Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the 2+2] photocycloaddition in the model system pseudo‐gem‐divinyl2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for 2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S₁ lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to 2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state 2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl2.2]paracyclophane, which has a lifetime of more than 500 ps in the S₁ state.