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. 

An increase in the ion annihilation exergonicity AG to values comparable to the excited triplet-state energies (AG I LT < 0) opens an additional electron transfer channel (T-route). In the simplest case, only one excited triplet 3 A or 3 D becomes accessible. Triplet emission can be directly observed from the ECL systems involving rare earth and transition metal complexes with allowed (due to extensive spin-orbit coupling) triplet-singlet electronic transition. 

Extremely high ECL efficiencies seem to be a common feature of the homoleptic-IrL3 as well as the heteroleptic-L2Ir(X) iridium(III) cyclometallated complexes. Extremely high ECL efficiencies (up to 0.55) were observed via ion annihilation between the electrochemically generated L2Ir(acac)+ or L2Ir(pico) + cations (where... 

One should not gain the impression from the foregoing that redox excitation is restricted to reactions between aromatic radical ions in aprotic solvents. Studies of such reactions have indeed dominated research because the optical and electrochemical properties of many aromatics are well known, but there are numerous cases of redox excitation outside this chemical domain. For example, singlet oxygen seems to arise from oxidations of superoxide ion in acetonitrile [94]. Similarly, luminescent tris(2,2 -bipyridyl)ruthenium(II) can arise in at least three ways (1) from a kind of ion annihilation in CH3CN [95] or DMF,... 

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. 

The excited state R can represent the lowest singlet state species ( R ) or triplet state species ( R ) according to the energy available during the annihilation reaction. If the enthalpy of the ion annihilation (AH) exceeds the energy required to produce the lowest excited states from the ground... 

The first co-reactant discovered was oxalate in 1977. The introduction of the co-reactant in ECL exhibits distinct advantage in comparison with the annihilation reaction (1) it can overcome the limited potential window of solvent and the poor stability of radical anions or cations (2) the coreactant ECL can be beneficial for some fluorescent compounds that have only a electrochemical reduction or oxidation (3) the use of co-reactant can produce more intense ECL emission when the annihilation reaction between oxidized and reduced species is not efficient (4) it can eliminate the oxygen quenching effect frequently encountered in ion annihilation reaction and facilitate the ECL in the air. All commercially available ECL analytical instruments are based on this pathway. According to the generated intermediates and the polarity of the applied potential, the corresponding coreactant ECL can be classified as oxidative-reduction ECL and reductive-oxidation ECL, respectively. 

During this ECL process, a powerful reductant (C02 ) was in situ generated due to the decomposition of intermediate (C204 ). The ECL can be achieved by two pathways (1) reaction between the C02 and Ru(bpy) (Eqn (10)), (2) ion annihilation reaction between Ru(bpy)3 and Ru(bpy)3 (Eqn (12)). The intermediate C204 can be formed upon oxidation by Ru(bpy)3 or be directly oxidized at the electrode surface (Eqn (8)). For example, in acetonitrile (MeCN) media, oxalate is easier to be oxidized than Ru(bpy)3 " complex and both the reactants are oxidized during the light emission. In addition, the applied potential, the concentration of C204 and the electrode surface properties influence the direct oxidation of oxalate to the overall ECL behaviour. ... 

Ru(bpy)3 (scheme 4). And then the excited state Ru(bpy)3 forms through three different reactions (1) ion annihilation reaction between Ru(bpy)3 " and Ru(bpy)3+ [generated from the reduction of Ru(bpy)3 by... 

The excited singlet state RJ is formed in an ion annihilation reaction involving the ion radicals R] and R2, where Ri and R2 can be either the same or two different precursors. In energy-sufficient systems, the formation of the excited state is energetically accessible to the redox process, and the ion annihilation occurs via the S-route ... 

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