Mechanistic study of the unimolecular decomposition of 1,2-dioxetanedione Luís Pinto da Silva a and Joaquim C. G. Esteves da Silva a * The unimolecular decomposition of 1,2-dioxetanedione, the high-energy intermediate of the chemiluminescence peroxyoxalate reaction, was studied by theoretical means for the first time. Our calculations have provided results in line with the experimental data regarding this compound. 1,2-Dioxetanedione decomposes due to a step-wise biradical mechanism. In the biradical region of the decomposition path, there is a path for singlet chemiexcitation. Interactions between the singlet ground and excited states with triplet states can explain the weak unimolecular chemiluminescence of 1,2-dioxetanedione. Copyright © 2013 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this paper. Keywords: 1,2-dioxetanedione; chemiluminescence; CIEEL; ICIC; interstate crossing; peroxyoxalate system; step-wise biradical; unimolecular decomposition INTRODUCTION Firefly bioluminescence is the emission of light, in the fireflies, due to a luciferase catalyzed reaction. [1–5] This reaction is com- posed of two different steps: the first is an adenylation step, in which firefly luciferin is converted into luciferyl-adenylate due to the reaction with adenosine-5’-triphosphate; the second step is an oxidation, in which luciferyl-adenylate is converted into ex- cited state oxyluciferin by reaction with molecular oxygen. [6–8] This system has been gaining numerous practical applications in the pharmaceutical, biomedical and bioanalytical areas, among others, due to very interesting characteristics. [9,10] Excited state oxyluciferin is thought to be formed due to the formation and subsequent decomposition of firefly dioxetanone (Fig. 1). [6] Schuster and co-workers have proposed a chemically initiated electron exchange luminescence (CIEEL) mechanism, first to explain the efficient chemiluminescence of diphenoyl peroxide and dimethyl-1,2-dioxetanone (activated by perylene), and latter to explain the efficient firefly bioluminescence. [11–16] The initiating step of this mechanism is an electron transfer from an easily oxidizable electron-rich moiety to an acceptor energy- rich moiety (denominated as the high-energy intermediate, HEI). Due to this transfer, the electron acceptor decomposes via bond cleavage to form a radical ion pair with the electron donor. The radical ion pair can then transfer an electron backwards in order to generate excited state products. If the electron-rich moi- ety is part of the molecule on which the HEI is found, we use the term intramolecular CIEEL (the case of firefly bioluminescence), but if the electron-rich moiety (now called the activator, ACT) is not part of the same molecule as the HEI, the term intermolecular is used (the case of diphenoyl peroxide and di- methyl-1,2-dioxetanone, where the ACT was perylene). [6,11–17] However, revaluation of the intermolecular CIEEL-based chemiluminescence of diphenoyl peroxide and dimethyl-1,2- dioxetanone revealed that the efficiency of this process is much lower than expected. [18,19] The analysis of an ACT-activated chemiluminescence of two novel 1,2-dioxetanones also revealed an inefficient intermolecular CIEEL-based chemiluminescence efficiency. [19] Thus, this findings indicates that CIEEL mechanism must be reassessed, as the study of model compounds of this theory presented inefficient chemiluminescence contrary to the postulated by Schuster and co-workers. In this case, it is fair to say that the application of the CIEEL theory to the explanation of the efficient firefly bioluminescence should be questioned. The only known intermolecular CIEEL-based efficient chemilu- minescence is that of the peroxyoxalate reaction (Fig. 2). [20] In this reaction, it is thought that 1,2-dioxetanedione is formed as the HEI. Subsequently, this molecule forms a supramolecular charge transfer complex with an ACT. Electron and back electron transfers between the ACT and the HEI will provoke a bond cleavage step and chemiexcite the ACT to a singlet excited state, which then decays to the ground state with emission of light. [20–23] 1,2-Dioxetanedione was also found to emit weak chemiluminescence, in a unimolecular decomposition reaction in the absence of an ACT. [20,23] Given this, it appears to be very important to study the ACT- catalyzed chemiluminescence of 1,2-dioxetanedione, in order to fully understand the features of a efficient intermolecular CIEEL-based chemiluminescence. Besides giving insights of an efficient chemiluminescence reaction, this type of study can also give some information regarding the application of the CIEEL theory to the efficient firefly bioluminescence. However, as 1,2-dioxetanedione presents an intrinsic and non-catalyzed chemiluminescence, it is of most importance to characterize the unimolecular decomposition of this HEI, in order to better understand its ACT-catalyzed chemilumines- cence. Thus, the objective of this study is the characterization * Correspondence to: J. C. G. Esteves da Silva, Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. E-mail: jcsilva@fc.up.pt a L. Pinto da Silva, J. C. G. Esteves da Silva Centro de Investigação em Química, Departamento de Química e Bioquímica, Faculdade de Ciências da Universidade do Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal Research Article Received: 5 February 2013, Revised: 02 April 2013, Accepted: 17 May 2013, Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/poc.3149 J. Phys. Org. Chem. 2013 Copyright © 2013 John Wiley & Sons, Ltd.