New perspectives on electrocyclic photochemistry using ultrafast diffraction probes*

Electrocyclic chemistry has been fascinating chemists for many decades due to the concerted formation and cleavage of multiple bonds and the resulting stereospecificity of electrocyclic reactions. They can proceed both via thermal and photochemical pathways with opposite stereospecificity as predicted by the famous Woodward-Hoffmann rules. The existence of photochemical pathways and their femtosecond timescales makes these reactions available for detailed studies using time-resolved methods. The availability of novel ultrafast experimental methods with direct structural sensitivity such as ultrafast electron diffraction and X-ray diffraction provides revolutionary insights into such reaction dynamics. Such methods allow for following the involved structural dynamics in real space and time on the level of individual atomic distance changes during the evolution from the reactant to the photoproduct.

We have investigated electrocyclic ring-opening reactions in a series of molecules sharing the structural backbone of 1,3-cyclohexadiene. In 1,3-cyclohexadiene, we follow the ring-opening reaction directly in space and time in the evolution of atomic pair distribution functions. Using the 1,3-cyclohexadiene derivative α-phellandrene, we confirm the Woodward-Hoffmann rules in real time by obtaining unambiguous signatures of a specific conformer of α-phellandrene evolving into the photoproduct isomer predicted by the Woodward-Hoffmann rules. In a further derivative,  α-terpinene, we observe the structural relaxation into the pericyclic minimum of the excited state, the photochemical “transition state” of the reaction in the vicinity of the conical intersection with the electronic ground state. The observed structural rearrangements suggest a new perspective on the origins of the stereoselectivity of electrocyclic reactions.