Mechanistic Investigation into Chemiluminescence from 1,4-Benzoquinone

Tetrahalogen-1,4-benzoquinone (THBQ) represents a category of H₂O₂-dependent substrates for chemiluminescence (CL), including tetrafluoro-, tetrachloro-, tetrabromo-, and tetraiodo-1,4-benzoquinone (TFBQ, TCBQ, TBBQ, and TIBQ). A deep understanding of the CL mechanism of THBQ is essential for all H₂O₂-dependent CL and even some bioluminescence. This article systematically investigates the CL process of THBQ by density functional theory and multireference state theory. The theoretical results confirm the generality of the CL mechanism previously proposed in studies on TCBQ and TBBQ. The dissociation steps producing the emitter of light from dihalogenquinone dioxetane (DHD) and its anion (DHD^-), formed by the oxidation of THBQ, were carefully considered. Findings show that the dissociation of DHD/DHD^- follows the entropy trap/gradually reversible charge-transfer-induced luminescence (GRCTIL) mechanisms. The dissociation of DHD^- is kinetically more advantageous compared with that of DHD. At the practical experimental pH value, the decrease in the electron-withdrawing inductive effect from F to I substituents results in the decrease in the proportions of easily dissociated DHD^-, and the increase in the heavy-atom effect from F to I substituents leads to the increase in the phosphorescence emission. These combined factors successively decrease the CL intensity from TFBQ to TCBQ, TBBQ, and TIBQ. The conclusions are verified by the previous experiments on TCBQ and TBBQ, and they are expected to be confirmed by future experiments on TFBQ and TIBQ.