Near-Infrared Dioxaborine-Containing Polymethines for Light-Activated Anticancer Activity

Polymethine dyes incorporating a dioxaborine ring within the π-chain are heavy-atom-free near-infrared (NIR) fluorophores combining strong absorption, high fluorescence efficiency, and tunable electronic structures. Here, we investigated three dioxaborine-containing polymethines with distinct electronic structures (neutral merocyanine and anionic derivatives) as potential photosensitizers with light-activated cytotoxicity. The dyes exhibit NIR emission and singlet oxygen generation, highlighting their theranostic potential. In the dark, the dyes exhibit low cytotoxicity toward F98 and B16F10 cell lines only at higher concentrations (15 and 20 μM). Upon illumination with a 660 nm LED, cell viability decreased by up to 88% at dye concentrations as low as 1 μM. Flow cytometry and confocal fluorescence microscopy indicated stronger intracellular fluorescence for In33 and M25In compared to B232, with predominantly cytoplasmic localization and no detectable nuclear accumulation. Intracellular reactive oxygen species (ROS) generation was further confirmed using the ROS-sensitive probe H₂DCFDA combined with confocal fluorescence imaging. These results demonstrate that dioxaborine-containing polymethines provide an effective heavy-atom-free platform that integrates NIR fluorescence with potent light-activated anticancer activity.