Structure-Activity Relationships in Methyl-Driven Overhauser Effect DNP.

Dynamic nuclear polarization (DNP) enables dramatic sensitivity enhancements in solid-state nuclear magnetic resonance studies of materials. Most DNP approaches rely on the cross effect mechanism that typically employs nitroxide biradicals that are chemically incompatible with many samples and whose efficiency decreases with increasing magnetic field strength. The Overhauser effect DNP mechanism is a potential alternative that allows the use of a broader range of radical polarizing agents but requires further development. Mixed-valence and methyl-functionalized conjugated radicals have both been shown to enable Overhauser DNP, yet general design rules remain elusive. Here, we studied a series of 14 methyl-functionalized Blatter-type radicals to elucidate key structure-activity relationships. We found that radicals with sterically unhindered methyl groups are active, a feature readily identifiable from a Kekulé structure. Substitutions that alter molecular conformation, and thus the spin density distribution, predictably reduce performance. Addition of trifluoromethyl groups, commonly employed to enhance stability, also decreases activity by withdrawing spin density from Overhauser-active sites. These findings provide design principles that accelerate the discovery of efficient methyl-driven Overhauser effect DNP polarizing agents.