The photochemical ring-opening reaction of 1,3-cyclohexadiene: complex dynamical evolution of the reactive state

TL;DR

This study reveals that the photochemical ring-opening of 1,3-cyclohexadiene involves a more complex electronic state evolution than previously thought, emphasizing the importance of high-lying states and diabatic dynamics in the reaction pathway.

Contribution

The paper combines experimental and computational methods to demonstrate the complex role of high-lying electronic states and diabatic dynamics in the ring-opening reaction of 1,3-cyclohexadiene.

Findings

High-lying electronic states influence the reaction pathway.

Diabatic states are crucial for understanding the dynamics.

The reaction involves a smooth energy decrease of an excited state.

Abstract

The photochemically induced ring-opening isomerization reaction of 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT) is a textbook example of a pericyclic reaction, and has been amply investigated with advanced spectroscopic techniques. The generally accepted description of the isomerization pathway starts with a valence excitation to the lowest-lying bright state, followed by a passage through a conical intersection to a dark doubly excited state, and finally a branching between either the return to the ground state of the cyclic molecule or the actual ring-opening reaction leading to the open-chain isomer. It was traditionally assumed that the dark reactive state corresponds to the second excited state of CHD at the Franck-Condon geometry. Here in a joint experimental and computational effort we demonstrate that the evolution of the excitation-deexcitation process is much more complex than usually described. In particular, we show that an initially high-lying electronic state smoothly decreasing in energy along the reaction path plays a key role in the ring-opening reaction. The conceptual basis of our work is that the dynamics to consider here is determined by diabatic states, whose populations are the ones closely related to the observed photoelectron signal.