Branching and competition of ultrafast photochemical reactions of cyclooctatriene and bicyclooctadiene

•Cyclooctatriene forms a mono-E-isomer and undergoes ring opening and ring cleavage.•The paths are barrierless on the spectroscopic and dark surfaces.•The Woodward-Hoffmann forces are activated with delay on the spectroscopic surface.•Hula-twist isomerization is stimulated by pretwist of a conjugated single bond.•Also the ring cleavage in bicylooctadiene is concerted.

The main primary photoproducts of cycloocta-1,3,5-triene (COT) are a strained mono-E isomer, Z,Z-octatetraene (OT, from electrocyclic ring opening) and benzene + ethylene. We investigated the excited-state dynamics of COT by time-resolved mass spectroscopy, probing by near-IR photoionization. Unexpectedly, we found only one reaction channel. We assign it to the pericyclic reactions. Evidence for an early branching between this and the Z-E channel is taken from previous resonance Raman data. This channel confirms previously formulated rules on the excited states involved, the reaction path and driving forces and contributes to their rationalization. Bicyclo[4.2.0]octa-2,4-diene undergoes only two pericyclic reactions: ring opening to OT and cleavage to benzene + ethylene. We investigated it briefly in its equilibrium mixture with COT. The data are consistent with a common path on the excited surfaces. Suggestions are made for structures of conical intersections, and driving forces are considered. All processes were found to be barrierless.

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