Stable radical ions

Bipolar Molecular Glasses. Recently, bipolar molecular glasses have been described that allow both injection of holes and electrons (Fig. 3.30). 2- 4-[bis(4-methylphenyl)amino]phenyl -5-(dimesitylboryl)thiophene (PhAMB-lT, 68) and 2- 4-[bis(9,9-dimethylfluorenyl)amino]phenyl -5-(dimesitylboryl)thiophene (F1AMB-1T, 69) show oxidation potentials of 0.62 and 0.58 V, and reduction potentials of —2.13 and —2.01 V vs. Ag/0.01 Ag+, respectively [145]. Oxidation as well as reduction leads to stable radical ions. With the conversion rules given above, the HOMO and LUMO levels can be estimated to be approximately at —5.3 and —2.8 eV. In comparison, for the bipolar compound 70, consisting of triarylamine and oxadiazole moieties, the values are —5.5 and — 2.7eV [129]. However, in this case no data on the stability of the radical ions are available. 

The advent of ESR opened several classes of moderately stable radical ions to scrutiny, namely, ketyls, semidiones, semiquinones, and those of aromatic sys-... 

Electron transfer reactions (with formation of stable radical ions or ions). 

As mentioned in the preceding section, few organic electron-transfer processes occur alone. It is possible to prepare certain relatively stable radical-ion salts by one-electron oxidation (Bard et al., 1976 Holy, 1974), but by far the greatest... 

Early studies of radiative charge recombination processes made use of chemical oxidation and reduction to create the reactive radical ion species. Despite the limitation of the use of chemically stable radical ions, a significant body of literature developed that discussed radiative charge recombination for cross-electron-transfer reactions. In particular, many studies involved reactions of organic radical anions with amine cation radicals [5, 15, 18, 19]. 

At least in some cases, a low Eox facilitates the generation and/or injection of a positive charge carrier from the CGL (see above). A low qx also makes it unlikely that an impurity might have a much lower value and act as a trap for holes [44h. For obvious reasons, the cation radicals of these compounds must be stable in the CTL environment, and some of the better donors indeed form remarkably stable radical-ion salts, even in fluid solution [44a]. The CTM must be highly soluble in the polymeric host and the solvent used to fabricate the CTM. It must provide a high concentration of electrically active moieties without plasticizing the host polymer excessively. Compounds that meet these requirements are typically rather large and flexible, and they may contain more than one electrically independent moiety per molecule, e.g., 1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) in Scheme 2. 

For the efficient generation of annihilation ECL, three conditions should be met as follows (1) sufficient energy in the electron transfer reaction to produce the excited state for ECL emission, (2) stable radical ions of the precursor molecules in the electrolyte, and (3) good PL efficiency of a product of the electron transfer reaction. 

A large number of radical ions have been studied by ESR since its applications to free radical chemistry started in the middle of the 1950s. They were generally prepared in the liquid phase by redox (reduction-oxidation) reactions, electrolysis or photolysis, and rather stable radical ions possessing conjugation system such as aromatics have been studied. The results have been summarized, for example, in a book Radical Ions by Kaiser and Kevan [30]. Since the middle of the 1960s a... 

Depending on their molecular structure, the (electro)chemical oxidation of thiophenes and oligothiophenes (see below) might lead to stable radical-ions as intermediate reacting species for following oligo- and... 

Typical Applications Experiments performed with stable radical ions in DMF, such as those produced from the reduction of nitrobenzene, azobenzene, and 9,10-anthraquinone, indicated that the observed signal was stable for 20-30 s after generation by a current step, and after which natural convection set in, suggesting that kinetic measurements are possible from the magnitude of this constant steady state EPR signal, within the 20 s time limit [43]. 

With these measures an EPR spectrum with hyperfine structure can be easily recorded if the half-life of the free radical is not less than 10 min. For less stable radical ions the cell can be used in a vacuum [55]. 

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