BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T.

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, α,γ-bisdiphenylene-β-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field Al and Si DNP surface enhanced NMR spectra of amorphous aluminosilicates and O NMR on silica nanoparticles.