Boosting Non‐Radiative Decay of Boron Difluoride Formazanate Dendrimers for NIR‐II Photothermal Theranostics

The development of small molecular dyes excitable in the second near‐infrared window (NIR‐II, 1000~1700 nm) is crucial for deep‐tissue penetration and maximum permissible exposure in cancer photothermal theranostics. Herein, we employed a dendrimer engineering strategy to develop the boron difluoride formazanate (BDF) dye BDF‐8OMe for photoacoustic imaging‐mediated NIR‐II photothermal therapy. BDF‐8OMe, characterized by an increased molecular branching degree and extended π‐conjugation, exhibited broad absorbance peaked at 905 nm, with the absorption tail extending to 1300 nm. Additionally, reorganization energy calculation, molecular dynamics simulation, and femtosecond transient absorption spectroscopy demonstrated that the multiple identical dendritic units of BDF‐8OMe significantly enhanced the molecular motions, enabling the nanoparticles (NPs) to rapidly release 94.4% of the excited state energy through non‐radiative decay at a rate of 11.7 ps. Under 1064 nm photoirradiation, BDF‐8OMe NPs achieved a high photothermal conversion efficiency of 62.5%, facilitating NIR‐II photothermal theranostics. This work highlights the potential of the dendrimer‐building strategy in developing NIR‐II excitable small molecular dyes for efficient photothermal theranostics.