Abstract:

To elucidate the effect of precise sulfur-atom positioning on the performance of organic photothermal agents, this work compares two conjugated donor-acceptor （D-A） small molecules, S-1 and S-2, which differ only in the position of a single sulfur atom in the thiophene [3, 4-b] unit. Theoretical calculations revealed that compared to S-1, the altered sulfur atom position in S-2 reduced the dihedral angle, increased molecular planarity, and decreased the HOMO-LUMO gap （1.90 eV for S-2 and 2.10 eV for S-1）. Optical characterization showed that S-2 exhibited a maximum absorption peak at 770 nm in tetrahydrofuran, which was red-shifted to 780 nm when it was prepared as nanoparticles （S-2 NPs）. Photothermal performance tests demonstrated that S-2 NPs achieved a 52% photothermal conversion efficiency under 808 nm laser irradiation, surpassing the 44% of S-1 NPs. In antibacterial experiments, under laser irradiation, S-2 NPs at 12 μmol/L achieved a 99.1% bacterial killing rate against Staphylococcus aureus （43.8% for S-1 NPs）, 15 μmol/L S-2 NPs reached 99.9% against Escherichia coli （43.4% for S-1 NPs）, and 20 μmol/L S-2 NPs attained 96.0% against Candida albicans （83.8% for S-1 NPs）, effectively eliminating Staphylococcus aureus biofilms （survival rate was only 16% at 40 μmol/L）. Cell experiments indicated that S-2 NPs maintained over 80% L929 cell viability at 40 μmol/L. Thus, S-2 NPs are considered a highly promising photothermal agent. The study demonstrates that precise control over individual sulfur atom positions is an effective strategy for enhancing photothermal conversion efficiency and antibacterial performance in organic photothermal agents. With superior photothermal antibacterial properties and excellent biocompatibility, S-2 NPs can be incorporated into cosmetic face creams, lotions, or facial masks to impart photothermal antibacterial functionality, offering a novel strategy for extending product shelf life and improving skin health.

Key words: organic photothermal agent, sulfur atom, energy gap, photothermal conversion efficiency, antibacterial, cosmetics