Energy Transfer in Aqueous Light Harvesting Antennae Based on Brush-like Inter-Conjugated Polyelectrolyte Complexes

Conjugated polyelectrolytes (CPEs) have the potential to serve as building blocks of artificial light-harvesting systems. This is primarily due to their delocalized electronic states and potential for hierarchical self-assembly. We showed previously that inter-CPE complexes composed of oppositely charged exciton-donor and exciton-acceptor CPEs displayed efficient electronic energy transfer. However, near ionic charge equivalence, complexed CPE chains become net-neutral and thus experience a precipitous drop in aqueous solubility. To increase the stability and to rationally manipulate the phase behavior of inter-CPE complexes, we synthesized a series of highly water-soluble exciton-donor CPEs composed of alternating ionic and polar nonionic fluorene monomers. The nonionic monomer contained oligo(ethyleneglycol) sidechains of variable length. We then formed exciton donor-acceptor complexes and investigated their relative energy transfer efficiencies in the presence of a fixed exciton-acceptor CPE. We find that, even when the polar nonionic sidechains become quite long (nine ethyleneglycol units), the energy transfer efficiency is hardly affected so long as the inter-CPE network retains a net polyelectrolyte charge. However, near the onset of spontaneous phase separation, we observe a clear influence of the length of the oligo(ethyleneglycol) sidechains on the photophysics of the complex. Our results have implications for the use of polyelectrolyte phase separation to produce aqueous light-harvesting soft materials

Influence of Molecular Excluded Volume and Connectivity on the Nanoscale Morphology of Conjugated Polymer/Small Molecule Blends

Over the past couple of decades, organic photovoltaics (OPV) based on conjugated polymer/fullerene derivative bulk heterojunctions have been extensively studied, resulting in single-junction efficiencies of order 10%. The need to push the efficiency toward 15% has resulted in the synthesis of a large number of non-fullerene electron acceptors with ever-increasing absorption coefficients in the red. Though some new acceptors have recently begun to be competitive with the fullerene, there is very little systematic understanding of which molecular geometries and spatial frontier orbital extent correlate with improved performance. One of the most important factors that determines the OPV efficiency is the nanoscale, phase-separated morphology of the blend. In this article, using a combination of resonant elastic X-ray scattering and elemental mapping, we investigate the influence of relatively small chemical changes to a nonplanar conjugated small molecule on the nanoscale morphology of the resulting polymer/molecule blend. We find that subtle modifications of the number and placement of peripheral functional groups can have an enormous influence on the length scale of phase separation. We then quantify the extent of phase separation by using the generalized indirect Fourier transform to convert resonant scattering intensities to pair-distance distribution functions. Our results point toward the large influence of the molecular excluded volume as a major morphology determinant. This work has implications for synthetic efforts to create non-fullerene electron acceptors that can substantially outcompete fullerenes in OPV devices

Excitonically Coupled Simple Coacervates via Liquid/Liquid 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