Bright, Color-Tunable Coumarino[3,4-b]BODIPY Fluorophores in the Visible-Near-Infrared Region for Lipid Droplets Imaging

Lipid droplets are now understood to be dynamic signaling hubs, with their morphological and polarity changes linked to obesity and related metabolic disorders. However, visualizing these processes is challenging, as most of existing probes suffer from short emission, small Stokes shifts, or poor photostability. Herein, we report a new family of color-tunable coumarino[3,4- b ]BODIPY dyes (C[ b ]B) that function as high-performance probes for lipid droplet imaging in the visible to near-infrared (NIR) region. The key to their properties lies in the fusion of a coumarin donor to the BODIPY acceptor core, creating a strong intramolecular charge-transfer (ICT) system. These dyes display high molar absorptivity (logε > 4.8), and bright, tunable emission (Φ F up to 95%) across the visible-near-infrared region. Peripheral donor tailoring narrows the HOMO–LUMO gap by raising the HOMO (−5.80 → −4.90 eV) while the LUMO remains fixed near −3.55 eV, compressing the electrochemical gap from 2.20 to 1.35 eV and red-shifting absorption from 531 to 711 nm. X-ray crystallography reveals nearly planar cores that switch to twisted conformations (up to 18°) upon styryl extension. Applied in live HeLa cells, the probes selectively stain lipid droplets with low cytotoxicity across the visible-NIR spectrum.