Organic photosensitizers are versatile chromophores capable of absorbing light and transferring energy to surrounding molecules or mediating photochemical transformations. Relative to widely used inorganic photocatalysts, a key advantage of organic systems is the ability to finely tune their photophysical properties through rational molecular design. Photosensitizers are central to photodynamic therapy (PDT),¹,² an emerging, non-invasive clinical modality for the treatment of malignant tissues and pathogenic bacterial infections that offers high spatiotemporal control with reduced side effects.³
Increasing attention has been directed toward 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-derived photosensitizers owing to their excellent photostability and synthetic versatility.⁴ Established strategies to enhance the performance of BODIPY photosensitizers⁵ focus on improving intersystem crossing (ISC) and tailoring solubility, targeting, or emission properties, most commonly through heavy-atom substitution to increase spin–orbit coupling.⁶ An alternative and conceptually distinct approach to promoting ISC and accessing long-lived triplet excited states—thereby enhancing singlet oxygen generation—relies on spin–spin interactions between a photoexcited organic chromophore and a stable radical.⁷ Herein, we report the synthesis and evaluation of nitroxide-functionalized azaBODIPY photosensitizers for potential application in PDT.