A Comparative Study of Two Different Media Effect on the Electropolymerization of 2-(9-ethylcarbazol-3- yliminomethyl)phenol by Cyclic Voltammetry, Impedance Spectroscopy, XPS, UV-visible Measurements and DFT Calculation
A carbazole-based monomer, 2-(9-ethylcarbazol-3-yliminomethyl)phenol (SIC), was first synthesized and characterized. It was afterward polymerized by potentiostatic methods. Oxidative polymerization of SIC was carried out in two media. The first one (i.e. Bu4NBF4 (0.1M)/MeCN) gives the poly(2-(9- ethylcarbazol-3-yliminomethyl)phenol). However, the second one (LiClO4/MeCN + 35mM HClO4), leads to another polymer, the poly(2-(9-ethylcarbazol-3-yliminomethyl)cyclohexa-2,5-diene-1,4- dione).These two novel polymers will be abbreviated here after as PSIC and PDIC, respectively. If the formation of PSIC was expected, the formation of PDIC in acidic medium isn’t. It was explained by a hydroxyl function oxidation of SIC. The obtained polymers were then characterized by cyclic voltammetry, electrochemical impedance spectroscopy, XPS, IR and UV-Vis spectroscopies. Furthermore, the corresponding electrochemical and optical bandgap values were calculated in order to get an idea about the conductive properties. The related results show that PDIC and PSIC are good conductive polymers. The electrochemical polymerisation of SIC is performed in both protic and organic medium. During the anodic oxidation in protic medium containing LiClO4/MeCN + (35mM) HClO4, SIC undergoes hydrolysis around 1V, which modifies its structure before its polymerization. This reaction is very fast, and leads to the formation of two isomers, para-quinone and ortho-quinone (DIC) which oxidizes at the same potential and form PDIC. In the organic medium, the SIC retains its initial structure, and leads after oxidation to the corresponding polymer PSIC. The electrochemical behaviour of PSIC and PDIC supports the conductive character of both polymers and the difference between their structures. In an acidic medium, the imine bond undergoes a protonation which increases commonly the planarity of the polymeric material and subsequently, improves its electrical properties. The charge transfer resistance recorded for the PDIC film was much lower than the one recorded for the PSIC film. The electrochemical and the optical bandgaps of PDIC are lower than those of PSIC. The difference in structures of both polymers could explain these results. Compared to SIC, para-quinone and ortho-quinone compounds are formally electrons donor-acceptor (D-A) type moieties. In a D-A system, a charge transfer from the donor to the acceptor increases conjugation length through resonance which causes the decrease of its bandgap. Therefore, these findings indicate that PDIC is more conductive than PSIC.
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