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Abstract
A new kind of cross-effect (CE) dynamic nuclear polarization (DNP) using biradicals with exchange couplings larger than the g-anisotropy or g-difference is proposed. The strongly coupled biradicals can be described with a combination of singlet S0 and triplet T(0,±1) states. Despite being invisible to w irradiation and spectral detection for the singlet state, S0↔T0 singlet-triplet transition by hyperfine couplings can occur driving nuclear spin flip for cross-effect DNP process. The T0 state population and spin alignment order can be generated by selectively saturating one of the T0↔ T(±1) transitions. Introducing dipolar couplings and frequency differences between the two electrons can remove the degeneracy between the two triplet transitions enabling selective saturation to obtain singlet-to-triplet cross-effect (ST-CE) DNP.
