Luminol chemiexcitation

It must be pointed out that, despite the lack of direct experimental support for this hypothesis, the involvement of the electron transfer in luminol chemiexcitation is related to the chemiexcitation steps proposed for several highly efficient chemi- and biolu-minescent systems, such as activated peroxyoxalate chemiluminescence and the firefly luciferin/luciferase system (Section... 

In aprotic media, only molecular oxygen and a strong base are needed to produce chemiluminescence from luminol. In such media, an important intermediate in the reaction is the dianion of luminol, which can be oxidized by oxygen, resulting in the formation of the diazaquinone 27 and deprotonated hydrogen peroxide. The subsequent nucleophilic attack by hydrogen peroxide dianion to one of the diazaquinone carbonyls gives rise to the formation of a metastable peroxidic intermediate that, by several steps, results in chemiexcited 3-aminophthalate (Scheme 16)123,174, iso ... 

The attempt to prepare a compound analogous to endoperoxide 30 from the reaction of 1,4-dimethoxyphthalazines with singlet oxygen resulted in the formation of the corresponding phthalate by elimination of N2, albeit no chemiluminescence was observed . Even though no experimental evidence indicates its existence, the endoperoxide intermediate 30 is one of the most well-accepted HEI postulated for the chemiexcitation step in luminol chemiluminescence. 

A CIEEL approach can also be used to explain chemiexcitation in luminol chemilumi-nescence . Two possibilities arise (i) an electron transfer from the amino group to the peroxidic moiety in the antiaromatic peroxide 33, resulting in bond cleavage followed by intramolecular back-electron transfer and formation of excited 3-aminophthalate (Scheme 24) (ii) the equilibrium between the peroxycarboxylic aldehyde 34, formed after elimination of nitrogen, and the cyclic peroxy semiacetal 35 is shifted in the direction of 35, as the result of an electron transfer from the amino group to the cyclic peroxide moiety, followed by 0—0 bond cleavage . Back-electron transfer would result in chemiexcitation (Scheme 25). 

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