Boosting fluorescence efficiency of NIR-II dyes for multifunctional fluorescence imaging via hydrogen bonding†

Donor–acceptor–donor (D–A–D) type fluorophores with a planar conformation hold great promise for second near-infrared (NIR-II) fluorescence imaging due to their enhanced light absorption and red-shifted absorption/emission peaks. However, achieving high fluorescence efficiency remains challenging because of severe fluorescence quenching in the aggregate state. Herein, five 6,7-diphenyl-[1,2,5]thiadiazoloquinoxaline-based NIR-II dyes (TP-TQ1, TP-OH, TP-OMe, TP-F and TP-Acr) were synthesized by modifying the acceptor core with various substituents to create planar π-conjugated D–A–D structures. We systematically investigated the influence of the substituent effect on the dye's band gap, molecular conformation, absorption/emission wavelengths, fluorescence efficiency, and aggregation behaviors. The results indicate that hydroxyl-substituted TP-OH nanoparticles (NPs) possess the highest NIR absorption ability and fluorescence brightness. This is attributed to intermolecular hydrogen bonding, which effectively suppresses π–π stacking. Furthermore, the steric hindrance of substituents plays a prominent role in limiting the intramolecular potential energy. In vivo experiments demonstrated the potential of TP-OH NPs as NIR-II fluorescent contrast agents for gastrointestinal tract imaging, vascular imaging, and navigation of lymph node dissection. These findings suggest that hydrogen bonding functionalization on the acceptor offers a valuable strategy for significantly enhancing the NIR-II fluorescence performance of planarized D–A–D fluorophores.