Dependence of Cation Structures on Electrochemical Polymerization of Aniline in Phosphonium Ionic Liquids

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Ionic liquids (ILs) are known to possess unique physicochemical and abundant design properties. When ILs are used as electrolytes in the electropolymerization reactions of aromatic compounds to form conducting polymers, the grown film properties are significantly dependent of the combination of cations and anions. Typical researches on the IL-based electrolytes have been described for nitrogen-based ILs;1) on the other hand, phosphonium-based ILs have also attracted attention in recent years. We have reported the preliminary electrochemical investigation for electropolymerization of aniline (ANi), indicating that the phosphonium ILs were able to be employed as potential electrolytes.2) In this work, various phosphonium ILs ( Fig. 1) including typical two phosphonium ILs such as triethylpenthylphosphonium bis(trifluoromethanesulfonyl)amide (P2225-TFSA) and tributylmethylphosphonium bis(trifluoromethanesulfonyl)amide (P4441-TFSA) 3) are selected for the electrooxidative polymerization of ANi, in order to investigate the influence of the physicochemical properties of the phosphonium ILs on the electropolymerization processes and the grown polyaniline (PANi) films.The phosphonium ILs were synthesized and purified according to a previously published paper.3) The electrolytes were prepared by adding 2.0 M of ANi monomer and 1.0 M of 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl]methanesulfonamide (HTFSA) as a proton source. The electropolymerization was carried out by cyclic voltammetry (CV) using a three-electrode cell with a platinum disc or an indium tin oxide (ITO) as a working electrode, a Ag/Ag+ as a reference electrode and a platinum wire as a counter electrode (potential sweep rate: 0.1 V s-1, potential range: -0.5~+1.5 V). The morphology of the electropolymerized PANi films on the ITO electrode were observed by scanning electron microscopy (SEM).Fig. 2 shows the cyclic voltammograms for electrooxidative polymerization of ANi in typical P2225-TFSA and P4441-TFSA. It was found that the P4441-TFSA based electrolyte tended to give relatively low oxidation currents, forming the uniform PANi film on the electrode surface as shown in Fig. 3. The voltammograms for the electropolymerization measured in various phosphonium ILs will be discussed, demonstrating the influence of the transport property of the phosphonium IL electrolytes.