Chemistry of Singlet Oxygen with a Cadmium–Sulfur Cluster: Physical Quenching versus Photooxidation CaganDavid A. GarciaArman C. LiKin Ashen-GarryDavid TadleAbegail C. ZhangDong NelmsKatherine J. LiuYangyang ShallenbergerJeffrey R. StapletonJoshua J. SelkeMatthias 2018 We investigated the chemistry of singlet oxygen with a cadmium–sulfur cluster, (Me<sub>4</sub>N)<sub>2</sub>[Cd<sub>4</sub>(SPh)<sub>10</sub>]. This cluster was used as a model for cadmium–sulfur nanoparticles. Such nanoparticles are often used in conjunction with photosensitizers (for singlet oxygen generation or dye-sensitized solar cells), and hence, it is important to determine if cadmium–sulfur moieties physically quench and/or chemically react with singlet oxygen. We found that (Me<sub>4</sub>N)<sub>2</sub>[Cd<sub>4</sub>(SPh)<sub>10</sub>] is indeed a very strong quencher of singlet oxygen with total rate constants for <sup>1</sup>O<sub>2</sub> removal of (5.8 ± 1.3) × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> in acetonitrile and (1.2 ± 0.5) × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup> in CD<sub>3</sub>OD. Physical quenching predominates, but chemical reaction leading to decomposition of the cluster and formation of sulfinate is also significant, with a rate constant of (4.1 ± 0.6) × 10<sup>6</sup> M<sup>–1</sup> s<sup>–1</sup> in methanol. Commercially available cadmium–sulfur quantum dots (“lumidots”) show similar singlet oxygen quenching rate constants, based on the molar concentration of the quantum dots.