Investigation of electrochemical oxidative coupling of 3 and 6 substituted carbazoles

Abstract

• Investigation of carbazole reactivity and electropolymerization mechanism. • Combination of fast electrochemistry experiments and molecular orbital calculations. • Investigation of the stability of radical cations and polarons formed by electro-oxidation of carbazoles. • Comparison of the stability of the dimers depending on the coupling positions. In this study, electrochemical behavior of different functionalized carbazoles has been studied through classical cyclic voltammetry, computational calculations, electrochemical simulations and more originally fast cyclic voltammetry. Indeed, fast electrochemistry is rarely used to study reactivity of such compounds even if it can bring very interesting information as shown in this work. Carbazoles were substituted with ethyl, tert -butyl and phenyl groups at 3 and 6 positions. Oxidation of 3-ethylcarbazole leads to very fast dimerization of formed radical cations (k dim = 1.5.10 7 M −1 .s −1 ) and electrochemical polymerization of this compound leads to a thick adherent conducting polymer film. This was also the case for 3-phenylcarbazole and 3,6-diphenylcarbazole when potential was brought over 1.7 V/SCE because of formation of very reactive radicals after oxidation process involving phenyl groups. For 3,6-diethylcarbazole, 3- tert -butylcarbazole, 3,6-ditert-butylcarbazole, as well as 3-phenylcarbazole and 3,6-diphenylcarbazole when potential is kept under 1.4 V/SCE, dimerization of radical cations is slower (k dim ≤ 1.5.10 6 M −1 .s −1 ) and no efficient electropolymerization occurs. A theoretical study of the same substituted carbazoles in their neutral and oxidized states was done using molecular orbital calculations (DFT method) to obtain additional information about carbazole reactivity. These numerical calculations were in perfect accordance with results of fast electrochemistry and electrochemical simulations regarding stability of radical cations. They demonstrated that monosubstituted carbazole dimers are probably obtained through 6-6′ coupling while 1-1′ coupling is the most favorable for disubstituted carbazole dimers.