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Scholarly Works (4 results)

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UC Santa Cruz Electronic Theses and Dissertations (2012)

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.

    Cover page: Nitroxides: Profluorescent Sensors and Functionalized Alkoxyamine Initiators for Nitroxide Mediated Radical Polymerization
    • Article
    • Peer Reviewed
    UC Santa Cruz Previously Published Works (2013)
    Monomers or N-alkoxyamine initiators containing protected thiol groups are utilized to prepare polymers via nitroxide-mediated radical polymerization. Following thiol deprotection, the macromolecular properties of these polymers are manipulated, by adjusting the redox conditions to either form or cleave disulfide bonds, or irreversibly cap free thiols by the rapid addition to a maleimide Michael acceptor. Formation of disulfide bonds under dilute conditions results in intramolecular disulfide formation, resulting in internal polymer collapse. Alternatively, disulfide formation under high concentration results in intermolecular crosslinking of polymers to form networked macromolecular assemblies.© 2013 Elsevier Ltd. All rights reserved.
      Cover page: Reversible crosslinking of polymers bearing pendant or terminal thiol groups prepared by nitroxide-mediated radical polymerizationCreative Commons 'BY-NC' version 4.0 license
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      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2010)
      Quantum dot (QD) fluorescence is effectively quenched at low concentration by nitroxides bearing amine or carboxylic acid ligands. The association constants and fluorescence quenching of CdSe QDs with these derivatized nitroxides have been examined using electron paramagnetic resonance (EPR) and fluorescence spectroscopy. The EPR spectra in the non-protic solvent toluene are extremely sensitive to intermolecular and intramolecular hydrogen bonding of the functionalized nitroxides. Fluorescence measurements show that quenching of QD luminescence is nonlinear, with a strong dependence on the distance between the radical and the QD. The quenched fluorescence is restored when the surface-bound nitroxides are converted to hydroxylamines by mild reducing agents, or trapped by carbon radicals to form alkoxyamines. EPR studies indicate that photoreduction of the nitroxide occurs in toluene solution upon photoexcitation at 365 nm. However, photolysis in benzene solution gives no photoreduction, suggesting that photoreduction in toluene is independent of the quenching mechanism. The fluorescence quenching of QDs by nitroxide binding is a reversible process.
        Cover page: Distance-Dependent Fluorescence Quenching and Binding of CdSe Quantum Dots by Functionalized Nitroxide RadicalsCreative Commons 'BY-NC' version 4.0 license
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        • Peer Reviewed
        UC Santa Cruz Previously Published Works (2015)
        Chloromagnesium dimethylaminoborohydride (ClMg+ [H3BNMe2]-, MgAB) is an analogue of the versatile lithium dialkylaminoborohydrides (LAB reagents), prepared by the reaction of dimethylamine-borane with methylmagnesium chloride. MgAB is a partial reducing agent for Weinreb amides under ambient conditions and is complementary to the commonly utilized lithium aluminum hydride (LiAlH4) and diisobutylaluminum hydride (DIBAL) reagents, while exhibiting enhanced chemoselectivity. To prevent over-reduction, the aldehyde products are readily isolated in good yields by forming the sodium bisulfite adducts. Aldehyde products can both be stored and later used as the bisulfite adducts, or can be regenerated from the bisulfite adducts by treatment with aqueous formaldehyde.
          Cover page: Reduction of Weinreb amides to aldehydes under ambient conditions with magnesium borohydride reagents
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