<p>Amino acid radicals are key redox intermediates in several natural enzymes including Cytochrome c peroxidase, DNA photolyase, ribonucletide reductase, cytochrome c oxidase and photosystem II. Electron transfer from amino acids is often coupled to deprotonation and this thesis concerns the coupling of electron transfer from tyrosine and tryptophan to trisbipyridineruthenium(III) with deprotonation in model complexes. Specifically the mechanisms for these proton coupled electron transfer reactions have been studied and the controlling parameters have been identified, the possible mechanisms being stepwise electron transfer followed by deprotonation and deprotonation followed by electron transfer or concerted electron transfer/deprotonation.</p><p>Proton coupled electron transfer reactions have been studied using nano-second flash photolysis in water solution and the effect of pH, temperature, reaction driving force, deuteration and nature of the amino acid has been determined. I have shown that the rate constant for the concerted reaction depends intrinsically on the mixing entropy of the released proton and that the pH-dependence can be used as an experimental tool for mechanistic discrimination. Moreover I have shown that the concerted reaction inherently has a high reorganisation energy due to the coupling of the electron motion with deprotonation. Hydrogen bonding to the transferring proton however significantly reduces this reorganisation energy. The concerted reaction also has a relatively high driving force counteracting the high reorganisation energy in the competition between the concerted reaction and the stepwise electron transfer first reaction. The relative importance of the high reorganisation energy and the high driving force for the concerted reaction determines the mechanistic outcome of the reaction, the stepwise reaction being favoured by high over-all driving forces and the concerted reaction by high pH.</p><p>By comparing my results from model complexes with tyrosineZ oxidation in photosystem II, I give strong evidence for a concerted electron transfer/deprotonation mechanism.</p>

Amino acid radicals are key redox intermediates in several natural enzymes including Cytochrome c peroxidase, DNA photolyase, ribonucletide reductase, cytochrome c oxidase and photosystem II. Electron transfer from amino acids is often coupled to deprotonation and this thesis concerns the coupling of electron transfer from tyrosine and tryptophan to trisbipyridineruthenium(III) with deprotonation in model complexes. Specifically the mechanisms for these proton coupled electron transfer reactions have been studied and the controlling parameters have been identified, the possible mechanisms being stepwise electron transfer followed by deprotonation and deprotonation followed by electron transfer or concerted electron transfer/deprotonation. Proton coupled electron transfer reactions have been studied using nano-second flash photolysis in water solution and the effect of pH, temperature, reaction driving force, deuteration and nature of the amino acid has been determined. I have shown that the rate constant for the concerted reaction depends intrinsically on the mixing entropy of the released proton and that the pH-dependence can be used as an experimental tool for mechanistic discrimination. Moreover I have shown that the concerted reaction inherently has a high reorganisation energy due to the coupling of the electron motion with deprotonation. Hydrogen bonding to the transferring proton however significantly reduces this reorganisation energy. The concerted reaction also has a relatively high driving force counteracting the high reorganisation energy in the competition between the concerted reaction and the stepwise electron transfer first reaction. The relative importance of the high reorganisation energy and the high driving force for the concerted reaction determines the mechanistic outcome of the reaction, the stepwise reaction being favoured by high over-all driving forces and the concerted reaction by high pH. By comparing my results from model complexes with tyrosineZ oxidation in photosystem II, I give strong evidence for a concerted electron transfer/deprotonation mechanism.

Photoinduced electron transfer is a fundamentally interesting process; it occurs everywhere in the natural world. Studies on electron transfer shed light on questions about the interaction between molecules and how the dynamics of these can be utilized to steer the electron transfer processes to achieve a desired goal. The goal may be to get electrons to the electrode of a solar cell, or to make the electrons form an energy rich fuel such as hydrogen, and it may also be an input or output for molecular switches. The importance of electron transfer reactions will be highlighted in this thesis, however, the main motivation is to gain a better understanding of the fundamental processes that affect the rate and direction of the electron transfer. A study of photoinduced electron transfer (ET) in a series of metallophorphyrin/bipyridinium complexes in aqueous solution provided fresh insight concerning the intimate relationship between vibrational relaxation and electron transfer. The forward electron transfer from porphyrin to bipyridinium as well as the following back electron transfer to the ground state could be observed by femtosecond transient absorption spectroscopy. Both the reactant and the product states of the ET processes were vibrationally unrelaxed, in contrary to what is assumed for most expressions of the ET rates. This could be understood from the observation of unrelaxed ground states. The excess energy given by the initial excitation of the porphyrin does not relax completely during the two steps of electron transfer. This is an unusual observation, not reported in the literature prior the studies presented in this thesis. This study also gave the first clear evidence of electronically excited radical pairs formed as products of intramolecular electron transfer. Signs of electronically excited radical pairs were seen in transient spectra, and were further verified by the observation that the rates followed a Marcus normal region behavior for all excitation wavelengths, despite the relatively large excess energy of the second excited state. This thesis also concerns electron transfer in solar cell dyes and mixed valence complexes. In the ruthenium polypyridyl complex Ru(dcb)2(NCS)2, where dcb = 4,4’-dicarboxy-2,2’-bipyridine, inter-ligand electron transfer (ILET) in the 3MLCT state was followed by means of femtosecond transient absorption anisotropy that was probed in the mid-IR region. Unexpectedly, ILET was not observed because electron density was localized on the same bpy during the time-window allowed by the rotational lifetime.

Le développement de sources d’énergie renouvelables telles que les combustibles solaires est une question cruciale dans le contexte actuel du réchauffement de la planète. L'eau est une source abondante, respectueuse de l'environnement, bon marché et abondante en électrons et en protons nécessaires à la production de combustible. Par conséquent, l'oxydation de l'eau activée par la lumière est une étape clé de la photosynthèse artificielle et le développement de catalyseurs efficaces, robustes et durables constitue un objectif important pour les chimistes. Dans la première partie de cette étude, nous nous concentrons sur le développement de tels catalyseurs basés sur des complexes métalliques à base de métaux de la première série des éléments de transition tel que le cuivre pour cette étude. L'électrocatalyse et la photocatalyse par oxydation de l'eau ont été étudiées. La deuxième partie du travail concerne la formation de paires d'ions entre les espèces à double charge opposée du catalyseur complexe et de l'accepteur d'électrons et du photosensibilisant et du catalyseur complexe. Cette étude devrait apporter des preuves solides de l'influence de chaque composant du photosystème par l'association et la dissociation de paires d'ions.Dans la troisième partie, nous étudions un système synthétique sensibilisant-catalyseur capable de photoactiver une molécule d’eau liée à l’unité catalytique par le biais d’une oxydation à deux électrons et à deux protons, réalisant toute la caractérisation photophysique de la dyade. Par conséquent, l’étude des complexes moléculaires pour la photosynthèse artificielle fournit diverses orientations pour développer le rendement d’utilisation de l’énergie solaire. / Development of renewable energy sources like solar fuels is a crucial issue in the actual context of global warming. Water is an environmentally friendly, cheap and abundant source of the electrons and protons needed for fuel production. Therefore, light-activated water oxidation is a key step in artificial photosynthesis and the development of efficient, robust and sustainable catalysts is an important goal for chemists. In the first part of this study, we focus on the development of such catalysts based on earth abundant copper complexes. The water oxidation electrocatalysis and photocatalysis were investigated. The second part of the work concerns the ion pair formation between the oppositely double charged species of complex catalyst and electron acceptor and Photosensitizer and complex catalyst are investigated. This study should bring solid evidence on the influence of each component in photosystem through the ion pair association and dissociation. In the third part, we study a synthetic sensitizer-catalyst system that can photoactivate a water molecule bound to the catalytic unit through a two-electron, two-proton abstraction, performed all the photophysical characterization of the dyad. Therefore, studying molecular complexes for artificial photosynthesis provides diverse direction to develop the utilization efficiency of solar energy.

No description available.

Nanoelectrochemistry explores the differences in chemical behaviour at the nanoscale as compared to the macro-scale. This thesis is concerned with nanoelectrochemistry and aims to develop and apply novel experiments for the unambiguous identification of changed electrode kinetics at the nanoscale. This is challenging since electrochemical responses are controlled by diverse factors like enhanced mass transport and adsorption as well as electron transfer kinetics. A joint computational and experimental strategy is employed. Chapter 1, 2 and 3 cover essential introductory material and basic experimental details relevant to all experiment. Fuller descriptions and details are given in the following chapters as and when needed. Chapter 4 reports the development of an electrochemical characterization method, to achieve a fast and simple quantification of the average particle size and the number of nanoparticles deposited on a glassy carbon electrode. The method consists of surface area characterization by underpotential deposition of lead particles and the determination of the amount of gold from anodic stripping in HCl. This method is also proven to be effective by comparing the results with SEM measurements. Next, in chapter 5, a generic strategy combining computation and experimental approach is developed in order to study the electron transfer kinetics of gold nanoparticles. The modelling part considers the kinetics of the electrochemical process on the bulk materials for different regions in the electrode, that is, the substrate (glassy carbon) and the nanoparticles (gold). Comparison of experimental and theoretical results enables the detection of changes in the electrode kinetics at the nanoscale. This approach is applied into the electro-oxidations of nitrite and L-ascorbic acid for gold nanoparticles from 20 - 90 nm. In the former, analysing the system shows that no change in electron transfer kinetics is involved in the process, even though a decrease of the over-potential and an increase in the peak current are observed. But these changes reflect mass transport effects, not electrocatalysis. A case where an authentic enhanced electron transfer kinetic change occurs is shown for the ascorbic acid system. Finally, in chapter 6 , the above strategy is exploited further to apply a quantitative study of electron transfer kinetics for various sizes of gold nanoparticles in the oxygen reduction reaction system in sulphuric acid at 298 K. The latter is at the heart of energy transformation techniques (fuel cells, battery and so on). Compared with the electron transfer kinetics on macro gold electrodes, there is no change at gold nanoparticles from size 5 nm to 40 nm. However, in the presence of Pb(II), a strong enhancement of electron transfer kinetics is observed on 5 nm citrate capped gold nanoparticles for ORR. On the other hand, a significant decrease of electron transfer kinetics has been found for gold nanoparticles of size 2 nm for ORR. The latter observation of strong negative electrocatalysis is also observed for the hydrogen evolution reaction (HER). This represents the first report of such effects with the HER system. Overall the thesis has established a rigurous, theoretical basis for evaluating electrocatalysis in nanoparticulate system.

<p>This thesis deals with the impact of hydrogen bonding on the properties of phenols. The possibility for tyrosine to form hydrogen bonds to other amino acids has been found to be important for its function as an electron transfer mediator in a number of important redox enzymes. This study has focused on modeling the function of tyrosine in Photosystem II, a crucial enzyme in the photosynthetic pathway of green plants.</p><p>Hydrogen bonds between phenol and amines in both inter- and intramolecular systems have been studied with quantum chemical calculations and also in some solid-state structures involving phenol and imidazole.</p><p>Different phenols linked to amines have been synthesized and their possibilities of forming intra- and intermolecular hydrogen bonds have been studied as well as the thermodynamics and kinetics of the generation of phenoxyl radicals via oxidation reactions.</p><p>Since carboxylates may in principle act as hydrogen bond acceptors in a manner similar to imidazole, proton coupled electron transfer has also been studied for a few phenols intramolecularly hydrogen bonded to carboxylates with the aim to elucidate the mechanism for oxidation. Electron transfer in a new linked phenol—ruthenium(II)trisbipyridine complex was studied as well.</p><p>The knowledge is important for the ultimate goal of the project, which is to transform solar energy into a fuel by an artificial mimic of the natural photosynthetic apparatus</p>

This thesis examines the synthesis of conformationally constrained β-amino acids and β- peptides, and the electron transfer properties of the latter when immobilised on gold. Additionally, cross metathesis on gold was investigated as a method for surface functionalisation. Chapter One introduces the concepts of electron transfer in nature, how it is facilitated by the secondary structure in α-peptides, and why β-peptides might be useful for studying electron transfer. This is followed by a discussion of the properties of β-peptides, including the enhanced stability and variety of helical secondary structures and the greater potential for functionalisation of the peptide backbone when compared to α-peptides. Finally, the conformational constraints of ring-systems on cyclic amino acids is discussed, with reference to the stabilising effect of these compounds on peptide secondary structures. Chapter Two describes the electrochemical analysis of β-hexapeptides immobilised on gold. The chapter is prefaced by a discussion of the important electron transfer mechanisms for peptides, the fabrication of peptide-gold self-assembled monolayer (SAM) interfaces, and the electron transfer in helical α-peptides. β-Peptides containing an electroactive ferrocene moeity were immobilised on gold and studied using cyclic voltammetry and chronoamperometry. The latter method was used to examine the dependence of the electron transfer rate on overpotential, thereby determining the likely mode of electron transfer through the β-peptides SSβ₆Fc, Fcβ₆SS and SC₁₅β₆Fc. These peptides exhibited very weak dependence on overpotential, characteristic of electron transfer behaviour of an electron hopping mechanism (which is also thought to occur in helical α-peptides). Both the dipole moment of the peptides and the structure of the sulfurlinker group were found to be important in determining the rate of electron transfer. Conversely, the equivalent α-peptide SSα₆Fc exhibited electron transfer behaviour characteristic of the less efficient tunnelling mechanism, which is thought to operate in strand-like peptides. Chapter Three examines the application of cross metathesis, using a Grubbs' second generation catalyst, as a means to functionalise olefin-terminated self-assembled monolayers on gold. Abstract iv Firstly, an introduction into the limited published research on cross metathesis on both planar surfaces and nanoparticles is given. Olefin-terminated thiol 3.18, suitable for immobilisation on gold, and solution phase olefin-terminated ferrocene 3.10 were synthesised as reactants for cross metathesis studies. An analytical methodology was developed involving the cross metathesis of surface-immobilised 3.18 with ferrocene 3.10 in dichloromethane, whereby the concentration of electroactive cross metathesis product 3.22 was monitored electrochemically as a function of time. The concentration of surface-immobilised product 3.22 was determined by integration of the oxidation peak area and found to be highly dependent on both the concentration of immobilised olefin reactant 3.18 and reaction time. Furthermore, the surface concentration of ferrocenyl model disulfide 3.21 and thiol 2.18 decayed markedly upon addition of Grubb's catalyst, as revealed by the decrease in the oxidation peak area, which suggested that catalystmediated desorption was occurring. Chapter Four details the solution-phase synthesis of ferrocene- and thiol-functionalised β- hexapeptides used in both the electron transfer studies described in chapter two, and in the determination of secondary structure using circular dichroism and NMR techniques. The synthesis of simple model compounds 4.14, 4.16 and 4.18 established the incompatibility of the deprotection of methyl and benzyl ester protecting groups with protected-thiol and disulfide linkers, leading to the use of N-hydroxysuccinmide-activated sulfur-linkers 4.20 and 4.22 in further synthesis. A number of β-hexapeptides were synthesised by amide coupling of β- tripeptides functionalised at the N- and C-termini. Structural studies of the methanol soluble β- hexapeptide 4.60 suggested that the covalent attachment of ferrocene moeity to the C-terminus of a β-peptide did not disrupt the formation of a 14-helix in solution. β-peptides containing functionality at both the C- and N-termini (such as SSβ₆Fc, SSβ₆Et and acetyl-protected SC₁₅β₆Fc) were not suitable for solution phase structural studies; however, molecular modelling suggested that helical conformations are the most stable these β-peptides in solution phase. Chapter Five outlines the synthesis of novel cyclic β-amino acids by two different general synthetic routes. The first uses an efficient conjugate addition/fluorination reaction of α,β- unsaturated esters with lithiated chiral secondary amines to prepare the novel cyclopentyl- and cyclohexyl-based fluorinated β-amino acids 2.43a and 2.43b. The high diastereoselectivity of this reaction, which introduces two stereocentres into the achiral unsaturated esters, is directed by the configuration of the attacking amine. The second methodology utilizes the versatile ringclosing metathesis reaction in the synthesis of novel cyclic β-amino acids. A stereoselective Abstract v trans-alkylation of olefinic β-amino acids gave the required β-dienes 5.62 and 5.77. Optimised cyclisation yields were achieved with a Grubb's 2nd generation catalyst for diene 5.62 and Grubb's 1st generation catalyst for diene 5.77, to give the trans-cycloheptyl- and cyclooctylbased β-amino acids 5.63 and 5.78, respectively. The attempted synthesis of cyclononyl-based β-amino acid 5.87 using both catalysts yielded only cyclic dimer products 5.88 and 5.89. The trans configuration of the 5.62 diene was confirmed by x-ray crystallography. Chapter Six is an experimental chapter and outlines the electrochemical setup and analysis, and the synthesis, purification and characterisation of compounds described in this thesis.

No description available.

The theoretical model of Beratan and Onuchic predicts a large attenuation of ET rates through hydrogen bonds; however, the effect of individual hydrogen bond on electron transfer reaction has not been systematically studied. The organic complexes in this study are a series of crown ether/ammonium salt, which incorporate a redox partner on each component of the complex. The dimethoxynaphthalene redox donor was attached to the crown ether and a series of ammonium salts was synthesized which bear substituted quinone and naphthoquinone acceptor. The complexes characterization and preliminary electron transfer rate measurement were completed with UV/Vis and steady-state emission spectroscopy.