Tunable Wavelength, High-Brightness, and NIR-II Emission Phenanthridinium-Based Heptamethine Dyes for High-Contrast Vascular and Intestinal Imaging In Vivo

The second near-infrared window (NIR-II, 900–1700 nm) fluorescence imaging has garnered considerable attention for its deep tissue penetration and high contrast. However, the development of small molecular fluorophores that encompass the NIR-II wavelength, high brightness, and large Stokes shift presents a formidable challenge. Herein, we report a series of phenanthridinium-based fluorophores, named YH dyes, which exhibit high brightness and large Stokes shift. Among these dyes, YH-1c demonstrates the best NIR-II fluorescence quantum yield (2.06% in DMSO) and excellent photostability. As a proof to concept, stunningly rich vascular details in mice were obtained through tail-vein injection of YH-1c NPs, where even under 5-fold magnification, clear visualization of deep abdominal microvessel details down to a diameter of ca. 10 μm was achieved, with a signal-to-background ratio reaching 5.21. This study not only provides a promising tool for high spatiotemporal resolution imaging but also imparts valuable insights for the design of novel NIR-II molecular scaffolds.