A CASSCF/CASPT2 approach to the decomposition of thiazole-substituted dioxetanone: Substitution effects and charge-transfer induced electron excitation

The gas-phase decompositions of a thiazole-substituted dioxetanone, in both the natural and anionic forms, were investigated theoretically in a CASSCF/CASPT2 study. The neutral conjugated thiazole (with or without a hydroxyl group) substitution on the dioxetanone has no evident effect on the dissociation; however, a subsequent deprotonation – invoking charge-transfer excitations from the thiazole to the dioxetanone moiety – will dramatically change the reaction mechanism from stepwise to concerted, and reduce the activation barrier by ∼8 kcal mol−1. These findings are helpful for the better understanding of dioxetanone chemiluminescence and the charge-transfer induced electron excitation in chemi- and bioluminescence processes.