Bright Near-Infrared Emitting Organic Nanoparticles Based on Small-Molecule Ionic Isolation Lattices

Near infrared (NIR) fluorescence imaging has received significant attention. However, it remains challenging to develop NIR emitters with high brightness. Herein, we report a supramolecular approach to formulate NIR fluorescent molecular nanoparticles from cationic NIR fluorophores and cyanostar receptors using their charge-by-charge packing in small-molecule, ionic isolation lattices (SMILES). Four types of cationic NIR fluorophores were successfully used to form NIR SMILES nanoparticles. The NIR nanoparticles show fluorescence quantum yields (FQYs) of ~1% and are over 60 times brighter than single fluorophores. Four different surface capping agents were examined and found to generate stable nanoparticles with no change in FQY. Femtosecond transient absorption spectroscopy reveals the low FQY originates from nonradiative decay introduced by inter-fluorophore coupling. Introduction of inert molecular cations into the SMILES structures raises the FQY to ~7%. The NIR SMILES nanoparticles were used for in-vivo tumor imaging and are thus promising for bioimaging applications.