Alkyl chain elongation engineering: Unlocking high-brightness NIR-II O-xanthene dyes for in vivo bioimaging

NIR-II fluorescent probes are powerful tools for advanced imaging, with performance governed by molar extinction coefficient (ε), quantum yield (Φ), and brightness (ε × Φ). However, achieving high brightness remains a major challenge. Here we report SCU-OX-Hex, a D-A-D probe with an O-xanthene acceptor and an N-alkylated aniline donor. Elongating the N-alkyl chain enhances Φ and brightness while suppressing aggregation-caused quenching. SCU-OX-Hex exhibits ε = 1.71 × 10⁵ M^-1 cm^-1, Φ = 0.57%, and a brightness of 974.7 M^-1 cm^-1-among the highest reported for organic NIR-II probes. In vivo, it enables high-resolution imaging of the abdominal vasculature, the gastrointestinal tract, and tumor angiogenesis. In a lymph node metastasis model, it provides superior contrast and resolution compared with ICG, enabling precise intraoperative identification. This work establishes SCU-OX-Hex as a high-performance NIR-II agent and offers a broadly applicable design strategy for next-generation probes.