The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

Abstract

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Ultrafast Dynamics of 1,3-Cyclohexadiene in Highly Excited States

“…Of particular interest are the geometric changes occurring in the molecule after excitation into the 1B-valence state and as the molecule travels through the electronic states, as such structural motions have eluded direct experimental observation. As has been pointed out, [5,6,8,9] the wave packet remains quite narrow during the reaction, implying that "structure" is a well-defined quantity even though it rapidly evolves. Moreover, it is unknown if higher-lying electronic states follow ring-opening pathways similar to that following the 1B excitation, or if other unique mechanisms emerge.…”

“…Upon excitation to the 1B valence state, the reaction path carries the molecule within about 140 fs along the ring-opening coordinate through the 1B/2A and 2A/1A conical intersections back to the ground state [3][4][5][6][7]. But even while these curve-crossing transitions are now well understood, numerous questions remain.…”

“…The 4.63 eV photon places CHD on a steeply repulsive part of the 1B potential energy surface, which accelerates it along the ring-opening coordinate, yielding 1,3,5-hexatriene (HT). Along this reaction path the wave packet bypasses a cusp in a symmetry-breaking direction and enters the 2A 1 surface, from where it decays to the electronic ground state [4][5][6][7]. Ionization out of 2A 1 by two Dω 2 probe photons passes through the Rydberg states that are seen in the spectrum [7].…”

The Dynamophore – Localization of Excited State Dynamics Studied by Time–Resolved Photoelectron Spectroscopy

“…Ethylene exhibits a characteristic set of motions to return from the excited *-state to the electronic ground state: It twists about its double bond and either pyramidalizes at one of the two carbon atoms or forms a H-bridged structure which can lead to a [1,2]-hydrogen migration [2][3][4]. Other examples of dynamophores are the allyl-group (CH=CH-CH2) which performs an [1,3]-H shift or the cyclohexa- [1,3]-diene group exhibiting a ring opening reaction [5]. The concept of a dynamophore allows for the classification of a wide range of dynamical processes in terms of a small set of prototypical types.…”

Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1,3-cyclohexadiene ring opening

“…The molecule has been studied extensively [3--13] (for a recent review see [14] and the references therein), and is suitable for testing new experimental techniques. Ion fragment time--of--flight (TOF) mass spectra of the parent molecule CHD and the product HT differ significantly when short intense 800 nm radiation is used for fragmentation [3,4,15].…”