Azulenes electrochemical oxidation

Determination of electrochemical oxidation potentials and electrochemical reduction of 13 p-phosphorylated acyclic nitrones shows that phosphorylated compounds have a clear anodic shift of potentials of both, oxidation (Ep 1.40 to 2.00 V versus SCE in CH3CN) and reduction (Ep—0.94 to —2.06 V). This is caused by a strong electron-acceptor influence of the diethoxyphosphoryl group (430). In contrast, a reversible one-electron oxidation of azulene nitrones (233) (Scheme 2.80) occurs 0.6 V below the Ep potential of PBN, that is at the value one observes the oxidation of AH -imidazole-1,3-dioxides (219) (428, 429). In other words, the corresponding RC (234) is 14 kcal more stable than the RC of PBN. Although the EPR spectrum of RC (234) was not recorded, RC (236) from dinitrone (235) turned out to be rather stable and gave an EPR spectrum (170). 

The electrochemical oxidation of a wide range of conjugated monomers results in the deposition of an electrically conductive film on the working electrode (Waltman and Bargon, 1986). This method has mostly been used for the polymerisation of heteroatom-containing monomers, e.g. pyrrole, thiophenef aniline, indole, etc., and polycyclic monomers, e.g. azulene, fluorene, pyrene , etc. The example of pyrrole is illustrative of the inherent advantages and problems of electrochemical polymerisation. 

F. X. Redl, O. Kothe, K. Roekl, W. Bauer, and J. Daub, Azulene-appended cellulose Synthesis, optieal and ehiroptical properties, film formation by electrochemical oxidation, Macromol. Chem. Phys., 201 (2000) 2091-2100. 

Chemical or electrochemical oxidation of numerous resonance-stabihsed aromatic molecules, such as pyrrole (9), thiophene (10), aniline (11), furan (12), carbazole (13), azulene (14) and indole (15), produces electronically conducting polymers (2,17-21,53-55) (see Electrically Active Polymers). 

The electrochemical reduction of azulene with carbon, platinum, lead or zinc cathode does not give any product, whereas that with magnesium electrode yields a dimeric compound as the only reduction product, though the dimeric compound is easily transformed to the corresponding monomeric compound by a mild oxidation as shown in equation 2825. 

We have established the conversion between the two colored species by electrochemical reaction utilizing the concept of a Wurster type violene-cyanine hybrid. Dications 222+ and 232+ showed significant changes in their absorption spectra in different oxidation states. Therefore, dications 222+ and 232+ could function as new violene-cyanine hybrids, in which the four end groups (X and Y) in the general structure are azulenes (Figure 4). 

The improved electrochemical synthesis (7) of poly pyrrole has led to its use as coating for the protection of n-type semiconductors against photocorrosion in photoelectrochemical cells. (8,9) Recently, it was announced that pyrrole was not the only five-membered heterocyclic aromatic ring compound to undergo simultaneous oxidation and polymerization. Thiophene, furan, indole, and azulene all undergo electrochemical polymerization and oxidation to yield oxidized polymers of varying conductivities (5 x 10 3 to 102 cm- ). (10-13) The purpose... 

By the electrochemical method, the oxidation potential of the polymerization reaction can be controlled and the quality of the polymer optimized moreover, the polymers are obtained in the doped state as films suitable for further electrochemical modifications. Therefore, in this field, electrochemistry plays both a synthetic and an analytical role. Soon the electrochemical polymerization was extended to other aromatic compounds such as furan, indole [12], carbazole, azulene, pyrene [13] benzene [14], and fluorene [15], and electrochemical synthesis is now the most widely used technique for the preparation of conducting PHCs. 

The simultaneous polymerization and oxidation of azulene with bromine or iodine in acetonitrile have recently been reported. The resultant slightly soluble poly(azulene)-bromine and insoluble poly(azulene)-iodine complexes have lower electrical conductivities than the electrochemically produced polymer, 5x10 and 10 S/cm, respectively. Removal of soluble oligomers from the former leads to a slight improvement of the electrical conductivity. [119,127]. 

During a typical electropolymerization, it is nearly always noted that the net charge transfer is a little in excess of that indicated stoichiometrically, due to the additional oxidation (and doping) that occurs during electrochemical preparation of CPs. For example, while P(Py) and poly(azulene) both stoichiometrically require 2 electrons per monomer for electropolymerization, experimentally a charge of the order of 2.3 electrons per monomer is foimd to be consumed. The excess 0.3 is used to effect a 30% doping of the polymer. 

Radical anions have also been obtained from such condensed systems as azulene [122, 123], 4,6,8-trimethylazulene [122], acenaphthylene [124], and sym-dibenzocycloctatetraene [125]. Radical cations of phenanthrene [117] and of 9,10-diphenylanthracene [69] have been obtained by oxidative electrochemical generation at platinum electrodes. 

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