The unique physical properties of photoexcited triplet states have been explored in numerous spectroscopic studies employing electron paramagnetic resonance (EPR). So far, however, no quantum interference effects were found in these systems in the presence of a magnetic field. In this study, we report the successful EPR detection of nuclear quantum oscillations in an organic triplet state subject to an external magnetic field. The observed quantum coherences can be rationalized using an analytical theory. Analysis suggests that the nuclear spins are actively involved in the intersystem crossing process. The novel mechanism also acts as a source of oscillatory nuclear spin polarization that gives rise to large signal enhancement in nuclear magnetic resonance (NMR). This opens new perspectives for the analysis of chemically induced dynamic nuclear polarization in mechanistic studies of photoactive proteins.
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