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

Conjugated polyelectrolytes (CPEs), which combine conjugated backbones with charged sidechains, show unique electronic properties. This study focuses on two key strategies for controlling the conductivity of CPEs: sidechain engineering and dopant selection. We synthesized and characterized two derivatives of poly(cyclopentadithiophene-alt-thiophene) (PCT) with different sidechains: cationic (PCT-NBr) and anionic (PCT-SO₃Na). Both polymers were found by solution-phase small-angle X-ray scattering (SAXS) measurements to form rod-like micelles in aqueous solutions.

Dopants compatible with the side-chain charge of both PCT derivatives were found: FeCl₃ for PCT-NBr, and benzoquinone (BQ) for PCT-SO₃Na. A detailed analysis suggests that the counterion is Cl^– in FeCl₃-doped PCT-NBr and the anionic -SO₃^– headgroups in BQ-doped PCT-SO₃Na. Red-shifted polaron absorption features for PCT-SO₃Na relative to PCT-NBr suggest the headgroup-stabilized polarons in PCT-SO₃Na are more delocalized than in small ion-stabilized PCT-NBr. SAXS measurements revealed that the low pH required for doping disorders cationic PCT-NBr micelles but straightens anionic PCT-SO₃Na micelles, another factor benefitting carrier mobility. As a result, BQ-doped PCT-SO₃Na thin films show reasonable conductive at 30 S/cm, a new benchmark for CPEs, while films made from doped PCT-NBr barely conduct. Overall, our findings emphasize the critical role of sidechain engineering and dopant selection in tuning CPE properties, offering valuable insights for the development of aqueous conductive polymer inks.