Overcoming Nuclear Spin Diffusion Barrier in DNP via Electron-Electron Flip-Flop

The study introduces a new mechanism of nuclear spin diffusion in the context of Dynamic Nuclear Polarization (DNP) with magic angle spinning (MAS) under high magnetic fields. Electron-electron ($e$-$e$) coupling interactions, particularly the electron spin flip-flop interaction, play a significant role in enhancing nuclear spin diffusion in DNP polarizing agents with substantial hyperfine couplings. Using a four-spin system model, both theoretical analysis and numerical simulations suggest that the $e-e$ interactions eliminate the spin diffusion barrier in the $\alpha\beta\$ and $\beta\alpha$ coupled electron spin manifolds when the electron and nuclear states are degenerate through a concurrent four-spin flip-flop mechanism, known as the Electron-Assisted Spin Diffusion (EASD) mechanism. Experimental DNP buildup curves measured at 14.1 T under MAS conditions validate the EASD model, since a radical system with larger $e-e$ coupling shows faster spin diffusion rates. Our results provide fresh perspectives with the potential to greatly improve DNP transfer under MAS conditions. They elucidate how polarization can be diffused out of polarizing agents that otherwise would not diffuse to the bulk nuclei and lay the groundwork for designing and synthesizing more efficient DNP polarizing agents. In particular, this suggests engineering bis-nitroxides with ideal $e-e$ coupling for the cross-effect and EASD, along with narrow-line bis-BDPA and bis-trityl for solid-effect DNP.