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, (MeN)[Cd(SPh)]. 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 (MeN)[Cd(SPh)] is indeed a very strong quencher of singlet oxygen with total rate constants for O removal of (5.8 ± 1.3) × 10 M s in acetonitrile and (1.2 ± 0.5) × 10 M s in CDOD. 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 M s 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.