Fig. 1.

Molecular design and spectra properties of NIR-II BTCs. a Schematic illustration shows the general strategies for tuning absorption wavelength of cyanines: lengthening polymethine chain (n denotes the number of methine unit) leads to bathochromic shift; however, enhancing donor (D) strength by heterocycle modification is a more-effective strategy to afford NIR-II absorbing/emitting cyanines. b, c Chemical structure b and absorption spectra c of IR26 in dichloromethane (DCM), dimethyl sulfoxide (DMSO) and H₂O (phospholipid micelle formation). The ideal state, namely cyanine limit, presenting equalized charge and minimal bond length alternation in a symmetric cyanine structure, is characterized by intense and narrow absorption in nonpolar solvents such as DCM. Polar solvents such as DMSO and H₂O break the symmetry, leading to weak and broad absorption, corresponding to a quenching state called as beyond the cyanine limit. d–f Rational design of pentamethine benzothiopyrylium cyanines (d, BTC980, e, BTC982, and f BTC1070) by shortening polymethine chain (red) and introducing electron-donating diethylamino groups (blue). g–i Intense and sharp absorption spectra of BTC980 g, BTC982 h, and BTC1070 i (5 μm) in dichloromethane, dimethyl sulfoxide, and H₂O (phospholipid micelle formation). j Normalized fluorescence spectra of BTC980, BTC982, and BTC1070 in DCM. The ~ 1150 nm dips are caused by solvent absorption (Supplementary Figure 6). k NIR-II (1000–1700 nm) brightness comparison of equimolar (10 μm) BTC980, BTC982, BTC1070, IR26, and ICG in PBS (pH 7.4) on an InGaAs camera. Inset: fluorescence images of capillaries filled with fluorophores. Color bar ranges from 0 to 30,000 for each image. The detailed imaging parameters for each image are listed in Supplementary Table 1. Source data are provided as a Source Data file. l Photostability comparison of all fluorophores (5 μm) in PBS (pH 7.4) under continuous-wave laser exposure (ICG: 808 nm, BTC980: 915 nm, BTC982: 940 nm, BTC1070 and IR26: 1064 nm) at a fluence rate of 2.3 W cm⁻². The bars in k represent mean ± s.d. derived from n = 3 replicated measurements of every pixel of the capillaries