Efficient and stable NIR‐II phosphorescence of metallophilicmolecular oligomers for in vivo single‐cell tracking and time‐resolved imaging

Abstract Molecular phosphorescence in the second near‐infrared window (NIR‐II, 1000–1700 nm) holds promise for deep‐tissue optical imaging with high contrast by overcoming background fluorescence interference. However, achieving bright and stable NIR‐II molecular phosphorescence suitable for biological applications remains a formidable challenge. Herein, we report a new series of symmetric isocyanorhodium(I) complexes that could form oligomers and exhibit bright, long‐lived (7–8 μs) phosphorescence in aqueous solution via metallophilic interaction. Ligand substituents with enhanced dispersion attraction and electron‐donating properties were explored to extend excitation/emission wavelengths and enhanced stability. Further binding the oligomers with fetal bovine serum (FBS) resulted in NIR‐II molecular phosphorescence with high quantum yields (up to 3.93 %) and long‐term stability in biological environments, enabling in vivo tracking of single‐macrophage dynamics and high‐contrast time‐resolved imaging. These results pave the way for the development of highly‐efficient NIR‐II molecular phosphorescence for biomedical applications.