Excited-state dynamics and photo-reaction pathways of 4-nitrothiophenol in solution investigated by time-resolved soft X-ray absorption spectroscopy

Understanding the molecular pathways underlying light-driven reactions is essential for developing efficient and sustainable photochemical processes. Here, we investigate the photoinduced dynamics and reactivity of deprotonated 4-nitrothiophenol (4-NTP) in aqueous solution using transient nitrogen and oxygen K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with density functional theory (DFT) and equation-of-motion coupled-cluster (EOM-CC) calculations. Valence-state calculations identify the lowest bright excited state S1 as a ππ∗ state with partial charge-transfer character, consistent with the experimentally observed absorption maximum near 408 nm. Following excitation at 343 nm, transient NEXAFS reveals the formation and reaction pathways of both singlet and triplet excited states. The combined experimental and theoretical results indicate that intersystem crossing proceeds from the vibrationally hot S1 state through the higher-lying T2 state, followed by rapid internal conversion to T1. The S1 state additionally undergoes photodissociation on the nanosecond timescale, producing NO2 and a thiophenyl radical. In parallel, the T1 state reacts with dissolved O2, leading to formation of the 4-NTP radical and reactive oxygen species, including hydroxyl radicals. oxygen K-edge measurements provide direct evidence for the formation and persistence of these oxygen-centered reactive intermediates. These results establish distinct singlet-and triplet-mediated reaction channels in aqueous 4-NTP and demonstrate the capability of elementspecific transient X-ray spectroscopy, supported by high-level EOM-CC calculations, to resolve competing excited-state pathways and photochemical reactions in solution.