Structure–Activity Relationship of Hemicyanine Molecules: Mitochondrion-Targeting Hemicyanine Fluorescent Molecular Probes for HSO3– Recognition and Near-Infrared Image-Guided Photothermal/Photodynamic Synergetic Therapy

Hemicyanine molecules have unparalleled potential in the fields of fluorescence sensing, bioimaging, and disease therapeutics due to their excellent optical properties, cell penetration, potential mitochondrial targeting, and photosensitivity. Herein, three dual-cation hemicyanine molecular probes named DCy, PDCy, and TDCy were developed. All of them could detect HSO₃^–, and PDCy could recognize HSO₃^– under 365 nm ultraviolet light or sunlight. In addition, TDCy is a multifunctional molecule, which has the following advantages: simple synthesis, red and near-infrared dual-channel mitochondrial imaging, and photothermal/photodynamic synergistic therapy capabilities. Upon analysis of the correlation between the structures of the three hemicyanine molecules and HSO₃^– recognition, photosensitivity, photothermal activity, cell imaging, and cytotoxicity, the structure–activity relationship of the hemicyanine molecules could be summarized, which could provide guidance for subsequent research and development.