Direct polarization transfer to remote nuclei: Expanding the reach of cross-effect Dynamic Nuclear Polarization

Dynamic Nuclear Polarization (DNP) has revolutionized the field of solid-state NMR spectroscopy by significantly improving the sensitivity of nuclear magnetic resonance experiments. Conventionally, cross-effect DNP relies on biradicals to transfer polarization from coupled electron spins to nearby nuclear spins and subsequent relay to target nuclei via a spin diffusion mechanism. However, direct transfer of electron spin polarization to distant nuclei remains a significant challenge due to the small magnitude of effective Hamiltonian, limiting applicability of DNP in various contexts. In this work, we investigate a biradical design concept that involves a very strong electron-electron coupling, with a magnitude of hundreds of MHz, which could enable direct polarization transfer from coupled electron spins to nuclear spins over much longer distances, exceeding 2.0 nm. The concept is experimentally supported using a 14.1 T MAS DNP setup for various nuclei. The use of ASYMPOL-POK, a strongly coupled biradical, results in up to a four-fold increase in long-range ¹H DNP enhancement compared to AMUPOL, a commonly used standard polarizing agent in traditional MAS DNP. We also discuss the potential of tailored biradicals in scenarios where conventional spin diffusion mechanisms are inefficient or where direct nuclear spin polarization enhancement or sensing through electron spin interactions is desired. Our study presents an avenue for expanding the scope of cross-effect DNP in solid-state NMR spectroscopy of ¹H, ¹⁹F and ³¹P nuclei, commonly found in various biological and material systems.