Chemically induced 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). 

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. 

Figure 2. Mechanism of chemically induced electron-exchange bioluminescence (CIEEL)...

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... 

Dixon and Schuster (1979, 1981) have investigated both the thermal and electron-donor induced reactions of 1-phenylethyl peroxyacetate [28] and a series of substituted 1-phenylethyl peroxybenzoates [29a-29e]. They report the direct generation of electronically excited states from unimolecular thermo-lyses, as well as generation of light by the chemically initiated electron-exchange luminescence mechanism. 

We have seen that electron-transfer reactions can occur at one charged plate. What happens if one takes into account the second plate There, the electron transfer is from the solution to the plate or electronic conductor. Thus, if we consider the two electronic conductor-ionic conductor interfaces (namely, the whole cell), there is no net electron transfer. The electron outflow from one electronic conductor equals the inflow to the other that is, a purely chemical reaction (one not involving net electron ttansfer) can be carried out in an electrochemical cell. Such net reactions in an electrochemical cell turn out to be formally identical to the familiar thermally induced reactions of ordinary chemistry in which molecules collide with each other and form new species with new bonds. There are, however, fundamental differences between the ordinary chemical way of effecting a reaction and the less familiar electrical or electrochemical way, in which the reactants collide not with each other but with separated charge-transfer catalysts, as the two plates which serve as electron-exchange areas might well be called. One of the differences, of course, pertains to the facility with which the rate of a reaction in an electrochemical cell can be controlled all one has to do is electroiucally to control the power source. This ease of control arises because the electrochemical reaction rate is the rate at which the power source pushes out and receives back electrons after their journey around the circuit that includes (Figs. 1.4 and 1.5) the electrochemical cell. 

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