Ultrafast fluorescence spectroscopy unravels efficient exciton transport within dye-loaded organic nanoparticles

In this work, we utilize time-resolved fluorescence spectroscopy to investigate the exciton diffusion properties of polymeric, organic nanoparticles loaded with fluorescent dyes, which mimic the role of natural light-harvesting complexes found in photosynthetic organisms. We employ polarization-resolved fluorescence up-conversion spectroscopy to track the kinetics of fluorescence anisotropy decay, unravelling the timescales of homo- Exciton Energy Transfer (EET). Additionally, we employ photoluminescence spectroscopy to study the fluence-dependent population decay kinetics, uncovering the Singlet-Singlet exciton Annihilation (SSA) mechanism. Moreover, we explore the population kinetics of donor dyes when co-encapsulated with a fluorescent acceptor at low concentrations within the ONPs. From the measured parameters, we deduce a diffusion constant of ~0.5 nm^2/ps, resulting in a diffusion length as large as 70 nm, i.e. twice as large as the ONP diameter.