Subcellular Aggregation of Photosensitizers Overrides Reactive Oxygen Species Generation in Photodynamic Therapy Effects

The efficacy of photodynamic therapy (PDT) is conventionally correlated with the total yield of reactive oxygen species (ROS). However, we demonstrate here that the subcellular location of ROS generation is a more critical determinant of therapeutic outcome than its absolute quantity. Using two tailored aggregation‐induced emission photosensitizers (PSs)—hydrophobic Me‐PyTPA (targeting endoplasmic reticulum, ER) and amphiphilic Sulfo‐PyTPA (anchored to the cell membrane)—we show that despite its lower ROS yield, Me‐PyTPA induces significantly higher phototoxicity. Precise ER localization enables Me‐PyTPA to trigger rapid apoptosis via ER stress, even under low‐dose white‐light irradiation. In contrast, the higher‐yielding but membrane‐restricted Sulfo‐PyTPA is less effective. Cell imaging and molecular dynamics simulations reveal that hydrophobicity dictates deep membrane penetration and ER accumulation. This work establishes a “location‐over‐yield” principle for PDT, advocating for the design of organelle‐specific PSs to maximize therapeutic efficiency.