Exploring the Chemical Doping of a Water-Soluble Cylindrical Micelle-Forming Conjugated Polyelectrolyte

The chemical doping of water-soluble conjugated polyelectrolytes (CPEs) offers a promising pathway for the direct printing of semiconducting polymer films by using environmentally friendly solvents. In this study, we explored the chemical doping of the cationic cylindrical micelle-forming CPE poly(cyclopentadithiophene-alt-thiophene) (PCT-NBr) in aqueous solution using two Fe(III)-halide dopants, FeCl₃ and FeBr₃. Treatment with nonoxidizing salts (KCl and KBr) showed that polymer micelles preferentially interact with Br^- ions over Cl^- ions, resulting in a more rigid micelle and spectroscopic evidence of Br^- ion accumulation around the polymer. Doping with both FeCl₃ and FeBr₃ was followed using UV-visible-near IR absorption spectroscopy, which indicated that the polymer micelles could be stably doped with both iron compounds. FeCl₃ was shown to be a stronger dopant due to differences in the lability of Cl^- and Br^- ligands in water. Compared at similar concentrations, FeCl₃ induces higher doping levels, while FeBr₃ generates more delocalized charge carriers, as evidenced by spectral shifts in the polaronic bands, likely due to weaker counterion Coulombic trapping. Small-angle X-ray scattering was used to confirm that a micellar structure was preserved at all doping levels of PCT-NBr, but the data also indicate increased structural disorder in doped polymer micelles, likely due to partial loss of the polymer's amphiphilic character and ion-polymer interactions. Films spin-cast directly from FeBr₃-doped polymer solutions exhibited a stable conductivity of 1.0 S/cm, demonstrating the viability of using doped micellar CPE solutions as a route to single-step deposition of conductive polymer films.