Generation of Radical Anions

The electron from the antibonding orbital of the radical anion is transferred to the antibonding orbital of the radical cation. The transition of this electron to thejrround state produces the fluorescence spectrum. 

Bard etaL 5S6 5571 and Visco etaL 558) have quantitatively analyzed the intensity of pulsed ECL of 9,10-diphenylanthracene, tetraphenylpyrene and rubrene. By computer simulation of the electrode process and the subsequent chemical reactions the rates for chemical decay of the radical ions could be determined. Weaker ECL with fluorescence emission 559 or electrophosphorescence S60) occurs if the radical anion R - reacts with a dissimilar radical cation R,+ of insufficient high oxidation potential to gain enough energy for fluorescence emission, that is, if ht fluorescence) 23.06 (Ej +. -Ej -.), e.g., in the annihilation of the anthracene radical anion with Wurster s blue. For these process the following schemes are assumed (Eq. (242) )  

The triplet emission can be totally suppressed by adding an efficient quencher, e.g., 1,3,5-hexatriene 560 . 

6-Tris (f-butyl) phenoxy-, 4,4 -methylenebis (2,4- di-f-butyl) phenoxy- or 4,4 -thiobis (2-t-b utyl-6-methyl) phenoxy-radicals have been prepared for ESR observation by oxidation of the corresponding phenolates at a graphite electrode in acetonitrile 561). Dimroth etaL S62 determined reversible le -oxidation potentials for a series of substituted phenols in basic medium. (CF3)2NOH has been oxidized in alkaline medium on platinum or magnetite in quantitative yield to the pink violet hexafluorodimethylnitroxide (CF3)2NO 563  

Radical anions 189 (Eq. (243) ) are generated by electron transfer from the cathode or a reducing agent to the lowest empty orbital (LUMO) of a neutral substrate. The electrochemical generation of the radical anion (or cation) is superior to chemical reduction (or oxidation). Advantages are the use of a wider range of solvents, the applicability of an electrode potential that can be regulated at will 

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Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Mechanistic studies on the reduction of the isomeric bipyridines (1) have been done.31,32 The generation of radical anions and dianions in liquid ammonia was studied and a mechanism was proposed for the electrolysis of 2,2 -bipyridine (1) in basic solution. In addition, the macroscale reduction of the symmetrical isomers of 1 was studied, and the ease of reduction followed the order 2,2 - =4,4 - > 3,3 -(l),33 Various electrode materials were examined. The reduction products of 2,2 - and 3,3 -bipyridine (1) at various cathodes were found to consist of roughly equal amounts of both the threo and erythro stereoisomers. Reduction of anabasine (4) gave the unsymmetrical 2,3 -bipiperidyl (2).34... 

The photodechlorination of 2,2/,3,3/,6,6/-hexachlorobiphenyl and of three commercial mixtures of polychlorinated biphenyls solubilized in an aqueous solution of poly(sodium styrenesulphonate-co-2-vinylnaphthalene) was studied with the use of solar-simulated radiation426,427. The reaction was found to be photosensitized by the naphthalene antenna units present in the copolymer. Exciplex formation and generation of radical anions lead to dechlorination. 

The first and the second waves on the polarogram correspond to irreversible and reversible electron transfer, respectively. The addition of the first electron results in the generation of radical anions, which break up on the N-H-bond with elimination of atomic hydrogen and the formation of anions. At the second half-wave potential the nitroazole anions are reduced to the corresponding radical dianions registered by the ESR method [852-854],... 

Nonphotochemical Generation of Radical Anions of Aromatic Halides. The second method involved using an arene radical anion as a convenient electron donor. In order to avoid side reactions, discovered when lithium naphthalenide was employed, presumably arising from coupling reactions between the donor and radical derived from acceptor radical anion, lithium p,p -d -tert-butylbiphenylide (LiDBB) [46] was used as donor. The presence of the cert-butyl groups is known to prevent the side reactions encountered with naphthalene [46]. Treatment of 1 with LiDBB in THF gave the three isomers of tetrachloro-benzene as products as shown in Eq. 17. 

Moore E, ChigneU CF, Sik RH, Motten AG. Generation of radical anions from metronidazole, misonidazole and azathioprine by photoreduction in the presence of EDT A. Int J Radiat Biol 1986 50 885-891. 

An alternative procednre for the prodnction of negative ions is electron-captnre negative ionization (ECNl). It is as a highly selective ionization method, as only a limited number of analytes are prone to efficient electron capture, e.g., fluorinated compounds or derivatives. It takes place by captnre by the analyte molecules of thermal electrons, and resnlts in the generation of radical anions. The process must be performed in a medinm-pressnre ion sonrce in order to slow down the electrons and to remove excess energy from the radical anion formed upon electron attachment. The formation of negative ions by electron capture can occur by two mechanisms ... 

FIGURE 7.3 Generation of radical anions and cations atthe electrodes of an OLED. The ions migrate to the center of the material where they meet on the same chain segment to form emissive singlet (shown here) or triplet excited states. 

Fig. 2. Diagram of cell for internal electrochemical generation [32], W is a wave guide R is the resonant cavity M is the magnet C is the cell H is the cell holder E i, E2, E3 are the electrodes for electrochemical generation of radical anions. E3 is connected to the mercury drop, the surface of which forms the cathode.

It is more logical to compare electrochemical generation with methods such as the photochemical or chemical methods. The photochemical method for generation of radical anions in combination with EPR spectrometry has not yet received greater attention owing to difficulty in arranging the direct action of light on the specimen in the resonance cavity of the spectrometer [96-105]. 

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