Silicon Flavylium Polymethine Dyes for Shortwave Infrared Imaging

The shortwave infrared (SWIR, 1000-2000 nm) region offers significant advantages for in vivo fluorescence imaging, including reduced scattering, minimal autofluorescence, and deep-tissue penetration. However, the development of small-molecule fluorophores with high brightness and maximum emission above 1100 nm remains a key challenge. Here, we report a new class of silicon-containing flavylium (SiliFlav) polymethine dyes that address this limitation. By incorporating a silicon heteroatom into the established Flav scaffold, we achieve unprecedented bathochromic shifts in emission up to 200 nm, while maintaining respectable fluorescence quantum yields (up to 0.30%). A modular synthetic route enables diverse functionalization at the 1-, 2-, and 7-positions of the heterocycle, allowing for systematic tuning of photophysical properties and improved dye performance. The lead fluorophore, SiliFlav5, formulated into canola oil nanoemulsions, enables bright SWIR emission above 1300 nm. In vivo imaging in mice demonstrates high-resolution visualization of the vasculature and spleen, confirming the utility of SiliFlav polymethine dyes as a new platform for deep-tissue SWIR imaging.