Electro-Optically Active Polyaniline-Polypyrrole Double Layer Polymer Semiconducting Composite
Authors: Komaba Kyoka , Goto Hiromasa 
Published: February 2020

Optically active polyanilines have been synthesized in the presence of chiral camphorsulfonic acid (CSA) by electrochemical polymerization in water medium. Polyaniline forms various electronic states such as emeraldine base, emeraldine salt, and pernigraniline. As prepared polyaniline in the presence of (+)-CSA is called as emeraldine salt. In this case, (+)-CSA shows two functions as 1) electro-dopant and 2) chiral inducer for the polyaniline, producing radical cation (polarons) and diction (bipolarons) as charge carriers and one-handed helically twist for the main chain.

In the present research, we succeeded to obtain double layer conductive polymer film consisting of polyaniline and polypyrrole with electrochemical method. Fig. 1 shows the structure of the double layer polymer film. The optical activity of the polymer film was examined with optical rotatory dispersion (ORD). The double layer supports strength of the polymer film form. Electrical conductivity of the polymer film is ca. 0.25 S/cm evaluated with four-probe method.

We carried out synthesis of a single layer polyaniline with (+)-CSA with potentiostat with three electrode system, Pt counter electrode saturated calomel electrode (SCE) as reference, and the polymer as working electrode. Fig.2 shows the results. Five scans afford to produce green polymer film deposited on the indium-tin oxide (ITO) coated grass electrode. The polymer shows redox activity conformed with cyclic voltammetry (CV). Increase of scan rate provides increase of current density in the electrochemical measurement because of increase of diffusion of ionic species in the electrolyte solution. The single layer polyaniline film displays electrochemical driven change in optical rotation evaluated with in-situ ORD/CV.

The double layer polymer film was synthesized as followed. First, electrochemical polymerization of aniline as a monomer was carried out in concentrated optically active (+)-CSA with two electrode system. An application of electric filed (3.0 V) between electrodes in the electrolyte solution was carried out. The electrochemical polymerization reaction is rapidly progressed for formation of polymer thin film onto the anode side. Continuously, pyrrole as another kind of monomer was added to the electrolyte solution. The sequential electrochemical polymerization process affords to yield polypyrrole layer deposited onto the polyaniline layer.

ORD measurement revealed that the double layer polymer film thus obtained is optically active. The bottom layer of polyaniline can be formed helical structure due to combination with optically active (+)-CSA. The top layer of polypyrrole may also form helical structure with aid of (+)-CSA as chiral doping agent. Chemical structure of the double layer polymer (composite) is examined with Fourie transform infrared optical absorption spectroscopy measurements. This result indicates that the polymer composite consists of polyaniline, polypyrrole, and CSA. The composite shows intense and wide range optical absorptions from invisible to infrared range due to delocalized charge carriers along the main-chain.

The advantages of double conductive polymer film are redox activity, tunable optical activity, and color coordination. Top layer of polypyrrole is black in color with no polish and good redox character, and polyaniline as the bottom layer displays electro-optical activity. This paper reports convenient method for preparation of double layer system with electrochemical method. Further variation can be expected using different type of monomers for construction of multi-layer conductive polymer composite. This concept can be a first example as an electro-optically active conducting polymer composite.

Figure 1