Radicals electrochemiluminescence

The mechanism of chemiluminescence is still being studied and most mechanistic interpretations should be regarded as tentative. Nevertheless, most chemiluminescent reactions can be classified into (/) peroxide decomposition, including biolurninescence and peroxyoxalate chemiluminescence (2) singlet oxygen chemiluminescence and (J) ion radical or electron-transfer chemiluminescence, which includes electrochemiluminescence. 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

In this paper the electtode anodic reactions of a number of dihydropyridine (DHP) derivatives, quantum-chemical calculations of reactions between DHP cation-radicals and electrochemiluminescers anion-radicals (aromatic compounds) and DHP indirect ECL assay were investigated. The actuality of this work and its analytical value follow from the fact that objects of investigation - DHP derivatives - have pronounced importance due to its phaiTnacology properties as high effective hypertensive medical product. 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Emission spectra have been recorded for four aryl-substituted isoindoles rmder conditions of electrochemical stimulation. Electrochemiluminescence, which was easily visible in daylight, was measured at a concentration of 2-10 mM of emitter in V jV-dimethylformamide with platinum electrodes. Emission spectra due to electrochemi-luminescence and to fluorescence were found to be identical, and quantum yields for fluorescence were obtained by irradiation with a calibrated Hght source. Values are given in Table X. As with peak potentials determined by cyclic voltammetry, the results of luminescence studies are interpreted in terms of radical ion intermediates. ... 

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. 

The most versatile technique for producing emission by the generation of ion radicals followed by their oxidation or reduction in situ is the electrochemical method. The emission produced by this technique has been termed electrochemiluminescence (ECL). Chemical oxidants and reductants have also been widely used, although their employment, especially in quantitative work, is more cumbersome. This review attempts to describe the current knowledge, both experimental and... 

Several detailed studies of electrochemiluminescence emission under controlled potentials have been conducted.11 13,63"67 These have been double potential step experiments where one of the ion radicals was first generated at a potential slightly more negative (or positive, for cation generation) than its half-wave potential for a short time period, and then the potential at the electrode was switched to some... 

By media variables we mean the solvent, electrolyte, and electrodes employed in electrochemical generation of excited states. The roles which these play in the emissive process have not been sufficiently investigated. The combination of A vV-dimethylformamide, or acetonitrile, tetra-n-butylammonium perchlorate and platinum have been most commonly reported because they have been found empirically to function well. Despite various inadequacies of these systems, however, relatively little has been done to find and develop improved conditions under which emission could be seen and studied. Electrochemiluminescence emission has also been observed in dimethyl sulfite, propylene carbonate, 1,2-dimethoxyethane, trimethylacetonitrile, and benzonitrile.17 Recently the last of these has proven very useful for stabilizing the rubrene cation radical.65,66 Other electrolytes that have been tried are tetraethylam-monium bromide and perchlorate1 and tetra-n-butylammonium bromide and iodide.5 Emission has also been observed with gold,4 mercury,5 and transparent tin oxide electrodes,9 but few studies have yet been made1 as to the effects of electrode construction and orientation on the emission character. 

Feldberg68,69 has made a valuable analysis of the relationship of the light produced in a double potential step electrochemiluminescence experiment to the current, time, and kinetic parameters involved. The analysis presumes that the reaction which produces excited states is cation-anion radical annihilation which occurs when the radical ions, separately produced, diffuse together in the solution near the electrode. The processes that Feldberg initially considered were eqs. (7)—(13). The assumptions involved are that decay of the excited state... 

As described above, spectroelectrochemical methods are useful in studying the reactivity of radical anions and cations in non-aqueous solutions. Related to this, electrochemiluminescence (ECL), which is often caused by the reaction between radical... 

Peak Potentials, Estimated Lifetimes of Radical-Ions, and Electrochemiluminescence (ecl) Emission Data for Some Benzo[c]furans (in jV,JV -Dimethylformamide) ... 

Academic and commercial interest in electrochemiluminescence = ECL gave a great impetus to research in elechtrochemically generated radical ions. The simplified scheme for ECL is shown by Eq. (240) ... 

Certain aromatic hydrocarbons, such as 9,10-diphenylanthracene, give relatively stable radicals and cation radicals upon electrochemical reduction and oxidation, respectively. If one arranges to have the radical ions from both processes mixed, either by normal DC electrolysis in a suitably designed cell or by using an alternating current for the electrolysis, the phenomenon of electrochemiluminescence appears (Hercules, 1971 McCapra, 1973). 

A slightly different application is where species produced electrochem-ically lead to photon emission in the visible spectrum, via the formation of organic radicals by homogeneous reaction from electrochemically generated precursors. The electrode controls the quantity of precursor, enabling quantitative parameters of the homogeneous reaction to be elucidated. This is known as electrogenerated chemiluminescence or electrochemiluminescence (ECL). 

Elect regenerated chemiluminescence (ECL) — (-> electrochemiluminescence or electrochemically generated chemiluminescence) The generation of light in an electrochemical cell by an energetic electron transfer reaction, often between radical ions in an aprotic solvent. In a typical experiment in a solution of rubrene (R) and N,N,N, N -tetramethyl-p-phenylenediamine (TMPD) in dimethylformamide initially radical anions of rubrene are formed by electroreduction... 

Proof of highly unstable radical intermediates by electrochemiluminescence — A special mechanism of Electrochemiluminescence is observed when both reacting species of the electron transfer are generated simultaneously at the same potential at the same electrode (so-called DC-ECL or ECL with a coreactant). This is possible only when a chemical step is involved in the electrode process of the coreactant (CH). A typical example is the cleavage of its primarily formed ion radical into a stable ion and a strongly reducing or oxidizing... 

The formation of electron donor-acceptor complexes from excited singlet states can lead to triplet formation. In highly polar solvents where radical-ion formation readily occurs, triplets may be produced by recombination (25) of solvent-separated radical ions and of geminate radical ions (Schulten et al., 1976) A- + Dt-> A + D. Such an electron-transfer reaction occurs in many of the electrochemiluminescent reactions discussed earlier. There is also evidence that, in some solvents of medium polarity, triplet production occurs via an exciplex (Orbach and Ottolenghi, 1975). The extent to which each of these processes contributes is obviously highly dependent upon the solvent. 

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