Carbazole anion-radicals

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Coulometry is generally difficult with in situ EPR experiments because of convection and iR drop in the long, thin layer of solution. In order to determine the current efficiency of carbazole anion formation, an in situ measurement was carried out [29,30], Carbazoles can either form soluble products or polymerize forming a conductive layer on the electrode. Results of the measurement showed that there was between 0.2 and 2% current efficiency for radical formation depending on the carbazole used. The system described, however, can only be used for coulometry over short periods of time because of the problems discussed earlier. 

Solvations of the charged species accelerate the transfer of electrons and the ionizations are enhanced by polar solvents.Charge transfer reaction studies with tetracyanoethylene, an acceptor, and A -vinyl carbazole, a donor, in benzene solution demonstrated that both cation radicals and anion radicals form. This can be used in a subsequent cationic polymerizations ... 

The carbazole polymer anions behave quite differently. Electron transfer quenching of the present polymers by 1,4-diazabicyclo[2,2,2loctane (Dabco) results in the formation of the carbazole anion and the Dabco cation. The measured spectrum is common to all model compounds and polymers, and no interchromophoric interaction operates between anionic and neutral chromophores, even in the polymer system. The electron is trapped as a monomer anion radical. 

The use of the carbazole derivative (75) has been made in some SET reactions of benzoate derivatives. The reaction depends on electron transfer from the carbazole to the benzoate to afford the corresponding radical anion and it is suggested that the removal of the benzoate group occurs most efficiently with the m-trifluorophenyl benzoate derivatives. Typically the yields are high for example the conversion of (76) into (77) which takes place in 84% yield. 

In addition to free radical polymerizations, however, cationic and anionic polymerizations may, depending on solvent used, also be observed. The anodic discharge of acetate ions into the homogeneous phase, for example, produces a free radical polymerization of styrene or acrylonitrile. The anodic discharge of perchlorate or boron tetrafluoride ions, on the other hand, leads to the cationic polymerization of styrene, iV-vinyl carbazole, and isobutyl vinyl ether. In contrast, the cathodic decomposition of tertaalkyl ammonium salts induce acrylonitrile to polymerize anionically. 

It has been considered that a coordinated nitrene may give the same reaction and work in our group has shown that this is indeed possible in at least one case [260] (see also Chapter 5). However, it should be also noted that we now know that most reactions of nitro- and nitroso compounds with a metal complex occur through an intermediate electron transfer to the organic compound vide supra). Thus the formation of the radical anion of o-nitro- and/or o-nitrosobiphenyl should be considered as probable during the reaction. The reactivity of such radicals is virtually unknown and it cannot be excluded that these species, and not a nitrene intermediate, are responsible for the carbazole formation. The validity of carbazole formation as an indication for nitrene intermediates has been questioned very early [261]. It was shown that other reactions (including oxidation of 2-amino-biphenyl) can afford carbazole. Thus the results of this test should be taken with caution. 

It was first prepared by Reppe and co-workers in 1934 [381]. The polymerization reaction as well as the structure and properties of NVK and PVK are largely controlled by the electronic and steric influence of the carbazole group. The nitrogen atom donates its unshared electron pair, thus creating an electron-rich double bond. NVK can be polymerized by radical initiators and by cationic initiators but fails to polymerize anionically. 

Cation radicals of carbazolyl chromophore can be formed by quenching its excited state by the electron acceptor, DMTP, in DMF. The measured absorption spectra were composed of the carbazole cation and the counter acceptor anion. The former is observed in the wavelength region above 600 nm, while the latter is seen below 600 nm. The molecular structures of PVCz, Poly(N-carbazolylethyl vinyl ether) and polyurethanes studied and the absorption spectra of their cation radicals are shown in Fig. 8. The spectra of the polyvinyl-ethers and polyurethanes are very broad and show a peak at 780 nm (25, 28-30). PVCz has a maximum at 770 nm and no shoulder is detected. It is worth noting that this spectral shape is common to all PVCz polymer cations with different n-values from 4 to 1100 and is observed even for PVCz-4. 

Results obtained in the many copolymerizations of carbazole-containing monomers with different chiral comonomers may be summarized as follows i) real copolymer macromolecules are obtained in the cationically and free radically initiated polymerization with alkyl vinyl ethers, acrylic and methacrylic derivatives, and butene-dioic acid diesters ii) homopolymer mixtures are obtained in copolymerization runs with a-olefms in the presence of Ziegler-Natta catalysts, indicating that the conventional anionic coordinate mechanism is not effective in the polymerization of carbazole monomers... 

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