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

TL;DR

This study uses ultrafast X-ray spectroscopy and simulations to elucidate the photochemical pathways of Norrish type I reactions in aromatic carbonyls, revealing detailed excited-state dynamics and population transfer mechanisms.

Contribution

It provides the first detailed time-resolved spectroscopic and computational analysis of the excited-state dynamics in Norrish type I reactions, clarifying the population flow and decay pathways.

Findings

Population transfer from $^1\pi\pi^*$ to $^1n\pi^*$ state occurs in 0.13 ps

Intersystem crossing leads to a long-lived $^3n\pi^*$ state within 3.17 ps

Spectroscopy combined with simulations accurately maps excited-state dynamics

Abstract

Norrish type I reactions selectively cleave carbon-carbon bonds directly adjacent to carbonyl groups. Despite their broad use in combination with aromatic carbonyls for additive manufacturing and dental UV curing applications, the nature of the photochemically active state and its population mechanism remain insufficiently understood. Detailed mechanistic insight requires mapping of the photoexcited population flow involving internal conversion and intersystem crossing. We present a time-domain study of gas phase acetophenone as a prototypical aromatic carbonyl combining soft X-ray time-resolved near-edge X-ray absorption fine structure (TR-NEXAFS) spectroscopy at the oxygen K-edge with ab initio multiple spawning (AIMS) simulations. Exploiting the specific sensitivity of TR-NEXAFS spectroscopy to states with $n\pi^*$ character, we observe population transfer from the initially excited $^1\pi\pi^*$ state to the $^1n\pi^*$ state with a time constant of $(0.13 \pm 0.02)$ ps after an initial induction period of $(0.12 \pm 0.02)$ ps without population transfer, in quantitative agreement with the AIMS simulations. The population in the $^1n\pi^*$ state subsequently decays via intersystem crossing, likely mediated by a $^3\pi\pi^*$ state, within $(3.17 \pm 0.66)$ ps to a long-lived $^3n\pi^*$ state, which is presumed to be active towards Norrish type I chemistry.