9D nonadiabatic quantum dynamics through a four-state conical intersection: investigating the homolysis of the O-O bond in anthracene-9,10-endoperoxide.

The excited state dynamics of anthracene-9,10-endoperoxide is investigated using quantum wavepacket dynamics. APO is an aromatic endoperoxide which, upon excitation to S(1), shows a cleavage of the oxygen-oxygen bond, leading to rearrangement products. It was shown that the dynamics of the O-O cleavage is modulated by a four-state degeneracy [D. Mollenhauer, I. Corral, and L. González, J. Phys. Chem. Lett. 1, 1036 (2010)]. The most important mode to describe the O-O cleavage is the opening of the O-O bond. Excitation to higher excited states S(n) (n ≥ 2) leads to the release of singlet molecular oxygen. For this release, the twist of the oxygen atoms around the molecular axis is an important mode. These two degrees of freedom were employed to calculate two-dimensional potential energy surfaces for the four singlet states which become degenerate. Further modes were included in terms of harmonic oscillators. Using the multiconfigurational time-dependent Hartree method, quantum dynamic simulations were performed in up to nine degrees of freedom. Moreover, the nine branching space vectors, which prove the degeneracy to be a four-state conical intersection (4CI), were calculated and included in the wavepacket propagations. The resulting dynamics show that the 4CI is reached very fast (in less than 30 fs after excitation) and the wavepacket distributes over all states. The degree of distribution into the states is very much dependent on which modes are included in the simulation.