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    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. 
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  5. Abstract

    Photoexcited organic chromophores appended to stable radicals can serve as qubit and/or qudit candidates for quantum information applications. 1,6,7,12‐Tetra‐(4‐tert‐butylphenoxy)‐perylene‐3,4 : 9,10‐bis(dicarboximide) (tpPDI) linked to a partially deuterated α,γ‐bisdiphenylene‐β‐phenylallyl radical (BDPA‐d16) was synthesized and characterized by time‐resolved optical and electron paramagnetic resonance (EPR) spectroscopies. Photoexcitation of tpPDI‐BDPA‐d16results in ultrafast radical‐enhanced intersystem crossing to produce a quartet state (Q) followed by formation of a spin‐polarized doublet ground state (D0). Pulse‐EPR experiments confirmed the spin multiplicity ofQand yielded coherence times ofTm=2.1±0.1 μs and 2.8±0.2 μs forQandD0, respectively. BDPA‐d16eliminates the dominant1H hyperfine couplings, resulting in a single narrow line for both theQandD0states, which enhances the spectral resolution needed for good qubit addressability.

     
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  6. Abstract

    Photoexcited organic chromophores appended to stable radicals can serve as qubit and/or qudit candidates for quantum information applications. 1,6,7,12‐Tetra‐(4‐tert‐butylphenoxy)‐perylene‐3,4 : 9,10‐bis(dicarboximide) (tpPDI) linked to a partially deuterated α,γ‐bisdiphenylene‐β‐phenylallyl radical (BDPA‐d16) was synthesized and characterized by time‐resolved optical and electron paramagnetic resonance (EPR) spectroscopies. Photoexcitation of tpPDI‐BDPA‐d16results in ultrafast radical‐enhanced intersystem crossing to produce a quartet state (Q) followed by formation of a spin‐polarized doublet ground state (D0). Pulse‐EPR experiments confirmed the spin multiplicity ofQand yielded coherence times ofTm=2.1±0.1 μs and 2.8±0.2 μs forQandD0, respectively. BDPA‐d16eliminates the dominant1H hyperfine couplings, resulting in a single narrow line for both theQandD0states, which enhances the spectral resolution needed for good qubit addressability.

     
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