(a) Structures of BF2 formazanate dyes BF1-BF8 in this study. (b) HOMOs and LUMOs calculated for BF1. (c) The normalized absorbance spectra of BF1-BF8 (20 μM) solution in DMSO. Inset shows photos of the solutions. (d) The normalized fluorescence spectra of BF1-BF8 (20 μM) in DMSO. (e) False-colored images of BF1-BF8 at 50 μM collected through long-pass (LP) filters with cut-on wavelengths of 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, and 1400 nm illumination was at 808 nm laser (45 mW cm−2, 250 ms exposure). (f) Stokes shift of BF1 with a concentration of 10 μM in DMSO and FBS. (g) NIR-II fluorescence images of BF1 (10 μM) in deionized water (DI), PBS buffer, and FBS. (h) Stability of BF1 and ICG (10 μM) under 808 nm light radiation within 180 minutes in FBS and DMSO. (i) Fluorescent stability of BF1 (10 μM) in PBS (1% FBS) buffer with different pH and ROS species.