Access Type

Open Access Dissertation

Date of Award

January 2024

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Long L. Luo

Abstract

This dissertation presents metal chalcogenide quantum dots (QDs), a group of emerging visible light photocatalysts, for unique organic chemical transformations that are impossible using conventional molecular photocatalysts. The first part of this dissertation focuses on the comparison between ligand-capped CdS QD and CdS gel. CdS QD gel is a three-dimensional network of interconnected QD. CdS gels were found to show more facile charge transfer with substrates than QDs, due to the partial removal of ligands from its surface during gelation. Our study showed that the photocatalytic activity of CdS gels was indeed superior to trioctylphosphine (TOP)- and thioglycolic acid (TGA)-capped CdS QDs for dehalogenation, α-amine, and α-amide arylation reactions. More interestingly, we discovered that the QD gels also offered new chemical reactivity, for example, aryl cyanide isomerization and tetrahydroisoquinoline ring-opening via C-N scission, which was not previously observed using conventional molecular photocatalysts. In the second part of the dissertation, we report that CdS QD gel can act as a direct hydrogen atom transfer (d-HAT) photocatalyst for C(sp3)-H bond functionalization. We observed that photoexcited QD gels exhibited unusual reactivities for generating various radicals, including α-amido radicals, heterocyclic radicals, acyl radicals, and benzylic radicals from their stable molecular precursors such as amides, ethers, and aldehydes with high redox potentials. Such board reactivities enable a large substrate scope and provide easy access to a variety of important synthons. The mechanistic study reveals that photocatalytic radical generation should undergo a direct HAT pathway on the QD gel surface rather than a sequential electron/proton transfer pathway. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy with a Brønsted slope α = 0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H functionalization.

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