Exciton Transfer and Emergent Excitonic States in Oppositely-Charged Conjugated Polyelectrolyte Complexes

Abstract

Photosynthetic organisms have mastered the use of "soft" macromolecular assemblies for light absorption and concentration of electronic excitation energy. Nature's design centers on an optically inactive protein-based backbone that acts as a host matrix for an array of light-harvesting pigment molecules. The pigments are organized in space such that excited states can migrate between molecules, ultimately delivering the energy to the reaction center. Here we report our investigation of an artificial light-harvesting energy transfer antenna based on complexes of oppositely charged conjugated polyelectrolytes (CPEs). The conjugated backbone and the charged side chains of the CPE lead to an architecture that simultaneously functions as a structural scaffold and an electronic energy "highway". We find that the process of ionic complex formation leads to a remarkable change in the excitonic wavefunction of the energy acceptor, which manifests in a dramatic increase in the fluorescence quantum yield. We argue that the extended backbone of the donor CPE effectively templates a planarized acceptor polymer, leading to excited states that are highly delocalized along the polymer backbone.

Recent progress in the science of complex coacervation

“…These enhanced properties have been attributed to a planar polymer architecture that leads to excited states that are highly delocalized along the polymer backbone. 279 This idea of using molecular-level interactions and self-assembly to tune materials properties is emblematic of the future potential of coacervate-based materials.…”

Control of Photoinduced Charge Separation in Conjugated Polyelectrolyte Complexes through Microstructure-Dependent Exciton Delocalization

“…Conjugated polymers remain attractive structures for light-harvesting applications, as they offer continuous intrachain “highways” for exciton and charge migration. − Conjugated polyelectrolytes (CPEs), − which are π-conjugated polymers functionalized with charge-bearing substituents, make it possible to introduce conjugated polymers into water-soluble complexes. The formation of conjugated polyelectrolyte complexes (CPECs) in solution is driven by electrostatic interactions between oppositely charged ionic head groups, with potential for π–π interactions between composite chromophores. , Solution-based aggregation through ionic and π–π interactions has been applied to impact CPE photophysical properties for various applications: CPEs have been used extensively for chemo- and biosensing applications based on amplified quenching. ,− CPEs have also been used to control the optoelectronic properties of conjugated materials for organic semiconductors; for example, CPEs were paired with charge-bearing fullerenes to producing uniform electron donor–acceptor architectures at the molecular level in thin films for use in organic photovoltaic devices . More recently, it has been demonstrated that energy donor–acceptor systems can be built with oppositely charged CPEs, , while CPEs and electrolytic electron acceptors can be combined to make complexes that support long-lived photoinduced charge pairs …”

“…The formation of conjugated polyelectrolyte complexes (CPECs) in solution is driven by electrostatic interactions between oppositely charged ionic head groups, with potential for π–π interactions between composite chromophores. , Solution-based aggregation through ionic and π–π interactions has been applied to impact CPE photophysical properties for various applications: CPEs have been used extensively for chemo- and biosensing applications based on amplified quenching. ,− CPEs have also been used to control the optoelectronic properties of conjugated materials for organic semiconductors; for example, CPEs were paired with charge-bearing fullerenes to producing uniform electron donor–acceptor architectures at the molecular level in thin films for use in organic photovoltaic devices . More recently, it has been demonstrated that energy donor–acceptor systems can be built with oppositely charged CPEs, , while CPEs and electrolytic electron acceptors can be combined to make complexes that support long-lived photoinduced charge pairs …”

“…Pairing CPEs and electrolyte chromophores with oppositely charged head groups consequently has great potential for creating ordered, aqueous-soluble complexes for artificial photosynthetic and light-harvesting applications. A very important question is how complexation impacts the microstructures and therefore light-harvesting and energy- or charge-transport properties of CPEs. , Electrolytic polythiophenes are a compelling class of CPEs for investigating this question given the strong connection between polythiophene microstructure and excitonic coupling. ,,,,,,− Recently, Ayzner demonstrated that complexing a poly­(alkylcarboxythiophene) derivative (PTAK), shown in Chart , with an oppositely charged poly­([fluorene]- alt - co -[phenylene]) electrolyte (PFPI) changes the prevalent PTAK excitonic coupling from H- to J-like; complex formation, specifically at high PFPI/PTAK charge ratios, unfurls the PTAK chain, eliminating intra- and interchain PTAK–PTAK interactions and imposing a high degree of structural and conformational regularity within PTAK chains . PFPI emission was observed to be quenched by energy transfer to PTAK in these complexes, while ultrafast spectroscopic measurements revealed very rapid energy-transfer timescales (∼200 fs); both data reveal saturation of PTAK–PFPI interactions at a ∼2:1 PFPI/PTAK charge ratio.…”

Significantly Enhanced Visible‐Light H2 Evolution of Polyfluorene Polyelectrolyte by Anionic Polyelectrolyte Doping

“…This phenomenon suggest that excitation energy transfer had occurred from PFN-Br to the encapsulated PCP-2F-Li, which has been confirmed by similar CPE complexes in recent studies. [30,31] Electrochemical impedance spectroscopy (EIS) was performed to investigate the charge transport of PFN-Br before and after doping with PCP-2F-Li, which resulted in the semicircular Nyquist plots for both PFN-Br and a significantly decreased diameter was observed for polyelectrolyte blends as compared to PFN-Br (Figure 1c). The reduced diameter indicates improved charge transport in the photoelectrodes, which is also confirmed by the enhanced photocurrent response generated on polyelectrolyte blend.…”