Probing Inter- and Intra-chain Excitonic Coupling in Crystalline Polythiophene Nanofibers and Nanoparticles

We summarize some recent new results probing inter- and intra-chain coupling in aggregated P3HT in isolated nanoparticles and nanofibers. Time-resolved photoluminescence studies show interesting correlations between amplitude and decay constant for different decay components that are tied to both polymer regio-regularity and nanoparticle processing conditions. In the frequency domain, we observe distinct signatures of both H- and J-aggregate type exciton coupling, manifested as different vibronic progressions with different electronic origins, linewidths, and Huang-Rhys factors. We show how the extent of this H/J composite coupling can be tuned to a certain extent by changes in molecular parameters (polymer molecular weight and regioregularity) and by solvent processing conditions. Finally we discuss recent results of near-field optical absorption probes of nanoparticles and nanofibers where optical contrast is afforded by the different absorption cross-section (at 532 nm) for aggregated vs. unaggregated P3HT.