We study exciton coupling and interconversion between neutral and charged states of different spin in pi-stacked
conjugated polymer aggregates. Rigorous self-assembly approaches are used to prepare aggregate nanofibers that permit
reliable control of polymer chain conformational and packing (intra- and interchain) order within these structures.
Exciton coupling can be tuned between the H- and J-aggregate limits, which has important implications for determining
the fates of excitons and polarons. Single molecule intensity modulation spectroscopy was performed on individual
nanofibers and large quenching depths of emissive singlet excitons by triplets are found in J-aggregate type structures.
We propose that high intrachain order leads to exciton delocalization that effectively lowers singlet-triplet energy
splittings thus increasing triplet yields. Exciton-polaron and polaron-polaron interactions are next investigated in both
H- and J-type nanofibers where polarons are injected by charge transfer doping. We find that the enhanced intrachain
order of J-aggregates enables efficient intrachain polaron transport and leads to significantly larger doping efficiencies
than less ordered H-aggregates. As polaron densities increase, signatures of spin-spin interactions between polarons on
adjacent chains become appreciable leading to the formation of a spinless bipolaron. Overall, these studies demonstrate
the potential for controlling and directing exciton and polaron interactions via tuning of subtle intra- and interchain
ordering characteristics of aggregates, which could benefit various polymeric optoelectronic applications.
John K. Grey, Alan K. Thomas, and Jian Gao, "Exciton and polaron interactions in self-assembled conjugated polymer aggregates," Proc. SPIE 9549, Physical Chemistry of Interfaces and Nanomaterials XIV, 95490V (Presented at SPIE Nanoscience + Engineering: August 11, 2015; Published: 20 August 2015); https://doi.org/10.1117/12.2190229.
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