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Description
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.
ContributorsBergkamp, Jesse J (Author) / Moore, Ana L (Thesis advisor) / Mariño-Ochoa, Ernesto (Thesis advisor) / Gust, Devens J (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2013
Description
Solar energy is a promising alternative for addressing the world's current and future energy requirements in a sustainable way. Because solar irradiation is intermittent, it is necessary to store this energy in the form of a fuel so it can be used when required. The light-driven splitting of water into oxygen and hydrogen (a useful chemical fuel) is a fascinating theoretical and experimental challenge that is worth pursuing because the advance of the knowledge that it implies and the availability of water and sunlight. Inspired by natural photosynthesis and building on previous work from our laboratory, this dissertation focuses on the development of water-splitting dye-sensitized photoelectrochemical tandem cells (WSDSPETCs). The design, synthesis, and characterization of high-potential porphyrins and metal-free phthalocyanines with phosphonic anchoring groups are reported. Photocurrents measured for WSDSPETCs made with some of these dyes co-adsorbed with molecular or colloidal catalysts on TiO2 electrodes are reported as well. To guide in the design of new molecules we have used computational quantum chemistry extensively. Linear correlations between calculated frontier molecular orbital energies and redox potentials were built and tested at multiple levels of theory (from semi-empirical methods to density functional theory). Strong correlations (with r2 values > 0.99) with very good predictive abilities (rmsd < 50 mV) were found when using density functional theory (DFT) combined with a continuum solvent model. DFT was also used to aid in the elucidation of the mechanism of the thermal relaxation observed for the charge-separated state of a molecular triad that mimics the photo-induced proton coupled electron transfer of the tyrosine-histidine redox relay in the reaction center of Photosystem II. It was found that the inclusion of explicit solvent molecules, hydrogen bonded to specific sites within the molecular triad, was essential to explain the observed thermal relaxation. These results are relevant for both advancing the knowledge about natural photosynthesis and for the future design of new molecules for WSDSPETCs.
ContributorsMéndez-Hernández, Dalvin D (Author) / Moore, Ana L (Thesis advisor) / Mujica, Vladimiro (Thesis advisor) / Gust, Devens J. (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2014
Description
[FeFe]-hydrogenases are enzymes for the reduction of protons to hydrogen. They rely on only the earth abundant first-row transition metal iron at their active site (H cluster). In recent years, a multitude of diiron mimics of hydrogenases have been synthesized, but none of them catalyzes hydrogen production with the same exquisite combination of high turnover frequency and low activation energy as the enzymes. Generally, model complexes fail to include one or both of two features essential to the natural enzyme: an intricate array of outer coordination sphere contacts that constrain the coordination geometry to attain a catalytically optimal conformation, and the redox non-innocence of accessory [FeS] clusters found at or near the hydrogen-activating site. The work presented herein describes the synthesis and electrocatalytic characterization of iron-dithiolate models designed to incorporate these features. First, synthetic strategies are developed for constructing peptides with artificial metal-binding motifs, such as 1,3-dithiolate and phosphines, which are utilized to append diiron-polycarbonyl clusters onto a peptide. The phosphine-functionalized peptides are shown to be better electrocatalysts for proton reduction in water/acetonitrile mixtures than in neat acetonitrile. Second, we report the impact of redox non-innocent ligands on the electrocatalytic properties of two types of [FeFe]-hydrogenase models: dinuclear and mononuclear iron complexes. The bidentate, redox non-innocent α-diimine ligands (N-N), 2,2'-bipyridine and 2,2' bipyrimidine, are used to create complexes with the general formula (μ-SRS)Fe2(CO)4(N-N), new members of the well known family of asymmetric diiron carbonyls. While the 2,2'-bipyridine derivatives can act as electrocatalysts for proton reduction, surprisingly, the 2,2'-bipyrimidine analogues are found to be inactive towards catalysis. Electrochemical investigation of two related Fe(II) complexes, (bdt)Fe(CO)P2 for bdt = benzene-1,2-dithiolate and P2 = 1,1'-diphenylphosphinoferrocene or methyl-2-{bis(diphenylphosphinomethylamino}acetate, related to the distal iron in [FeFe]-hydrogenase show that these complexes catalyze the reduction of protons under mild conditions. However, their reactivities toward the external ligand CO are distinguished by gross geometrical differences.
ContributorsRoy, Souvik (Author) / Jones, Anne K (Thesis advisor) / Moore, Thomas (Committee member) / Trovitch, Ryan (Committee member) / Arizona State University (Publisher)
Created2013
Description
Natural photosynthesis dedicates specific proteins to achieve the modular division of the essential roles of solar energy harvesting, charge separation and carrier transport within natural photosynthesis. The modern understanding of the fundamental photochemistry by which natural photosynthesis operates is well advanced and solution state mimics of the key photochemical processes have been reported previously. All of the early events in natural photosynthesis responsible for the conversion of solar energy to electric potential energy occur within proteins and phospholipid membranes that act as scaffolds for arranging the active chromophores. Accordingly, for creating artificial photovoltaic (PV) systems, scaffolds are required to imbue structure to the systems. An approach to incorporating modular design into solid-state organic mimics of the natural system is presented together with how conductive scaffolds can be utilized in organic PV systems. To support the chromophore arrays present within this design and to extract separated charges from within the structure, linear pyrazine-containing molecular ribbons were chosen as candidates for forming conductive linear scaffolds that could be functionalized orthogonally to the linear axis. A series of donor-wire-acceptor (D-W-A) compounds employing porphyrins as the donors and a C60 fullerene adduct as the acceptors have been synthesized for studying the ability of the pyrazine-containing hetero-aromatic wires to mediate photoinduced electron transfer between the porphyrin donor and fullerene acceptor. Appropriate substitutions were made and the necessary model compounds useful for dissecting the complex photochemistry that the series is expected to display were also synthesized. A dye was synthesized using a pyrazine-containing heteroaromatic spacer that features two porphyrin chromophores. The dye dramatically outperforms the control dye featuring the same porphyrin and a simple benzoic acid linker. A novel, highly soluble 6+kDa extended phthalocyanine was also synthesized and exhibits absorption out to 900nm. The extensive functionalization of the extended phthalocyanine core with dodecyl groups enabled purification and characterization of an otherwise insoluble entity. Finally, in the interest of incorporating modular design into plastic solar cells, a series of porphyrin-containing monomers have been synthesized that are intended to form dyadic and triadic molecular-heterojunction polymers with dedicated hole and electron transport pathways during electrochemical polymerization.
ContributorsWatson, Brian Lyndon (Author) / Gust, Devens (Thesis advisor) / Gould, Ian (Committee member) / Moore, Ana L (Committee member) / Arizona State University (Publisher)
Created2013
Description
Thiol functionalization is one potentially useful way to tailor physical and chemical properties of graphene oxides (GOs) and reduced graphene oxides (RGOs). Despite the ubiquitous presence of thiol functional groups in diverse chemical systems, efficient thiol functionalization has been challenging for GOs and RGOs, or for carbonaceous materials in general. In this work, thionation of GOs has been achieved in high yield through two new methods that also allow concomitant chemical reduction/thermal reduction of GOs; a solid-gas metathetical reaction method with boron sulfides (BxSy) gases and a solvothermal reaction method employing phosphorus decasulfide (P4S10). The thionation products, called "mercapto reduced graphene oxides (m-RGOs)", were characterized by employing X-ray photoelectron spectroscopy, powder X-ray diffraction, UV-Vis spectroscopy, FT-IR spectroscopy, Raman spectroscopy, electron probe analysis, scanning electron microscopy, (scanning) transmission electron microscopy, nano secondary ion mass spectrometry, Ellman assay and atomic force microscopy. The excellent dispersibility of m-RGOs in various solvents including alcohols has allowed fabrication of thin films of m-RGOs. Deposition of m-RGOs on gold substrates was achieved through solution deposition and the m-RGOs were homogeneously distributed on gold surface shown by atomic force microscopy. Langmuir-Blodgett (LB) films of m-RGOs were obtained by transferring their Langmuir films, formed by simple drop casting of m-RGOs dispersion on water surface, onto various substrates including gold, glass and indium tin oxide. The m-RGO LB films showed low sheet resistances down to about 500 kΩ/sq at 92% optical transparency. The successful results make m-RGOs promising for applications in transparent conductive coatings, biosensing, etc.
ContributorsJeon, Kiwan (Author) / Seo, Dong-Kyun (Thesis advisor) / Jones, Anne K (Committee member) / Yarger, Jeffery (Committee member) / Arizona State University (Publisher)
Created2013
Description
The first chapter reviews three decades of artificial photosynthetic research conducted by the A. Moore, T. Moore, and D. Gust research group. Several carotenoid (Car) and tetrapyrrole containing molecules were synthesized and investigated for excitation energy transfer (EET), photoregulation, and photoprotective functions. These artificial photosynthetic compounds mimicked known processes and investigated proposed mechanisms in natural systems. This research leads to a greater understanding of photosynthesis and design concepts for organic based solar energy conversion devices. The second and third chapters analyze the triplet energy transfer in carotenoid containing dyads. Transient absorption, time-resolved FTIR and resonance Raman spectra revealed that in a 4-amide linked carotenophthalocyanine dyads the Car triplet state is shared across the larger conjugated system, which is similar to protein complexes in oxygenic photosynthetic organisms. In a carotenopurpurin dyad (CarPur) a methylene ester covalent bond prevents the purpurin (Pur) from influencing the Car triplet based on the transient absorption, time-resolved FTIR and resonance Raman spectra. Thus CarPur resembles the antenna proteins from anoxygenic photosynthetic bacteria. Additional examples of carotenoporphyrin dyads further demonstrates the need for orbital overlap for ultrafast triplet energy transfer and the formations of possible intramolecular charge transfer state. The fourth chapter studies a 4-amino phenyl carotenophthalocyanine and its model compounds using high temporal resolution transient absorption spectroscopy techniques. EET from the Car second excited (S2) state to the phthalocyanine (Pc) was determined to be 37% and a coupled hot ground state (S*)/Pc excited state spectrum was observed. Excitation of the tetrapyrrole portion of the dyad did not yield any kinetic differences, but there was an S* signal during the excited states of the dyad. This demonstrates the EET and photoregulating properties of this artificial photosynthetic compound are similar to those of natural photosynthesis. The last chapter covers the synthesis of silicon Pc (SiPc) dyes and the methods for attaching them to gold nanoparticles and flat gold surfaces. SiPc attached to patterned gold surfaces had unperturbed fluorescence, however the selectivity for the gold was low, so alternative materials are under investigation to improve the dye's selectivity for the gold surface.
ContributorsWongCarter, Katherine (Author) / Moore, Ana L (Thesis advisor) / Gust, Devens (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2014
Description
Redox reactions are crucial to energy transduction in biology. Protein film electrochemistry (PFE) is a technique for studying redox proteins in which the protein is immobilized at an electrode surface so as to allow direct exchange of electrons. Establishing a direct electronic connection eliminates the need for redoxactive mediators, thus allowing for interrogation of the redox protein of interest. PFE has proven a versatile tool that has been used to elucidate the properties of many technologically relevant redox proteins including hydrogenases, laccases, and glucose oxidase.
This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from Pyrococcus furiosus (PfSHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis.
This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from Pyrococcus furiosus (PfSHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis.
ContributorsKwan, Patrick Karchung (Author) / Jones, Anne K (Thesis advisor) / Francisco, Wilson (Committee member) / Moore, Thomas (Committee member) / Arizona State University (Publisher)
Created2014
Description
Increased global demand for energy has led to prolific use of fossil fuels, which produce and release greenhouse gases, such as carbon dioxide. This increase in atmospheric carbon dioxide affects the global weather system and has been cited as a cause for global warming. For humans to continue to meet demands for energy while reducing greenhouse emission, a sustainable, carbon-neutral energy source must be developed. The sun provides energy for the majority of life on earth, as well as the energy stored in the chemical bonds of fossil fuels. This dissertation investigates systems inspired by the biological mechanism of solar energy capture and storage. In natural photosynthesis, organisms use chlorophyll as a chromophore to absorb the sun's energy. Bio-inspired systems use close analogues like porphyrins and phthalocyanines. In this dissertation, a soluble, semiconducting porphyrin is reported. The polymer was synthesized via a Buchwald-Hartwig style coupling of porphyrin monomers which produced a polyaniline-like chain with porphyrins incorporated into the backbone. Spectroscopic and electrochemical studies were performed, which show evidence of excited state charge transfer and a first oxidation state of 0.58 V (vs SCE). These properties suggest that the polymer could be involved in excited state electron donation to fullerenes and other electron acceptors, which could be beneficial in organic photovoltaics, sensors, and other applications. Molecular dyads and triads capable of charge separation have been studied for decades, and the spectroscopic properties of two novel systems are reported in this dissertation. A peripherally-connected zinc-phthalocyanine-C60 dyad was studied, and showed excited state electron transfer from the phthalocyanine excited state to the C60, with a long-lived charge separated state. An axially-linked carotene-Si-pthalocyanine-C60 triad was studied, showing excited state electron transfer from the phthalocyanine to the C60, but fast recombination before hole transfer can occur to the carotene. Analogues of the electron transport mechanisms used in many biological systems use iron-sulfur clusters to shuttle electrons from donors to acceptors. In this dissertation, the spectroscopic properties of a de novo protein were studied. Nanosecond transient absorption was used to characterize the electron and energy transfer of an excited water-soluble porphyrin to the oxidized [FeS] clusters incorporated in the de novo protein. The triplet state of the porphyrin was strongly quenched with the holo-protein without a rise in porphyrin plus signal, suggesting that only Dexter-type energy transfer occurs between the sensitized porphyrin and the [FeS] clusters.
ContributorsSchmitz, Robert (Author) / Gust, John D (Thesis advisor) / Jones, Anne K (Committee member) / Buttry, Daniel (Committee member) / Arizona State University (Publisher)
Created2014
Description
Molybdenum (Mo) is a key trace nutrient for biological assimilation of nitrogen, either as nitrogen gas (N2) or nitrate (NO3-). Although Mo is the most abundant metal in seawater (105 nM), its concentration is low (<5 nM) in most freshwaters today, and it was scarce in the ocean before 600 million years ago. The use of Mo for nitrogen assimilation can be understood in terms of the changing Mo availability through time; for instance, the higher Mo content of eukaryotic vs. prokaryotic nitrate reductase may have stalled proliferation of eukaryotes in low-Mo Proterozoic oceans. Field and laboratory experiments were performed to study Mo requirements for NO3- assimilation and N2 fixation, respectively. Molybdenum-nitrate addition experiments at Castle Lake, California revealed interannual and depth variability in plankton community response, perhaps resulting from differences in species composition and/or ammonium availability. Furthermore, lake sediments were elevated in Mo compared to soils and bedrock in the watershed. Box modeling suggested that the largest source of Mo to the lake was particulate matter from the watershed. Month-long laboratory experiments with heterocystous cyanobacteria (HC) showed that <1 nM Mo led to low N2 fixation rates, while 10 nM Mo was sufficient for optimal rates. At 1500 nM Mo, freshwater HC hyperaccumulated Mo intercellularly, whereas coastal HC did not. These differences in storage capacity were likely due to the presence in freshwater HC of the small molybdate-binding protein, Mop, and its absence in coastal and marine cyanobacterial species. Expression of the mop gene was regulated by Mo availability in the freshwater HC species Nostoc sp. PCC 7120. Under low Mo (<1 nM) conditions, mop gene expression was up-regulated compared to higher Mo (150 and 3000 nM) treatments, but the subunit composition of the Mop protein changed, suggesting that Mop does not bind Mo in the same manner at <1 nM Mo that it can at higher Mo concentrations. These findings support a role for Mop as a Mo storage protein in HC and suggest that freshwater HC control Mo cellular homeostasis at the post-translational level. Mop's widespread distribution in prokaryotes lends support to the theory that it may be an ancient protein inherited from low-Mo Precambrian oceans.
ContributorsGlass, Jennifer (Author) / Anbar, Ariel D (Thesis advisor) / Shock, Everett L (Committee member) / Jones, Anne K (Committee member) / Hartnett, Hilairy E (Committee member) / Elser, James J (Committee member) / Fromme, Petra (Committee member) / Arizona State University (Publisher)
Created2011
Description
The work described in the thesis involves the synthesis of a molecular triad which is designed to undergo proton coupled electron transfer (PCET) upon irradiation with light. Photoinduced PCET is an important process that many organisms use and the elucidation of its mechanism will allow further understanding of this process and its potential applications. The target compound designed for PCET studies consists of a porphyrin chromophore (also a primary electron donor), covalently linked to a phenol-imidazole (secondary electron donor), and a C60 (primary electron acceptor). The phenol-imidazole moiety of this system is modeled after the TyrZ His-190 residues in the reaction center of Photosystem II (PS II). These residues participate in an intermolecular H-bond between the phenol side chain of TyrZ and the imidazole side chain of His-190. The phenol side chain of TyrZ is the electron transfer mediator between the oxygen evolving complex (OEC) and P680 (primary electron donor) in PSII. During electron transfer from TyrZ to P680*+, the phenolic proton of TyrZ becomes highly acidic (pKa~-2) and the hydrogen is preferentially transferred to the relatively basic imidazole of His-190 through a pre-existing hydrogen bond. This PCET process avoids a charged intermediate, on TyrZ, and results in a neutral phenolic radical (TyrZ*). The current research consists of building a molecular triad, which can mimic the photoinduced PCET process of PSII. The following, documents the synthetic progress in the synthesis of a molecular triad designed to investigate the mechanism of PCET as well as gain further insight on how this process can be applied in artificial photosynthetic devices.
ContributorsPatterson, Dustin (Author) / Moore, Ana L (Thesis advisor) / Gust, Devens (Committee member) / Skibo, Edward B (Committee member) / Arizona State University (Publisher)
Created2011