Chemiluminescence intramolecular electron transfer

Intramolecular electron transfer initiated peroxide decomposition. . 1236 HIGH-EFFICIENCY ORGANIC CHEMILUMINESCENT REACTIONS INVOLVING PEROXIDE INTERMEDIATES. 1238... 

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 involvement of the CIEEL process in the thermolysis of [21] immediately offers new insight into many previously perplexing proposals of dioxetane or dioxetanone intermediacy in various chemi- and bio-luminescent reactions. For example, the discovery of activated chemiluminescence for [21], and the finding that intramolecular electron transfer can generate a very high yield of electronically excited singlet (Horn et al., 1978-79), prompts speculation that an intramolecular version (34) of the CIEEL mechanism is... 

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. 

H02 " as nucleophile [73]. The intramolecular electron transfer is in fact a combination of the hypothesis for the firefly dioxetanone decomposition (p. 152) and the (intramolecular) chemiluminescent activation of secondary peresters (p. 35). However it is only one hypothesis among many which lack confirmatory evidence in this exceedingly complicated reaction. 

The decomposition of the endoperoxide via the o-xylene derivative (48) in an intramolecular electron transfer mechanism would also give the substituents a decisive role in the excitation step itself - not only affecting the fluorescence efficiency of the phthalate dianion. A high fluorescence efficiency is of course a necessary, but not sufficient, requirement in luminol type chemiluminescence [9, 16],... 

Nonetheless, a dioxetan decomposition mechanism for lucigenin chemiluminescence, based on the exergonic processes described in Chap. V, seems well established [3]. A direct demonstration of the intermediacy of this dioxetane was first made [4] in 1969 by treating 10,10 -dimethyl-9,9 -biacrylidene (4) with singlet oxygen from several sources. Emission from N-methyl acridone was unequivocally shown. The lifetime of the intermediate was characteristic of the supposed dioxetane. Intramolecular electron transfer has been suggested as the excitation mechanism in the decomposition of this and other electron-rich dioxetans. 

Although there are many components in a mechanistic description of a chemiluminescent reaction, the heart of the matter is the actual excitation step. Several such steps have been identified. Some are molecular in character e. g. the decomposition of dioxetans and some are intermolecular electron transfer steps. There is an intermediate class in which the step can be formulated as an /n ramolecular electron transfer. Many luminescent reactions have been ascribed to this category with varying degrees of confidence. Cyclic hydrazides such as luminol belong rather uncertainly here. Electron rich dioxetans and dioxetanones and the luciferins with such intermediates on the pathway are a little more reasonably assigned to an intramolecular electron transfer mechanism. Even here however caution is required in that direct evidence for discrete electron transfer will by its very nature be almost impossible to obtain and will probably remain circumstantial. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

The imbedded nature of the potential curves in Figure 6 for electron transfer in the inverted region is a feature shared with the nonradiative decay of molecular excited states. In fact, in the inverted region another channel for the transition between states is by emission, D,A -> D+,A + hv, which can be observed, for example, from organic exciplexes,74 chemiluminescent reactions,75 or from intramolecular charge transfer excited states, e.g. (bipy)2Rum(bipyT)2+ - (bipy)2Run(bipy)2+ + hv. 

The title paper was enormously important by itself, but in addition it was the first step (and the cornerstone) in a long series of papers on electron-transfer reactions which were published by Marcus from 1956 to 1965. During those years he extended [3, 4] the theory to include, for instance, intramolecular vibrational effects, numerically calculated rates of self-exchange and cross reactions, electrochemical electron-transfer reactions (i.e. including electrodes), chemiluminescent electron transfers, the relation between nonequilibrium and... 

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