Mechanistic insights into the efficient intramolecular chemiexcitation of dioxetanones from TD‐DFT and multireference calculations

Abstract

AbstractDespite decades of research, it is still not clear what is the mechanism behind the efficient chemiexcitation of dioxetanones in chemiluminescent and bioluminescent reactions. In fact, long‐standing theories (charge transfer‐initiated luminescence and chemically induced electron‐exchange luminescence) have been demonstrated to not be able to explain this phenomenon. Herein, a theoretical approach using reliable and up‐to‐date methodology was used to address this problem, by focusing on model dioxetanones. Time‐dependent (TD)‐Density functional theory (DFT) and multireference complete‐active‐space second‐order perturbation theory (CASPT2) calculations provided evidence that points to efficient intramolecular chemiexcitation being the result of the reacting molecules having access to a long zone of the Potential energy surface (PES), within the biradicalar region, where S0 and S1 are degenerate. Molecules with inefficient chemiexcitation are unable to reach this zone of degeneracy. Our main finding is that access to the region of degeneracy appears to be given due to increased interaction between the keto and CO2 moieties, as supported by the use of the activation strain model and Born‐Oppenheimer molecular dynamics, which extends the biradical region by delaying the rupture of the peroxide ring. Increased interaction derives from attractive electrostatic interactions between the moieties of dioxetanone. Thus, we hypothesize that efficient chemiexcitation results not only from electron/charge transfer and subsequent charge annihilation but is instead based on the degree of interaction between the keto and CO2 moieties, which controls the access to a region of degeneracy between the ground and excited states.