Two Conical Intersections Control Luminol Chemiluminescence

Luminol, the first discovered and man-made effective chemiluminescence (CL) system, is the best known and one of the most widely used CL materials. The chemiluminescent process of the luminol CL has not yet been fully elucidated, although the decomposition of 1,2-dioxane-3,6-dione dianion (CP^2-) is verified to be the key step to produce light emitter. However, the mechanisms of the CP^2- decomposition and the effective singlet chemiexcitation are totally unknown, which is the outstanding obstacle to comprehending luminol CL. In the present work, by means of the state-of-the-art multireference computation and the nonadiabatic molecular dynamics (NAMD) simulation, the decomposition mechanism of CP^2- is clearly revealed. A stepwise single electron transfer from the aminophthaloyl to the O-O bond initiates the decomposition of CP^2-, and the light emitter is produced via the two crossings of the potential energy surfaces of the ground state (S₀) and the first singlet excited state (S₁). The NAMD simulated quantum yield of the light emitter demonstrates that the located two conical intersections control an effective nonadiabatic pathway in luminol CL. The proposed mechanism of "two conical intersections" is suitable not only to luminol but also to other CL materials with cyclic peroxide as the chemical energy provider.