Efficient Dynamic Nuclear Polarization up to 230 K with Hybrid BDPA-Nitroxide Radicals at a High Magnetic Field

Pairing the spectral resolution provided by high magnetic fields at ambient temperature with the enhanced sensitivity offered by dynamic nuclear polarization (DNP) is a major goal of modern solid-state NMR spectroscopy, which will allow one to unlock ever-challenging applications. This study demonstrates that, by combining HyTEK2, a hybrid BDPA-nitroxide biradical polarizing agent, with ortho-terphenyl (OTP), a rigid DNP matrix, enhancement factors as high as 65 can be obtained at 230 K, 40 kHz magic angle spinning (MAS), and 18.8 T. The temperature dependence of the DNP enhancement and its behavior around the glass transition temperature (T-g) of the matrix is investigated by variable-temperature EPR measurements of the electron relaxation properties and numerical simulations. A correlation is suggested between the decrease in enhancement at the passage of the T-g and the concomitant drop of both transverse electron relaxation times in the biradical.

Automated summary

Pairing high magnetic fields with DNP in solid-state NMR spectroscopy aims to unlock challenging applications. This study demonstrates that combining HyTEK2 with OTP can achieve high enhancement factors, with investigations into the temperature dependence of DNP enhancement and behavior around T-g. A correlation between enhancement decrease at T-g passage and the drop in electron relaxation times in the biradical is suggested.