Bright “D–A–D” semiconducting small molecule aggregates for NIR-II fluorescence bioimaging guiding photothermal therapy†

Donor–acceptor–donor (D–A–D) semiconducting small molecule nanoparticles have emerged as high-performance NIR-II fluorophores for real-time bioimaging. However, due to their intrinsic defects in aggregation-caused quenching (ACQ) and “energy gap law”, D–A–D semiconducting small molecule nanoparticles typically exhibit low NIR-II fluorescence quantum yields (QYs). Herein, both the strategies of aggregation induced emission (AIE) and intermolecular charge transfer (CT) have been incorporated into the design of new D–A–D semiconducting small molecules. AIE enhances the NIR-II fluorescence intensity of NIR-II fluorophore aggregates in nanoparticles, while intermolecular CT increases both NIR absorption and NIR-II emission, thereby further improving their NIR-II fluorescence QYs. Four D–A–D semiconducting small molecules (TD, TT, TC, and TCD) were designed. Due to the combination of intermolecular CT and AIE of TCD aggregates, the NIR absorption and NIR-II fluorescence signals of TCD NPs were stronger than those of TD NPs and TT NPs with a single AIE property or TC NPs with strong intermolecular CT. Furthermore, TCD NPs demonstrated excellent performance in in vivo NIR-II fluorescence bioimaging guiding photothermal therapy.