Aminophthalic dianion

Steinfatt proposed an alternative mechanism for the formation of excited aminophth-alate, based on the concept of dioxirane-carbene mediated chemiexcitation, which is also attributed to other chemiluminescent systems ° °. After the attack of hydrogen peroxide on the diazaquinone 27 carbonyl carbon, a perhydrolysis step is postulated to result in the intramolecular dioxirane-carbene system (32) in the excited state ° ° . This species presumably rearranges to 3-aminophthalate dianion while still in the singlet-excited state (Scheme 23). Although this is a very interesting mechanistic proposal, it is based on experimental evidence obtained with indirect phthaloyl peroxide chemiluminescence and no further evidence corroborates this proposal. 

Fio. 15. Postulated mechanism for the chemiluminescence of luminol, taken from McCapra and Beheshti (M2I). In this sequence, ferricyanide acts as a one-electron oxidant. The products are nitrogen and an excited-state aminophthalate dianion that decays to the ground state with concomittant light emission. 

The dianion of luminol undergoes a reaction with molecular oxygen to form a peroxide of unknown structure. This peroxide is unstable and decomposes with the evolution of nitrogen gas, producing the 3-aminophthalate dianion in an electronically excited state. The excited dianion emits a photon that is visible as light. One very attractive hypothesis for the structure of the peroxide postulates a cyclic endoperoxide that decomposes by the following mechanism ... 

Although the nature of the peroxide is still debatable, the remainder of the reaction is well understood. The chemical products of the reaction have been shown to be the 3-aminophthalate dianion and molecular nitrogen. The intermediate that emits light has been identified definitely as the excited state singlet of the 3-aminophthalate dianion. Thus, the fluorescence emission spectrum of the 3-aminophthalate dianion (produced by photon absorption) is identical to the spectrum of the light emitted from the chemiluminescent reaction. However, for numerous complicated reasons, it is believed that the... 

The excited state of the 3-aminophthalate dianion may be quenched by suitable acceptor molecules, or the energy (about 50-80 Kcal/mol) may be transferred to give emission from the acceptor molecules. Several such experiments are described in the following procedure. 

The maximum of luminol chemiluminescence in aqueous medium is at 425 nm and in aprotic media (DMF, DMSO etc.) at 480 nm. In DMSO/water mixtures both maxima are observed [5]. These different maxima are ascribed to different species of the aminophthalate dianion (15) and (16). 

In summary, there is no reasonable doubt that 3-aminophthalate dianion and... 

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