Chiral Charge Carriers Chiralions for Polyaniline Examined with Optical Rotation Spectroelectrochemistry

Abstract

In this study, the optical active polyaniline film was examined with optical rotatory dispersion (ORD) through change in potential with spectroelectrochemical method. Polyaniline is one of the representative conducting polymers. Preparation of polyaniline is convenient because synthesis of polyaniline can perform in water medium. Conducting polymers shows electrochromism via change in the electronic state upon electrochemical redox process. Polyaniline forms various electronic states such as emeraldine base, emeraldine salt, and pernigraniline. This change allows to color tuning through electrochemical method. Further, optically active polyanilines have been synthesized in the presence of (+) or (-)-camphorsulfonic acid (CSA) by electrochemical polymerization or chemical polymerization with ammonium persulfate (oxidizing agent). Both electrochemical method and chemical method produce polyaniline through radical polymerization process. Polyaniline emeraldine salt as prepared form of polyaniline is doped with CSA, showing optical activity in the circular dichroism. In this research, we carried out to synthesis of polyaniline in the presence of optically active CSA, according to previously reported method (by Wallace et al). Here, we have found that the polyaniline thus prepared showed optically active electrochromism via electrochemical redox process in circular dichroism spectroscopy measurements. In our previous study, we calculated the ORD spectra for the polymer by Kramars - Kronich (K-K) conversion from circular dichromism. In the present research, we successfully obtained experimental data of optical rotation-spectroelectrochemical result, as a first example of optical rotation control of natural optical activity for conjugated polymers based on electrochemical method. In the case of low potential for the polymer in the electrochemical redox process, the polymer shows intense optical rotation at visible wavelength range. On the other hands, higher potential at around 1 V affords to give doping process for the polymer, resulting generation of polaron as charge carrier, which shows optical activity at near infrared region. We referred this optical charge carrier as “chiral charge carrier chiralions”. The electrochemical oxidation equivalents to electrochemical doping from the polymer. In other words, electrochemical oxidation gives electrochemical doping which results change in degree of optical rotation for the polymer. The control of optical ration for materials can be driven by magnetic field (Faraday rotation), and electric field (Kerr rotation). We propose a new phenomenon as ‘‘electrochemical optical rotation’’. The electro-conductivity of the polymer is measured with four-probe method. The polymer which is electronic doped state with (+)-CSA as electric dopant for production of radical cation (polarons) or dication (bipolarons) as charge carriers and chiral inducer for twisting main-chain. The polymer film was synthesized as followed. First, electrochemical polymerization of aniline as a monomer was carried out in excessive optically active (+)-CSA with three electrode system consisting of working electrode (indium-tin oxide coated glass), counter electrode (platinum wire), and reference electrode (SCE) in water. The polymerization reaction is rapidly progressed with formation of polymer thin film onto the counter electrode. The polymer was washed with excess amount of water, acetone. The electroactive polymer film was thus deposited on the electrode was used for optical activity measurements. Cyclic voltammetry (CV) measurements were carried out for the resultant polymer (monomer free) deposited on to the ITO coated glass electrode. The CV results with change in scan rate from 10-100 mV/s indicates electroactive. Increase of scan rate increases current density for the polymer due to increase of diffusion of ion in the electrolyte solution. Oxidation signals of the polymer are observed due to change in electronic state of the polymer. Corresponding reduction signals against oxidation process are observed, indicating reversible redox activity for the polymer. ORD measurement reveled that the polymer film thus obtained is optically active. The polyaniline can be formed helical structure due to doping of optically active CSA. We carried out in-situ ORD measurement with potential change, evaluating electrochemical driven change in optical rotation. Chemical structure of the double layer polymer is examined with Fourie transform infrared optical absorption spectroscopy measurements which reveled this polymer is polyaniline, and CSA as a dopant. Ultra-violet optical absorption measurements for the polymer film indicates intense and wide range absorptions from invisible to infrared range as charge carriers the delocalized along the main-chain, indicating the polymers are conductive materials. The advantages of the polymer film are formation of self-standing redox activity, tuning of optical activity and color coordination. This paper reports novel function of control optical rotation with electrochemical method examined with spectroelectrochemistry. This concept can be a first example as optical rotation effect of electro-optically active conducting polymers. This paper reports the charge carries of helical polyaniline as chiralions can tune via electrochemical method.