Electron Transfer in Conjugated Polymer Electrolyte Complexes: Impact of Donor–Acceptor Interactions on Microstructure, Charge Separation, and Charge Recombination

Abstract

Conjugated polyelectrolyte complexes (CPECs) are an artificial light-harvesting platform formed by pairing oppositely charged conjugated polyelectrolytes in solution. We demonstrate that selective pairing of poly[3-(potassium-4-alkanoate)thiophene-2,5-diyl] (PTAK) of various regioregularity and side-chain lengths with either methyl viologen or an electrolytic naphthalene diimide electron acceptor supports different PTAK microstructures based on specific donor−acceptor stacking relationships. Alteration in microstructure is signaled by distinct signatures of excitonic coupling in steady-state absorption spectra. More ordered PTAK microstructures are obtained in CPECs formed with regioregular PTAK and when the distance between charged groups on the acceptor and PTAK matches. Photoinduced dynamics in these CPECs are characterized by sub-100-fs PTAK-to-acceptor electron transfer with polaron-pair generation in PTAK quenched in higher-order complexes. Rates of subsequent multiphasic charge recombination on picosecond-to-nanosecond timescales are determined by structural characteristics associated with specific donor− acceptor pairings and acceptor-dependent driving force for recombination, with longer-lived charge pairs observed in highly ordered CPEC microstructures. CPECs also demonstrate structure-dependent sensitivity to excitation energy and intensity; excitation energies significantly exceeding PTAK band energy increase exciton delocalization, particularly in complexes with higher structural order. The structure dependence of charge-transfer behaviors in CPECs provides insights for inducing long-range charge separation in related materials for light-harvesting applications.

Energy-Dependent Charge Separation in Conjugated Polymer Electrolyte Complexes

“…In separate work, we examined ultrafast charge generation in CPECs of PTAK with small-molecule electrolytes, including methyl viologen (MV 2+ ) and an electrolytic naphthalene diimide (ENDI). A bathochromic red shift of the PTAK absorption band is observed in small-molecule/polymer complexes, indicating that these pairings afford significant changes in polymer confirmation as well as intermolecular orbital mixing. , Hence, orbital mixing between polymer donors and small-molecule acceptors yields ultrafast charge generation upon excitation of PTAK in these CPECs . Notably, we see an increased efficiency for ultrafast polaron formation in these complexes with increased excitation energy, which we attribute to greater coupling of the local PTAK exciton with the CT excitons at higher energies. ,, Further, we infer from polaron signatures that are formed on sub-ps timescales that the primary photophysical dynamic in the small-molecule/polymer systems is direct charge separation rather than exciton migration, despite the potential for significant exciton delocalization.…”

“…We focus specifically on the photoinduced dynamics of CPECs composed of the energy donor–acceptor pair poly­([fluorene]-alt-co-[phenylene]) (PFPI) and poly­[3-(potassium-4-alkanoate)­thiophene-2,5-diyl] (PTAK), presented in Chart , to provide further insight into charge generation in combination with energy transfer. The shapes and energies of absorption and emission spectra of PTAK in CPECs are determined by the prevalent excitonic coupling between thiophene monomers, providing a window into polymer microstructure. − Hence, CPECs containing PTAK have been valuable for discerning how the character, rates, and efficiencies of photophysical processes depend on polymer microstructure. , Prior work has determined that PFPI:PTAK CPECs form structurally ordered complexes at a 60:40 charge ratio that exhibit extended (ordered) regions of polymer chains and decreased PTAK π-stacking. , We observed that PFPI-to-PTAK Förster Resonant Energy Transfer (FRET) occurs on a timescale of ∼200 fs (or faster) in these complexes . We found that excited PTAK (singlet) persists on longer timescales in complexes at this charge ratio compared to free PTAK.…”

“…Notably, we see an increased efficiency for ultrafast polaron formation in these complexes with increased excitation energy, which we attribute to greater coupling of the local PTAK exciton with the CT excitons at higher energies. ,, Further, we infer from polaron signatures that are formed on sub-ps timescales that the primary photophysical dynamic in the small-molecule/polymer systems is direct charge separation rather than exciton migration, despite the potential for significant exciton delocalization. In contrast, the steady-state spectra of CPECs formed with regioregular butanoate PTAK and PFPI do not exhibit signatures in their steady-state absorption indicating significant charge transfer or orbital interaction, ,, suggesting weaker electronic coupling between polymer strands that should result in slower charge separation dynamics.…”

Methylated precursor leads to carbon nitride (CNx) with improved interfacial interactions for enhanced photocatalytic performance

“…However, the activity appears to be more dependent on the density of trap states rather than on its distribution which points to trap lling playing an important role in the generation of active and mobile charges. 76,77 Next, we focus on the impact of particle size and surface area on photoactivity. Typically, semiconductor materials with smaller particle sizes tend to have large surface area, indicating potential reaction sites and demonstrate high light absorption properties which may favour efficient photoactivity.…”

“…However, the activity appears to be more dependent on the density of trap states rather than on its distribution which points to trap filling playing an important role in the generation of active and mobile charges. 76,77…”