Chemiluminescence electron exchange

Chemical off—on switching of the chemiluminescence of a 1,2-dioxetane (9-benzyhdene-10-methylacridan-l,2-dioxetane [66762-83-2] (9)) was first described in 1980 (33). No chemiluminescence was observed when excess acetic acid was added to (9) but chemiluminescence was recovered when triethylamine was added. The off—on switching was attributed to reversible protonation of the nitrogen lone pair and modulation of chemically induced electron-exchange luminescence (CIEEL). Base-induced decomposition of a 1,2-dioxetane of 2-phen5l-3-(4 -hydroxyphenyl)-l,4-dioxetane (10) by deprotonation of the phenoHc hydroxy group has also been described (34). 

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. 

The assumptions, equations and several applications of a recently formulated theory of electron transfer reactions of solvated electrons are outlined. The relationship of the reorganization terms to those of ordinary electron exchange and electrochemical reactions is described, together with the role played by an effective standard free energy of reaction. Applications include prediction of conditions under which chemiluminescence might be found and description of conditions under which reactions might not be diffusion-controlled. 

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... 

Despite the clear implication of the involvement of intramolecular electron transfer in the chemiluminescence of certain dioxetanes, there have been no clear examples of intermolecular electron exchange luminescence processes with dioxetanes. In a recent note, however, Wilson (1979) reports the observation of catalysis of the chemiluminescence of tetramethoxy-1,2-dioxetane by rubrene and, most surprisingly, by 9,10-dicyanoanthracene. While catalysis by the added fluorescers was not kinetically discernible, a lowering of the activation energy for chemiluminescence was observed. These results were interpreted not in terms of an actual electron transfer with the formation of radical ions, but rather in terms of charge transfer interactions between fluorescer and dioxetane in the collision complex. In any event, these results certainly emphasize the need for caution in considering the fluorescer as a passive energy acceptor in dioxetane chemiluminescence. 

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. 

Recently,the electron-transfer theory was extended in order to incorporate the slow and reversible chemically induced electron-exchange reactions, as observed for the fluorescer-catalyzed chemiluminescent decomposition of a-peroxylactones. It was argued that electron transfer is complete in the transition state for such a slow and irreversible endergonic electron-transfer reaction, but that the typically small slopes (— a/RT where a is about 0.3) of the In (intensity) vs. oxidation potential plot was due to the fact that only a fraction (a) of the total free-energy change manifests itself in the activation energy. 

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. 

Adam, W., Cadilla, C., Cueto, O., and Rodriguez, L. O., Fluorescer-enhanced chemiluminescence of a cyclic peroxalate via electron exchange. J. Am. Chem. Soc. 102, 4802-4805... 

The electrogenerated chemiluminescence (eel) is therefore specific of the triplet-triplet annihilation for A-type compounds. In such processes, the triplet formation, on which the eel phenomenon is based is in competition with the expected reactions already established for redox catalysis, namely homogeneous electron exchange reactions (SET and then disproportionation reactions) ... 

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