Single‐Atom C‐to‐N Editing Unlocks Bright NIR‐II J‐Aggregates for Imaging Lymphatic Metastasis

The development of near-infrared II (NIR-II) J-aggregates is hindered by a lack of rational design guidelines. Here we show that single-atom C-to-N skeletal editing in hemicyanine dyes precisely regulates intramolecular charge distribution and intermolecular packing, thereby allowing efficient formation of bright NIR-II J-aggregates. Systematic acceptor engineering reveals clear structure-assembly-property relationships governing J-aggregate formation and fluorescence brightness. The optimized dye HZOE9 forms stable J-aggregates with emission beyond 1000 nm and a quantum yield surpassing most reported cyanine-based NIR-II fluorophores. Mechanistic studies further elucidate that phenolic hydroxyl modification can tune J-aggregation and NIR-II emission, highlighting its key role in modulating supramolecular excitonic coupling. Encapsulation into biocompatible nanoparticles enables high-resolution lymphatic imaging, real-time metastasis visualization, fluorescence-guided surgery, and therapeutic monitoring in vivo. This work establishes a single-atom skeletal editing strategy for developing hemicyanine-based NIR-II fluorophores and provides a conceptual framework for the future development of activatable NIR-II fluorophores via controllable supramolecular assembly.