Nuclear magnetic resonance spectroscopy is undoubtedly one of the most versatile techniques to investigate matter at the atomic scale. Nonetheless, low sensitivity always limited the application of magnetic resonance experiments to very diluted samples. To circumvent this issue, some of us recently reported the dissolution of optically polarized naphthalene crystals doped with pentacene for the hyperpolarization of small molecules in liquids at low field.[1] This innovative method exploits the intermolecular nuclear Overhauser effect (NOE) to transfer magnetization from the extremely polarized naphthalene protons to the other species in solution, enhancing their signal by up to three orders of magnitude. Indeed, despite the very low efficiency of intermolecular cross relaxation, the starting polarization of the naphthalene protons is so high that a considerable amount of magnetization is exchanged during the process. More recent experiments confirm that it is possible to obtain significant enhancements also at higher fields. Although the use of optically polarized naphthalene provides a completely new and general way for hyperpolarization in organic solvents, it also poses some practical challenges, especially because of its huge residual magnetization that causes detrimental effects such as radiation damping and receiver saturation. Not only, because of the size dependence of longitudinal relaxation at high fields, strategies for efficient polarization of medium and large molecules in the T₁ minimum range must be used.

Here we demonstrate that conventional signal suppression sequences can be successfully implemented to overcome both radiation damping and receiver saturation. We also show that polarization of large systems at high field can be performed by tuning the experiment or by exploiting additional interactions between the source and the target species.

[1] Eichhorn, et al., J. Am. Chem. Soc., 2022, 144, 2511–2519