Enhancing Doping Efficiency in Self-Assembled Cylindrical Micelles of Conjugated Polyelectrolytes via Sidechain Engineering

Conjugated polyelectrolytes (CPEs), which combine conjugated backbones with charged sidechains, exhibit unique electronic properties, including tunable optical features, solution processability, low cost, and biocompatibility. This makes them interesting materials for a range of applications. This study focuses on two key strategies for optimizing the self-assembly and optoelectronic properties of CPEs: sidechain engineering and dopant selection. We specifically synthesized and characterized two derivatives of poly(cyclopentadithiophene-alt-thiophene) (PCT) with different sidechains that were designed as model systems that would self-assemble into cylindrical micelles with the polymer chains running along the micelle axis: cationic (PCT-NBr) and anionic (PCT-SO₃Na). Solution phase Small-Angle X-ray Scattering (SAXS) measurements confirmed the formation of rod-like micelles in aqueous solutions, with subtle variations in shape and dimensions due to differences in the sidechains and their respective headgroup areas. Spectroscopic analysis showed a red-shifted absorption band and enhanced J-aggregate formation in the anionic polymer (PCT-SO₃Na), indicating sidechain effects on optical properties.
We then investigated doping of these polymers, starting with acid doping in PCT-SO₃Na. Acid doping is induced by the high local concentration of protons near the polymer backbone due to the sulfonate head group, and can be facilitated by exciton formation and trapping from the sidechain electric field. We further explored the effects of chemical doping on polymer properties. FeCl₃ efficiently doped PCT-NBr while maintaining the micellar structure. PCT-SO₃Na is not stable in the presence of FeCl₃, but benzoquinone (BQ) dopes PCT-SO₃Na via a proton-coupled two-electron transfer redox process and is more compatible with anionic sidechains. Our detailed analysis of ion speciation and absorption spectra suggested that Cl^– is the counterion in FeCl₃-doped PCT-NBr and occupies voids in the sidechain area, whereas the anionic -SO₃^– headgroups in PCT-SO₃Na act as the counterion. The large distance between charge carriers on the backbone and counterions in sidechain headgroups reduces the coulombic trapping of charge carriers and thus promotes carrier delocalization, evidenced by a red shift in the polaron absorption spectrum of doped PCT-SO₃Na.
Both dopants require an acidic environment, and SAXS measurements revealed that decreasing pH decreases the order of micelles in cationic PCT-NBr but straightens the micelles in anionic PCT-SO₃Na. This straightening effect in BQ-doped PCT-SO₃Na could potentially reduce traps in the conjugated core of micelles and improve delocalization of charge carriers as well. In agreement with this idea, BQ-doped PCT-SO₃Na thin films are more conductive than films made from doped PCT-NBr, reaching a conductivity of 30 S/cm, which is a new benchmark for CPEs. Overall, our findings emphasize the critical role of sidechain engineering and dopant selection in tuning CPE properties, offering valuable insights for the development of high-performance semiconductive polymers for a wide range of organic electronic applications.