Ion annihilation ECL

Classical ECL involves the formation of an excited state as a result of an energetic electron transfer between electrochemically generated species, often radical ions, at the surface of 

Comparison of photoluminescence (PL), chemiluminescence (CL) and electrochemiluminescence (ECL), in terms of an analytical technique 

No light source needed 2. No scattered light 3. No interference from lumineseent impurities 1. ECL reactions are localized spatially and temporally and controllable 2. Application is possible through turnover of reactions 

Equation (13.5) is often expressed using CV peak potentials (E in volts), 

If —AH° is nearly marginal to E,., the T-route can contribute to the formation of A in addition to the S-route, and the reactions are said to follow the ST-route. A typical system with such a route is the rubrene anion-cation annihilation (115-117). 

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For a given chemical species, three criteria are generally required for efficient ion annihilation ECL to occur (10) (i) stable radical ions of the precursor molecules in the electrolyte of interest (as seen in the CV response, e.g., DPA in Figure 13.3), (ii) good PL efficiency of a product of the electron-transfer reaction, which often can be evaluated from the fluorescent experiment, and (iii) sufficient energy in the electron-transfer reaction to produce the excited state (see below for details). 

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. 

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... 

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. 

In the annihilation pathway, the reduced and oxidized species are both generated in the vicinity of the electrode surface by alternate pulsing of the electrode potential. The corresponding process is outlined in the Eqn (1-4). The annihilation ECL can also be achieved in the mixed systems . The ECL is achieved via cross-reactions between the radical ions of the different species. 

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. ... 

The enthalpy is lower than the energy required to produce the lowest excited state, still exceeding the triplet state energy, 3R, which in turn produces IR by subsequent annihilation of 3R (triplet-triplet annihilation, TTA). ECL of ruthenium tris-bipyridyl-type derivatives falls in this category. Additionally, ion annihilation can also follow E-route, ensuing the formation of excimers (excited dimers) and exciplexes (excited complexes). 

Several types of cells with three conventional electrodes have been used for ECL studies. The most commonly used ECL cell for ion annihilation study in Bard s group at the University of Texas at Austin is shown in Figure 13.7a. This type of cell can be easily fabricated from an ACE (www.aceglass.com) internal threaded glass connector... 

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