Design strategies and biomedical applications of organic NIR-IIb fluorophores

The introduction of fluorescence imaging (FLI) in near-infrared II sub-channels (NIR-IIb, 1500-1700 nm) has revolutionized the ability to explore complex patho-physiological settings in vivo. Despite the transformative potentials, the development of organic NIR IIb dyes encounters considerable difficulties, and only a limited number of such fluorophores have been developed so far. This review systematically introduces design strategies of organic NIR-IIb fluorophores classified by molecular scaffolds, mainly including cyanine dyes and D-A-D small molecule dyes. The design strategies of cyanine dyes involve repurposing of the existing NIR dyes, conjugate reinforcement and regulation of the aggregation state. For D-A-D small molecule dyes, strategies mainly incorporate the extension of the conjugate skeleton, introduction of shielding units, and acceptor/donor engineering. We further describe recent biomedical applications including biomedical imaging and imaging-guided therapy, and conclude by clarifying the current challenges and prospects of NIR-IIb FLI.