Polymethine Thiopyrylium Fluorophores with Absorption beyond 1000 nm for Biological Imaging in the Second Near-Infrared Subwindow

Small-molecule fluorescence
imaging in the second near-infrared
(NIR-II, 1000–1700 nm) window has gained increasing interest
in clinical application. Till now, very few studies have been exploited
in the small-molecule fluorophores with both excitation and emission
in the NIR-II window. Inspired by the indocyanine green structure,
a series of polymethine dyes with both absorption and emission in
the NIR-II window have been developed for NIR-II imaging, providing
the feasibility to directly compare optical imaging in the NIR-IIa
(1300–1400 nm) subwindow under 1064 nm excitation with that
in the NIR-II window under 808 nm excitation. The signal–background
ratio and the tumor–normal tissue ratio achieved great improvement
under 1064 nm excitation in the imaging of mouse blood pool and U87
glioma tumors. Our study not only introduces a broadband emission
fluorophore for both NIR-II and NIR-IIa imaging, but also reveals
the advantages of NIR-II excitation over NIR-I in in vivo imaging.