School of Chemistry - Theses
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ItemSpectroelectrochemistry of Semiconductor Nanocrystals( 2021)Semiconductor nanocrystals exhibit well-known, size-dependent optical and electronic properties. Control over the charge carriers in semiconductor nanocrystals enables the possibility to tune the optical response. One way to achieve this is through electrochemistry. Carrier modulation through electrochemical methods allows more precise control over electron transfer compared to methods such as photocharging and chemical redox reactions. By combining electrochemistry with spectroscopic techniques, the charged states in semiconductor nanocrystals can be studied in detail. A spectroelectrochemical setup has been developed to study the charging of semiconductor nanocrystals in solution and its influence on absorption and photoluminescence (PL). A negative trion state can be generated in CdSe quantum dots (QDs) and stabilised for hours under an applied cathodic potential. By monitoring both the absorbance and fluorescence changes, one can determine whether charge carriers are free or trapped. The total number of electrons injected into the QDs can be estimated from current and coulometry measurements. Hole injection into CdSe QDs induces corrosion of the lattice, whereas injection into nanocrystals shelled with CdS induces bleaching. Coupling the spectroelectrochemical setup with time-resolved PL measurements reveals the trion lifetime of CdSe/CdS QDs as a function of shell thickness. In the last section of the thesis, the effects of charge injection on CdSe nanoplatelets (NPLs) is explored. In contrast to QDs, hole injection into the NPLs enhances the photoluminescence.
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ItemNo Preview AvailableVisible-Light Promoted Carbonylation of Unactivated Alkyl Halides and Inert C(sp3)-H Bonds in Continuous Flow( 2021)Multicomponent carbonylations with carbon monoxide gas is an increasingly important strategy to generate carbonyl-rich scaffolds, such as amides and ketones. Traditionally, carbonylation reactions are catalysed by transition metals in a 2-electron pathway. While the carbonylation of aromatic substrates is well established, methodology to C(sp3) hybridised substrates remains underdeveloped. The p-acidity of carbon monoxide renders the metal catalyst less reactive towards an oxidative addition to alkyl halides and completive b-hydride elimination precludes product formation. Successful C(sp3) carbonylation has been achieved via transition-metal catalysed C(sp3)-H activation. These reactions, however, require elevated temperatures which could negatively affect selectivity and cause degradation of the starting materials and/or products. Additionally, this approach generally requires prolongated reaction times and stoichiometric amounts of external oxidants. Radical chemistry has emerged as an important alternative to overcome the limitation of the 2- electron carbonylation reactions. In this approach, the reactivity of carbon centred radicals towards carbon monoxide to form carbonyl compounds is explored. In the context of carbonylation reactions, alkyl radicals have been generated mainly from alkyl halides by means of explosive and toxic radical initiators or UV radiation. Direct C(sp3)-H alkylation is also reported, however, available methodologies required the use of toxic lead salts and prolonged reaction times. With the advent of visible light photoredox catalysis, safer and milder conditions to generate radicals have emerged. In the context of carbonylation reactions, the low redox potential of alkyl halides (Ered = -1.90 to -2.90 V vs SCE) greatly limited the use of this technique. This constrain of traditional photoredox methods can be overcome by the recently developed multiphoton catalytic systems and electrochemical methods, which both remain underexplored. Additionally, a visible light photocatalysed method for direct carbonylation of C(sp3)-H bonds remains unreported. In this PhD thesis, visible light multiphoton photoredox catalysis was explored to discover new C(sp3) carbonylation reactions. Implementation of continuous flow technology allowed for short residence times, safe handling of toxic gases and ready scalability. In chapter one, an introduction to transition-metal catalysed and radical C(sp3) carbonylation is provided. In this section, the current examples and limitations are highlighted. Next, the principles of conventional and multiphoton photoredox catalysis are discussed. The general objectives of this PhD thesis are subsequently elaborated followed by an introduction to flow chemistry. In chapter two, a new visible light promoted aminocarbonylation of unactivated alkyl iodides with carbon monoxide gas is reported. The reaction harnesses the highly reducing capabilities of the multiphoton tandem catalytic cycle of the [Ir(ppy)2(dtb-bpy)]+ photocatalyst to engage energy demanding substrates in an oxidative quenching cycle. The reaction presented excellent functional group tolerance and a broad substrate scope. This new methodology allowed for the late-stage functionalisation of natural products, in which five new cholesterol derivatives were synthetized in good to excellent yields. Steady state quenching experiments confirmed operation of the tandem photocatalytic cycle and DFT calculations led to the conclusion that the reaction proceeds via radical chain propagation. The use of continuous flow allowed short reaction times, safe handling of CO gas and scalability of the reaction. In chapter three, the tandem photoredox of the [Ir(ppy)2(dtb-bpy)]+ photocatalyst was explored in the first visible-light promoted radical carbonylative hydroacylation of alkenes with unactivated alkyl iodides and bromides. Optimisation studies showed this transformation benefits from the presence of water in the reaction mixture. The reaction showed broad substrate scope, good functional group tolerance and was high yielding. Deuterium labelling experiments studies demonstrated that this transformation proceeds via a radical polar cross over mechanism. The anionic intermediate was subsequently trapped with carbon dioxide leading to the synthesis of 1,4-keto esters and furanones via a 4 component multi-gas reaction. In chapter four, various substrates and photocatalytic systems were investigated to promote the carbonylation of inert C(sp3)-H bonds via an intramolecular hydrogen atom transfer process. The desired product could be obtained, however in low yield. The limitation of the photocatalytic methods motivated investigation of an electrochemical approach. During this study, it was discovered a new unexpected electrochemical arylation of inert C(sp3)-H bonds, which was fully developed in the subsequent chapter. In chapter five, a new electrochemical and catalyst-free arylation of arylation of inert C(sp3)-H bonds of aryl sulfonamides is reported. This reaction harnesses the ability of electron deficient cyanoarenes to function as both mediators and arylating reagent. Mechanistic studies demonstrated that the cathodic cyanoarene radical anion promotes single electron reduction of the halogenated aryl sulfonamide leading to aryl radical formation. Subsequent intramolecular hydrogen atom transfer led to the arylation of the remote C(sp3)-H arylation. An extensive study was conducted to optimise this reaction. Substrate scope was limited as the reaction was very sensitive to the nature of the cyanoarene and substitution on the sulfonamide.
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ItemAssessing the Efficacy of Bisdipyrrins as Ligands for in Vivo Applications in Disease( 2020)Porphyrins are tetrapyrrolic macrocycles which form highly stable metal complexes crucial to many biological processes. These complexes are characterised by a high kinetic barrier to decomplexation, which is a desirable quality in the design of new radiopharmaceuticals. Positron Emission Tomography (PET) imaging makes use of radionuclides such as copper-64 (half life of 12.7 hours) to aid in the diagnosis of disease. Current reports of porphyrin ligands radiolabelled with copper-64 are limited, due to the typically forcing conditions required to achieve sufficient radiolabelling for application. 2,2’-Bisdipyrrins are acyclic tetrapyrroles that form charge-neutral square-planar complexes with divalent metal cations. They are structurally analogous to porphyrins, although they are suggested to offer superior complexation kinetics under milder conditions. Accordingly, they hold promise as an alternative to porphyrins as potential radiotherapeutics. To date, there have been no reported attempts to translate these ligands into biological applications. This thesis presents the synthesis and characterisation of new (2,2’-bisdipyrrinato) metal(II) complexes and attempts to assess their efficacy for applications in the diagnostic imaging of disease. A number of (2,2’-bisdipyrrinato) copper(II) complexes have been synthesised and shown to be charge-neutral and approximately square-planar in structure based on crystallography. Cyclic voltammetry has elucidated a one-electron transfer process at ca. –1.1 V in all copper(II) complexes, and retention of this process with slight variations in potential for the corresponding Pd(II) and Ni(II) complexes evidences that this process is largely ligand-based in character. Slight shifts between the free base ligand and complexes are associated with structural variations following metal ion coordination. A (2,2’-bisdipyrrinato) copper(II) complex was determined to have a KD of 6 x 10-15 M at pH 7.4 and 25 degrees celcius, and is resistant to copper(II) removal following administration of a high affinity copper(II) chelator at up to 80 degrees Celcius with 50 equivalents of competitor. Interactions of the model ligand with bovine serum albumin (BSA) were demonstrated using fluorescence spectroscopy, and the protective effects of BSA following administration of a reducing agent on the complex supports the formation of BSA-complex interactions in situ. Preliminary work has also shown the aforementioned ligand based electrochemistry is potentially sufficient to mediate some decomplexation. Cell studies involving four different cell lines have shown that membrane permeability is significantly affected by the peripheral substitution of the 2,2’-bisdipyrrin. Oligo(ethylene glycol) substituents and negative charges limit the membrane permeability, while charge neutral or positively charged derivatives with comparatively small substituents were shown to cross membranes with the highest efficacy. Preliminary radiolabelling of the 2,2’-bisdipyrrins is efficient under mild conditions, although the presence of the oligo(ethylene glycol) substituents inhibits this. Biodistribution of a representative complex labelled with copper-64 in mice is distinct from the biodistribution of the unchelated [64Cu]Cu(OAc)2, indicating in vivo stability following administration. The stability studies in conjunction with the membrane permeability and biodistribution of selected (2,2’-bisdipyrrinato) copper(II) complexes suggest this new family of complexes are superior to porphyrins for applications in diagnostic imaging.