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Molecular Basis of the Chemiluminescence Mechanism of Luminol: A DFT and SO-CASPT2 Study

Dr. Daniel Roca-Sanjuán ( Universitat de València, Spain)

Category: Séminaires de l'équipe CT

Lieu et heure:Bibliothèque recherche, 3 eme étage, bâtiment Lavoisier, 14h.

Intervenant: : Dr. Daniel Roca-Sanjuán,

Institut de Ciència Molecular, Universitat de València, Spain


Molecular Basis of the Chemiluminescence Mechanism of Luminol: A DFT and SO-CASPT2 Study

Light emission of luminol is probably one of the most popular chemiluminescence reactions due to its use in forensic science. Moreover, it has potential applications on the treatment of cancer in deep tissues. The mechanism is however very complex, involving many steps, as compared to other chemiluminescence reactions, and being sensitive to the experimental conditions. Experimental procedures have allowed to identify some key aspects proposing plausible mechanisms involving the formation or not of diazaquinone, peroxide or superoxide molecular oxygen, and concerted or stepwise processes. To clearly determine the mechanisms, such proposals require further studies by tools able to accurately determine the electronic-structure of the species and ground and excited states involved. By efficiently combining density functional theory (DFT) and spin-orbit completeactive-space second-order perturbation theory (SO-CASPT2) methodologies, we have systematically studied the in-vaccuo chemiluminescence mechanism in three steps, (i) luminol oxygenation to generate the chemiluminophore, (ii) chemiexcitation step, and (iii) generation of the light emitter. High differential correlation nature is found in the chemiexcitation step, which has prompted to use the fully correlated CASPT2 method for both energy and geometry determinations. The mechanism of spin inversion and activation of oxygen is found to take place via a single electron transfer from the luminol double deprotonated dianion and in a concerted manner to reach a bicyclic endoperoxide which subsequently eliminates molecular nitrogen and gives rise to a cyclic peroxide. The peroxide bond in comparison to other isoelectronic chemical functionalities (-NH−NH-, -N −−N − -, and -S−S-) is found to have the best chemiexcitation efficiency, which allow to rationalize the oxygenation requirement. Electron transfer from a  orbital of the aniline ring to the  antibonding orbital of the OO bond activates the excitation process creating a nO* state which is found to decay in a radiationless manner. To produce light emission, high-energy excitation must take place able to produce a proton transfer from the amino group to the carbonyl which generate a * state with a vertical emission of 541 nm in agreement with the experiments in DMSO. Triplet chemiexcitation might take place contributing to decrease the fluorescence yield. In this talk, we shall present all the details of the findings briefly listed above, which have allowed us to clarify the mechanism related to the oxygenation, to demonstrate the need to substitute the nitrogen bond by peroxide, and to determine the electronic nature of the chemiexcitation process and the path leading to the light emitter.


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