Transforming non-photosensitizing fluorophores into ROS photogenerators via radical-promoted intersystem crossing

Designing photosensitizers with efficient intersystem crossing (ISC) and long-lived triplet excited state is critically essential for photodynamic therapy in biomedical applications. To achieve this goal, exploring new molecule design principles to enhance photosensitization remains an urgent need. Herein, we propose a facile and rational strategy to design a series of guanidinium-modified photosensitizers that exhibit prolonged triplet excited state lifetimes and considerable reactive oxygen species (ROS) production, in contrast to unmodified fluorophores which show intense fluorescence and negligible ROS production. The design strategy is not limited to specific molecular structures, thus demonstrating its generality. Through electron paramagnetic resonance spectroscopy and high-resolution mass spectrometry, we identify stable radicals on guanidinium substitutes that play a pivotal role in converting the intrinsically non-photosensitive fluorophores into effective ROS-generating photosensitizers. Mechanistic studies suggest that the nitrogen-centered radical cation could be stabilized by the p-π conjugation effect of guanidinium, which favors enhanced ISC and prolonged triplet excited state. In vitro and in vivo experiments demonstrate that the guanidinium-modified photosensitizers can elicit anti-tumor immunity by inducing immunogenic cell death, thereby achieving potent anti-tumor effects. Overall, this work provides a new perspective as a universal and facile strategy for designing organic photosensitizers through the introduction of stable radical cation-containing building blocks.