Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Subject: search.filters.subject.Photochemistry

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    Mechanistic Studies of Photochemical Reactions: Photoacid Generators, Photoreleaseable Protecting Groups, and Diarylnitrenium Ions
    (2021) Zeppuhar, Andrea; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The use of light to drive chemical reactions is becoming increasingly popular due to the enhanced spatial and temporal control provided. Because of this, it is important to understand how these photochemical transformations occur from a mechanistic viewpoint in order to aid in the improvement of existing systems as well as in the development of new systems. The work presented in this dissertation will examine the mechanisms of several photochemical systems including photoacid generators, photoreleaseable protecting groups, and diarylnitrenium ions. Chapter 1 will begin with an introduction to organic photochemistry and describe some of the excited state reactions that will be encountered throughout this text. It will also describe laser flash photolysis, a technique critical to studying the reactive intermediates generated in photochemical reactions. Chapter 2 will describe the design and synthesis of photoacid generators that are activated via sequential two-photon absorption. The experiments conducted support a mechanism involving triplet re-excitation providing a more favorable bond scission. Chapter 3 will explore the applications of these newly developed photoacid generators, specifically for photopolymerization. It is shown that these compounds are capable of initiating both cationic and radical polymerizations depending on the intensity of visible light irradiation used. Chapter 4 will examine the 9-phenyl-9-tritylone photoreleaseable protecting group for alcohols to understand the details of its release mechanism. It is shown that the tritylone anion radical is required for alcohol photorelease. Chapters 5 and 6 will explore the behavior of diarylnitrenium ions in aqueous media. Chapter 5 will examine the reactivity of diarylnitrenium ions toward guanosine and it is shown that there is a rapid reaction to generate the C8 adduct, suggesting potential carcinogenicity. Chapter 6 will examine the reactivity of diarylnitrenium ions under acidic aqueous conditions. Under these conditions, a long-lived species is formed, and the experiments conducted indicate this species is the cation radical derived from the diarylnitrenium ion. Mechanistic analysis supports formation via a pathway separate from the nitrenium ion, suggestive of a triplet mechanism.
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    Applications of Photoinduced Electron Transfer Chemistry: Photoremovable Protecting Groups and Carbon Dioxide Conversion
    (2016) Denning, Derek Michael; Falvey, Daniel; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Traditional organic chemistry has long been dominated by ground state thermal reactions. The alternative to this is excited state chemistry, which uses light to drive chemical transformations. There is considerable interest in using this clean renewable energy source due to concerns surrounding the combustion byproducts associated with the consumption of fossil fuels. The work presented in this text will focus on the use of light (both ultraviolet and visible) for the following quantitative chemical transformations: (1) the release of compounds containing carboxylic acid and alcohol functional groups and (2) the conversion of carbon dioxide into other useable chemicals. Chapters 1-3 will introduce and explore the use of photoremovable protecting groups (PPGs) for the spatiotemporal control of molecular concentrations. Two new PPGs are discussed, the 2,2,2-tribromoethoxy group for the protection of carboxylic acids and the 9-phenyl-9-tritylone group for the protection of alcohols. Fundamental interest in the factors that affect C–X bond breaking has driven the work presented in this text for the release of carboxylic acid substrates. Product analysis from the UV photolysis of 2,2,2-tribromoethyl-(2′-phenylacetate) in various solvents results in the formation of H–atom abstraction products as well as the release of phenylacetic acid. The deprotection of alcohols is realized through the use of UV or visible light photolysis of 9-phenyl-9-tritylone ethers. Central to this study is the use of photoinduced electron transfer chemistry for the generation of ion diradicals capable of undergoing bond-breaking chemistry leading to the release of the alcohol substrates. Chapters 4 and 5 will explore the use of N-heterocyclic carbenes (NHCs) as a catalyst for the photochemical reduction of carbon dioxide. Previous experiments have demonstrated that NHCs can add to CO2 to form stable zwitterionic species known as N-heterocylic-2-carboxylates (NHC–CO2). Work presented in this text illustrate that the stability of these species is highly dependent on solvent polarity, consistent with a lengthening of the imidazolium to carbon dioxide bond (CNHC–CCO2). Furthermore, these adducts interact with excited state electron donors resulting in the generation of ion diradicals capable of converting carbon dioxide into formic acid.
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    N-METHYL-4-PICOLINIUM IODIDE ESTERS AS VISIBLE LIGHT PHOTOREMOVABLE PROTECTING GROUPS FOR CARBOXYLIC ACIDS
    (2015) Kunsberg, David Joseph; Falvey, Daniel E.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Facile photorelease of carboxylic acids is performed through charge-transfer excitation of N-methyl-4-picolinium iodide esters. Photolysis reactions are carried out under mild, biphasic solvent conditions using a household LED lamp. Release is quantified by 1H NMR analysis and carboxylic acid release is reported in high yields. The viability of this method for synthetic chemistry is demonstrated through a macroscale photolysis. Additionally, the potential for solvent-independent NAP ester charge-transfer photolysis is explored.
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    Photolytic Studies of Aryl and Heteroaryl Nitrenium Ions: Laser Flash Photolysis Studies
    (2006-05-07) Thomas, Selina Ivan; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The objective of the thesis was to understand the chemical and kinetic behavior of arylnitrenium ions and a heteroaromatic nitrenium ion through photolytic studies. N-(4,4'-dichlorodiphenyl) nitrenium ion and N-(4,4'-dibromodiphenyl) nitrenium ion are the halogenated counterparts of diphenylnitrenium ions and are generated photochemically from their respective N-(4,4'-dihalogenated diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate salts. The halogenated diarylnitrenium ions are ground state singlets that live for more than 1 x 105 ns in acetonitrile. In the absence of nucleophiles, these ions decay to form a dimerized hydrazine. These ions react with nucleophiles such as water and alcohol at a rate constant of 104 - 105M-1s-1 and at diffusion limit with chlorides. With arenes, these ions react via electron transfer mechanism and nucleophilic addition process. The rate constants for the electron transfer mechanisms are between 105 - 109 M-1 s-1 and depend on the Eox of the arenes. Arenes with Eox above 1.78 V showed no reactivity towards the ion. Unlike other diarylnitrenium ions, the halogenated diarylnitrenium ions react with hydrogen atom donors via a hydrogen atom transfer mechanism. The triplet behavior of these ions is attributed to singlet-triplet intersystem crossing facilitated by the lower singlet-triplet energy gap. Therefore, it has been concluded that substituting halogens in diphenylnitrenium ion lowers the singlet-triplet energy gap and increases the lifetime of these ions. (N-Methyl-N-4-biphenylyl) nitrenium ion generated by photolysis was reacted with amino acids and proteins to determine their reactivity with these ions. Eight amino acids were observed to react with the ion at a rate constant of 107 - 109 M-1 s-1. The rate constants depend on the nucleophilicity of the side chains of the amino acids. In addition, this ion also reacts rapidly with proteins with a rate constant of 108 M-1 s-1, comparable to their reactions with ss-DNA. Investigations on generating the quinoline N-oxide nitrenium ions showed that the transient species from the photolysis of 4-azidoquinoline N-oxide (4-AzQO) shows more characteristics of a nitrenium ion. The formation of 4-aminoquinoline N-oxide upon photolysis of 4-AzQO in acetonitrile with 10% sulphuric acid and the electron transfer reactions observed with arenes, indicate that the transient species generated could be a heteroaromatic nitrenium ion. However, more experiments are needed to confirm the assignment.