Extended Fused Carbazole-BODIPY, High Brightness NIR Organic Dyes

Fluorescence-based bioimaging enables noninvasive visualization of molecular and cellular processes with high sensitivity and without ionizing radiation. However, conventional fluorophores emitting in the visible or near-red infrared I (NIR-I) (650-800 nm) regions suffer from limited tissue penetration and scattering. Extending fluorescence emission into the deeper NIR region represents a promising strategy to overcome these drawbacks, yet achieving high brightness and stability in organic dyes remains a major challenge. We report an original family of hetero-substituted-fused boron-dipyrromethene (BODIPY) dyes bearing carbazole and thienyl donors that exhibit record brightness and emission maxima up to 852 nm in toluene. The synthetic route combines successive Stille couplings from a 2,6-dibromo-3,5-diiodo-BODIPY precursor and an unprecedented silver(I)-mediated oxidative cyclization, affording high yields and suppressing undesired chlorination. The resulting dyes display intense absorption (ε = 1.8-2.5 × 10⁵ M^{- 1} cm^{- 1}) and exceptional fluorescence quantum yields (Φ up to 0.73). Encapsulation in silica nanoparticles (NPs) preserves their photophysical properties and enables efficient NIR-II in vivo imaging in mice, allowing tumor detection at doses as low as 0.2 nmol with tumor-to-muscle ratios > 4. These fused BODIPY derivatives rank among the brightest NIR fluorophores reported to date and open new avenues for high-contrast deep-tissue imaging and image-guided surgery.