Photothermal therapy (PTT) is a promising approach to cancer treatment. Heptamethine cyanine (Cy7) is an attractive photothermal reagent because of its large molar absorption coefficient, good biocompatibility, and absorption of near-infrared irradiation. However, the photothermal conversion efficiency (PCE) of Cy7 is limited without ingenious excitation-state regulation. In this study, the photothermal conversion ability of Cy7 is efficiently enhanced based on photo-induced electron transfer (PET)-triggered structural deformation. Three Cy7 derivatives, whose Cl is replaced by carbazole, phenoxazine, and phenothiazine at the meso-position (CZ-Cy7, PXZ-Cy7, and PTZ-Cy7), are presented as examples to demonstrate the regulation of the energy release of the excited states. Because the phenothiazine moiety exhibits an obvious PET-induced structural deformation in the excited state, which quenches the fluorescence and inhibits intersystem crossing of S1 →T1 , PTZ-Cy7 exhibits a PCE as high as 77.5%. As a control, only PET occurs in PXZ-Cy7, with a PCE of 43.5%. Furthermore, the PCE of CZ-Cy7 is only 13.0% because there is no PET process. Interestingly, PTZ-Cy7 self-assembles into homogeneous nanoparticles exhibiting passive tumor-targeting properties. This study provides a new strategy for excited-state regulation for photoacoustic imaging-guided PTT with high efficiency.
Keywords: cyanine; near-infrared therapy; photo-induced electron transfer; photothermal therapy; tumor therapy.
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