Transforming Non-Photosensitizing Fluorophores into ROS Photogenerators via Radical-Promoted Intersystem Crossing
Designing photosensitizers with efficient intersystem crossing (ISC) and long-lived triplet excited states is critical for photodynamic therapy (PDT). However, conventional molecular design principles often rely on heavy-atom effects or specific donor-acceptor architectures, limiting generality. Here, we report a facile and rational strategy to convert intrinsically non-photosensitizing fluorophores into effective reactive oxygen species (ROS) generators by introducing guanidinium substituents. The modified photosensitizers exhibit prolonged triplet excited state lifetimes and considerable ROS production, in stark contrast to unmodified fluorophores which show intense fluorescence and negligible ROS generation. Electron paramagnetic resonance spectroscopy and high-resolution mass spectrometry confirm the formation of stable nitrogen-centered radical cations on the guanidinium moiety, stabilized by p-π conjugation. Mechanistic studies indicate that these radicals promote ISC and prolong triplet state lifetimes. In vitro and in vivo experiments demonstrate that guanidinium-modified photosensitizers induce immunogenic cell death (ICD) and elicit potent anti-tumor immunity. This work provides a universal and facile strategy for designing organic photosensitizers through stable radical cation-containing building blocks, expanding the scope of PDT agents.