Intermolecular Excitonic Coupling in Molecular Aggregates Facilitates Polariton Formation.

J-aggregates of cyanine dyes are optimal organic materials for polariton formation, enabling access to concentrated solution-processable thin films, extreme transition dipole strength due to superradiance, and ultranarrow line widths. In the polaritonic literature, J-aggregates are often viewed as large molecules with narrow line widths. However, we emphasize that this is not the case, as intermolecular excitonic coupling produces a unique band structure that reshapes cavity coupling to the J-aggregate exciton. Here, we elucidate the effects of intermolecular dipolar coupling on polariton formation by comparing cavity coupling of a monomeric and J-aggregated cyanine dye at similar concentrations. The cavities display similar apparent Rabi splitting of 168 ± 4 meV (monomer) and 203 ± 1 meV (J-aggregate), yet possess distinctly different spectra, rendering the monomer intermediately coupled and the J-aggregate strongly coupled. Our results suggest that excitonic coupling facilitates polariton formation (even in highly disordered organic thin films), reinforcing J-aggregates as favorable molecular substrates for strong coupling.