LAUSR

The design of photosensitizers (PSs) with fluorescence in the second near‐infrared (NIR‐II, 1000–1700 nm) window remains a challenge, as the introduction of donor or acceptor units with excessively strong electron‐withdrawing or donating ability leads to longer‐wavelength emission but insufficient production of singlet oxygen (1O2). In this study, a series of acceptor‐donor‐acceptor‐donor‐acceptor‐type PSs are designed by adjusting the steric hindrance of the molecules. Compound BNET forms a dihedral angle of 88° with a nearly vertically twisted backbone to show that the intensity of local emission in the first near‐infrared (750–900 nm) region declines in the aggregated state, while the emission peaks of twisted intramolecular charge transfer span over 1000 nm with significant enhancement. The albumin‐bound NIR‐II PS nanoparticles exhibit efficient 1O2 generation, good photostability and biocompatibility, and negligible dark toxicity. The nanoparticles demonstrate high specific NIR‐II fluorescence imaging of tumor lesions as well as effective image‐guided photodynamic therapy in mice bearing orthotopic colon cancer or pancreatic cancer. The designed NIR‐II PS nanoparticles show great potential for biomedical applications.