Mechanistic Insight into pH-Dependent Luminol Chemiluminescence in Aqueous Solution

Luminol is one of the best known chemiluminescent cyclic hydrazides used in basic solution. Owing to the complexity of luminol oxidation, the mechanism of luminol chemiluminescence (CL), especially in aqueous solution, has not yet been fully elucidated. Recent theoretical computations have confirmed that luminol CL originates from the chemiexcitation of a 1,2-dioxane-3,6-dione dianion (CP^2-). This seems to be inconsistent with the luminol oxidation in aqueous solutions, where only the decomposition of a monoanionic peroxyketal (LOOH) is confirmed to yield CL. In this work, we theoretically investigated the complete decomposition of LOOH and the pK_a of key intermediates in aqueous solutions using (time-dependent) density functional theory. LOOH first cyclizes to an endoperoxide monoanion (EP^-). When pH < pK_a (EP^-), EP^- directly decomposes by a retro-Diels-Alder (rDA) reaction to an aminophthalate monoanion (AP^-) without CL activity. Moreover, when pH > pK_a (EP^-), EP^- deprotonates to dianionic EP^2-, which rapidly eliminates N₂ to CP^2-, inducing chemiexcitation. This conclusion is supported by the pK_a (EP^-) ≈ 8 estimated in this work, which is consistent with the pH-dependent profile of luminol CL observed in previous experiments. Thus, the pH-dependent CL determined by the acidity of endoperoxides may provide a theoretical basis to design new cyclic hydrazides with CL activity at different pH.