Lanthanide-Boosted Singlet Oxygen from Diverse Photosensitizers along with Potent Photocatalytic Oxidation

Abstract

Singlet oxygen (¹O₂) plays a central role in photochemical and photobiological research. Although many photosensitizers for efficient ¹O₂ generation were reported, further improving its yield and oxidation power is still highly desirable. Instead of developing new ones, current photosensitizers might be boosted by mediators to facilitate energy transfer. Taking advantage of the long triplet state lifetime of lanthanide ions (Ln³⁺), we herein demonstrate their roles as potent oxidation mediators. Using oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) at neutral pH as a difficult model reaction, based on the fluorescence, phosphorescence, and metal-binding properties, various dyes and nanomaterials were classified into four types. The ¹O₂ emission of carbon dots and rose bengal was enhanced 4 times in the presence of Ce³⁺. Some nonphosphorescent, but strongly fluorescent dyes that are not known as photosensitizers can still be mediated by Ln³⁺ to produce ¹O₂, but metal-chelating calcein was not enhanced. Finally, nonemissive dyes failed to show activity. As mediators, the excited Ln³⁺ can migrate a long distance and transfer energy to O₂, resulting in high ¹O₂ yield. Since redox-active Ce³⁺ and Eu³⁺ had the highest activity, participation of oxidation involving excited lanthanides might be possible too. In addition, Ln³⁺ also enhanced the activity of graphene quantum dots, graphene oxide, and <i>g</i>-C₃N₄. Rapid degradation of organic dyes was demonstrated, further supporting a high photocatalytic activity of the Ln³⁺-mediated system.