Influence of the radical properties in triplet-radical spin-polarization transfer.

Photogenerated triplet states enable non-equilibrium electron spin polarization in solution via the radical-triplet pair mechanism (RTPM). Here, we systematically investigate how nitroxide radical structure affects chemically-induced dynamic electron polarization (CIDEP) under ambient conditions. A series of structurally modified nitroxides is studied with rose bengal as photosensitizer using transient and pulse EPR spectroscopy. Pronounced differences in polarization efficiency are observed across the series. The experimental results combined with quantum chemical calculations identify spin relaxation as a key determinant of CIDEP efficiency, while the structural accessibility of the nitroxide may emerge as an additional factor influencing productive triplet-radical encounters. Overall, these findings could inform molecular design principles for radicals in liquid-state hyperpolarization applications.