Chemically initiated electron exchange Chemiluminescence

Schuster, G. B. (1979). Chemiluminescence of organic peroxides. Conversion of ground-state reactants to excited-state products by chemically initiated electron-exchange luminescence mechanism. Acc. Chem. Res. 12 366-373. 

Chemiluminescence. The mechanisms behind this phenomenon, as induced by the reaction of, e.g. diphenoyl peroxide and an easily oxidized fluorescent molecule has been brilliantly illuminated by Schuster and co-workers (Schuster, 1979b Koo and Schuster, 1978) who proposed the CIEEL pathway (Chemically Initiated Electron Exchange Luminescence) according to (12). Note that two electron-transfer steps are postulated, the... 

The proposed mechanism for the activator-catalyzed chemiluminescence of dimethyldioxetanone is the general mechanism identified as chemically initiated electron-exchange luminescence (Schmidt and Schuster, 1978a Adam et al., 1978). The CIEEL sequence as applied to dimethyldioxetanone is shown in Fig. 8. In short, the light-generating sequence is initiated by electron transfer from the activator (act) to the dioxetanone. Subsequent decarboxylation gives acetone radical anion. Annihilation of acetone radical anion and activator radical cation generates the excited state of the activator. 

Fio. 8 Mechanism of the activator-catalyzed chemiluminescence of dimethyldioxetanone chemically initiated electron-exchange luminescence (CIEEL)... 

The chemiluminescent reaction of diphenoyl peroxide [26] with easily oxidized, aromatic hydrocarbons, reported by Koo and Schuster (1977b, 1978), was the first well-defined example of an electron-exchange chemiluminescent reaction of an organic peroxide. Its study led to the postulation of chemically initiated electron-exchange luminescence as a generalized mechanism for efficient chemical light formation (Schuster, 1979 Schuster et al., 1979). 

CIEEL (Chemically initiated Electron Exchange Luminescence) A type of luminescence resulting from a thermal electron-transfer reaction. Also called catalyzed chemiluminescence. 

The 1,2-dioxetanes are another important group of chemiluminescent compounds. These compounds are oxidatively cleaved thermally in a concerted fashion to yield two carbonyl moieties, one of which is excited. The mechanism of this cleavage has been described as a chemically initiated electron-exchange... 

While they are all undoubtedly chemiluminescent, the compounds and reactions illustrated above, with few exceptions, generate extremely weak light emission. Even when the emission is amplified by the presence of fluorescent molecules, as in reactions involving chemically initiated electron-exchange luminescence... 

This was further elaborated upon by Schuster and co-workers (K21, S23, S24) and by Schaap s group at Wayne State University (S6, S8, SIO, Sll, Z2, Z3). Thus, the observation that some hydroxy-substituted aromatic dioxetanes show high chemiluminescent efficiencies at alkaline pH (phenolic anionic form) led to the formulation of a third mechanism for chemiluminescent decomposition of dioxetanes. This mechanism, known initially as intramolecular electron transfer (Ml9, Z2) and subsequently as chemically initiated electron exchange luminescence, or CIEEL (FI, K20), can be best illustrated by reference to the dioxetane shown in Fig. 37, where the chemiluminescence is triggered by the addition of fluoride ions. 

A related mechanism for chemiluminescence is called chemically initiated electron exchange luminescence (CIEEL). Here, electron transfer from a donor to an acceptor initiates a thermal reaction that releases enough energy to place one of the reactants in an excited state. One case studied by Schuster is shown in Eq. 16.76, where electron transfer initiates CO2I0SS. This is followed by back electron transfer, giving the excited state of 9,10-diphenyl-anthracene, which then emits. 

S. P. Schmidt and G. B. Schuster (1978), Dioxetanone chemiluminescence by the chemically initiated electron exchange pathway. Efficient generation of excited singlet states. J. Amer. Chem. Soc. 100, 1966-1968. 

Since the minimum description of an excited state involves the separation of electrons into bonding and anti-bonding orbitals, radical processes have often featured in the explanations. However it is important to distinguish the various mechanisms since some are very much better understood than others. The most satisfactory mechanism to date is that named CIEEL by Schuster and anticipated by McCapra in his explanation for the chemiluminescence of the active oxalates. The essence of the chemically initiated electron exchange luminescence process is that there are two electron transfer steps. These steps take place within the first encounter complex formed from the two reacting molecules. The first transfer is strongly dependent on the difference in redox potential between the oxidant and reductant. This dependency is seen in the need for fluorescent molecules of low... 

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