Imaging the Photochemical Ring-Opening of 1,3-Cyclohexadiene by Ultrafast Electron Diffraction

TL;DR

This study uses ultrafast electron diffraction to directly observe the femtosecond-scale structural dynamics of the photochemical ring-opening of 1,3-cyclohexadiene, revealing detailed reaction pathways and timescales.

Contribution

It provides the first direct measurement of the ultrafast structural changes during the ring-opening of 1,3-cyclohexadiene using megaelectronvolt electron diffraction.

Findings

Observation of carbon-carbon bond dissociation dynamics.

Acceleration of ring-opening after internal conversion.

Transformation into ethylene rotation in the photoproduct.

Abstract

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Here, we present direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and sub-{\AA}ngstr\"om length scale by megaelectronvolt ultrafast electron diffraction. We follow the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by direct measurement of time-dependent changes in the distribution of interatomic distances. We observe a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the sub-picosecond timescale. Our work demonstrates the potential of megaelectronvolt ultrafast electron diffraction to elucidate photochemical reaction paths in organic chemistry.