Ultrabright 1650 nm-Emitting Biodegradable Organic Luminophores for NIR-IIbc Fluorescence Imaging

Near-infrared IIbc (NIR-IIbc, 1500-2000 nm) fluorescence imaging permits unprecedented resolution for deep-tissue visualization, driving the high demand for organic NIR-IIbc materials due to their tunable optical properties, versatile functionalization, and excellent biocompatibility. However, nearly all existing organic probes emit peaks below 1500 nm in H₂O with emission tails extending only to 1600 nm and raise biosafety concerns due to slow hepatobiliary clearance. These limitations hinder the full potential of NIR-IIbc imaging. Herein, we report a planar ambipolar unit-acceptor design to develop biodegradable organic semiconducting polymer (PTZ) nanoprobes emitting a peak at ∼1650 nm in H₂O. PTZ nanoprobes enable ultrahigh-resolution imaging of near-single-cell cancer (as few as approximately 20 cells), tiny blood vessels (∼100 μm), and ultradeep-seated bone marrow in vivo. Furthermore, the PTZ permits the development of NIR-IIbc molecular probes for ultrahigh-resolution imaging of deep-seated orthotopic liver tumors as small as ∼1.5 mm in vivo. Importantly, PTZ-based probes are biodegradable and can be cleared through hepatobiliary and renal pathways within 18 days, alleviating long-term toxicity concerns. Our work reveals the molecular design toward ultrabright organic NIR-IIbc fluorophores and highlights new opportunities for high-resolution deep-tissue imaging.