Nitroxides: Profluorescent Sensors and Functionalized Alkoxyamine Initiators for Nitroxide Mediated Radical Polymerization
Nitroxides have played a vital role in many areas in chemistry, due to their persistent radical nature, and facile redox properties. In this thesis, nitroxides are used to quench the fluorescence of organic fluorophores and quantum dots, as well as to prepare end-functionalized polymers using nitroxide mediated radical polymerization (NMRP).
Quantum dot (QD) fluorescence is effectively quenched by binding to functionalized nitroxides. The association constants and fluorescence quenching of CdSe QDs with nitroxides bearing a ligand coordinated to the QD surface have been examined using electron paramagnetic resonance and fluorescence spectroscopy. Quenching of QD fluorescence is dependent on the distance between the radical and the QD, and the binding affinity. The quenched fluorescence is restored when the surface-bound nitroxide is converted to a hydroxylamine by a mild reducing agent, or trapped by a carbon radical to form an alkoxyamine. A fluorescence quenching mechanism is proposed.
The phenomenon of fluorescence quenching was extended to organic fluorophores. Nitroxides were synthesized bearing a tethered photosensitive species that undergo photocleavage to form a radical, which is then intramolecularly trapped by the nitroxide. When a fluorescent tag is appended to these nitroxides, the fluorescence is restored upon photoexcitation. A profluorescent nitroxide sensor of mild reductants was also developed, which undergo cyclization upon formation of the hydroxylamine. Unlike existing methodologies, this sensor is non-reversible since it cannot be reoxidized in the presence of oxygen.
Thiol-derivatized N-alkoxyamines, functionalized with a free thiol, disulfide, or trityl-protected thiol group were synthesized and employed in NMRP. Deprotection of the trityl group and mild oxidation provided disulfide linked polystyrene of double the molecular weight. Reduction of this polystyrene, followed by trapping with N-phenylmaleimide (NPM) resulted in a NPM-protected thiol terminated polystyrene with the same molecular weight as prior to oxidation.