Photoexcited dynamics of the valence states of norbornadiene

The non-radiative decay of photoexcited norbornadiene, which together with its isomer quadricyclane forms a molecular photoswitch, is investigated using surface-hopping non-adiabatic dynamics. The simulations are performed using four levels of electronic structure theory: CASSCF(2,2), CASSCF(4,4), XMS-CASPT2(2,2), and XMS-CASPT2(4,4). These electronic structure models yield two distinct classes of excited-state reaction pathways, with different quantum yields for the isomerization. This illustrates the significance of the potential energy surfaces when simulating photoexcited dynamics. The nature of the two reaction pathways is related to topographical features on the surfaces, suggesting potential "design rules" for chemical modification via substituent groups. How the molecule approaches the conical intersection is also shown to play a decisive role in the reaction outcome.