Fullerene-Nitroxide Derivatives as Potential Polarizers for Dynamic Nuclear Polarization (DNP) in Liquid State

Abstract

Nuclear magnetic resonance spectroscopy and imaging (NMR/MRI) are widely applied methods for non-destructive analysis of biological systems, materials and medical applications. Dynamic nuclear polarization (DNP) is a method to enhance NMR signal by means of polarization transfer from unpaired electrons to nuclei through saturation the electron spin transition by microwave (MW). DNP in liquids is governed by Overhauser mechanism which results from the simultaneous flipping of electron and nuclear spins. The DNP efficiency decreases with increasing an external magnetic field. Since many organic molecules, which are potentially interesting for DNP as polarizers, are not soluble in water, the Overhauser DNP analysis has been done for toluene solutions. Also nonpolar solvents, in contrast to water, suffer less from dielectric losses and this allows using larger volume of the investigated samples. These experiments have demonstrated higher DNP efficiency of nitroxide radicals in toluene solution than in aqueous solution at low radical concentration. The reason is higher Heisenberg spin exchange rate between different hyperfine EPR transitions due to lower viscosity of toluene. The theoretical model of the relaxation dynamic for the electron-nuclear hyperfine coupled system with S=1/2 and I=1 has been analytically solved and the analytical expressions for the saturation factors of the individual hyperfine lines have been obtained. The electron-electron double resonance (ELDOR) method has been extended for the experimental measurements of the saturation factors and the polarization recoveries for radicals with more than 2 hyperfine lines. The series of experiments have demonstrated that the DNP enhancement increases with increasing of molecular size and does not depend on the nitroxide radical concentration in the case of dominant nitrogen nuclear relaxation between hyperfine EPR transitions. Several nitroxide derivatives with the covalently linked fullerene C60, as the dye with high quantum yield of the triplet state, have been synthetized. The transient EPR experiments of the photoexcited fullerene-nitroxide derivatives have demonstrated the feasibility to generate high non-equilibrium electron spin polarization by light. The photoexcited electron polarization, if successfully transferred to solvent nuclei, may provide a new method to perform DNP in liquids without MW irradiation.