Theoretical Study on Chemiluminescence of H₂O₂-dependent Tetrachloro-1,4-benzoquinone

Compared with O₂-dependent chemiluminescence (CL) and bioluminescence (BL), H₂O₂-dependent ones are far from being completely understood. A two-step mechanism for H₂O₂-dependent CL production from tetrachloro-1,4-benzoquinone (TCBQ) was proposed based on experimental evidence of detecting the formation of several intermediates and products. This mechanism is not yet supported by theoretical evidence, and its details remain unknown. In the present paper, we performed multireference and (time-dependent) density functional theory calculations on the complete reaction process of TCBQ with H₂O₂ to produce CL. The calculations reproduced the experimentally observed two-step CL. Although the reactants are different, the first and second CLs follow very similar reaction processes and mechanisms. First, an anionic dioxetane is formed via five sequential reactions. The intrinsically produced ^•OH is crucial for forming dioxetane. Subsequently, the anionic dioxetane decomposes to produce an anionic excited-state (S₁) product. A conical interaction of the ground and the S₁-state potential energy surfaces is responsible for producing the S₁-state product. Finally, the S₁-state anionic product changes to its neutral form, and the latter emits light as an actual light emitter. This mechanism could be extended to luminescent systems of all H₂O₂-dependent tetrahalogenated quinoids, including acorn worms, because TCBQ/H₂O₂ is a typical representative of these luminescent systems.