymeric particles with ultrasound and the acoustic hologram; and 3) the characterization of antibubbles as novel contrast agents that can carry a fluid load. Organ phantoms serve as tools in medical fields to train and plan medical procedures. However, current organ phantoms miss important features or are not realistic. Current models tend to possess a Young’s modulus that is much higher than that of tissue. Furthermore, many of the current models do not show the correct contrast in a medical imaging setting. This thesis presents high fidelity organ phantoms that possess the correct elasticity, compliance, optical appearance, and correct ultrasound contrast. One model is developed for cystoscopy (CY) of the bladder. Another phantom for the transurethral resection of the prostate (TURP). The quality of the phantoms is validated by medical practitioners. For CY, the execution time of the medical practitioners is recorded to completely map the inside of the bladder phantom while localizing tumor models that have been embedded in the bladder wall. For TURP, the quality of the resection is compared with ultrasound imaging before and after the surgical simulation. Parameters are defined to quantify the success of the procedure. The phantoms developed as part of this thesis have received high satisfaction scores from medical practitioners. The parameters reflect the experience of the surgeons. In assembling soft matter, one challenge is that existing 3D printing methods are slow. In contrast, the use of ultrasound patterns shaped with a recently invented acoustic hologram allows objects to be built at once. In this thesis, polydimethylsiloxane (PDMS) particles have been assembled into two-dimensional shapes with ultrasound. To fix the assembly, the PDMS has been physically functionalized with an initiator using swelling. Suitable swelling solutions have been determined based on their solubility. The stability of the physisorbed initiators is evaluated, and the functionalized PDMS particles are fixed via photopolymerization after assembly in aqueous polyethylene glycol dimethacrylate (PEG-DMA) solutions. The fabrication steps can be repeated to increase the thickness of structures that are mechanically stable. The antibubble is an emerging ultrasound contrast agent. It has an inverse form to a conventional bubble in that a substance in the core is surrounded by a gaseous layer. The antibubble is acoustically responsive and, compared to conventional microbubbles, can carry a much greater load. In this thesis, the structure of antibubbles is examined. In particular, the volume of the load is quantified, and the amount of gas per bubble is estimated. The stability of the core substance against diffusion is investigated and shown to be stable for over 11 h.Item Open Access Validierung von Reaktionsmechanismen für biogene Kraftstoffkomponenten(Stuttgart : Deutsches Zentrum für Luft- und Raumfahrt, Institut für Verbrennungstechnik, 2016) Schuler, Dominik Friedemann; Zabel, Friedhelm (Prof. Dr.)In der vorliegenden Arbeit werden die Verbindungen Butanol, 2-Butanol, iso-Butanol und 2,5-Dimethylfuran bei Bedingungen, die für die technische Verbrennung relevant sind, untersucht (p=8.20 bar, T=1190-1520 K). Diese Verbindungen sind mögliche Komponenten zukünftiger Kraftstoffe. Die Experimente werden in Abwesenheit von Sauerstoff (Pyrolyse) in einem speziell für diese Untersuchungen konfigurierten Single-Pulse-Stoßrohr hinter reflektierten Stoßwellen durchgeführt. Die Experimente werden mit der Simulationsumgebung Chemkin II und kinetischen Reaktionsmodellen aus der Literatur simuliert, um die Produktverteilungen aus Experiment und Simulation gegenüber zu stellen. Zur besseren Charakterisierung der Unterschiede zwischen den Modellen werden Reaktionsfluss- und Störungsanalysen durchgeführt. Mit Hilfe dieser Daten kann Verbesserungspotential in den Mechanismen identifiziert werden und Ansätze für eine Optimierung können aufgezeigt werden. Bei der Pyrolyse der Butanolisomere wurden die Hauptprodukte Ethen, Ethin, Ethan, Methan und Propen quantifiziert. In geringeren Konzentrationen wurden die Produkte Ethanal, 1,3-Butadien, 1,3-Butadiin, 2-Buten, iso-Buten, Propen, Propin und Allen nachgewiesen. Bei den Simulationen zeigten sich einerseits teils erstaunlich gute Vorhersagen der experimentellen Daten, andererseits bei einigen Produktspezies auch eine große Diskrepanz der Vorhersagen der Mechanismen untereinander sowie zum Experiment. Bei der Pyrolyse von 2,5-Dimethylfuran wurden die Hauptprodukte Kohlenstoffmonoxid, Ethan, Ethen, Ethin und 1,3-Butenin nachgewiesen. Weitere quantifizierte Produkte sind Allen, Benzol, 1,3-Butadien, 1,3-Butadiin, 2-Buten, Butin, Cyclopentadien, 2-Methylfuran, Propen, Propin und Toluol. Die Mechanismen zeigen in den Hauptprodukten eine gute Übereinstimmung untereinander und mit den experimentell gewonnenen Daten. Bei den Nebenprodukten kommt es jedoch auf Grund der Unterschiede in den Mechanismen zu unterschiedlichen Vorhersagen. Neben den Untersuchungen im Single-Pulse-Stoßrohr wurde in einem weiteren Stoßrohr hinter reflektierten Stoßwellen zeitaufgelöst die Bildung von Kohlenstoffmonoxid bei der Pyrolyse von 2,5-Dimethylfuran gemessen. Kohlenstoffmonoxid wurde dabei mittels Absorptionsmessungen bei einer Wellenlänge von 151,0 nm detektiert. Das ebenfalls auf dieser Wellenlänge absorbierende Edukt 2,5-Dimethylfuran und das Produkt Ethin wurden bei der Vorhersage der Absorption berücksichtigt. Im Rahmen der Messgenauigkeit decken sich die gefundenen Absorptionen mit den von den Modellen vorhergesagten Werten, so dass die Messungen die Reaktionsmodelle unterstützen.Item Open Access Integration of molecular quantum bits with semiconductor spintronics(2022) Kern, Michal; Slageren, Joris van (Prof. Dr.)