J‐Aggregate Promoting NIR‐II Emission for Fluorescence/Photoacoustic Imaging‐Guided Phototherapy

Abstract J‐aggregate is a promising strategy to enhance second near‐infrared window (NIR‐II) emission, while the controlled synthesis of J‐aggregated NIR‐II dyes is a huge challenge because of the lack of molecular design principle. Herein, bulk spiro[fluorene‐9,9′‐xanthene] functionalized benzobisthiadiazole‐based NIR‐II dyes (named BSFX‐BBT and OSFX‐BBT) are synthesized with different alkyl chains. The weak repulsion interaction between the donor and acceptor units and the S…N secondary interactions make the dyes to adopt a co‐planar molecular conformation and display a peak absorption >880 nm in solution. Importantly, BSFX‐BBT can form a desiring J‐aggregate in the condensed state, and femtosecond transient absorption spectra reveal that the excited states of J‐aggregate are the radiative states, and J‐aggregate can facilitate stimulated emission. Consequently, the J‐aggregated nanoparticles (NPs) display a peak emission at 1124 nm with a high relative quantum yield of 0.81%. The efficient NIR‐II emission, good photothermal effect, and biocompatibility make the J‐aggregated NPs demonstrate efficient antitumor efficacy via fluorescence/photoacoustic imaging‐guided phototherapy. The paradigm illustrates that tuning the aggregate states of NIR‐II dye via spiro‐functionalized strategy is an effective approach to enhance photo‐theranostic performance.