Expanding the polarizing agent toolbox for liquid-state dynamic nuclear polarization: stability and performance at...

Dynamic nuclear polarization (DNP) in liquids is emerging as a potential technique to increase the NMR signal of routine NMR spectroscopy at high magnetic fields. Monoradicals are utilized as polarizing agents and microwave irradiation is used to transfer the high electron spin polarization to nuclei on nearby molecules. Recently, narrow-line radicals have gained significant interest as polarizing agents because they can efficiently polarize a broad set of nuclei (¹H, ¹³C, ¹⁹F, ³¹P) in various systems, such as neat liquids, viscous liquids, and heterogeneous media. However, these radicals can be unstable in solutions, which can make data reproducibility difficult. Here, we studied the persistence of narrow-line radicals in organic solvents typically used as matrices in liquid DNP. We observed that the persistence of monoradicals varies significantly depending on the monoradical, the solvent, and the storage condition of the sample. Our results aim to serve as a guideline for sample preparation and data reproducibility. Furthermore, we expand the viable class of polarizing agents by introducing two radicals that are new in the context of DNP in liquids driven by the Overhauser effect at high magnetic fields; verdazyl and tris(pentafluorophenyl)methyl (PF-trityl). Our ¹⁹F-DNP experiments at 14.1 T show that PF-trityl can polarize aromatic and aliphatic fluorinated molecules equally well, a feature that has not yet been reported and that may be ascribed to the fluorination of the radical itself.