Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: chemical oxidation; In-Situ; TCE. Includes bibliographical references (leaves 50-53).

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Hydrophobicity is the generalized characteristic of non-polar substances that brings about their exclusion from aqueous phases. This property, entropic in its nature, drives key self-assembly and phase separation processes in water. Protein folding, the formation of DNA double helix, the existence of lipid bilayers and the wetting properties of leaf surfaces are all due to hydrophobic interactions. Inspired by Nature, we aimed to use hydrophobicity for creating novel and improved catalytic systems. (I) A number of fluorous amphiphilic star block-copolymers containing a tris(benzyltriazolylmethyl)amine motif have been prepared. These polymers assembled into well-defined nanostructures in water, and their mode of assembly could be controlled by changing the composition of the polymer. The polymers were used for enzyme-inspired catalysis of alcohol oxidation. (II) An enzyme-inspired catalytic system based on a rationally designed multifunctional surfactant was developed. The resulting micelles feature metal-binding sites and stable free radical moieties as well as fluorous pockets that attract and preconcentrate molecular oxygen. In the presence of copper ions, the micelles effect chemoselective aerobic alcohol oxidation under ambient conditions in water, a transformation that is challenging to achieve nonenzymatically. (III) Development of a facile means of photo/electrocatalytic water splitting is one of the main barriers to establishing of a solar hydrogen economy. Of the two half-reactions involved in splitting water into O2 and H2, water oxidation presents the most challenge due to its mechanistic complexity. A practical water oxidation catalyst must be highly active, yet inexpensive and indefinitely stable under harsh oxidative conditions. Here, I shall describe the synthesis of a library of molecular water oxidation catalysts based on the Co complex of tris(2-benzimidazolylmethyl)amine, (BimH)3. A wide range of catalysts differing in their electronic properties, surface affinity, and steric bulk was explored. We identified hydrophobicity as the key variable in mediating the catalytic competence of Co-(BimH)3 complexes. The change in this parameter correlates both with the conformational mobility of the ligand core and the structural changes in the local solvent environment around the catalytic metal site. The optimal ligand identified is superhydrophobic due to three fluorinated side chains. The corresponding Co complex catalyzes water electrooxidation efficiently, with an onset potential equal to that for the well-established CoPi heterogeneous system, albeit with a dramatically higher turnover frequency (TOF) and in the absence of soluble Co salts. As an added benefit, the hydrophobic catalyst can be immobilized through physisorption, and remains stable after prolonged controlled-potential electrolysis. A DFT calculation was also performed to understand the catalytic pathway.

An increasing global energy demand requires alternative fuel sources. A promising method is artificial photosynthesis. Although, the artificial processes are different from the natural photosynthetic process, the basic principles are the same, i.e. to split water and to convert solar energy into chemical energy. The energy is stored in bonds, which can at a later stage be released upon combustion. The bottleneck in the artificial systems is the water oxidation. The aim of this research has been to develop catalysts for water oxidation that are stable, yet efficient. The molecular catalysts are comprised of organic ligands that ultimately are responsible for the catalyst structure and activity. These ligands are often based on polypyridines or other nitrogen-containing aromatic compounds. This thesis describes the development of molecular ruthenium catalysts and the evaluation of their ability to mediate chemical and photochemical oxidation of water. Previous work from our group has shown that the introduction of negatively charged groups into the ligand frameworks lowers the redox potentials of the metal complexes. This is beneficial as it makes it possible to drive water oxidation with [Ru(bpy)3]3+-type oxidants (bpy = 2,2’-bipyridine), which can be photochemically generated from the corresponding [Ru(bpy)3]2+ complex. Hence, all the designed ligands herein contain negatively charged groups in the coordination site for ruthenium. The first part of this thesis describes the development of two mononuclear ruthenium complexes and the evaluation of these for water oxidation. Both complexes displayed low redox potentials, allowing for water oxidation to be driven either chemically or photochemically using the mild one-electron oxidant [Ru(bpy)3]3+. The second part is a structure–activity relationship study on several analogues of mononuclear ruthenium complexes. The complexes were active for water oxidation and the redox potentials of the analogues displayed a linear relationship with the Hammet σmeta parameter. It was also found that the complexes form high-valent Ru(VI) species, which are responsible for mediating O–O bond formation. The last part of the thesis describes the development of a dinuclear ruthenium complex and the catalytic performance for chemical and photochemical water oxidation. It was found that the complex undergoes O–O bond formation via a bridging peroxide intermediate, i.e. an I2M–type mechanism.

Solar water-splitting is a potentially transformative renewable energy technology. Slow kinetics of the oxygen evolution reaction (OER) limit the efficiency of solar-water-splitting devices, thus constituting a hurdle to widespread implementation of this technology. Catalysts must be stable under highly oxidizing conditions in aqueous electrolyte and minimally absorb light. A grand goal of OER catalysis research is the design of new materials with higher efficiencies enabled by comprehensive understanding of the fundamental chemistry behind catalyst activity. However, little progress has been made towards this goal to date. This dissertation details work addressing major challenges in the field of OER catalysis. Chapter I introduces the current state-of-the-art and challenges in the field. Chapter II highlights work using ultra-thin films as a platform for fundamental study and comparison of catalyst activity. Key results of this work are (1) the identification of a Ni0.9Fe0.1OOH catalyst displaying the highest OER activity in base to date and (2) that in base, many transition-metal oxides transform to layered oxyhydroxide materials which are the active catalysts. The latter result is critical in the context of understanding structure-activity relationships in OER catalysts. Chapter III explores the optical properties of these catalysts, using in situ spectroelectrochemistry to quantify their optical absorption. A new figure-of-merit for catalyst performance is developed which considers both optical and kinetic losses due to the catalyst and describes how these factors together affect the efficiency of composite semiconductor/catalyst photoanodes. In Chapter IV, the fundamental structure-composition-activity relationships in Ni1-xFexOOH catalysts are systematically investigated. This work shows that nearly all previous studies of Ni-based catalysts were likely affected by the presence of Fe impurities, a realization which holds significant weight for future study of Ni-based catalyst materials. Chapter V discusses the synthesis of tin-titanium oxide nanoparticles with tunable lattice constants. These materials could be used to make high-surface-area supports for thin layers of OER catalysts, which is important for maximizing catalyst surface area, minimizing the use of precious-metal catalysts, and optimizing 3D structure for enhanced mass/bubble transport. Finally, Chapter VI summarizes this work and outlines directions for future research. This work contains previously published and unpublished co-authored material. / 2015-03-29

<p>The solar energy accumulated by photosynthesis over billions of years is the sole source of energy available on Earth. Photosystem II (PSII) uses the sunlight to split water, an energetically unfavorable reaction where electrons and protons are extracted from water and oxygen is released as a by-product. Understanding this process is crucial for the future development of clean, renewable and unlimited energy sources, which can use sunlight to split water and produce hydrogen and electricity. In order to do so we need to understand how this is solved in plants.</p><p>I have been focusing on the role of two lumenal proteins associated with the thylakoid membrane PsbO and Cah3, in the water oxidation process. Convincing evidences have been presented supporting the hypothesis that bicarbonate acts as a proton acceptor in the water splitting process in PSII and the lumenal carbonic anhydrase, Cah3, supplies bicarbonate required for this function. The PsbO protein, an important constituent of the water-oxidizing complex, however, its function is still unknown. The PsbO protein undergoes a pH dependent conformational change that in turn influences its capacity to bind calcium and manganese, forming a catalytic Mn4Ca cluster in PSII. We propose that light-induced structural dynamics of the PsbO is of functional relevance for the regulation of proton release and for forming a proton sensing - proton transporting pathway. The cluster of conserved glutamic and aspartic acid residues in the PsbO protein acts as buffering antennae providing efficient acceptors of protons derived from substrate water molecules. Both proteins, Cah3 and PsbO have a conserved S-S bridge, required for proper folding and activity; therefore they are potential targets for red-ox regulation in lumen.</p> / <p>Solenergi som omvandlats av fotosyntesen under miljarder av år är basen för nästan all energi på jorden. Fotosystem 2 använder solljuset till att oxidera vatten, ur energisynpunkt en ofördelaktig process, där elektroner och protoner extraheras från vattenmolekyler vilket ger upphov till syrgas som biprodukt. Förståelsen av denna process är viktig för att vi i framtiden skall kunna utveckla rena och förnyelsebara energislag i obegrensad mängd. Genom att efterlikna fotosyntesprocessen skulle vi i framtiden kunna utvecka artificiella system som använder solljuset till att sönderdela vatten för att producera vätgas eller elektrisitet. För att kunna göra det så måste vi kunna förstå hur dessa processer fungerar i växterna.</p><p>Min forskning har fokuserat på att förstå funktionen hos två av de proteiner, PsbO och Cah3, som deltar i sönderdelningen av vatten. Jag har visat, för första gången, att ett lumen karboanhydras, Cah3, deltar i regleringen av den process där vatten spjälkas. Jag postulerar att Cah3 underlättar bort transporten av protoner från det vattenoxiderande komplexet genom att generera bikarbonat lokalt, som kan fungera som proton transportör. PsbO proteinet genomgår en pH beroende konformationsförändring vilket påverkar dess kapacitet and binda calcium och mangan som i sin tur formar ett katalytiskt Mn4Ca center i fotosystem 2. Jag föreslår att en ljusberoende strukturförändring av Psbo är av funktionell betydelse för regleringen av protonfrigörandet och formar ett proton-avkännande och proton-transporterande system. Ett kluster av konserverande glutamat- och aspartat-aminosyror i PsbO proteinet fungerar som ett buffrande nätverk för protoner som frigörs vid oxidering av vatten. Båda dessa proteiner innerhåller S-S bryggor ock kan därför vara red-ox reglerade i lumen.</p>

The solar energy accumulated by photosynthesis over billions of years is the sole source of energy available on Earth. Photosystem II (PSII) uses the sunlight to split water, an energetically unfavorable reaction where electrons and protons are extracted from water and oxygen is released as a by-product. Understanding this process is crucial for the future development of clean, renewable and unlimited energy sources, which can use sunlight to split water and produce hydrogen and electricity. In order to do so we need to understand how this is solved in plants. I have been focusing on the role of two lumenal proteins associated with the thylakoid membrane PsbO and Cah3, in the water oxidation process. Convincing evidences have been presented supporting the hypothesis that bicarbonate acts as a proton acceptor in the water splitting process in PSII and the lumenal carbonic anhydrase, Cah3, supplies bicarbonate required for this function. The PsbO protein, an important constituent of the water-oxidizing complex, however, its function is still unknown. The PsbO protein undergoes a pH dependent conformational change that in turn influences its capacity to bind calcium and manganese, forming a catalytic Mn4Ca cluster in PSII. We propose that light-induced structural dynamics of the PsbO is of functional relevance for the regulation of proton release and for forming a proton sensing - proton transporting pathway. The cluster of conserved glutamic and aspartic acid residues in the PsbO protein acts as buffering antennae providing efficient acceptors of protons derived from substrate water molecules. Both proteins, Cah3 and PsbO have a conserved S-S bridge, required for proper folding and activity; therefore they are potential targets for red-ox regulation in lumen. / Solenergi som omvandlats av fotosyntesen under miljarder av år är basen för nästan all energi på jorden. Fotosystem 2 använder solljuset till att oxidera vatten, ur energisynpunkt en ofördelaktig process, där elektroner och protoner extraheras från vattenmolekyler vilket ger upphov till syrgas som biprodukt. Förståelsen av denna process är viktig för att vi i framtiden skall kunna utveckla rena och förnyelsebara energislag i obegrensad mängd. Genom att efterlikna fotosyntesprocessen skulle vi i framtiden kunna utvecka artificiella system som använder solljuset till att sönderdela vatten för att producera vätgas eller elektrisitet. För att kunna göra det så måste vi kunna förstå hur dessa processer fungerar i växterna. Min forskning har fokuserat på att förstå funktionen hos två av de proteiner, PsbO och Cah3, som deltar i sönderdelningen av vatten. Jag har visat, för första gången, att ett lumen karboanhydras, Cah3, deltar i regleringen av den process där vatten spjälkas. Jag postulerar att Cah3 underlättar bort transporten av protoner från det vattenoxiderande komplexet genom att generera bikarbonat lokalt, som kan fungera som proton transportör. PsbO proteinet genomgår en pH beroende konformationsförändring vilket påverkar dess kapacitet and binda calcium och mangan som i sin tur formar ett katalytiskt Mn4Ca center i fotosystem 2. Jag föreslår att en ljusberoende strukturförändring av Psbo är av funktionell betydelse för regleringen av protonfrigörandet och formar ett proton-avkännande och proton-transporterande system. Ett kluster av konserverande glutamat- och aspartat-aminosyror i PsbO proteinet fungerar som ett buffrande nätverk för protoner som frigörs vid oxidering av vatten. Båda dessa proteiner innerhåller S-S bryggor ock kan därför vara red-ox reglerade i lumen.

El objeto principal de mi tesis es el desarrollo de nuevos complejos polipiridílicos de rutenio, mas concretamente complejos Ru-aguo. Una de las propediades mas interesantes que presentan este tipo de complejos es su capacidad para llevar a cabo un proceso de ”Proton Coupled Electron Transfer”. Este proceso le permite acceder a estados de oxidación elevados. Este es un requisito para ejecutar catálisis redox. Como interesante aplicaciones podemos nombrar la transformación de sulfitos a sulfoxidos (sulfoxidación) y la oxidación de agua a oxígeno molecular. Al principio del manuscrito se presentan complejos mononucleares de rutenio que contienen el ligando DMSO. Los resultados obtenidos respecto la isomerización de enlace están conectados con los resultados obtenidos durante la sulfoxidación utilizando los correspondientes complejos Ru(H2O). El segundo campo de interés esta basado en la oxidación de agua a oxigeno molecular. Una variedad de complejos Ru(H2O) mono- y dinucleares son presentados. Estos se distinguen entre si en sus propiedades estéricas y electrónicas. Estas propiedades están realizadas con su reactividad respeto la oxidación de agua a oxígeno molecular. Los resultados obtenidos permiten tener un mayor conocimiento sobre el mecanismo, formando una base para el futuro desarrollo de nuevos catalizadores para la oxidación de agua. Un punto culminante de esta tesis es la presentación de un sistema capaz de oxidar agua a oxigeno utilizando la irradiación con luz visible. Este sistema consiste en uno de los complejos Ru(H2O) anteriormente descritos, un “photosensitizer” ([Ru(bpy)3]2+) y un aceptor de electrones ([CoCl(NH3)5]2+). Este tipo de reacción juega un papel importante como reacción en el ánodo / The main topic of my thesis is the development of new Ruthenium based polypyridyl complexes, with the focus on Ru-aquo compounds. These Ru-aquo compounds present the interesting property of undergoing proton coupled electron processes. This allows such compounds to reach high oxidation states, which is a requirement in redox catalysis. Applications of interest involve the transformation of sulfites into sulfoxides (sulfoxidation) and the oxidation of water to molecular oxygen (water oxidation). At first different mononuclear ruthenium compounds containing one or two DMSO ligands are presented. The obtained findings concerning S- to O-linkage isomerization are transferred and connected to results from catalysis using Ru(H2O)-compounds as catalysts. The second field of interest concerns water oxidation. A variety of mono- and dinuclear Ru(H2O)-compounds differing in electronic and steric properties are described. These differences are connected to the catalytic activity towards water oxidation reaction. New insights in the underlying reaction mechanisms are provided, which constitute a strong basis for the development of future water oxidation catalysts. One highlight of this thesis is the introduction of mononuclear Ru(H2O)-compounds combined with a [Ru(bpy)3]2+ and a CoIII sacrificial electron acceptor, which successfully produce molecular oxygen upon light irradiation. Light driven water oxidation as anode reaction and the recombination of the electrons and protons from the oxidation process at the cathode form a device called solar fuel cell, that once fully mature may play an important role in a future renewable energy system.

abstract: Natural photosynthesis features a complex biophysical/chemical process that requires sunlight to produce energy rich products. It is one of the most important processes responsible for the appearance and sustainability of life on earth. The first part of the thesis focuses on understanding the mechanisms involved in regulation of light harvesting, which is necessary to balance the absorption and utilization of light energy and in that way reduce the effect caused by photooxidative damage. In photosynthesis, carotenoids are responsible not only for collection of light, but also play a major role in protecting the photosynthetic system. To investigate the role of carotenoids in the quenching of the excited state of cyclic tetrapyrroles, two sets of dyads were studied. Both sets of dyads contain zinc phthalocyanine (Pc) covalently attached to carotenoids of varying conjugation lengths. In the first set of dyads, carotenoids were attached to the phthalocyanine via amide linkage. This set of dyads serves as a good model for understanding the molecular "gear-shift" mechanism, where the addition of one double bond can turn the carotenoid from a nonquencher to a very strong quencher of the excited state of a tetrapyrrole. In the second set of dyads, carotenoids were attached to phthalocyanine via a phenyl amino group. Two independent studies were performed on these dyads: femtosecond transient absorption and steady state fluorescence induced by two-photon excitation. In the transient absorption study it was observed that there is an instantaneous population of the carotenoid S1 state after Pc excitation, while two-photon excitation of the optically forbidden carotenoid S1 state shows 1Pc population. Both observations provide a strong indication of the existence of a shared excitonic state between carotenoid and Pc. Similar results were observed in LHC II complexes in plants, supporting the role of such interactions in photosynthetic down regulation. In the second chapter we describe the synthesis of porphyrin dyes functionalized with carboxylate and phosphonate anchoring groups to be used in the construction of photoelectrochemical cells containing a porphyrin-IrO2&middot;nH2O complex immobilized on a TiO2 electrode. The research presented here is a step in the development of high potential porphyrin-metal oxide complexes to be used in the photooxidation of water. The last chapter focuses on developing synthetic strategies for the construction of an artificial antenna system consisting of porphyrin-silver nanoparticle conjugates, linked by DNA of varied length to study the distance dependence of the interaction between nanoparticles and the porphyrin chromophore. Preliminary studies indicate that at the distance of about 7-10 nm between porphyrin and silver nanoparticle is where the porphyrin absorption leading to fluorescence shows maximum enhancement. These new hybrid constructs will be helpful for designing efficient light harvesting systems. / Dissertation/Thesis / Ph.D. Chemistry 2011

abstract: Photosynthesis, one of the most important processes in nature, has provided an energy basis for nearly all life on Earth, as well as the fossil fuels we use today to power modern society. This research aims to mimic the photosynthetic process of converting incident solar energy into chemical potential energy in the form of a fuel via systems capable of carrying out photo-induced electron transfer to drive the production of hydrogen from water. Herein is detailed progress in using photo-induced stepwise electron transfer to drive the oxidation of water and reduction of protons to hydrogen. In the design, use of more blue absorbing porphyrin dyes to generate high-potential intermediates for oxidizing water and more red absorbing phthalocyanine dyes for forming the low potential charge needed for the production of hydrogen have been utilized. For investigating water oxidation at the photoanode, high potential porphyrins such as, bis-pyridyl porphyrins and pentafluorophenyl porphyrins have been synthesized and experiments have aimed at the co-immobilization of this dye with an IrO2-nH2O catalyst on TiO2. To drive the cathodic reaction of the water splitting photoelectrochemical cell, utilization of silicon octabutoxy-phthalocyanines have been explored, as they offer good absorption in the red to near infrared, coupled with low potential photo-excited states. Axially and peripherally substituted phthalocyanines bearing carboxylic anchoring groups for the immobilization on semiconductors such as TiO2 has been investigated. Ultimately, this work should culminate in a photoelectrochemical cell capable of splitting water to oxygen and hydrogen with the only energy input from light. A series of perylene dyes bearing multiple semi-conducting metal oxide anchoring groups have been synthesized and studied. Results have shown interfacial electron transfer between these perylenes and TiO2 nanoparticles encapsulated within reverse micelles and naked nanoparticles. The binding process was followed by monitoring the hypsochromic shift of the dye absorption spectra over time. Photoinduced electron transfer from the singlet excited state of the perylenes to the TiO2 conduction band is indicated by emission quenching of the TiO2-bound form of the dyes and confirmed by transient absorption measurements of the radical cation of the dyes and free carriers (injected electrons) in the TiO2. / Dissertation/Thesis / Ph.D. Chemistry 2013