Conjugated polyelectrolyte complexes (CPECs) are promising aqueous-compatible materials for artificial light-harvesting applications that possess continuous one-dimensional pathways for exciton and charge delocalization and migration. We demonstrate how donor−acceptor composition in complexes of electrolytic polythiophene (PTAK) and naphthalene diimide (ENDI) impacts the microstructure of the polymer and donor−acceptor interaction, with consequences for photoinduced chargepair formation and recombination. PTAK evolves from microstructures with H-like to random-coil to J-like excitonic coupling character with increasing ENDI/PTAK charge ratio, while ENDI exhibits weak J-like coupling at high acceptor densities that reflects ordering along PTAK strands. We observe sub-100-fs charge separation between PTAK and ENDI that implies close donor−acceptor orbital proximity and hot-exciton relaxation via chargetransfer exciton states. Multiphasic recombination is observed and reflects a distribution of charge-pair separation distances that is correlated with degree of exciton delocalization, which can be controlled with CPEC composition. Recombination timescales and the fraction of long-lived charge pairs increase with higher excitation energies, consistent with energy-dependent coupling between hot polymer and delocalized charge-transfer excitons. These results indicate that CPEC structure is characterized by an ordered donor−acceptor interface that could be used to induce long-range charge separation for the benefit of applications in artificial light harvesting and photosynthesis.

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.

F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is used widely as a hole-doping agent in photoresponsive organic semiconducting materials, yet relatively little is known about the photoresponses of the F4TCNQ·– anion generated via doping. Furthermore, there is still relatively little systematic exploration of how the properties of the local material or chemical environment impacts the driving force for generating these charge-transfer complexes. Here we present spectroscopic and photophysical studies of F4TCNQ in charge-transfer complexes (CTCs) with the electron donor N,N′-diphenyl-N-N′-di-p-tolylbenzene-1,4-diamine (MPDA) both in dichloroethane solution and polystyrene matrices. Integer charge transfer (ICT) between donor and acceptor occurs readily in dichloroethane solvent to form F4TCNQ·–:MPDA+ CTCs, due to a ∼150 mV difference in MPDA+/MPDA and F4TCNQ/F4TCNQ·– reduction potentials. Ultrafast spectroscopic studies of the CTC as well as electrochemically generated F4TCNQ·– and MDPA+ reveal that the photoresponses of these CTCs are dominated by that of the dopant anion, including rapid deactivation (800 fs) after excitation to the anion D1 excited state, followed by slower (∼10 ps) vibrational cooling in the anion D0 state. Excitation to the higher-lying D2 state results in a rapid relaxation to the D1 state, in contrast to direct D2 → D0 relaxation previously observed for F4TCNQ·– in the gas phase. CTCs embedded in polystyrene (PS) matrices are observed to lose their integer charge-transfer character upon evaporation of solvent, as evidenced by changes to electronic and vibrational absorption features associated with F4TCNQ·–. This change is attributed to the loss of solvent stabilization of the ion pair formed through the charge-transfer reaction. Ultrafast spectral measurements reveal that the photoresponses of the partial charge-transfer (PCT) species embedded in PS are still highly similar to those of the ICT species and unlike that of neutral F4TCNQ, implying the electronic properties of the PCT state are likewise dominated by properties of the reduced acceptor molecule. We conclude that excitation of ICT or PCT states introduces optical losses for photoresponses of doped organic semiconductor materials due to the large anion absorption cross section and its rapid, dissipative deactivation dynamics.

Supramolecular complexes have great potential as lightharvesting materials, as intermolecular structural organization can be manipulated through steric or electrostatic interactions to impact electronic coupling between energy and charge donors and acceptors. Here, we examine the relative rates and efficiencies of charge transfer in conjugated polymer electrolyte complexes (CPECs) based on a polythiophene electron donor (PTAK) and polyfluorene electron acceptor (PFPI). These CPECs are characterized by ordered polymer microstructures, as evident from spectral signatures of strong excitonic coupling within the PTAK component from steady-state UV−vis absorption spectra. We find that PTAK polarons are generated within tens of picoseconds through a combination of prompt and delayed charge separation following direct photoexcitation or energy transfer from PFPI. Further, we find that decreasing the length of charged PTAK side chains or increasing excitation energy increases the driving force for electron transfer to increase charge separation rates and yields for polarons, with the greatest relative yields observed at excitation energies that initiate PFPI-to-PTAK energy transfer. Charge separation between components can be rationalized from a canonical Marcus picture, whereby excess vibrational energy effectively lowers the barrier for PTAK-to-PFPI charge separation. This contrasts with the recently reported ultrafast (<100 fs) charge separation in small-molecule/polythiophene electrolyte complexes that is attributed to strong orbital mixing that gives rise to charge generation via CT exciton states. These results provide insights into conditions for realizing charge separation in concert with energy transfer in CPECs as light-harvesting materials.