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We investigate electronic excitation energy transport within 40-nm organic nanoparticles (ONPs) loaded with cationic rhodamine dyes (R18). Bulky counterions act as spacers which prevent dyes aggregation and quenching, up to dye concentrations as large a 300 mM - i.e. ~1 nm inter-dye distance - and similar to the effective dye concentration in natural, photosynthetic light-harvesting antennas. In this disordered, rigid solution of dyes, the inter-dye interaction remains relatively weak suggesting an incoherent energy hoping mechanism within the dyes.
We implement femtosecond fluorescence up-conversion spectroscopy to monitor the ultrafast fluorescence anisotropy decay of the ONPs dispersed in water solution. Hoping times as fast as 90 fs are obtained. Alternatively, we use a streak camera to monitor the nanoparticles fluorescence decay kinetics due to exciton-exciton annihilation (1) or to energy transfer to a few energy acceptors (traps) embedded within the ONPs. All three types of experiments provide indirect measurements of the exciton diffusion constant and consistently reveal a diffusion length of 80 to 120 nm - i.e. longer than the particles’ diameter. These results rationalize the remarkable “antenna effect” already reported for the nanoparticles, which are applied to bio-sensing and single molecule detection.(2)
(1) Gharbi, A. M.; Biswas, D. S.; Crégut, O.; Malý, P.; Didier, P.; Klymchenko, A.; Léonard, J. Exciton Annihilation and Diffusion Length in Disordered Multichromophoric Nanoparticles. Nanoscale 2024, 16 (24), 11550–11563. https://doi.org/10.1039/D4NR00325J.
(2) Trofymchuk, K.; Reisch, A.; Didier, P.; Fras, F.; Gilliot, P.; Mely, Y.; Klymchenko, A. S. Giant Light-Harvesting Nanoantenna for Single-Molecule Detection in Ambient Light. Nature Photonics 2017, 11 (10), 657–663. https://doi.org/10.1038/s41566-017-0001-7.
