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.

The ability to assemble artificial systems that mimic aspects of natural light‐harvesting functions is fascinating and attractive for materials design. Given the complexity of such a system, a simple design pathway is desirable. Here, we argue that associative phase separation of oppositely charged conjugated polyelectrolytes (CPEs) can provide such a path in an environmentally benign medium: water. We find that complexation between an exciton–donor and acceptor CPE leads to formation of a complex fluid. We interrogate exciton transfer from the donor to the acceptor CPE within the complex fluid and find that transfer is highly efficient. We also find that excess molecular ions can tune the modulus of the inter‐CPE complex fluid. Even at high ion concentrations, CPEs remain complexed with significantly delocalized electronic wavefunctions. Our work lays the rational foundation for complex, tunable aqueous light‐harvesting systems via the intrinsic thermodynamics of associative phase separation.

The ability to assemble artificial systems that mimic aspects of natural light-harvesting functions is fascinating and attractive for materials design. Given the complexity of such a system, a simple design pathway is desirable. Here, we argue that associative phase separation of oppositely charged conjugated polyelectrolytes (CPEs) can provide such a path in an environmentally benign medium: water. We find that complexation between an exciton-donor and acceptor CPE leads to formation of a complex fluid. We interrogate exciton transfer from the donor to the acceptor CPE within the complex fluid and find that transfer is highly efficient. We also find that excess molecular ions can tune the modulus of the inter-CPE complex fluid. Even at high ion concentrations, CPEs remain complexed with significantly delocalized electronic wavefunctions. Our work lays the rational foundation for complex, tunable aqueous light-harvesting systems via the intrinsic thermodynamics of associative phase separation.

Viscoelastic liquid coacervate phases that are highly enriched in nonconjugated polyelectrolytes are currently the subject of highly active research from biological and soft-materials perspectives. However, formation of a liquid, electronically active coacervate has proved highly elusive, since extended π-electron interactions strongly favor the solid state. Herein we show that a conjugated polyelectrolyte can be rationally designed to undergo aqueous liquid/liquid phase separation to form a liquid coacervate phase. This result is significant both because it adds to the fundamental understanding of liquid/liquid phase separation but also because it opens intriguing applications in light harvesting and beyond. We find that the semiconducting coacervate is intrinsically excitonically coupled, allowing for long-range exciton diffusion in a strongly correlated, fluctuating environment. The emergent excitonic states are comprised of both excimers and H-aggregates.

The ability to form robust, optoelectronically responsive, and mechanically tunable hydrogels using facile processing is desirable for sensing, biomedical, and light-harvesting applications. We demonstrate that such a hydrogel can be formed using aqueous complexation between one conjugated and one nonconjugated polyelectrolyte. We show that the rheological properties of the hydrogel can be tuned using the regioregularity of the conjugated polyelectrolyte (CPE) backbone, leading to significantly different mesoscale gel morphologies. We also find that the exciton dynamics in the long-time limit reflect differences in the underlying electronic connectivity of the hydrogels as a function CPE regioregularity. The influence of excess small ions on the hydrogel structure and the exciton dynamics similarly depends on the regioregularity in a significant way. Finally, electrical impedance measurements lead us to infer that these hydrogels can act as mixed ionic/electronic conductors. We believe that such gels possess an attractive combination of physical-chemical properties that can be leveraged in multiple applications.