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

Abstract

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.

Aqueous Formulation of Concentrated Semiconductive Fluid Using Polyelectrolyte Coacervation

“…3 was also reported to occur in aqueous high-salt media, with the mixture's phase behavior and photophysical properties showing a strong dependence on the identity of the cation salt. 27 Studies on CPE−CPE complexation in salt-free water, 28,29 focusing on dilute regimes and the solid-state complexes, have further elucidated how the inclusion of the conjugated moieties fundamentally affects complexation thermodynamics. Similar to the CPE−polyelectrolyte coacervate, complexation of two CPEs in the dilute regime was shown to induce extension and planarization of the CPE backbone.…”

“…Despite this large body of literature on nonconjugated polyelectrolytes, coacervation has only been reported in a few conjugated polymer systems. , For example, the coacervation between a polythiophene-based CPE and an insulating polyelectrolyte in THF/water solvent mixtures has been shown to increase the CPE’s conjugation length and photoluminescence yield . CPE–polyelectrolyte coacervation was also reported to occur in aqueous high-salt media, with the mixture’s phase behavior and photophysical properties showing a strong dependence on the identity of the cation salt . Studies on CPE–CPE complexation in salt-free water, , focusing on dilute regimes and the solid-state complexes, have further elucidated how the inclusion of the conjugated moieties fundamentally affects complexation thermodynamics.…”

Conjugated Polyelectrolyte‐Based Complex Fluids as Aqueous Exciton Transport Networks

“…When supplied with a photoinduced electron acceptor, CPE excitons that efficiently sample the underlying network will be able to find an interface to dissociate into spatially separated electron‐hole pairs, thus storing photon energy as electrochemical energy. To this end, we previously showed that the quintessential electron acceptor, a C 70 derivative, with negligible aqueous solubility may be readily incorporated into the aqueous CPE‐based concentrated phase [26] . Therefore, we hypothesize that the yield of electron/hole pairs in such a complex fluid system could be tuned by varying the excess salt concentration.…”

“…We have recently shown that complexation of a CPE with an oppositely charged non-conjugated polyelectrolyte led to aqueous associative phase separation and formation of concentrated, CPE-rich phases. [26] The photophysics within the polymerrich phase were found to be strongly coupled to the ionic atmosphere. The Segalman and Chabinyc groups have also shown that oppositely charged systems with one conjugated and one non-conjugated polyelectrolyte can be used to form semiconducting and conducting fluids.…”

Modeling the Interplay of Conformational and Electronic Structure in Conjugated Polyelectrolytes

“…The coupled electronic and ionic functionality characteristic of CPEs has prompted their use in biological, sensing, optical, and electronic applications. [1][2][3] In dilute solutions and coacervates, CPEs' conformation dependent optical properties make them effective for the single molecule detection of oppositely charged biomolecules (e.g. DNA, ATP, proteins) and as inks.…”