Rationalizing the role of electron/charge transfer in the intramolecular chemiexcitation of dioxetanone-based chemi-/bioluminescent systems

[Display omitted] •The chemiexcitation step of dioxetanones is tuned by their ionization state.•The activation barrier of thermolysis is decreased by electron transfer processes.•The barrier decrease is caused by mitigating repulsions between reaction fragments.•Chemiexcitation transitions are local excitations without charge-transfer character.•Neutral dioxetanones present more efficient chemiexcitation profiles. The thermolysis of dioxetanones is a key process in the intramolecular chemiexcitation step of several chemi- and bioluminescent reactions. This step is generally explained with mechanisms based on either electron transfer (ET), such as the Chemically Initiated Electron-Exchange Luminescence (CIEEL) mechanism, or charge transfer (CT), such as the Charge Transfer-Initiated Luminescence (CTIL) mechanism. Here, we have used a TD-DFT approach to characterize the thermolysis and chemiexcitation steps of model dioxetanones, to rationalize the role of ET/CT in those intramolecular processes. Our results showed that ET/CT can reduce the activation barrier of the thermolysis reaction, by reducing the repulsion between the reacting fragments (ketone and CO2 moieties) that originate during peroxide bond breaking. However, in terms of singlet chemiexcitation profiles, those of non-CIEEL/CTIL-based dioxetanones appear to be more efficient than of CIEEL/CTIL-based ones. Furthermore, the ground state to singlet excited state transitions were found to be local excitations, without CT between the peroxide ring and the electron-rich moiety. So, ET/CT appear to be responsible for tuning the activation barrier of the thermolysis reaction, without playing a role in efficient singlet chemiexcitation itself.