Carbazoles anodic oxidation

Pyrrole derivatives substituted in positions 1-, 3-, or 4- have also been electrochemically polymerized (positions 2- and 5- must be free for polymerization). Besides homopolymers, copolymers can also be prepared in this way. Other nitrogen heterocycles that have been polymerized by anodic oxidation include carbazole, pyridazine, indole, and their various substitution derivatives. 

The formation of bicarbazoles can also be achieved electrolytically in acetonitrile solution once again, at the anode, the carbazole cation radical is produced and dimerizes. The further oxidation of 3,3 -bicar-bazole under these conditions with loss of two electrons and formation of a dication-diradical was demonstrated. Anodic oxidative coupling occurs at nitrogen best in the presence of collidine, presumably via deprotonation. 

The conversion of substituted diphenylamines and triphenylamines to carbazoles at platinum anodes in CH3CN-Et4NC104 takes place if the intermediate cation-radical is fairly stable. Thus the anodic oxidation of (V-ethylbis(p-fert-butylphenyl)amine (87) gave 3,6-di-ferf-butyl-Af-ethyl-carbazole (88) in 15% yield152 [Eq. (72)]. 

The formation of carbazole cation-radicals upon anodic oxidation of di-and triarylamines has been reported.474 The main requirement is that the amine used should form a persistent cation-radical (and hence not dimerize) so that it may be oxidized to the dication. It is this species which cyclizes to the carbazole. The carbazole formed is then oxidized to its cation-radical. This is the observed product if the reactive positions in the carbazole are blocked. In general, those substituents which confer persistence on the amine cation-radical are also those which confer it on the carbazole cation-radical. 

Tail-to-tail coupling of radicals obtained in the anodic oxidation of triphenyl-amines results in the formation of tetraphenylbenzidines. Oxidation of triarylamines to the di-cation results in the formation of the carbazoles, as observed for Ai -alkyl-p,p -disubstituted diphenylamines [1-3, 78]. The cation radicals of triarylamines with substituents in the para position of the aryl groups, which can protect them against nucleophilic attack, are very stable and can be used as organic redox catalysts for indirect electrochemical oxidation reactions. Depending on the substitution pattern on the phenyl group the oxidation potentials of the triarylamines can be tuned over a wide range [Eoy. = 0.7-2.0 V) and many of these have been used as redox catalysts in numerous indirect electrochemical reactions [1-3, 79-83]. 

Carbazoles are oxidized at controlled potential at a platinum anode in acetonitrile to cation radicals that are stable when the 3-, 6-, and 9-positions are blocked [223]. The radical cation from carbazole dimerizes predominantly at the 3-position to 3,3-dicarbazyl, which is further oxidizable to the quinoidal dication at the potential used. In the presence of pyridine, which may cause a rapid deprotonation of the N-H proton 9,9 -dicarbazyl is the isolated product. 

Intramolecular coupling or nucleophilic substitutions sometimes occur upon oxidation of the cation radical the new product frequently then undergoes an additional anodic electron-transfer reaction to yield a new cation radical. For example, tri-p-substituted triphenylamines form stable cation radicals upon oxidation at their first anodic wave. Reynolds et alt (1974) have shown, however, that further oxidation of the cation radicals leads to the appearance of a new reversible redox couple at potentials slightly positive with respect to the first anodic wave. This new couple was shown to occur at potentials where the corresponding carbazole is oxidized, so that the overall process involves conversion of the amine to the carbazole cation radical by reaction scheme (85). 

The direct electrochemistry of redox proteins such as ferredoxin and blue copper protein was studied. The additional effect of poly(L-lysine) on the redox behaviour of horse heart cyt c at functional electrodes has been reported Electropolymerized films such as PAn undergo redox reactions producing a colour change. This is described in Sect. 5.2. The anodic oxidation of poly(iV-vinylcarbazole) films was shown to involve initially the cross-linking of the polymer chains by oxidation of the carbazole moieties and dimerization of the resulting pendant carbazole cation radicals The resulting dimeric carbazole unit is more easily oxidized than the monomer and undergoes a further (reversible) two-electron oxidation. 

Several other organic systems have been stndied as potential electrochromes because of their redox behaviour. These include carbazoles, methoxybiphenyls, fluorenones, benzoquinones, naphthaquinones and anthraqninones, tetracyanoquinodimethane, tetrathiafnlvalene and pyrazolines. ° Of particnlar interest are the 1,4-phenylenedi-amines, which form highly colonred species on oxidation. These, known as Wurster s salts, exemplified by Wnrster s Bine (1.97), are anodically colouring and this type of material has found nse in composite electrochromic systems for car rearview mirrors (see 1.5.4.1). 

Electrochemical doping of insulating polymers has been attempted for polyacetylene, polypyrrole, poly-A/-vinyl carbazole and phthalocyaninato-poly-siloxane. Significantly, Shirota et al. [91] claim to have achieved the first synthesis of electrically conducting poly(vinyl ferrocene) by the method of electrochemical deposition (ECD) [91]. This is based on the insolubilization of doped polymers from a solution of neutral polymers. A typical procedure applied [91] for polyvinyl ferrocene is to dissolve the polymer in dichlorometh-ane and oxidize it anodically with Ag/Ag+ reference electrode under selective conditions. The modified polymer [91] (Fig. 28) is a partially oxidized mixed valence salt containing ferrocene and ferrocenium ion pendant groups with C104 as the counter anion. 

Electrochemical and spectroscopic techniques have been used to study the oxidation of carbazole and 71 of its derivatives.207,208 Positions 3, 6, and 9 of the carbazole nucleus are the most reactive sites, as expected from Hiickel theory. The products isolated are symmetric carbon-carbon (3,3 ) and nitrogen-nitrogen (9,9 ) dimers. Substitution of carbazoles in the 3-, 6-, and 9-positions prevents anodic dimerization at these positions the electrochemical formation of a stable radical-cation is possible.209 The electrochemical oxidation of iminobibenzyl and several related compounds have been investigated in CH3CN-Bu4NC104 and their electrochemistry was compared with that of related carbazoles.210... 

Oxidative coupling has been observed for benzene (52), methyl substituted benzenes (53), triphenylethylene (54), triphenyl-amines (55-59), anilines (57), carbazoles (60,61), iminobibenzyls (62), and heterocyclic phenols (71,72). Intramolecular anodic coupling reactions are used for synthesizing specific ring structures (63-68). Both dimer and octamer of dibenzothiophene have been detected (69,70)... 

With the AFM tip as the cathode, nanowires of polycarbazole were also patterned by electric-field-induced cross-Unking and polymerization of carbazole units on an Au/mica or Si substrate (anode) spin-coated with insulating poly(vinylcarbazole) film [23,24]. Figure 10.9 shows a schematic of the experimental setup and Figure 10.10 shows an AFM image of the patterned CP nanostractures [24]. Similarly, electrochemical oxidation and cross-linking of carbazole and thiophene due to the flow of electrons from the conductive AFM tip to the polymer (polystyrene functionalized with carbazole and thiophene groups) precursor film on an Si substrate produced CP nanofeatures in the precursor polymer film [25]. 

Polypyrrole degradation (pyrolysis at 600°C or anodic over-oxidation) gave benzene, indole, carbazole, etc. fragments. 

Poly(carbazole) [20]. Solution of carbazole in acetonitrile may be electrochemically oxidized (counterion CIO4 or BF4) at a platinum anode to give electrically conductive films with poor mechanical stability. The polymers obtained by chemical coupling are mores stable. Poly(carbazole) has also been obtained by vacuum evaporation of carbazole and by chemical condensation. Doping with I2 or NOBF4 leads to conductivities an high as 1 S/cm. 

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