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Dynamic Nuclear Polarization (DNP) utilizing Electron Spin Clusters to achieve resonance matching with the nucleus and to generate an Asymmetric Polarization Elevation (ESCAPE-DNP, or ESC-DNP for short) by monochromatic microwave irradiation at a select frequency is debuted as a promising mechanism to achieve NMR signal enhancements with a wide design scope requiring low microwave power at high magnetic field. In this paper, we present the design for a trityl-based tetra-radical (TetraTrityl) to achieve DNP for 1H NMR at 7 Tesla, supported by experimental data and quantum mechanical simulations. A slow relaxing (T1e ≈ 1 ms) four electron spin cluster is found to require at least two electron pairs with e-e distances of 8 Å or below to yield any meaningful 1H ESC-DNP NMR enhancement, while squeezing the rest of the e-e distances to 12 Å or below gives rise to near maximum 1H ESC-DNP-NMR enhancements. For the more common case of a fast-relaxing spin cluster (T1e ≈ 1 μs), efficient ESC-DNP is found to require an asymmetric ESC that contains a cluster of strongly coupled narrow-line radicals coexisting with a weakly coupled narrow-line radical acting as a sensitizer to extract polarization from the cluster. This study highlights the untapped potential of utilizing strong coupling of narrow-line radical clusters to achieve microwave power-efficient DNP that extends design options beyond what is available today and offers great tunability at high magnetic field. 

Understanding the spatial distribution of the P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as polarization sources for dynamic nuclear polarization (DNP) quantum sensing and play a significant role in the relaxation of nitrogen vacancy (NV) centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized DNP and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric line shapes. The 13C DNP frequency profile at a high magnetic field revealed a pattern that requires an asymmetric EPR line shape of the P1 clusters with electron–electron (e–e) coupling strengths exceeding the 13C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e–e couplings. We employed a two-frequency pump–probe pulsed electron double resonance technique to show cross-talk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all of the P1 populations. Direct observation of clustered P1 centers and their asymmetric line shape offers a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for 13C and other nuclear spins of analytes. We propose that room temperature 13C DNP at a high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects. 

Dynamic nuclear polarization (DNP) can amplify the solid-state nuclear magnetic resonance (NMR) signal by several orders of magnitude. The mechanism of DNP utilizing α,γ- Bisdiphenylene-β-phenylallyl (BDPA) variants as Polarizing Agents (PA) has been the subject of lively discussions on account of their remarkable DNP efficiency with low demand for microwave power. We propose that electron spin clustering of SA-BDPA is responsible for its DNP performance, as revealed by the temperature-dependent shape of the central DNP profile and strong electron-electron (e-e) crosstalk seen by Electron Double Resonance. We demonstrate that a multi-electron spin cluster can be modeled with three coupled spins, where electron J (exchange) coupling between one of the e-e pairs matching the NMR Larmor frequency induces the experimentally observed absorptive central DNP profile, and the electron T1e modulated by temperature and magic angle spinning alters the shape between an absorptive and dispersive feature. Understanding the microscopic origin is key to designing new PAs to harness the microwave power-efficient DNP effect observed with BDPA variants. 

Dynamic nuclear polarization (DNP) by the cross effect (CE) has become a game changer for solid-state nuclear magnetic resonance (NMR) spectroscopy. The efficiency of CE-DNP depends on the strength of the electron–electron coupling in biradical polarizing agents. Hence, the focus lately has been on designing biradicals with a large net exchange ( J ) and dipolar ( D ) coupling. In this study, we reveal that the crucial factor for CE-DNP is not the large sum, J + D , but rather the relative magnitude of J and D , expressed as the J / D ratio. We show that the mechanistic basis of this interference lies in the isotropic vs. the anisotropic nature of the J and D couplings, respectively. This interference can lead to a small (effective) electron–electron coupling for many orientations even when J + D is large, resulting in non-adiabatic rotor-events. We find that when 0 < | J / D | < 1 the CE-DNP efficiency is attenuated for the majority of orientations, with greater attenuation observed at higher magnetic fields and faster magic-angle spinning (MAS) frequency. The interference effect of J and D coupling introduced in this study can explain why many biradicals with high or comparable J + D still show significantly divergent DNP performances. We debut J / D as a consequential criteria for designing efficient biradicals to robustly perform across a large range of B 0 fields and MAS frequencies.