Readily Accessible bTurea-Derived Nitroxide Polarizing Agents for Dynamic Nuclear Polarization: bcTCOOKs and bcTmols

Dynamic nuclear polarization (DNP) enhances the sensitivity of solid-state nuclear magnetic resonance (ssNMR) by transferring polarization from unpaired electron spins to nuclear spins. In this work, two series of bTurea-based biradicals, named bcTmols and bcTCOOKs, were synthesized through short and practical routes. Variation in the N-alkyl substitution on the urea bridge affected the electron-electron interactions and the overall DNP efficiency. Conformer analysis was carried out using density functional theory (DFT) to identify the dominant conformers along with the g-tensor orientations, dipolar couplings, and exchange interactions, which correlate with their electron paramagnetic resonance (EPR) and DNP properties. The results show that methyl substitution on the urea bridge restricts conformational flexibility, leading to more stable geometries and improved DNP performance. Among all the radicals, bcTCOOK-M2 provided the highest overall synthetic yield (8.3%) and signal enhancement (ca. 220) with a buildup time of 3.9 s at 14.1 T and 100 K, making it competitive with AsymPol-POK, including measurements in proton-rich media. The combination of straightforward synthesis, good solubility in aqueous solvents, and conformational stability makes bcTCOOK-M2 a promising polarizing agent for modern magic angle spinning (MAS) DNP applications.