Palladium-Catalyzed Functionalization of Shortwave Infrared Heptamethine Fluorophores Expands Their In Vivo Utility

Fluorescence imaging in the near-infrared (NIR, 700-1000 nm) and shortwave infrared (SWIR, 1000-2000 nm) regions is advantageous for studying mammals. This work applies palladium-catalyzed coupling methods to functionalize flavylium and chromenylium SWIR polymethine fluorophores, which are challenging substrates due to their small HOMO-LUMO gaps. These chemistries include Suzuki-Miyaura and Sonogashira couplings as well as an unprecedented coupling of alcohol substrates to ultimately achieve a panel of C-C_Ar, C-C_sp, and C-O-alkyl functionalized SWIR fluorescent heptamethine dyes. The photophysical properties of the resulting fluorophores are analyzed against Hammett parameters to produce predictive metrics for absorption maxima. These metrics are strategically applied in the design of laser-matched, SWIR-emissive, chromenylium heptamethine dyes. Added functionalities advance the utility of SWIR fluorophores by increasing brightness in micelle formulations, modulating lipophilicity for alternative delivery vehicles, and enabling bioconjugation to targeting moieties. Ultimately, three functionalized fluorophores are employed in concert to achieve multicolor excitation-multiplexed imaging in murine cancer models.