Succinct croconic acid-based near-infrared functional materials for biomedical applications

Croconic acid (CA), an organic pseudo-oxocarbon molecule with a highly strong electron-withdrawing character, can efficiently react with electron-rich groups and form a donor–acceptor-donor (D-A-D) conjugated structure. CA-based molecules have excellent photobleaching resistance, thermal stability, high molar absorption coefficient, and favorable spectral tunability. Consequently, the development of CA-based functional materials in various biomedical applications is significant for potential clinical transformation. Herein, we thoroughly review the development of CA-based near-infrared (NIR) materials in biomedical fields. Primarily, we introduce the rational structure design of CA molecules for excellent NIR optical properties, and the formation of CA dye-based nanomaterials for better bioavailability and stability. Subsequently, the biomedical applications such as photothermal therapy, photoacoustic imaging, phototheranostics, and multimodal treatment of CA-based functional materials are summarized. Finally, the perspectives and potential drawbacks of CA-based materials for future clinical transformation are systematically discussed.