Energy-Dependent Charge Separation in Conjugated Polymer Electrolyte Complexes

Supramolecular complexes have great potential as light-harvestingmaterials, as intermolecular structural organization can be manipulatedthrough steric or electrostatic interactions to impact electroniccoupling between energy and charge donors and acceptors. Here, weexamine the relative rates and efficiencies of charge transfer inconjugated polymer electrolyte complexes (CPECs) based on a polythiopheneelectron donor (PTAK) and polyfluorene electron acceptor (PFPI). TheseCPECs are characterized by ordered polymer microstructures, as evidentfrom spectral signatures of strong excitonic coupling within the PTAKcomponent from steady-state UV-vis absorption spectra. We findthat PTAK polarons are generated within tens of picoseconds througha combination of prompt and delayed charge separation following directphotoexcitation or energy transfer from PFPI. Further, we find thatdecreasing the length of charged PTAK side chains or increasing excitationenergy increases the driving force for electron transfer to increasecharge separation rates and yields for polarons, with the greatestrelative yields observed at excitation energies that initiate PFPI-to-PTAKenergy transfer. Charge separation between components can be rationalizedfrom a canonical Marcus picture, whereby excess vibrational energyeffectively lowers the barrier for PTAK-to-PFPI charge separation.This contrasts with the recently reported ultrafast (<100 fs) chargeseparation in small-molecule/polythiophene electrolyte complexes thatis attributed to strong orbital mixing that gives rise to charge generationvia CT exciton states. These results provide insights into conditionsfor realizing charge separation in concert with energy transfer inCPECs as light-harvesting materials.