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

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.