Date of Degree

9-2022

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Alexander Greer

Committee Members

Jianbo Liu

Lesley Davenport

Subject Categories

Analytical Chemistry | Chemistry | Materials Chemistry | Organic Chemistry | Physical Chemistry

Keywords

photosensitization; phototoxicity

Abstract

This thesis consists of four chapters as detailed below.

Chapter 1 discusses a singlet oxygen priming mechanism. Airborne singlet oxygen derived from photosensitization of triplet dioxygen is shown to react with an alkene surfactant (8-methylnon-7-ene-1 sulfonate) leading to ‘ene’ hydroperoxides that in the dark inactivate planktonic E. coli. The ‘ene’ hydroperoxide photoproducts are not toxic on their own, but they become toxic after the bacteria are pretreated with singlet oxygen. The total quenching rate constant (kT) of singlet oxygen of the alkene surfactant was measured to be 1.1 × 106 M1 s1 at the air/liquid interface. Through a new mechanism called singlet oxygen priming (SOP), the singlet oxygen toxin leads to the hydroperoxides then to peroxyl radicals, tetraoxide and decomposition products, which also disinfect, and therefore offer a “one two” punch. This offers a strong secondary toxic effect in an otherwise indiscernible dark reaction. The results provide insight to assisted killing by an exogenous alkene with dark toxicity effects following exposure from singlet oxygen.

Chapter 2 discusses the sensitized photooxidation of ortho-prenyl phenol, with evidence that solvent aproticity favors the formation of a dihydrobenzofuran [2-(prop-1-en-2-yl)-2,3-dihydrobenzofuran], a moiety commonly found in natural products. Solvent aproticity also increases the total quenching rate constant (kT) of singlet oxygen with prenyl phenol by ~5-fold compared to protic solvent. A mechanism is proposed with preferential addition of singlet oxygen addition to prenyl site due to hydrogen bonding with phenol OH group, which causes a divergence away from the singlet oxygen ‘ene’ reaction toward the dihydrobenzofuran as the major product. The reaction is a mixed photooxidized system since an epoxide arises by a type I sensitized photooxidation.

Chapter 3 describes a theoretical study to predict the significance of iso-hydroperoxide [R(H)O+–O] as a new intermediate in singlet oxygen chemistry. A physical-organic study is presented, implicating an O-transfer process to reach isopropenyl dihydrofuroacidone rutacridine in the singlet oxygenation of the natural product glycocitrine. Our DFT study predicts an interconversion between glycocitrine and a novel iso-hydroperoxide intermediate [R(H)O+–O] that provides a key path in the chemistry which then follows. Formation of allylic hydroperoxides is unlikely from a singlet oxygen ‘ene’ reaction. Instead, dihydrofuran rutacridine arises from an iso-hydroperoxide due to an intramolecular hydroxy directing singlet-oxygenation process. The iso-hydroperoxide is analogous to the iso species CH2+–I and CHI2+–I formed by UV photolysis of CH2I2 and CHI3. The theoretical results point to intermolecular process, in which the iso-hydroperoxide’s fate relates to O-transfer and H2O dehydration reactions for new insight to the biosynthesis of dihydrofuran natural products.

Chapter 4 describes a theoretical study on elemental nonmetals X2 (X = F, Cl, Br, I), S8, and P4 to relate their HOMO-LUMO energy difference to the difference between the ionization potential (IP) and electron affinity (EA). A unique approach is used to relate the HOMO-LUMO energy difference to the difference between the ionization potential (IP) and electron affinity (EA) to assist in deducing not only the colors, but also chromophores in elemental nonmetals. Our analysis focuses on compounds with lone pair electrons and σ electrons, namely X2 (X = F, Cl, Br, I), S8, and P4. For the dihalogens, the [IP – EA] energies are found to be: F2 (12.58 eV), Cl2 (8.98 eV), Br2 (7.90 eV), I2 (6.78 eV). We suggest that the interahalogen X–X bond itself is the chromophore for these dihalogens, in which the light absorbed by the F2, Cl2, Br2, I2 leads to longer wavelengths in the visible by a π → σ* transition. Trace impurities are a likely case of cyclic S8 which contains amounts of selenium leading to a yellow color, where the [IP – EA] energy of S8 is found to be 7.02 eV. Elemental P4 with an [IP – EA] energy of 9.09 eV contains a tetrahedral and σ aromatic structure. In future work, refinement of the analysis will be required for compounds with π electrons and σ electrons, such as polycyclic aromatic hydrocarbons (PAHs).

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