52 research outputs found

    Amphoteric dissolution of two-dimensional polytriazine imide carbon nitrides in water

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    Crystalline two-dimensional carbon nitrides with polytriazine imide (PTI) structure are shown to act amphoterically, buffering both HCl and NaOH aqueous solutions, resulting in charged PTI layers that dissolve spontaneously in their aqueous media, particularly for the alkaline solutions. This provides a low energy, green route to their scalable solution processing. Protonation in acid is shown to occur at pyridinic nitrogens, stabilized by adjacent triazines, whereas deprotonation in base occurs primarily at basal plane NH bridges, although NH 2 edge deprotonation is competitive. We conclude that mildly acidic or basic pHs are necessary to provide sufficient net charge on the nanosheets to promote dissolution, while avoiding high ion concentrations which screen the repulsion of like-charged PTI sheets in solution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'

    Photophysical and Photocatalytic Properties of Covalent Organic Frameworks

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    This dissertation is most interested in how a class of materials known as covalent organic frameworks (COFs) can be designed to capture photon energy to initiate chemical reactions. Different COF designs change how long the energy is held, how it migrates, and how it is dispersed – and these differences can be used to change their performance as artificial photosynthesis platforms. Thus, it is helpful to have an informative discussion about the processes behind natural photosynthesis, that is, nature’s light harvesting strategies and photocatalytic schemes (Section 1.2) and will lead into an introduction of COFs and why they possess unique potential as artificial photosynthesis platforms (Section 1.3). Their beneficial physical qualities are complemented by understanding their electronic structures from theoretically predicted properties with specific focus on topological symmetry (Section 1.4). Synthesizing and characterizing COF systems then becomes an important consideration (Section 1.5) along with how their excited state behaviors are probed and interpreted at reaction timescales by ultrafast spectroscopic techniques (Section 1.6). Finally, a look is taken at how COF structure versatility adds unique potential in catalyst engineering (Section 1.7). The main body of this dissertation will present five main research projects that seek to test theoretical predictions, assess the impact of COF planarity, or fine tune electronic structures. To test theoretical predictions, “Tuning Photoexcited Charge Transfer in Imine-Linked Two-Dimensional Covalent Organic Frameworks, which involves exploring nodal symmetry in topologically similar COFs by varying monomers, is reported. This work has implications on charge separation characteristics of COFs which is important to retain activated catalytic sites for chemical reactions. The second project, “Impact of πConjugation Length on the Excited-State Dynamics of Star-Shaped Carbazole-π-Triazine Organic Chromophores,” doesn’t directly probe COF systems, but looks at the role of dihedral angles on intersystem crossing (ISC) rates in organic chromophores with similar star-shaped motifs like those often found in COFs. Another study on planarity is “Conjugation- and Aggregation-Directed Design of Covalent Organic Frameworks as White-Light-Emitting Diodes” that explores planar and non-planar COFs and the how this affects the deactivation of their photoexcited states. “Wavelength Dependent Excitonic Properties of Imine-Linked Covalent Organic Frameworks,” explores how subtle changes in donor-acceptor arrangements can lead to differences in excited state populations. Finally, the seminal work in this dissertation, “Imine Reversal Mediates Charge Separation and CO2 Photoreduction in Covalent Organic Frameworks,” explores the effect of the imine bond on photophysical and photocatalytic properties

    Ultrastable glasses : new perspectives for an old problem

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    Altres ajuts: the ICN2 was funded by the CERCA programme / Generalitat de Catalunya.Ultrastable glasses (mostly prepared from the vapor phase under optimized deposition conditions) represent a unique class of materials with low enthalpies and high kinetic stabilities. These highly stable and dense glasses show unique physicochemical properties, such as high thermal stability, improved mechanical properties or anomalous transitions into the supercooled liquid, offering unprecedented opportunities to understand many aspects of the glassy state. Their improved properties with respect to liquid-cooled glasses also open new prospects to their use in applications where liquid-cooled glasses failed or where not considered as usable materials. In this review article we summarize the state of the art of vapor-deposited (and other) ultrastable glasses with a focus on the mechanism of equilibration, the transformation to the liquid state and the low temperature properties. The review contains information on organic, metallic, polymeric and chalcogenide glasses and an updated list with relevant properties of all materials known today to form a stable glass

    Density Functional Theory

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    Density Functional Theory (DFT) is a powerful technique for calculating and comprehending the molecular and electrical structure of atoms, molecules, clusters, and solids. Its use is based not only on the capacity to calculate the molecular characteristics of the species of interest but also on the provision of interesting concepts that aid in a better understanding of the chemical reactivity of the systems under study. This book presents examples of recent advances, new perspectives, and applications of DFT for the understanding of chemical reactivity through descriptors forming the basis of Conceptual DFT as well as the application of the theory and its related computational procedures in the determination of the molecular properties of different systems of academic, social, and industrial interest

    Optimising Stable Radicals for the Electrochemical Generation of Reactive Intermediates

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    This thesis concentrates on the electrochemical activation of stable-radical adducts to generate reactive intermediates for small molecule and polymer chemistry. The majority of this work concerns the computational modelling and design of such compounds using high-level, ab inito quantum chemistry methods. The main findings are as follows. It is first shown that adducts based on highly-stable Blatter and Kuhn-type radicals undergo mesolytic cleavage upon one-electron oxidation, generating reactive carbocations or carbon-centred radicals. Substituent effects are employed to optimise this chemistry, either to reduce the oxidation potential of the adduct to favour the production of radicals, or by altering the bond-dissociation free energy of mesolytic cleavage to control the rate of fragmentation. Computational chemistry is then used to explore the scope for stable-radical adducts as electrochemically activated alkylating agents. SN2-type methylations of pyridine are studied over a broad range of nitroxide, triazinyl, and verdazyl-based adducts (X-Me). Here, high oxidation potentials are found to render low SN2 barriers to methylation and thus more reactive agents, highlighting the suitability of commercially available, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), in this role. Modelling is also applied to study the triboelectrification of polymeric insulators. Here, material-specific charging properties and dissipation rates are found to be connected to the stability of anionic polymer fragments to oxidation, and cationic fragments to reduction. Computational methods are then used to study the low-frequency (Terahertz) vibrations in molecular crystals. A method benchmark is presented - identifying parameters that reliably produce accurate simulated spectra - along with several new analytical tools built for the assessment of spectral data

    QUANTUM MECHANICS-BASED COMPUTATIONAL CHEMISTRY HAS BECOME A POWERFUL PARTNER IN THE SCIENTIFIC RESEARCH OF NITROGEN-RICH COMPOUNDS, PAVING THE WAY FOR IMPORTANT ADVANCES IN BIOCHEMICAL, PHARMACOLOGICAL AND OTHER RELATED FIELDS

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    The Computational Chemistry of Nitrogen-Rich Compounds; Insight into Pioneering Research Nitrogen-rich functional groups have long been studied for their diversity; nitrogen can form single, double and triple bonds with itself, and will therefore exist in a very broad range of molecular arrangements. Poly-nitrogen compounds are highly energetic and electron rich, and many compounds display unique properties that allow participation in very specialized chemical reactions. Of import is their ubiquity in biological systems, and throughout the past century and currently, their biological relevance is deeply and widely explored in biochemistry and biomedicine, from their involvement in natural biological processes and complex biomolecules, to the harnessing of their intrinsic properties for drug development and bioimaging. Computational Chemistry constitutes a major area of scientific research, constantly developing since the mid 2Oth century, where the smallest components of atoms and molecules are studied through quantum mechanics, approximations and empirical data, providing energetic and geometric data to predict and elucidate their macro properties and behaviors. Computational analysis introduces extensive applications in investigating compounds and reactions, including but not limited to; biomedical applications, including drug design and development; gaining an understanding of chemical properties where experiments fail; and predicting the interactions and reaction pathways between compounds – the feasibility and energetics of reactants, potential products and intermediates. Computational chemistry is an extremely versatile field, in that it can provide singular insight into the intricacies of an individual molecule yet extends to the behavior and arrangements of a crystal lattice, for example. This thesis is an exploration of recent research devoted to the chemistry of azides, heterocycles, and other small nitrogen-containing molecules through quantum mechanics. Computational chemistry has emerged over the past decades as a fundamental partner in research and vital to its advancement. With selective studies, a window is provided into the computational chemistry approach to researching these compounds; covering important heterocyclic reactions including click chemistry; the broader application of those reactions in biological systems – bioorthogonal chemistry – ; the exploration and characterization of various intrinsic and fascinating properties of heterocycles; and finally, a comprehensive look at studies of complex biomolecules that feature heterocycles in their chemical makeup. The immense range of theoretical methods available to address countless aspects and characteristics of these compounds demonstrates the tremendous value in this evolving field

    Ligand design for Ru(II) photosensitizers in photocatalytic hydrogen evolution

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    This thesis was conducted as cotutelle-de-thĂšse between the UniversitĂ© de MontrĂ©al and the UniversitĂ€t WĂŒrzburg (Germany). Cette thĂšse a Ă©tĂ© rĂ©alisĂ©e en cotutelle de thĂšse entre l'UniversitĂ© de MontrĂ©al et l'UniversitĂ€t WĂŒrzburg (Allemagne).Cette thĂšse Ă©tudie la conception de diffĂ©rentes ligands pour les complexes de Ru(II) et leur activitĂ© comme photosensibilisateur (PS) dans l'Ă©volution photocatalytique de l'hydrogĂšne. Le systĂšme catalytique contient gĂ©nĂ©ralement un catalyseur, un donneur d'Ă©lectron sacrificiel (SED) et un PS, qui doit prĂ©senter une forte absorption et luminescence et un comportement redox rĂ©versible. Les substituants pyridine attracteurs d'Ă©lectrons sur le rĂ©cepteur d'ions mĂ©talliques terpyridine entraĂźnent une augmentation de la durĂ©e de vie de l'Ă©tat excitĂ© et du rendement quantique (Ί = 74*10-5; τ = 3.8 ns) et permettent au complexe III-C1 de prĂ©senter une activitĂ© en tant que PS. Bien que la frĂ©quence (TOFmax) et le nombre de cycle catalytique (TON) soient relativement faibles (TOFmax = 57 mmolH2 molPS-1 min 1; TON(44 h) = 134 mmolH2 molPS-1), le systĂšme catalytique a une longue durĂ©e de vie, ne perdant que 20% de son activitĂ© au cours de 12 jours. De maniĂšre intĂ©ressante, la conception hĂ©tĂ©rolytique dans III-C1 s'avĂšre ĂȘtre bĂ©nĂ©fique pour la performance en tant que PS, malgrĂ© des propriĂ©tĂ©s photophysiques et Ă©lectrochimiques comparables Ă  celles du complexe homoleptique IV-C2 (TOFmax = 35 mmolH2 molPS-1 min-1; TON(24 h) = 14 mmolH2 molPS-1). L'extinction rĂ©ductive de la PS excitĂ©e par le SED est identifiĂ©e comme l'Ă©tape limitant la vitesse dans les deux cas. Par consĂ©quent, les ligands sont modifiĂ©s pour ĂȘtre plus accepteurs d'Ă©lectrons, soit par N-mĂ©thylation des substituants pyridine pĂ©riphĂ©riques, soit par introduction d'un cycle pyrimidine dans le rĂ©cepteur d'ion mĂ©tallique, ce qui conduit Ă  une augmentation des durĂ©es de vie des Ă©tats excitĂ©s (τ = 9–40 ns) et des rendements quantiques de luminescence (Ί = 40–400*10-5). Cependant, le caractĂšre plus accepteur d'Ă©lectrons des ligands entraĂźne Ă©galement des potentiels de rĂ©duction dĂ©calĂ©s anodiquement, ce qui conduit Ă  un manque de force motrice pour le transfert d'Ă©lectrons du PS rĂ©duit au catalyseur. Ainsi, cette Ă©tape de transfert d'Ă©lectrons s'avĂšre ĂȘtre un facteur limitant de la performance globale du PS. Alors que des TOFmax plus Ă©levĂ©s dans les expĂ©riences d'Ă©volution de l'hydrogĂšne sont observĂ©s pour les PS contenant le motif pyrimidine (TOFmax = 300–715 mmolH2 molPS-1 min-1), la longĂ©vitĂ© de ces systĂšmes est rĂ©duite avec des temps de demi-vie de 2–6 h. L'expansion des ligands contenant le motif pyrimidine en complexes dinuclĂ©aires conduit Ă  une absorptivitĂ© plus forte (Δ = 100–135*103 L mol-1 cm-1), une luminescence accrue (τ = 90–125 ns, Ί = 210–350*10-5) et peut Ă©galement entraĂźner un TOFmax plus Ă©levĂ© si la force motrice est suffisante pour le transfert d'Ă©lectrons vers le catalyseur (1500 mmolH2 molPS-1 min-1). En comparant des complexes avec des forces motrices similaires, une luminescence plus forte se traduit par un TOFmax plus Ă©levĂ©. Outre les considĂ©rations thermodynamiques, les effets cinĂ©tiques et l'efficacitĂ© du transfert d'Ă©lectrons sont supposĂ©s avoir un impact sur l'activitĂ© observĂ©e dans l'Ă©volution de l'hydrogĂšne. En rĂ©sumĂ©, ce travail montre que la conception ciblĂ©e de ligands peut faire du groupe prĂ©cĂ©demment nĂ©gligĂ© des complexes de Ru(II) avec des ligands tridentĂ©s des candidats attrayants pour une utilisation comme PS dans l'Ă©volution photocatalytique de l'hydrogĂšne.This thesis investigates different ligand designs for Ru(II) complexes and the activity of the complexes as photosensitizer (PS) in photocatalytic hydrogen evolution. The catalytic system typically contains a catalyst, a sacrificial electron donor (SED) and a PS, which needs to exhibit strong absorption and luminescence, as well as reversible redox behavior. Electron-withdrawing pyridine substituents on the terpyridine metal ion receptor result in an increase of excited-state lifetime and quantum yield (Ί = 74*10-5; τ = 3.8 ns) and lead to complex III-C1 exhibiting activity as PS. While the turn-over frequency (TOFmax) and turn-over number (TON) are relatively low (TOFmax = 57 mmolH2 molPS-1 min-1; TON(44 h) = 134 mmolH2 molPS-1), the catalytic system is long-lived, losing only 20% of its activity over the course of 12 days. Interestingly, the heteroleptic design in III-C1 proves to be beneficial for the performance as PS, despite III-C1 having comparable photophysical and electrochemical properties as the homoleptic complex IV-C2 (TOFmax = 35 mmolH2 molPS-1 min-1; TON(24 h) = 14 mmolH2 molPS-1). Reductive quenching of the excited PS by the SED is identified as rate-limiting step in both cases. Hence, the ligands are designed to be more electron-accepting either via N-methylation of the peripheral pyridine substituents or introduction of a pyrimidine ring in the metal ion receptor, leading to increased excited-state lifetimes (τ = 9–40 ns) and luminescence quantum yields (Ί = 40–400*10-5). However, the more electron-accepting character of the ligands also results in anodically shifted reduction potentials, leading to a lack of driving force for the electron transfer from the reduced PS to the catalyst. Hence, this electron transfer step is found to be a limiting factor to the overall performance of the PS. While higher TOFmax in hydrogen evolution experiments are observed for pyrimidine-containing PS (TOFmax = 300–715 mmolH2 molPS-1 min-1), the longevity for these systems is reduced with half-life times of 2–6 h. Expansion of the pyrimidine-containing ligands to dinuclear complexes yields a stronger absorptivity (Δ = 100–135*103 L mol-1 cm-1), increased luminescence (τ = 90–125 ns, Ί = 210–350*10-5) and can also result in higher TOFmax given sufficient driving force for electron transfer to the catalyst (TOFmax = 1500 mmolH2 molPS-1 min-1). When comparing complexes with similar driving forces, stronger luminescence is reflected in a higher TOFmax. Besides thermodynamic considerations, kinetic effects and electron transfer efficiency are assumed to impact the observed activity in hydrogen evolution. In summary, this work shows that targeted ligand design can make the previously disregarded group of Ru(II) complexes with tridentate ligands attractive candidates for use as PS in photocatalytic hydrogen evolution.In dieser Arbeit werden verschiedene Liganden fĂŒr Ru(II)-Komplexe und die AktivitĂ€t der Komplexe als Photosensibilisatoren (PS) in der photokatalytischen Wasserstoffentwicklung untersucht. Das katalytische System besteht typischerweise aus einem Katalysator, einem Opferelektronendonator (SED) und einem PS, welcher eine starke Absorption und Lumineszenz sowie ein reversibles Redoxverhalten aufweisen sollte. Elektronenziehende Pyridin-Substituenten am Terpyridin-Metallionenrezeptor resultieren in einer Erhöhung der Lebensdauer des angeregten Zustands sowie der Quantenausbeute (Ί = 74*10-5; τ = 3.8 ns), was dazu fĂŒhrt, dass Komplex III-C1 als PS aktiv ist. WĂ€hrend die Wechselzahl (TOFmax) und der Umsatz (TON) relativ niedrig sind (TOFmax = 57 mmolH2 molPS-1 min-1; TON(44 h) = 134 mmolH2 molPS 1), ist das katalytische System langlebig und verliert im Laufe von 12 Tagen nur 20% seiner AktivitĂ€t. Das heteroleptische Design in III-C1 erweist sich als vorteilhaft fĂŒr die Leistung als PS, obwohl III-C1 vergleichbare photophysikalische und elektrochemische Eigenschaften besitzt wie der homoleptische Komplex IV-C2 (TOFmax = 35 mmolH2 molPS-1 min-1; TON(24 h) = 14 mmolH2 molPS-1). In beiden FĂ€llen erweist sich das reduktive Lumineszenzlöschen des angeregten PS durch den SED als geschwindigkeitsbestimmender Schritt. Daher werden die Liganden entweder durch N-Methylierung der peripheren Pyridin-Substituenten oder durch EinfĂŒhrung eines Pyrimidinrings in den Metallionenrezeptor elektronenziehender gestaltet, was zu erhöhten Lebensdauern des angeregten Zustands (τ = 9–40 ns) und Lumineszenzquantenausbeuten (Ί = 40–400*10-5) fĂŒhrt. Der stĂ€rker elektronenziehende Charakter der Liganden fĂŒhrt allerdings auch zu anodisch verschobenen Reduktionspotentialen, wodurch die treibende Kraft fĂŒr den Elektronentransfer vom reduzierten PS zum Katalysator reduziert wird. Daher erweist sich dieser Elektronentransferschritt als ein limitierender Faktor fĂŒr die Gesamtleistung des PS. WĂ€hrend höhere TOFmax in Wasserstoffproduktionsexperimenten fĂŒr Pyrimidin-haltige PS beobachtet werden (TOFmax = 300–715 mmolH2 molPS-1 min-1), ist die Langlebigkeit fĂŒr diese Systeme mit Halbwertszeiten von 2–6 h deutlich reduziert. Die Erweiterung der Pyrimidin-haltigen Liganden zu zweikernigen Komplexen fĂŒhrt zu einem stĂ€rkeren Absorptionsvermögen (Δ = 100–135*103 L mol-1 cm-1), erhöhter Lumineszenz (τ = 90–125 ns, Ί = 210–350*10-5) und kann bei ausreichender treibender Kraft fĂŒr den Elektronentransfer zum Katalysator auch zu einer höheren TOFmax fĂŒhren (TOFmax = 1500 mmolH2 molPS-1 min-1). Beim Vergleich von Komplexen mit Ă€hnlichen treibenden KrĂ€ften spiegelt sich die stĂ€rkere Lumineszenz in einem höheren TOFmax wider. Es wird angenommen, dass neben thermodynamischen Faktoren auch kinetische Effekte und die Effizienz des Elektronentransfers die beobachtete AktivitĂ€t bei der Wasserstoffentwicklung beeinflussen. Zusammenfassend zeigt diese Arbeit, dass gezieltes Ligandendesign die bisher vernachlĂ€ssigte Gruppe der Ru(II)-Komplexe mit tridentaten Liganden zu attraktiven Kandidaten fĂŒr den Einsatz als PS in der photokatalytischen Wasserstoffentwicklung machen kann

    Quantum Chemical Studies on Bonding and Reactivity at Hybrid Organic-Inorganic Interfaces

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    In this cumulative dissertation organic-inorganic hybrid interfaces with relevance for nanoelectronic applications are investigated with theoretical methods. An emphasis is put on the elucidation of interface reaction mechanisms and the employment of bonding analysis to better inform synthetic design choices. Furthermore, electronic structure related properties are calculated in order to explain experimental observations. The dissertation is organized in two parts. In the first part, the covalent attachment of organic layers on the (001) facet of the inorganic semiconductor silicon is studied. The second part is concerned with the creation of interface models for organic semiconductors on Ag(111). In the past it was shown that cyclooctyne is a particularly suitable platform molecule for the purpose of creating a contact layer with Si(001) due to its highly selective adsoption. Building on this prior work, the focus of this dissertation is hence the growth of a second layer on the contact layer. For this purpose, promising reactions schemes with “click”-characteristics (fast, excellent yield, solvent tolerant) are studied. These are the azide-alkyne cycloaddition (AAC), the enolether-tetrazine inverse electron demand Diels-Alder reaction (IEDDA) as well as the nucleophilic substitutions of an acid chloride mediated esterification (ACE) and the sulfonylfluoride exchange (SuFEx). The second main topic of this dissertation is the chemical interplay of the Ag(111) surface with nature inspired organic semiconductors based on pyrrole units. In comparison to silicon, the coinage metal surface has a comparatively low reactivity. This property is crucial for keeping the π-systems of the organic semiconductors intact after interface formation. Still, various bonding patterns can emerge at the interface. Besides van der Waals interactions, an exchange of electron density between the surface and the organic π-system is observed. The strength of this interaction is determined primarily by the frontier orbitals of the molecule. In summary, the collected work of this dissertation shows that quantum chemical methods are a valuable tool for better understanding the chemistry and electronic structure of hybrid interfaces. Insights gained from theory not only explain experimental observations but can also be used to guide synthetic efforts even though, different terminologies and concepts exist for metal and semiconductor surfaces. Furthermore, the studies presented here highlight that various types of interfaces can be described efficiently within an ab inito framework

    Complexes of N4 Substituted Thiosemicarbazones with Applications in Medicine and Catalysis

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    The presented work is based on the N4 derivatization of thiosemicarbazones and the influence within medical and catalytical applications. A total of 48 thiosemicarbazones were synthesized and characterized. Chapter 2.1 gives an insight into the electrochemical characteristics of the synthesized ligands and the comparison shows the influence of the substitution patterns. As expected, all thiosemicarbazones get reduced on the imine moiety and oxidized at the sulphur atom during the first electrochemical processes. To further understand the influence of the substitutions on the metal centre, Ir(III) complexes were synthesized in the chapter 2.2. Herein, the complexes bear several coligand combinations of which the hydride coligand acts as a sensor for the electron density at the metal centre. The influence of a N2 substitution on a metal centre appears is neglectable. Decomposition in DCM yielded oxalate bridged dinuclear iridium complexes which could be determined by single crystal XRD. 2-Pyrdinyl thiosemicarbazones were used to coordinate nickel group metals. A series of Pt(II) chloro complexes were synthesized, characterized, and conjugated to peptides by solid phase peptide synthesis. Antiproliferation studies showed that no toxicity arises from the peptide conjugate and the corresponding complexes. Since the complex [Pt(dpyTSCLp-sC18)Cl] was instable under HPLC conditions the chlorido coligand was exchanged for NCS− and CN−. The resulting complexes of the nickel group were investigated by NMR, CV and (SEC-)UV/Vis. The coligands did not follow any trend, yet the complex [Pt(dpyTSCLp-sC18)CN] showed high stability under HPLC conditions, was stable in aqueous media, and was not toxic. Tetradentate thiosemicarbazone complexes of Ni(II), Cu(II), Pd(II) and Pt(II) were synthesized and characterized in Chapter 2.7. The complexes showed high similarity in their electrochemical and photophysical properties, due to mainly ligand centred frontier orbitals with low contributions of the metal ions. Additionally, the functionalities of the ligands and complexes could be tuned to gain access to cation specific fluorophores, highly active C-C coupling catalysts. Also, the first U(IV) complexes of thiosemicarbazones could be synthesized and characterized in this work
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