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Abstract Generally, long wavelength absorbed near‐infrared II (NIR‐II) dyes have a low fluorescence efficiency in aggregate states for aggregate‐caused quenching effect, simultaneously enhancing efficiency and extending absorption is a challenging issue for NIR‐II dyes. Here, three benzo[1,2‐c:4,5‐c']bis[1,2,5]thiadiazole (BBT) derivatives (TPA‐BBT, FT‐BBT, and BTBT‐BBT) are used to clarify fluorescence quenching mechanisms. When the BBT derivatives are doped into a small molecule matrix, they show quite different fluorescence behaviors. Structure‐distorted TPA‐BBT displays fluorescence quenching originating from short‐range exchange interaction, while FT‐BBT and BTBT‐BBT with a co‐planar‐conjugated backbone exhibit concentration‐dependent quenching processes, namely changing from long‐range dipole‐dipole interaction to exchange interaction, which could be majorly ascribed to large spectral overlap between absorption and emission. By precisely tuning doping concentration, both FT‐BBT and BTBT‐BBT nanoparticles (NPs) present the optimal NIR‐II fluorescence brightness at ∼2.5 wt% doping concentration. The doped NPs have good biocompatibility and could be served as fluorescence contrast agents for vascular imaging with a high resolution under 980‐nm laser excitation. Those paradigms evidence that molecular doping can promote fluorescence efficiency of long wavelength‐absorbed NIR‐II fluorophores via suppressing long‐range energy migration. |
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