Water-soluble chromenylium dyes for shortwave infrared imaging in mice

In vivo imaging using shortwave infrared (SWIR) light (1,000–2,000 nm) benefits from deeper penetration and higher resolution compared with using visible and near-infrared wavelengths. However, the development of biocompatible SWIR contrast agents remains challenging. Despite recent advancements, small-molecule SWIR fluorophores are often hindered by their significant hydrophobicity. We report a platform for generating a panel of soluble and functional dyes for SWIR imaging by late-stage functionalization of a versatile fluorophore intermediate, affording water-soluble dyes with bright SWIR fluorescence in serum. Specifically, a tetra-sulfonate derivative enables clear video-rate imaging of vasculature with only 0.05 nmol dye, and a tetra-ammonium dye shows strong cellular retention for tracking of tumor growth. Additionally, incorporation of phosphonate functionality enables imaging of bones in awake mice. This modular design provides insights into facile derivatization of existing SWIR fluorophores to introduce both solubility and bioactivity toward in vivo bioimaging.