Insight into the mechanism of excited state effects of substituents and donor-acceptor conjugated bridges on highly efficient hemicyanine analogs as photosensitizers in solution.

Traditional cyanine dyes exhibit excellent near-infrared spectral characteristics. However, their ability to generate reactive oxygen species (ROS) and photothermal effects is relatively weak, which greatly limits their potential applications as photosensitizers in cancer treatment. Herein, we selected sulfonamide and triphenylamine derivatives as electron donor groups, as well as 1-ethylbenzene (c, d) as electron acceptor groups. A phorbol ester with a D-π-A (donor-π-acceptor) structure was designed, connected by an ethylene group as a "bridge". As the conjugation degree of the donor substituent increases, the absorption wavelength of the photosensitizer increases from 593 nm to 602 nm, and the emission wavelength increases from 633 nm to 645 nm. At the same time, the spin-orbit coupling constant increases from 0.33 cm-1 to 1.39 cm-1, promoting ISC and improving the efficiency of singlet oxygen production. In addition, the increase in the length of the donor-acceptor bridge extends the absorption wavelength range to 528 nm - 675 nm and the emission wavelength range to 651 nm - 757 nm. This is very beneficial for achieving near-infrared luminescence imaging. In addition, the decrease in excited-state energy polarity from 0.11 eV to 0.01 eV promotes energy transfer between excited states, thereby enhancing the effectiveness of photodynamic therapy and providing a theoretical basis for the development of efficient photosensitizers.