Photochemistry of Aromatic N‐Oxides in Water Probed by Time‐Resolved X‐ray Absorption Spectroscopy

Aromatic N-oxides have a rich photochemistry, but the primary steps have not been investigated in much detail. In this work pyridine N-oxide and pyridazine N-oxide are studied using steady-state and time-resolved X-ray absorption spectroscopy (XAS). Absorption changes at the nitrogen and oxygen K-edges following UV photoexcitation are recorded on a timescale of picoseconds to hundreds of nanoseconds. The spectral features are assigned to the presence of specific transient intermediates. Quantum chemical calculations indicate that the excited state dynamics in the S1 state are characterized by a fast deplanarization. After reaching a minimum energy crossing point (MECP), evolution on the ground state surface leads to the starting materials and metastable products. The primary photoproduct of pyridine N-oxide, oxaziridine 3, is stable on the sub-microsecond timescale. In the photochemistry of pyridazine N-oxide, oxaziridine and oxadiazepine intermediates are not observed, and (Z)-4-diazobut-2-enal is formed within 100 ps. The mechanistic details of the two different N-oxides uncovered through characteristic features in the near-edge X-ray absorption fine structure (NEXAFS) region serve as an example of how time-resolved XAS enables the characterization of photochemical dynamics beyond those accessible to more traditional spectroscopies.