Role of Solvent and Secondary Doping in Polyaniline Films Doped with Chiral Camphorsulfonic Acid: Preparation of a Chiral Metal

Abstract

We present the first study of chirality from the UV to the IR region for a conducting polymer, thereby probing the motion of the metallic “free carriers.” Through study of chiral and achiral camphorsulfonic acid (CSA) doped polyaniline (PAN) films cast from different solvents (m-cresol and chloroform) and correlations with secondary doping experiments, we demonstrate that the chirality lies in the order “metallic” regions of the polymer films. Samples were studied by absorption, UV−vis−IR circular dichroism, and charge transport (temperature-dependent dc and microwave frequency conductivity and dielectric constant) measurements. The metallic films (cast from m-cresol) show strong optical activity of the conducting electrons in the visible and IR regions, showing that a chiral metal can be prepared by the methods of organic chemistry. In contrast, the nonmetallic chloroform cast films show weaker optical activity of the polymer backbone and a completely different interaction with CSA compared to m-cresol cast films. However, little difference in temperature-dependent dc conductivity is observed with PAN doped with chiral vs achiral CSA. It is postulated that chiral regions of the polymer backbone segregate in the metallic regions of the polymer film upon evaporation of m-cresol. Secondary doping of chloroform cast films with the vapors of m-cresol yields an increase in electron delocalization but a decrease in optical activity of the polymer backbone, in accord with a picture of inhomogeneous order in these materials.