Radical ion annihilation

Much of the study of ECL reactions has centered on two areas electron transfer reactions between certain transition metal complexes, and radical ion-annihilation reactions between polyaromatic hydrocarbons. ECL also encompasses the electrochemical generation of conventional chemiluminescence (CL) reactions, such as the electrochemical oxidation of luminol. Cathodic luminescence from oxide-covered valve metal electrodes is also termed ECL in the literature, and has found applications in analytical chemistry. Hence this type of ECL will also be covered here. 

This mechanism has been formulated in analogy to the known electrochemiluminescence, in which radical-ion annihilation generated at opposite electrodes leads to the formation of the electronically excited state (Scheme 2) . The difference between the CIEEL mechanism and electrochemiluminescence is that, in the former, the radical ions—whose annihilation is responsible for the formation of the excited state—are formed chemically by electron transfer to high-energy peroxides and subsequent bond cleavage or rearrangements. 

Fig. 2 Electron-transfer chemiluminescence. Radical ion annihilation is depicted...

ECL experiments focused on radical ion annihilation are carried out in fairly conventional electrochemical apparatus, but procedures must be modified to allow the electrogeneration of two reactants, rather than one, as is more commonly true. In addition, one must pay scrupulous attention to the purity of the solvent/supporting electrolyte system. Water and oxygen are particularly harmful to these experiments. Thus, apparatus is constructed to allow transfer of solvent and degassing on a high-vacuum line or in an inert-atmosphere box. Other constraints may be imposed by optical equipment used to monitor the light. 

Here a radical anion (M ) is formed which reacts with the cation (M ) in the solution ( radical ion annihilation ). This process leads also to the production of excited molecules which are detected by luminescence measurements. The process can also be observed with metal electrodes. ... 

The strongly exothermic transfer of electrons between fluorescent organic molecules represents one of the most general mechanisms in chemiluminescence [50, 51]. It can be found in electroluminescence, radical ion annihilation and peroxide decomposition. The basic concept was introduced by Hercules [39] following the general theory of Marcus [52]. The reaction co-ordinate can be roughly indicated by the potential energy curves shown. 

Originally invoked for electroluminescent reactions, this idea has now been developed for the reaction of peroxides with fluorescent compounds of low ionisation potential [51]. Many of these reactions are discussed in Chap. XI, but Fig. 2 can be most succintly exemplified by the radical ion annihilation shown, where Ar is a fluorescent aromatic hydrocarbon such as diphenylanthracene, LUMO is the lowest, normally unoccupied molecular orbital and HOMO is the highest occupied molecular orbital. 

Singlet O2 can be produced from superoxide radical ion by radical ion annihilation (p. 140) using an appropriate oxidant, such as ferrocene [66]. Alternatively, it can be formed in the reaction of KO2 with diacyl peroxides such as dibenzoyl peroxide or dilauroyl peroxide in benzene in the presence of 18-crown-6 [67]. 

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