Nonadiabatic Photochemistry Induced by Inaccessible Conical Intersections.

Abstract

Conical intersections are ubiquitous in photochemical processes, where nonadiabatic transfer induces ultrafast nonradiative decay from an excited state. Although they eluded experimental detection until the 1990s, today three diagnostic attributes are generally associated with photochemical reactions through conical intersections: ultrafast electronic dynamics, negligible fluorescence, and coherent wavepacket transfer. Here, we use generalized quantum dynamics simulations to show that coherent nonadiabatic transfer of excited vibrational wavepackets can occur even without reaching the conical intersection region. Instead, the wavepacket remains distant from the conical intersection throughout. In some topographies, an energetically inaccessible conical intersection can be completely avoided, yet still induce substantial nonadiabatic transfer with ultrafast transfer efficiencies that are nearly identical to those of direct transfer through a conical intersection. These results reveal that the diagnostic properties of conical intersections are not actually specific to decay pathways traveling directly through the intersection funnel, as is the common interpretation, but can also arise from alternative pathways that do not reach the intersection. This suggests that the diagnostic features and experimental signals associated with conical intersections should be reassessed, and the concept of pathways through a conical intersection as the "paradigm of photochemistry" may need to be adjusted.