Elucidating Norrish type I reactive pathways by ultrafast X-ray absorption spectroscopy

Area of Science:

• Photochemistry
• Physical Chemistry
• Spectroscopy

Background:

• Norrish type I reactions are crucial for cleaving carbon-carbon bonds adjacent to carbonyl groups.
• Aromatic carbonyls are widely used in additive manufacturing and dental UV curing.
• The photochemically active state and population mechanisms in these reactions are not fully understood.

Purpose of the Study:

• To elucidate the mechanistic details of photoexcited population flow in aromatic carbonyls.
• To map internal conversion and intersystem crossing pathways.
• To understand the states involved in Norrish type I reactions.

Main Methods:

• Time-domain study using gas-phase acetophenone as a model compound.
• Soft X-ray time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy at the oxygen K-edge.
• Ab initio multiple spawning (AIMS) simulations.

Main Results:

• Observed population transfer from the initial ¹ππ* state to the ¹nπ* state with a time constant of (0.13 ± 0.02) ps.
• Quantitatively agreement between experimental TR-NEXAFS data and AIMS simulations.
• Population in the ¹nπ* state decays via intersystem crossing to a long-lived ³nπ* state within (3.17 ± 0.66) ps.

Conclusions:

• The ³nπ* state is presumed to be the active state for Norrish type I chemistry.
• Detailed mechanistic insight into photoexcited population flow has been achieved.
• The findings provide a foundation for optimizing applications utilizing aromatic carbonyl photochemistry.