Singlet Fission Dynamics Controlled by an Electronic Crossing Seam

Electronic degeneracies and near-degeneracies including conical intersections and avoided crossings, typically accompanied by strong vibronic couplings and nonadiabatic transitions, play fundamental roles in photochemical, photophysical, and photobiological processes. However, their implications on excited-state chemical reactivities are not fully understood. In this theoretical study, we demonstrate that the singlet fission dynamics in the hydrogen chain H4, previously identified as a minimal molecule for singlet fission, is controlled by the S1/S2 crossing seam: the nuclear wavepacket traces the seam and turns around at the end. Specifically, when the crossing seam is extended, the open reaction channel can be completely blocked. We provide a chemically intuitive picture to understand this effect from the quantum geometric perspective. Our results not only reveal a new mechanism for controlling photochemical reactions but may help elucidate the mechanism of singlet fission.