Molecular Electronic Coupling-Induced Photoacoustics for NIR-I/II Duplex in Vivo Imaging

Molecular electronic coupling has been found to play a crucial role in the light-induced processes of natural and artificial systems. The light-induced acoustic process (photoacoustic) is also considered to be one of the most essential characters of the light-absorbing molecules. Herein, we report an interesting observation of the molecular electronic coupling-induced photoacoustic effect (MECPA) in which the PA intensity is dominated by the molecular electronic coupling, rather than the absorption intensity as usually concerned. The molecular electronic coupling effect results in a significant exciton absorption suppression from the monomer to pentamer. However, the corresponding PA efficiency was found to be approximately 2 orders of magnitude greater for the latter. A well-behaved linear correlation between the molecular exciton absorbance and PA efficiency was observed. The surprisingly significant MECPA effect was realized via novel NIR-II squaraine-benzothiopyrylium dyes. Excellent photophysical properties of the novel NIR-II dyes were achieved, such as a large absorption extinction coefficient and high photostability. In vivo duplex PA imaging was demonstrated for tumor tissue and macrophage cells and blood and lymphoid vessels. The MECPA effect would open a new window for the design of PA contrast agent systems for in vivo imaging.