Molecular Aggregation Induced Photoacoustics for NIR-II in vivo Imaging

Molecular aggregation induced photo-properties alteration has been found to play a crucial role in the light induced processes, such as aggregation induced emission (AIE) and J aggregation induced dramatic absorption red shift. The light induced acoustic process (photoacoustic) is also considered to be one of the most essential characters of the light absorbing molecules. However, to the best of our knowledge, the molecular aggregation induced photoacoustic effect (MAIPA) has never been reported. Herein, we report the first MAIPA effect for which the PA intensity is dominated by the molecular aggregation, rather than by absorbance as usual concerned. Molecular aggregation induces a strong electronic coupling effect, resulting in significant absorption suppression from the individual state to highly aggregated state (around 5 molecules aggregated). However, the corresponding PA efficiency was found to be about 2-orders of magnitude greater for the latter. A well-behaved linear correlation between the molecular aggregation level and MAIPA effect was observed. The surprisingly significant MAIPA effect was realized via novel NIR-II squaraine-benzothiopyrylium dyes. Excellent photophysical properties of the novel NIR-II dyes were achieved, such as large absorption extinction coefficient and high photostability. Thanks to the relatively narrow FWHM and the high PA efficiency of SQN2@PMAOPEG and ZC825@BSA, in vivo multiplex PA imaging was demonstrated for tumor tissue and macrophage cells, blood and lymphoid vessels. ### Competing Interest Statement The authors have declared no competing interest.