Generation of Singlet Oxygen by Photoexcited Au<sub>25</sub>(SR)<sub>18</sub> Clusters

The generation of highly reactive singlet oxygen (<sup>1</sup>O<sub>2</sub>) 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 <sup>1</sup>O<sub>2</sub> can be efficiently produced through the direct photosensitization by Au<sub>25</sub>(SR)<sub>18</sub><sup>–</sup> clusters (H−SR = phenylethanethiol or captopril) without using conventional organic photosensitizers under visible/near-IR (532, 650, and 808 nm) irradiation. <sup>1</sup>O<sub>2</sub> was successfully detected by direct observation of the characteristic <sup>1</sup>O<sub>2</sub> emission around 1276 nm as well as three different <sup>1</sup>O<sub>2</sub>-selective probes. Water-soluble Au<sub>25</sub>(captopril)<sub>18</sub><sup>–</sup> clusters were explored for cytocompatibility and photodynamic activity toward cancer cells. In addition, selective catalytic oxidation of organic sulfide to sulfoxide by <sup>1</sup>O<sub>2</sub> was demonstrated on the photoexcited Au<sub>25</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>18</sub><sup>–</sup> clusters. It is suggested that the optical gap of Au<sub>25</sub>(SR)<sub>18</sub> clusters (∼1.3 eV) being larger than the energy of <sup>1</sup>O<sub>2</sub> (0.97 eV) allows for the efficient energy transfer to <sup>3</sup>O<sub>2</sub>. In addition, the long lifetime of the electronic excited states of Au<sub>25</sub>(SR)<sub>18</sub> and the well-defined O<sub>2</sub> adsorption sites are the key factors that promote energy transfer from Au<sub>25</sub>(SR)<sub>18</sub><sup>–</sup> to molecular oxygen, thus facilitating the formation of <sup>1</sup>O<sub>2</sub>. Finally, neutral Au<sub>25</sub>(SR)<sub>18</sub><sup>0</sup> can also produce <sup>1</sup>O<sub>2</sub> as efficiently as does the anionic Au<sub>25</sub>(SR)<sub>18</sub><sup>−</sup>.