Design, synthesis, and characterization of conjugated polymers and functional paramagnetic materials for dynamic nuclear polarization

Abstract

The design, synthesis, and characterization of a series of radicals and biradicals for use as dynamic nuclear polarization (DNP) agents is described. DNP is a method to enhance the S/N-ratio in solid-state nuclear magnetic resonance (SS-NMR) that involves transferring the polarization of electrons, which are more easily polarized due to their larger magnetic moment, to nuclei. Two strategies to improve the performance of DNP-agents have been explored. The first involves combining a carbon-centered radical (1,3-bisdiphenylene-2-phenylallyl (BDPA) radical), which has a narrow line width at high field, with a nitroxide radical (2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical), which has a broader line width at high field. The synthesis and characterization of a BDPA-TEMPO biradical is described and is a first step in testing whether polarization agents of this type will out perform the currently used biradicals. Additionally, the synthesis of a water-soluble derivative of BDPA is described. The second strategy involves designing dinitroxide biradicals that rigidly hold the radicals in an orthogonal geometry. The synthesis of dinitroxide radicals of this type is described, along with efforts to optimize aqueous solubility. The design, synthesis, and characterization of thioether-containing poly(paraphenylene-ethynylene) (PPE) copolymers are reported. The polymers show a fluorescence turn-on response when exposed to oxidants in solution, and the oxidized polymers show desirable thin-film properties, such as high quantum yields and increased photostability. Work towards the synthesis of electroactive conjugated polymers based on the BDPA free radical is also reported.