A Stability-oriented heavy-atom-free Nile red photosensitizer via heterocyclic thionation and D-A intensification for ferro-pyroptosis photodynamic immunotherapy.

Nile Red derivatives typically suffer from limited intersystem crossing (ISC) efficiency in photodynamic therapy. Conventional ISC-enhancement strategies, such as heavy-atom incorporation and carbonyl thionation, often compromise increased dark toxicity, shortened triplet lifetimes, or poor photostability. Herein, we report a heavy-atom-free molecular design strategy for Nile Red-based photosensitizers (PSs) that overcomes these limitations. The optimized PSs, NRS-N-Me, synergistically integrates heterocyclic sulfur incorporation with donor-acceptor (D-A) system intensification. Sulfur is introduced into the heterocyclic N-O framework, enhancing spin-orbit coupling (SOC) through orbital mixing while avoiding labile thione motifs. Concurrently, replacement of the native naphthol unit with a methylated quinolinium acceptor strengthens the intramolecular donor-acceptor (D-A) push-pull effect. This dual engineering minimizes singlet-triplet energy gap (ΔEST<0.1 eV) and enables cascade ISC via higher triplet states (T2/T3), affording a high singlet oxygen quantum yield of 25.4% with red-shifted absorption. Mitochondrial accumulation of cationic NRS-N-Me enables localized Type I/II ROS generation, triggering synergistic ferro-pyroptosis cell death that overcomes apoptosis resistance and elicits potent immunogenic cell death (ICD). In vivo, liposomal NRS-N-Me achieves 90.09% tumor inhibition and suppresses metastasis in a 4T1 tumor model. This work establishes a stability-oriented, heavy-atom-free design paradigm for Nile Red-based PSs by integrating D-A optimization with heterocyclic N-O framework thionation, which offers a general platform for next-generation photodynamic immunotherapy.