Ultrabright Fluorescent Organic Nanoparticles Based on Small‐Molecule Ionic Isolation Lattices**

Abstract Ultrabright fluorescent nanoparticles (NPs) hold great promise for demanding bioimaging applications. Recently, extremely bright molecular crystals of cationic fluorophores were obtained by hierarchical coassembly with cyanostar anion‐receptor complexes. These small‐molecule ionic isolation lattices (SMILES) ensure spatial and electronic isolation to prohibit aggregation quenching of dyes. We report a simple, one‐step supramolecular approach to formulate SMILES materials into NPs. Rhodamine‐based SMILES NPs stabilized by glycol amphiphiles show high fluorescence quantum yield (30 %) and brightness per volume (5000 M −1 cm −1 /nm 3 ) with 400 dye molecules packed into 16‐nm particles, corresponding to a particle absorption coefficient of 4×10 7 M −1 cm −1 . UV excitation of the cyanostar component leads to higher brightness (>6000 M −1 cm −1 / nm 3 ) by energy transfer to rhodamine emitters. Coated NPs stain cells and are thus promising for bioimaging.