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Synthesis of Stable Double Helical Diradical Dications and Nitronyl Nitroxide Blatter-Based Di- and Tri-Radicals
This dissertation is mainly focused on the design and synthesis of stable highspin (S ≥ 1) N-radicals: the first project is designed based on the π-conjugated double bisdiazahelicenes and the second project is designed based on the stable nitronyl nitroxide and 1,2,4-benzotriazinyl (Blatter) radicals. The design, synthesis, and the properties of the molecules, characterized by NMR, EPR, UV-vis-NIR spectroscopies, SQUID magnetometry, X-ray crystallography and thermogravimetric analysis, are described. The π-systems of helicenes enhance the stabilization of radicals. Spin filtering effect is discovered in both radicals and helicenes. Thus, high-spin helical diradicals are attractive to the discovery of spin filters. The challenge is to design stable helical diradicals with large singlet-triplet energy gap (ΔEST). In the first project, the first chiral air-stable high-spin diradical dication of D2-symmetric conjoined bisdiazahelicene, with ΔEST = 0.3 kcal mol-1, was synthesized, which possesses a half-life of more than 2 weeks at ambient conditions in the presence of excess oxidant. Additionally, a C2- symmetric conjoined bisdiazahelicene was synthesized. After methylation, its radical cation and diradical dication were generated with different oxidants. The radical cation of C2-symmetric dimethyl-conjoined bisdiazahelicene was isolated as crystal, which owns an onset of decomposition at 203 °C based on the thermogravimetric analysis. High-spin radicals (S ≥ 1) with great stability are important to the further applications. However, only limited stable high-spin di- and tri-radicals are reported. In the second project, a high-spin triradical composed of Blatter radical and nitronyl nitroxide, with doublet-quartet energy gap ΔEDQ ≈ 0.2 – 0.3 kcal mol-1, was synthesized by radical-radical Pd(0)-catalyzed cross-coupling reaction. The triradical possesses great thermal stability, having an onset of decomposition at 166 °C based on the thermogravimetric analysis. The triradical was used to form the thin film under control conditions. Moreover, utilizing the Pd(0)-catalyzed cross-coupling reaction, a carbazole-nitronyl nitroxide-Blatter diradical was synthesized to investigate its fluorescence properties.
Shu, Chan, "Synthesis of Stable Double Helical Diradical Dications and Nitronyl Nitroxide Blatter-Based Di- and Tri-Radicals" (2022). ETD collection for University of Nebraska - Lincoln. AAI29323637.