Charged Particle Imaging of the Deprotonated Octatrienoic Acid Anion: Evidence for a Photoinduced Cyclization Reaction.

Photoelectron spectroscopy of the deprotonated octatrienoic acid anion, [CH-CO], shows the formation of [CH] and loss of H at hν = 4.13 eV. Using velocity map imaging, the H fragment was characterized to have a Boltzmann-like kinetic energy distribution consistent with dissociation on a ground electronic state. Similar dynamics were not observed at hν = 4.66 eV even though there is clear evidence for recovery of the ground electronic state of [CH-CO]. In accord with supporting electronic structure calculations, the production of H at hν = 4.13 eV is explained by excited-state dissociation of CO to form [CH], which subsequently undergoes a ring-closure isomerization reaction to yield toluene and H. These data represent the first evidence for a photoinduced ring-closing isomerization reaction in an anionic polyene and provides an interesting example of the rich anion dynamics that can occur in the detachment continuum and that can influence photochemistry.