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.