Unraveling the dynamics of multiple excited states in a single-molecule transistor

Characterizing charge transport through single molecules provides a fundamental route to explore correlated quantum states, forming the basis for nanodevices in which quantum effects govern operation. During the non-equilibrium electron transfer, a rich manifold of excited states emerges, whose dynamics encode many-body interactions at the single-molecule level. Disentangling these dynamics is crucial for understanding such interactions, however, has remained elusive due to synchronously experimental challenge of the time and energy resolution. Here, we resolved the dynamics of multiple excited states during non-equilibrium charge transport through a single-molecule radical junction using a nanosecond differential conductance spectroscopy. The participation of singlet and triplet states is revealed in both time and energy domains, and a continuous energy relaxation of ~440 meV occurring within ~150 ns is observed, mediated by doublet states. This provides a direct window into transient intermediates that fundamentally reshape our understanding of charge transport through radicals at the single-molecule level, and establish ultrafast manipulation strategies for quantum devices.