Boron difluoride formazanate dye for high‐efficiency NIR‐II fluorescence imaging‐guided cancer photothermal therapy

The small molecular second near-infrared (NIR-II, 1000–1700 nm) dye-based nanotheranostics can concurrently combine deep-tissue photodiagnosis with in situ phototherapy, which occupies a vital position in the early detection and precise treatment of tumors. However, the development of small molecular NIR-II dyes is still challenging due to the limited electron acceptors and cumbersome synthetic routes. Herein, we report a novel molecular electron acceptor, boron difluoride formazanate (BDF). Based on BDF, a new small molecular NIR-II dye BDF1005 is designed and synthesized with strong NIR-I absorption at 768 nm and bright NIR-II peak emission at 1034 nm. In vitro and in vivo experiments demonstrate that BDF1005-based nanotheranostics can be applied for NIR-II fluorescence imaging-guided photothermal therapy of 4T1 tumor-bearing mice. Under 808 nm laser irradiation, tumor growth can be effectively inhibited. This work opens up a new road for the exploitation of NIR-II small molecular dyes for cancer phototheranostics.