Excitons and polarons play a central role in the electronic and optical properties of semiconducting polymers, molecular assemblies, and protein aggregates. The creation of neutral excitations (excitons) or positive or negative ions (polarons) in organic materials is associated with significant nuclear relaxation, the bulk of which occurs along the vinyl-stretching mode or the aromatic-quinoidal stretching mode when conjugated rings are present.<sup>1</sup> Within the crystalline or aggregated phase, the nuclear relaxation competes with energy and charge transfer mediated by intermolecular interactions; for (neutral) electronic excitations such interactions lead to extended excited states or excitons, while for charged excitations, interactions lead to delocalized polarons. The electronic coupling as well as the local coupling between electronic and nuclear degrees of freedom for both excitons and polarons can be effciently described using a Holstein Multiparticle Approach. However, although excitons and polarons derive from similarly structured Hamiltonians, their optical / infrared signatures are quite distinct, largely due to differing ground states and optical selection rules. In this talk, I will discuss the similarities and differences in the spectral response of excitons and polarons in organic polymers and molecular aggregates. Using a generic linear array of coupled units as a model host for both excitons and polarons in either a single conjugated polymer chain or a small-molecule π-stack, I will compare and contrast the optical response of both quasiparticles, with a particular focus on how simple optical probes like steady-state absorption and photoluminescence can be used to extract the exciton and polaron coherence lengths, quantities which are important for understanding energy and charge transport dynamics in energy materials.

Towards fundamental understanding of excitons and polarons in organic materials

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ACS Fall 2022

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