Small molecular fluorophores in the second near-infrared window (NIR-II) have aroused much interest because of their excellent performance. Herein, a new small molecular NIR-II fluorophore, FM1210, with maximal emission beyond 1200 nm is reported. Compared to the corresponding control fluorophore CF1065, FM1210 exhibits an increase of 145 nm in the emission maximum, which is ascribed to the simultaneous introduction of both a Se atom and amino groups into the benzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole) skeleton. This large increase in the maximal emission enables FM1210 to be capable of in vivo imaging with lower autofluorescence, higher signal-to-background ratio, and better resolution. Moreover, nanosized FM1210 encapsulated in liposomes possesses passive targeting ability and good water solubility, and is suitable for imaging a tumor and even its vasculature with high signal-to-background ratio.

Fluorescence bioimaging through the second nearinfrared window (NIR-II, 1000−1700 nm) has attracted much attention due to its deep penetration and high contrast. However, exploring new fluorescent materials, especially small molecular fluorophores with long wavelength and high brightness, is still quite challenging. By expanding π-conjugation and enhancing the intramolecular charge transfer effect, herein we report a series of new xanthene-based NIR-II dyes, named VIXs. Among these dyes, VIX-4 exhibits the best performance with fluorescence emission at 1210 nm and high brightness and has been used for dynamically imaging the blood flow of mice at 200 fps. By virtue of high spatiotemporal resolution of the dynamic imaging, we can distinguish directly the artery and vein through the blood flow direction and measure the blood flow volume by the videos. This study provides not only an effective tool for high spatial and temporal resolution bioimaging but also a new and promising conjugated skeleton for NIR-II dyes.