Tuning J-Aggregation Behavior of Fused Ring Acceptor Fluorophore within Nanoparticles for NIR-II Excitable Bioimaging with High Brightness

Fluorescence imaging in the near-infrared-IIb (NIR-IIb, 1500–1700 nm) window offers high signal-to-background ratios (SBRs). However, the development of bright NIR-IIb fluorophores remains challenging due to the trade-off between long-wavelength absorption and brightness. In this study, we present a molecular design strategy that bypasses this limitation by inducing J-aggregation to redshift the absorption while maintaining an optimized bandgap with a high radiative decay rate. A quinoidal thieno[3,4-b]thiophene π-bridge is incorporated to synthesize a fused-ring acceptor fluorophore, CTTIC-4F, affording J-aggregation in encapsulated nanoparticles (NPs) with enhanced brightness. The CTTIC-4F NPs display strongly red-shifted absorption peaked at 1017 nm and an improved fluorescence quantum yield of 0.44% in aqueous solutions, outperforming counterparts with conventional π-bridges. The molecular dynamics simulations indicate compact and spherical aggregates of CTTIC-4F due to strong π–π interactions in aqueous solutions, consistent with J-type packing. In vivo imaging demonstrates that the CTTIC-4F NPs achieve a high SBR of 8.26 in vascular imaging and ultrahigh SBRs for lymph system imaging under the 1064 nm laser excitation, enabling high-contrast NIR-IIb lymph system imaging and image-guided resection of tumor-draining sentinel lymph nodes. These results demonstrate the effectiveness of aggregation-regulated molecular design for NIR-IIb fluorophores.