Ultrafast [2 + 2]-cycloaddition in norbornadiene

Abstract

Excitation of norbornadiene (bicyclo[2.2.1]hepta-2,5-diene) at 200 nm populates two states in parallel, the second ππ* state and a Rydberg state. We monitored both populations by transient nonresonant ionization. From the ππ* state the molecule relaxes in consecutive steps with time constants 5, 31 and 55 fs down to the ground-state surface, whereas the Rydberg population merges to the other path on the ππ* surface within 420 fs. The relaxation steps are discussed in terms of conical intersections (CoIns) between different surfaces. Information on them is inferred from known spectroscopy and, for the last CoIn, from published calculations on Dewar benzene→prismane conversion and on ethylene photodimerization for which norbornadiene with its two nonconjugated double bonds is a model. The calculation predicts symmetry breaking for this CoIn, the two ethylenes forming a rhombus. Although this distortion is hindered in norbornadiene by ring strain, this CoIn seems easily accessible as indicated by the short time (<55 fs) found for passing through it.