Photoinduced Dissociation of Halobenzenes: Nonadiabatic Molecular Dynamics Simulations Reveal Key Pathways

Aryl halides are prototypical molecules for studying photodissociation, yet the role of spin–orbit coupling (SOC) in their dynamics remains incompletely understood. Using state-of-the-art ab initio calculations and excited-state dynamics simulations, we explore the photodissociation pathways of iodobenzene (PhI) and bromobenzene (PhBr). For PhI, two dissociation pathways, direct and indirect modes, are identified, consistent with gas-phase experiments. In contrast, photodissociation of PhBr occurs only after overcoming the energy barrier between bound and repulsive states, which requires activation of specific vibrational modes, particularly those associated with boat-like out-of-plane motion. While previous studies have suggested that SOC primarily accelerates intersystem crossing and photodissociation in heavier halogens, our results show that, in addition to SOC, the activation of specific vibrational modes also plays a crucial role in the dissociation process. These findings enhance our understanding of how SOC influences excited-state dynamics, providing insight into controlling photochemical reactivity in halogenated organic compounds.