Anisotropic Photophysical Properties of Plexcitons in Strongly Coupled Metal–Organic Thin Films

Exciton–plasmon polaritons have gained increasing interest over recent years due to their versatile properties emerging by the underlying light–matter coupling and making them potential candidates for new photonic applications. We have advanced this concept by studying thin films of laterally aligned J-aggregates of self-assembled tetra-bay phenoxy-dendronized perylene bisimide (PBI) molecules, arranged in a helical manner of three slip-stacked strands on a silver surface. As a result of the interaction between the uniformly aligned dipole moments and the surface plasmons of a thin silver layer underneath, the excitonic state at 1.94 eV evolves into dispersions in absorption and emission, both characterized by distinct anisotropy. The coupling constant defined by the scalar product of the transition dipole moment μ⃗ and the surface plasmon wavevector k⃗x shows a pronounced 2-fold rotational symmetry with values between almost 0 and 29.5 meV. Complementary time-dependent density functional theory (TD-DFT) calculations of the angular-dependent absorption and photoluminescence (PL) provide insights in the coherent energy exchange between the excitonic and plasmonic subsystems. Additionally, power-dependent PL studies yield first evidence that the diffusion length of the coupled exciton–plasmon polaritons exceeds that of the mere Frenkel state in neat PBI-based J-aggregates by at least 1 order of magnitude. Our results not only demonstrate the possibility to control the photophysical properties of strongly coupled states by their spatially anisotropic light–matter interaction but also reveal innovative strategies to influence optoelectronic device operation by the directional transport of hybrid-state energy.