Understanding and Characterizing Excited State Electron Transfers in Catalysis and Nanomaterial Synthesis
Over the past decade, there has been an ever-increasing interest in photochemical redox processes as a tool in organic syntheses. While the underlying mechanism of many of these transformations involves free radicals, the complex product mixtures and poor selectivity that are traditionally associate...
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| Language: | en |
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Université d'Ottawa / University of Ottawa
2016
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| Online Access: | http://hdl.handle.net/10393/35636 http://dx.doi.org/10.20381/ruor-593 |
| Summary: | Over the past decade, there has been an ever-increasing interest in photochemical redox processes as a tool in organic syntheses. While the underlying mechanism of many of these transformations involves free radicals, the complex product mixtures and poor selectivity that are traditionally associated with these types of reaction can be in most cases avoided. In fact there are numerous examples of photoredox catalysis, which demonstrate that under well-selected conditions it is possible to obtain relatively clean products in high yields. Furthermore, photo-mediated reactions can typically be performed under milder conditions than thermal reactions, as the energy is supplied by light and heating is not required.
Despite recent advancements in the field, many of the new discoveries are accomplished using relatively expensive transition metal complexes of Ru and Ir as photocatalyst and tend to lack in-depth investigations into the corresponding excited-state kinetics or underlying mechanism of these transformations. While there are excellent reasons for employing polypyridyl Ru and Ir catalysts, such as strong visible light absorption, high photostability, and well known and documented excited state lifetimes and redox potentials; one of the aims of this thesis is to demonstrate that in many cases these Ru and Ir photocatalysts can be replaced with cheaper and in some cases metal-free and heterogeneous alternatives.
Herein, one will find examples of the use of the organic semiconductor, α-sexithiophene, as photocatalyst in the reductive dehalogenation of vicinal-dibromides and the singlet oxygen mediated oxidation of amines to their corresponding imines. We have also demonstrated that the inorganic semiconductor P25 TiO2 can be used as an efficient heterogeneous photocatalyst for a variety of reductive transformations and that it is possible to employ it as photocatalyst under dual-photoredox/Ni catalysis conditions to perform the decarboxylative cross-coupling of carboxylic acids with aryl iodides.
In addition to this, the usefulness of kinetic analysis in improving and better understanding these transformations as well as the development of tools to characterize chain processes in these reactions are covered. Lastly, we also explore the role of excited state electron transfer from the triplet of benzophenone in the synthesis of colloidal gold nanoparticles.
Overall, the content of this thesis describes the development and improvement of several photoredox systems through either rational design, the development of new characterization tools, and in-depth kinetic and mechanistic analysis. |
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