Promoting near-infrared II fluorescence efficiency by blocking long-range energy migration

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.