Crucial Role of Trigonal Pyramidal and Rigid Planar Donors in Croconaine Nanoprobe for Tumor NIR-II Fluorescence Imaging and High-Efficiency Phototherapy

Multifunctional organic small molecules with near-infrared-II fluorescence (NIR-II FL) imaging and synergistic type-I photodynamic and photothermal therapy (PDT/PTT) have afforded growing attention on the theranostics of hypoxic and deep-seated tumors. However, the large conjugated structure, poor water solubility, and metabolic toxicity, and incomplete integration of imaging and therapeutic methods are restricting them from being ideal NIR-II phototheranostic agents. Herein, croconaine with benzo[e]indole and thiophene-bis(dimethylaminophenyl)amine (CR-652) has been constructed and synthesized. Thiophene-bis(dimethylaminophenyl)amine as a strong trigonal pyramidal donor with the twisted spatial conformation enhanced the effect of intramolecular charge transfer (ICT), which extended the absorption and emission of CR-652 and enhanced type-I photodynamic activity. The planar and rigid conjugated skeleton of croconaine with benzo[e]indole and the thiophene-bis(dimethylaminophenyl)amine with a strong ICT effect and the twisted spatial conformation resulted in high photothermal conversion efficiency (PCE) through non-radiative relaxation. Connected with the zwitterionic croconaine, the N atoms in two donors of benzo[e]indole and thiophene-bis(dimethylaminophenyl)amine improved the water solubility of CR-652 by hydrogen bonds with water molecules. CR-652 was functionalized with DSPE-PEG₂₀₀₀ and the tumor-targeting peptide and afforded CR-652 NPs. CR-652 NPs were internalized into cells through endocytosis with specific targeting to breast cancer cells by NIR-II FL/photoacoustic imaging. CR-652 NPs exhibited excellent efficiency of O₂^•- generation and a high PCE (82%) under irradiation (808 nm). In preclinical studies, CR-652 NPs exerted synergistic type-I PDT and amplified PTT. Overall, this work offers a promising perspective on the rational design of croconaine-based NIR-II phototheranostic agents for hypoxic-deep tumors by donor-planar and non-planar-engineering.