Investigation into the Photophysics and Phase Behavior of Conjugated Polyelectrolyte Complexes

Oppositely charged, water-soluble polyelectrolytes undergo a spontaneous complexation and phase separation process with applications in fields spanning food science, origin of life studies, biomedicine, underwater adhesives, and general materials science. Specifically, associative phase separation leading to complex coacervation of oppositely charged polyelectrolytes has been extensively studied to understand prebiotic organization and to inform research into synthetic cell mimics. However, the phase behavior of conjugated polyelectrolytes (CPEs), macromolecules analogous to chromophores and light-harvesting antennae found in photosynthetic organelles, has been investigated only minimally. Many questions remain regarding controlling of CPEs phase behavior, and how the formation of dense CPE-rich phases influences the resultant photophysics of theses molecular semiconductors. The work in this thesis primarily focuses on gaining a systematic understanding of the influence of ionic strength on the phase behavior of CPEs. The main tools used in this inquiry are steady-state absorption (or optical density (OD)) and photoluminescence (PL), time-resolved photoluminescence (TRPL), time resolved photoluminescence anisotropy, fluorescence microscopy, and fluorescence lifetime imaging (FLIM). The major findings of this work are as follows: the concentration and identity of cations in solution were found to manipulate the radiative decay rate and the exciton diffusion dynamics highlighting the importance of the interactions of the CPE complex with ions differing in polarizability and size; the influence of molecular ions leads to a transition from a two-phase system to an optically clear single phase with increased lifetime and decrease depolarization rate not accessible with similar concentrations of atomic salts; the chemical structure of a CPE can be rationally designed to undergo liquid/liquid phase separation to stabilize a semiconducting coacervate microstate; the interaction between ethyleneglycol units and K+ ions is critical in forming the coacervate phase; excited state energy transfer between the donor and acceptor CPEs occurs within the highly electronically coupled complex coacervate droplet phase