UV Photochemistry of the Energy-Storing Isomer of a Norbornadiene-Based Molecular Switch: Ring Opening, Rehybridised Intramolecular Charge Transfer, and Isomerisation into a Carbene Photoproduct

Abstract

Photoswitches based on the norbornadiene-quadricyclane (NBD-QC) isomer pair capture solar energy by undergoing the NBD → QC photoisomerisation reaction. The energy-storing QC isomer contains two highly strained three-membered rings, which implies that it may be susceptible to photodegradation through exposure to the ultraviolet (UV) component of sunlight. In the present study, we use nonadiabatic molecular dynamics simulations to investigate the UV-induced photochemistry of the QC isomer of a representative NBD-QC photoswitch with a push-pull substitution motif. A pattern recognition algorithm is employed to analyze and systematize the simulation results. We find that the predominant outcome of UV absorption, observed in 75 out of the 100 simulated trajectories, is the opening of the four-membered ring which belongs to the QC moiety, although in about half of the trajectories which follow this relaxation pathway, the ring promptly closes again. However, in a small fraction of trajectories, the QC moiety instead breaks up, leading to the formation of a carbene photoproduct. We hypothesize that that the breakup of the QC moiety is the first step in a reaction pathway which leads to the decomposition of the photoswitch.