Molecular Design of Highly Efficient Heavy‐Atom‐Free Near‐Infrared Cyanine Photosensitizers for Fluorescence Image‐Guided Photodynamic Therapy of Solid Tumors

Recent advancements in photodynamic therapy (PDT) have highlighted its potential as a non-invasive cancer treatment. However, to fully realize the clinical potential of PDT, the development of innovative photosensitizers is essential. In this study, efficient heavy-atom-free PSs (IR820-1-3) activated by 808 nm light irradiation were developed by incorporating electron-rich/sterically bulky groups at the meso-position of the IR820 scaffold, a novel indocyanine green derivative. Notably, experimental results and transient absorption spectroscopic studies indicated that substituent bulkiness plays a key role in promoting reactive oxygen species (ROS) generation. IR820-3 PS, with a highly twisted molecular structure, demonstrated superior ROS production via both type I and type II photochemical pathways. The self-assembled nanostructure of IR820-3 underwent partial disassembly upon interaction with albumin, resulting in enhanced fluorescence intensity and photodynamic efficiency. Interestingly, the cationic cyanine backbone facilitated the mitochondria-specific localization of IR820-3, which further contributed to IR820-3's effective PDT performance against cancer cells. Importantly, in vivo findings indicated that the IR820-3 exhibited excellent tumor-targeting ability and induced efficient tumor photoablation under 808 nm NIR irradiation. This study provides insights into the molecular design of a facile, "one-for-all" NIR photosensitizer based on a heptamethine cyanine platform, highlighting its potential for preclinical and clinical applications.