Near-infrared II cyanine fluorophores with large stokes shift engineered by regulating respective absorption and emission

Fluorescence bioimaging in the near-infrared II window is a promising area due to its deep tissue penetration and high contrast. However, efficient design strategies for near-infrared II fluorophores with large Stokes shifts are still scarce. Here, we develop a series of near-infrared II fluorophores (termed VIPIs) with large Stokes shifts (167-260 nm in chloroform) by conjugating p-aminostyryl to hemicyanines. Time dependent density functional theory calculation and transient absorption spectra reveal that the excitation process is predominantly localized within the cyanine moiety, whereas the emission process involves the charge transfer from the cyanine to styryl moiety. We demonstrate the applications of VIPIs in multicolor imaging and conjugatable modification. Finally, we show that VIPI-4 liposomes can image the fine bone structure of knee joint of female mice over 1300 nm. This work provides insights into the excited-state photophysical processes in near-infrared II window, offering inspiration for designing fluorophores with extended emission and large Stokes shifts.