Generation of Singlet Oxygen by Photoexcited Au₂₅(SR)₁₈ Clusters

The generation of highly reactive singlet oxygen (¹O₂) is of major importance for a variety of applications such as photodynamic therapy (PDT) for cancer treatment, water treatment, catalytic oxidation, and others. Herein, we demonstrate that ¹O₂ can be efficiently produced through the direct photosensitization by Au₂₅(SR)₁₈^– clusters (H−SR = phenylethanethiol or captopril) without using conventional organic photosensitizers under visible/near-IR (532, 650, and 808 nm) irradiation. ¹O₂ was successfully detected by direct observation of the characteristic ¹O₂ emission around 1276 nm as well as three different ¹O₂-selective probes. Water-soluble Au₂₅(captopril)₁₈^– clusters were explored for cytocompatibility and photodynamic activity toward cancer cells. In addition, selective catalytic oxidation of organic sulfide to sulfoxide by ¹O₂ was demonstrated on the photoexcited Au₂₅(SC₂H₄Ph)₁₈^– clusters. It is suggested that the optical gap of Au₂₅(SR)₁₈ clusters (∼1.3 eV) being larger than the energy of ¹O₂ (0.97 eV) allows for the efficient energy transfer to ³O₂. In addition, the long lifetime of the electronic excited states of Au₂₅(SR)₁₈ and the well-defined O₂ adsorption sites are the key factors that promote energy transfer from Au₂₅(SR)₁₈^– to molecular oxygen, thus facilitating the formation of ¹O₂. Finally, neutral Au₂₅(SR)₁₈⁰ can also produce ¹O₂ as efficiently as does the anionic Au₂₅(SR)₁₈⁻.