Chemiluminescence of Organic Peroxides

The chemiluminescent peroxyoxalate system: state of the art almost 50 years from its discovery

“…Chemiluminescence reactions can be generally divided in three main steps: (i) formation of a high-energy intermediate (HEI), in one or more chemical transformations of ground state molecules; (ii) unimolecular decomposition of the HEI or its interaction with other reagents, leading to electronically excited state formation (chemiexcitation step); (iii) decay of this excited state to the ground state accompanied by fluorescence or phosphorescence emission, depending on the multiplicity of the excited state. 9,11 With the synthesis of cyclic organic peroxides like 1,2-dioxetanones (4) and 1,2-dioxetanes (5), which are nothing more than isolated HEI, and detailed studies on their chemiluminescent decomposition, two distinct general chemiexcitation mechanisms could be outlined: (i) unimolecular cleavage or rearrangement of molecules with high energy content forming excited states, as in the unimolecular thermal decomposition of 1,2-dioxetanes; 12 or (ii) catalyzed decomposition of the high energy peroxide by a suitable activator (ACT), forming the excited state of the ACT, a mechanism known as CIEEL (Chemically Initiated Electron Exchange Luminescence), initially proposed by Schuster 13 (Scheme 1). Studies on the chemiluminescence properties of 1,2-dioxetanones 14 and diphenoyl peroxide 15 (6) revealed that the observed light emission rate constants (k obs ), as well as the chemiluminescence quantum yields (Φ CL ), increased proportionally with the concentration of added ACT.…”

“…Furthermore, a dependency of the k obs and the emission intensities with the oxidation potential of the added ACT could be taken as a clearcut evidence for the occurrence of an electron transfer from the ACT to the cyclic peroxide in the rate-limiting step. 9,11,[13][14][15] On the basis of these and other experimental observations, Schuster and coworkers postulated the CIEEL mechanism, where the first step comprises formation of a charge-transfer type encounter complex (K CT ) between the peroxide and the ACT, inside the solvent cavity (Scheme 1). 7,[13][14][15] Elongation of the relatively weak O-O bond by thermal activation within the charge-transfer complex results in the lowering of the antibonding orbital energy (σ * ), permitting the occurrence of an endothermic electron transfer from the ACT to the peroxide (k ET ), which is accompanied by the cleavage of the O-O bond.…”

“…Although not being energetically favorable, the electron transfer is possible, as the O-O bond cleavage occurs almost simultaneously, making the whole process essentially irreversible (Scheme 1). 7,11 Subsequently, the radical anion of the 1,2-dioxetanone, still inside the solvent cavity with the ACT, undergoes C-C bond cleavage (k CLEAV ), resulting in a neutral compound and a carbonyl radical anion. The new pair of radical ions, the carbonyl radical anion and the radical cation of the ACT, yet inside the solvent cavity, can now undergo an electron back-transfer (k EBT ), † which can lead to the formation of a singlet excited state (S 1 ) of the ACT.…”

“…The quantum yield for the diphenoyl peroxide / perylene system was shown to be three orders of magnitude lower than the value initially determined, 17,18 and since this is the prototype system for the CIEEL proposal, the validity of this mechanism was itself questioned. 9,11 Contrarily, 1,2-dioxetanes containing substituents with low oxidation potentials, such as…”

“…11 Despite the fact that the PO reaction was discovered -or better to say observedalmost fifty years ago, the mechanism of formation and the structure of the high energy intermediate (HEI) and the chemiexcitation pathway are still matter of discussion in the scientific community.…”