Electron spin-lattice relaxation of trityl radicals

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Tris(2,3,5,6-tetrathiaaryl)methyl radicals (trityls) belong to a family of persistent free radicals that have been widely used in a number of magnetic resonance applications because of their relatively narrow EPR line shape, long relaxation time and high stability. Trityls have been widely used as polarizing agents in a signal enhancing technique called dynamic nuclear polarization (DNP) for nuclear magnetic resonance (NMR). Under different DNP conditions, the enhancement result varies dramatically. The electron paramagnetic resonance (EPR) properties need to be taken into consideration. We examined the electron spin-lattice relaxation (T1e) of trityl radicals (Finland trityl and trityl OX063) frozen solutions at low temperatures in Chapter 3. The spin-lattice relaxation can be fitted with three classical relaxation processes: direct process, Raman process and Orbach-Aminov process. We also found fast-relaxing centers in high concentration Finland trityl and trityl OX063 frozen solutions. The spin diffusion through fast-relaxing centers plays an important role in spin-lattice relaxation. Chapter 4 shows two models that have been used to mimic the fast-relaxing centers, the trityl-micelle model and trityl-metal model. After studying the spin-lattice relaxation behavior, we found that the fast-relaxing centers could be divided into two different types, small clusters containing two or three mono-radicals for trityl-micelle model or large ‘crystal-like’ aggregations that involve a large number of radicals for trityl-metal model. Chapter 5 shows the spin-lattice relaxation results of trityl poly-radicals that contain more than one mono-radical core. In the poly-radical solution, there are two structural conformations of the poly-radical, extended and folded. The spin-lattice relaxation rate is mainly related to the concentration and the amount of extended or unfolded conformations. However, the spin diffusion is proportional to the number of fast-relaxing centers that correspond to compact, or folded, conformations of the poly-radicals. The folded formation of poly-radicals share similar relaxation properties with the fast-relaxing centers in mono-radicals that we found in Finland trityl and trityl OX063.

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