Biotin‐Engineered Phenothiazine Photosensitizer Boosts Type I Photodynamic Therapy via Singlet‐Excited‐State Electron Transfer

Comprehensive Summary Type II‐dominant photosensitizers often suffer from reduced photodynamic efficacy under tumor hypoxia. We report a biotin‐modified phenothiazine photosensitizer termed DHU‐Blue‐7, which converts type II methylene blue (MB)‐like photosensitization into a type I pathway via regulating excited‐state characteristics, while retaining its visible‐to‐near‐infrared absorption. The incorporated biotin moieties serve as both tumor‐targeting groups for cellular internalization and pendant electron donors. Under light irradiation, DHU‐Blue‐7 produced both 1 O 2 and O 2 •–, showing a 3.3‐fold stronger fluorescence response associated with O 2 •– generation than that of MB. DFT/TD‐DFT calculations and femtosecond transient absorption spectroscopy revealed that the folded conformation of DHU‐Blue‐7 enables singlet‐excited‐state photoinduced electron transfer from biotin moieties to the phenothiazine core, forming a relatively long‐lived charge‐separated state that favors type‐I reactive oxygen species (ROS) generation. At the cellular level, DHU‐Blue‐7 achieves efficient light‐activated elimination of cancer cells under both normoxic and hypoxic environments, while exhibiting negligible phototoxicity to normal cells. In the 4T1 tumor‐bearing mouse model, DHU‐Blue‐7 coupled with light irradiation markedly suppressed tumor proliferation, with no evident damage to major organs. This study verifies that biotin modification serves as a facile yet effective strategy to equip phenothiazine photosensitizers with hypoxia‐resistant type I photodynamic performance through conformation‐governed singlet‐excited‐state electron transfer.