Chemistry of Singlet Oxygen with a Cadmium–Sulfur Cluster: Physical Quenching versus Photooxidation

We investigated the chemistry of singlet oxygen with a cadmium–sulfur cluster, (Me4N)2[Cd4(SPh)10]. 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 (Me4N)2[Cd4(SPh)10] is indeed a very strong quencher of singlet oxygen with total rate constants for 1O2 removal of (5.8 ± 1.3) × 108 M–1 s–1 in acetonitrile and (1.2 ± 0.5) × 108 M–1 s–1 in CD3OD. 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) × 106 M–1 s–1 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.