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

Area of Science:

• Photochemistry
• Spectroscopy
• Quantum Chemistry

Background:

• Aromatic N-oxides exhibit complex photochemistry.
• Primary photochemical steps remain underexplored.
• UV photoexcitation initiates transient species formation.

Purpose of the Study:

• Investigate the primary photochemical steps of pyridine N-oxide and pyridazine N-oxide.
• Characterize transient intermediates using advanced spectroscopic techniques.
• Elucidate the excited state dynamics and reaction pathways.

Main Methods:

• Time-resolved and steady-state X-ray absorption spectroscopy (XAS) at N and O K-edges.
• UV photoexcitation.
• Quantum chemical calculations.
• Near-edge X-ray absorption fine structure (NEXAFS) analysis.

Main Results:

• Identified transient intermediates on picosecond to nanosecond timescales.
• Observed fast deplanarization in the S1 state, followed by ground state evolution.
• Pyridine N-oxide yielded a stable oxaziridine intermediate.
• Pyridazine N-oxide formed (Z)-4-diazobut-2-enal within 100 ps, without oxaziridine or oxadiazepine intermediates.

Conclusions:

• Time-resolved XAS effectively characterizes photochemical dynamics beyond traditional methods.
• Mechanistic details of N-oxide photochemistry differ significantly between pyridine and pyridazine derivatives.
• Excited state evolution and subsequent ground state reactions dictate product formation.