Overcoming the nuclear spin diffusion barrier in dynamic nuclear polarization via electron-electron flip-flop

The spin diffusion barrier is a well-known phenomenon that limits the efficiency of dynamic nuclear polarization (DNP) in enhancing nuclear magnetic resonance (NMR) sensitivity. In this study, we introduce a solution to overcome the nuclear spin diffusion barrier in DNP under magic-angle spinning (MAS) at high magnetic fields. Our finding highlights the crucial role of electron-electron ($e\text{\ensuremath{-}}e$) coupling interactions, specifically electron spin flip-flop processes, in accelerating nuclear spin diffusion in DNP polarizing agents with strong hyperfine couplings, which are essential for effective DNP. Through a combination of theoretical and numerical analyses on a four-spin system model, we demonstrate that $e\text{\ensuremath{-}}e$ interactions can overcome the spin diffusion barrier via a concurrent four-spin flip-flop mechanism, termed the electron-assisted spin diffusion (EASD) mechanism. This mechanism operates within the $\ensuremath{\alpha}\ensuremath{\beta}$ and $\ensuremath{\beta}\ensuremath{\alpha}$ manifolds of coupled electron spins under specific degeneracy conditions. Experimental DNP buildup profiles, measured at 14.1 T under MAS, corroborate the EASD model, revealing that radical systems with stronger inter- or intramolecular $e\text{\ensuremath{-}}e$ coupling achieve significantly faster DNP buildup. EASD insights open avenues for the development of advanced DNP transfer strategies under MAS, elucidating how polarization can be diffused out of polarizing agents that would otherwise remain trapped. This paper not only advances our understanding of DNP transfer dynamics but also offers practical guidelines for designing next-generation DNP polarizing agents. In particular, it informs the optimization of bis-nitroxides with tailored $e\text{\ensuremath{-}}e$ coupling for cross-effect DNP, as well as the development of narrow-line radicals for solid-effect DNP, both of which can leverage the EASD mechanism for enhanced performance in hyperpolarized NMR applications. EASD also paves the way for improving DNP of materials with intrinsic or doped paramagnetic centers and advancing quantum information science, particularly quantum sensing, by leveraging coupled electron spins.