Energy-Dependent Charge Separation in Conjugated Polymer Electrolyte Complexes

Abstract

Supramolecular complexes have great potential as light-harvesting 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.