Unveiling Gas-phase Photoisomerization of Azobenzene through Time-Resolved Ultrafast Electron Diffraction

Photoisomerization of azobenzene (AB), an important molecular photoswitch with wide-ranging implications in fields such as smart materials and optogenetics, has been well-studied mostly with spectroscopic methods. However, there is no direct molecular structure measurement exploring the structural dynamics of trans-cis isomerization of AB. In this study, we introduce an improved sample delivery method utilizing a high-temperature slit jet system to overcome challenges encountered in previous gas phase ultrafast electron diffraction (UED) experiments, such as limited gas density and thermal decomposition. This system is coupled with the time-resolved MeV UED technique, allowing for the direct observation of molecular structures for both the static AB and the intermediate steps involved in the isomerization process. We use 300 nm photon to initiate large amplitude structural changes in the electronically excited state of trans-AB. Our preliminary results showed an increased population feature at ~5 Å at sub picosecond time scale, suggesting the formation of cis-AB. We anticipate that further analysis will provide insights into the structural details of the intermediates, thereby elucidating the reaction mechanism underlying this isomerization process.