Higher-order effects and validity of the point-dipole approximation for conjugated extended molecular emitters near plasmonic nanostructures.

Rapid advancements in nanotechnology have allowed for the characterization of single molecules by placing them in the vicinity of nanoplasmonic structures that are known to confine light to sub-molecular scales. In this study, we introduce a theoretical framework that captures higher-order effects, and we explore the limits of the standard description of a molecular emitter as a point-dipole. We particularly focus on the role played by the emitter chain length and electron conjugation. Strong deviations are observed from the point-dipole approximation, demonstrating that higher-order effects are essential to fully capture the emission rate of extended molecules in the vicinity of nanoparticles. This deviation strongly depends on the orientation of the conjugated chain relative to the nanoplasmonic structure. Finally, we propose a simple rationalization that qualitatively assesses the difference from the point-dipole approximation.