Polyion Charge Ratio Determines Transition between Bright and Dark Excitons in Donor/Acceptor-Conjugated Polyelectrolyte Complexes

There is substantial urgency to create artificial light-harvesting systems that are relatively inexpensive and capable of absorbing a significant fraction of the solar spectrum. Molecular materials possess a number of attractive characteristics for this purpose, such as their light weight, spectral tunability, and the potential to use self-assembly to form large structures capable of executing multiple photophysical processes required for photoelectric energy conversion. In this work, we demonstrate that ionically assembled complexes composed of oppositely charged conjugated polyelectrolytes (CPEs) that function as excitonic donor/acceptor pairs possess 10 significant potential as artificial energy transfer antennae. We find that, upon complexation in water, excitation energy is transferred from the donor to the acceptor CPE in less than 250 fs-a timescale that is competitive with natural light-harvesting antennae. We further find that the state of CPE chain extension and thus spatial delocalization of the excited-state wavefunction can be readily manipulated using the relative polyion charge ratio, allowing us to tune the emission quantum yield of the CPE in a straight-forward manner. Collectively, our results point toward the fact that the extension of a CPE chain upon complexation is a cooperative phenomenon between multiple chains even at dilute polymer concentrations.