Achieving Ultrabright NIR-II Nanofluorophore for In Vivo Imaging by Inhibiting H-Aggregates Formation

Small molecules with an acceptor-donor-acceptor (A-D-A) structure, featuring a fused-ring core as the donor and two electron-withdrawing end groups as acceptor units, represent a potential option for NIR-II fluorophores, benefiting from their narrow bandgaps, superior light-harvesting capabilities, and exceptional photostabilities. However, their planar conformations predispose them to forming H-aggregates during self-assembly, leading to significantly reduced fluorescence quantum yield (QY) of the resulting nanofluorophores. Herein, we report a small molecule, PF8CN, with a terminal unit-A-D-A-terminal unit structure. The terminal units of 3,5-bis(octyloxy)phenyl group result in a twisted conformation for PF8CN, preventing face-to-face stacking and thereby inhibiting the formation of H-aggregates. Consequently, the NIR-II fluorescence QY of PF8CN NPs is 3.8 times that of the model nanofluorophore (F8CN NPs), which contains a substantial amount of H-aggregates. The NIR-II brightness of PF8CN NPs is 5.3- and 14.9-times that of F8CN NPs and ICG/FBS, respectively, at an equal molar concentration. Such ultrahigh NIR-II brightness of PF8CN NPs allows us to perform long-term and real-time NIR-II fluorescence imaging of cerebral and hindlimb vessels, as well as the thrombolytic process. This work provides an effective method for producing nanofluorophores with ultrahigh NIR-II brightness, positioning PF8CN NPs as a strong contender in the field of NIR-II nanofluorophores.