Associative Phase Separation of Aqueous π-Conjugated Polyelectrolytes Couples Photophysical and Mechanical Properties

The associative phase separation of water-soluble polyelectrolytes is important across many different fields including food science, biomedicine, materials science, and prebiotic organization. Specifically, associative phase separation leading to complex coacervation of oppositely charged polyelectrolytes has been extensively studied to inform research into synthetic cell mimics. However, the phase behavior of conjugated polyelectrolytes (CPEs), macromolecules analogous to chromophores found in light harvesting organelles, has been investigated only minimally. A systematic understanding of the influence of ionic strength on the phase behavior of CPEs could provide insights into the potential for these systems to form complex coacervates and improve control over the photophysical properties of these materials. In this study, the influence of increasing ionic strength (0-5.0 M) of three simple salts (LiBr, KBr, and CsBr) on the phase behavior of a cationic CPE [poly(fluorene-alt-phenylene)] and an anionic non-conjugated polyelectrolyte [poly(4-styrenesulfonate)] complex is interrogated. Associative phase separation into diluted and concentrated polyelectrolyte phases was found to occur regardless of salt type. We report on the phase composition and influence of the ion type on the photophysical properties of the concentrated phase, where the nature of the counter cation was found to manipulate the radiative decay rate and the exciton diffusion dynamics. Additionally, we demonstrate the ability of the polymer-rich phase to recruit a nonpolar, fullerene-based electron acceptor PC[70]BM, resulting in photoluminescence quenching likely due to photoinduced electron transfer. Our findings show promise for the formation of CPE-based coacervate-like phases and highlight the importance of the interactions of the complex with ions differing in polarizability and size. Additionally, the potential for these systems to form liquid electron donor/acceptor bulk heterojunctions has great implications for their use in optoelectronics.

Automated summary

The study investigates the influence of increasing ionic strength on the phase behavior of a cationic CPE and an anionic non-conjugated polyelectrolyte complex, finding that associative phase separation occurs regardless of the type of salt. The nature of the counter cation was found to manipulate the radiative decay rate and the exciton diffusion dynamics in the concentrated phase, indicating potential for forming CPE-based coacervate-like phases and implications for optoelectronics.