Acceptor engineering of semiconducting oligomers for improved NIR-II fluorescence imaging and therapy

Fluorescence imaging (FI) conducted over second near-infrared (NIR-II) window shows superior performance with deeper penetration, higher temporal and spatial resolution than that over the first near-infrared (NIR-I) window. Various attempts have been made to develop ideal fluorophores with high brightness, longer emission wavelength and good biocompatibility for NIR-II FI. Here, we design and synthesize three D-A-D type oligomers through acceptor engineering approach to compare their optical properties. The newly prepared oligomer (O3) based on thiadiazolobenzotriazole (TBZ) acceptor exhibits the highest NIR-II brightness compared with the counterparts because of the weakest intramolecular charge transfer transition. Water-dispersed O3 nanoparticles (NPs) are then fabricated via nanoprecipitation method, which demonstrate superior NIR-II imaging ability for in vivo vascular imaging and tumor-targeted imaging. Furthermore, O3 NPs exhibit significant photothermal performance, thus inducing obvious cancer cell elimination upon 808 nm light irradiation. Taken together, TBZ unit is a promising electron acceptor for constructing bright and functional NIR-II dyes. This acceptor engineering method has offered us a feasible strategy to develop versatile NIR-II fluorophores.