A Spiro-Based NIR-II Photosensitizer with Efficient ROS Generation and Thermal Conversion Performances for Imaging-Guided Tumor Theranostics

Organic photosensitizers (PSs) possessing NIR-II emission and photodynamic/photothermal effect have received a great sense of attention for their cutting-edge applications in imaging-guided multimodal phototherapy. However, it is highly challenging to design efficient PSs with high luminescence and phototherapy performance simultaneously. In this study, a spiro-functionalization strategy is proposed to alleviate aggregate-caused quenching of PSs and promote photodynamic therapy, and the strategy is verified via a spiro[fluorine-9,9ʹ-xanthene]-modified NIR-II PS (named SFX-IC) with an acceptor–donor–acceptor configuration. SFX-IC-based nanoparticles (NPs) display a high molar extinction coefficient of 7.05 × 10⁴ m^‒1 cm⁻¹ at 645 nm due to strong intramolecular charge-transfer characteristics. As expected, the as-prepared NPs show strong NIR-II emission with a fluorescence quantum yield of 1.1%, thanks to the spiro-configuration that suppressing excessively intermolecular π–π stacking. Furthermore, SFX-IC NPs not only efficiently generate ¹O₂ and O^∙−₂ under 660 nm laser irradiation, but also possess good photothermal effect with photothermal conversion efficiency of 47.14%. Consequently, SFX-IC NPs can be served as versatile phototheranostic agents for NIR-II fluorescence/photoacoustic imaging-guided phototherapy, manifesting that the spiro-functionalized strategy is a powerful tool to construct efficient NIR-II emitting PSs.