Strategic Establishment of Rhodamine-Decorated Rhodium(III) Complexes as Bioimaging Reagents and Controllable ROS Photosensitizers for Photodynamic Therapy

In this work, a series of rhodamine-containing rhodium(III) complexes exhibited moderate rhodamine fluorescence in solutions under ambient conditions and sensitized a considerable amount of singlet oxygen (1O2) upon photoexcitation. Time-resolved transient absorption (TA) spectroscopic results suggested a long-lived dark rhodamine triplet state as a result of the incorporation of the rhodium(III) center, and this lowest-lying state is responsible for the enhanced reactive oxygen species (ROS) photogeneration ability. An energy cascade pathway from rhodamine S1 to a rhodium-based triplet (T1’), and subsequently to the lowest rhodamine T1 was proposed. Interestingly, live-cell studies revealed that the complexes were localized in the mitochondria, and the photogeneration of ROS triggered a loss of mitochondrial membrane potential, leading to cell death. The complexes exhibited promising photocytotoxicity toward MCF-7 cells, via a combination of type I and type II ROS photosensitization mechanisms. This hybrid rhodamine–rhodium(III) system is anticipated to function as innovative theragnostic agents for both imaging and PDT properties.