Dissertations, Theses, and Capstone Projects

Date of Degree

2-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Alexander Greer

Committee Members

Ryan Murelli

Jianbo Liu

Subject Categories

Environmental Chemistry | Organic Chemistry | Other Chemistry | Physical Chemistry

Keywords

Photochemistry, Peroxides, Pterins, Surfactants

Abstract

Photosensitized oxidation reactions produce a number of intermediates species, which are generated in varying amounts over time. This complexity presents major challenges in the study of oxidation processes. Mechanistic efforts to separate and deliver reactive oxygen intermediates enable their controlled use in processes such as bacterial inactivation. This thesis describes a heterogeneous reaction approach taken to control the generation and delivery of reactive oxygen intermediates. The mechanistic details of photosensitized reactions were elucidated via synthetic, materials, and physical organic techniques to optimize the delivery of reactive oxygen intermediates. This thesis contains six chapters as described below.

Chapter 1 gives a short background on molecular organic photochemistry, to provide a sense of the current state of photochemistry research, as well as an outline of the thesis. Chapter 2 describes a physical-organic study on the photodecomposition of dicumyl peroxide co-adsorbed with sensitizers 4,4¢-dimethylbenzil or chlorin e6 on dry silica. Dicumyl peroxide was decomposed by heterogeneous photosensitization under UV and white lamp irradiation and monitored by the desorption of products acetophenone, 2-phenylpropan-2-ol, and α-methylstyrene using 1H NMR spectroscopy and GC/MS. Dicumyl peroxide and sensitizer were co-adsorbed on silica in 1:4 up to 200:1 ratios, a high peroxide destabilization occurring in a ratio of about 10:1. This increased photodecomposition corresponds to sensitizer–peroxide distances of up to 6–9 Å on silica. Furthermore, a higher photostability of dicumyl peroxide was observed on silica than in a homogeneous acetonitrile solution, where the surface attenuated the diffusion of alkoxy radical geminate pairs apart from each other. A mechanism is proposed that explains how the sensitizer and peroxide separation distance, and geminate recombination of alkoxy radical pairs lead to higher and lower peroxide O–O bond homolysis efficiencies on silica, respectively. This biphasic system can thus serve both to destabilize and stabilize a peroxide; this may be of practical use in a surface used for the delivery of alkoxy radicals for bacterial disinfection.

Chapter 3 describes the study of a new series of alkyl chain pterin conjugates using photochemical and photophysical methods, as well as theoretical DFT and solubility calculations. Reactivity patterns for the alkylation of pterin were examined both experimentally and theoretically. The theoretical calculations were carried out using density functional theory (DFT) methods. 2D NMR spectroscopy was used to characterize the pterin derivatives, clearly indicating that the decyl chains were coupled to either the O4 or N3 site on the pterin. At a temperature of 70 °C, the pterin alkylation regioselectively favored the O4 alkylation over the N3 alkylation. The O4 alkylation was also favored when using solvents in which the reactants had increased solubility, e.g., N,N-dimethylformamide and N,N-dimethylacetamide, rather than solvents in which the reactants had a very low solubility, e.g., tetrahydrofuran and dichloromethane. Two additional adducts were also obtained from an N-amine condensation of DMF solvent molecule as byproducts. In comparison to the natural product pterin, the alkyl chain pterins have reduced fluorescence quantum yields (ΦF) and enhanced singlet oxygen (1O2)quantum yields (Φ∆). The DMF-condensed pterins were found to be more photostable compared with the alkylated pterins bearing a free amine group. The alkyl chain pterins efficiently intercalate in large unilamellar vesicles; this is a good indicator of their potential use as photosensitizers in biomembranes. Our study serves as a starting point where the synthesis can be expanded to produce a wider series of lipophilic, fluorophilic, and photooxidatively active pterins.

Chapter 4 describes the synthesis of new chlorin e6 silica conjugates and interfacial photooxidation studies. Porous silica and nonporous fumed silica were used as solid supports to evaluate the effect of solid supports on 1O2 production. Chlorin e6 conjugated silica was embedded on to superhydrophobic surfaces to generate bi- and triphasic photocatalytic systems. Finally, photooxidation efficiencies of interfacial systems were evaluated for applications in bacteria inactivation.

Chapter 5 describes a photooxidation study on prenylsurfactants [(CH3)2C=CH(CH2)nSO3- Na+ (n = 7, 9, 11)] to probe the “ene” reaction mechanism of 1O2 at an air–water interface. Increasing the number of carbon atoms in the hydrophobic chain increased the regioselectivity for a secondary rather than a tertiary surfactant hydroperoxide, arguing for an orthogonal alkene on water. The prenylsurfactants and a photoreactor technique enabled a certain degree of interfacial control of the hydroperoxidation reaction on a liquid support, where the oxidant (airborne 1O2) is delivered as a gas.

Chapter 6 is a review of literature techniques developed so far to understand the delivery of 1O2. This chapter strives to push the idea of 1O2 delivery further by examining two types of delivery: First, the transport of 1O2 in the presence of physical and chemical quenchers is described. Second, the transport of 1O2 by carrier compounds is described. Singlet oxygenation examples include endoperoxides and hydroperoxides.

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