Rhodamine-Functionalized Rhodium(III) Complexes: Dual Role as Bioimaging Reagents and Controllable ROS Photosensitizers for Photocytotoxicity Applications

Cyclometalated rhodium(III) complexes are underexplored as bioimaging agents and photosensitizers, primarily due to the presence of non-emissive low-lying d–d excited states. In this work, a series of rhodamine-containing rhodium(III) complexes was designed and synthesized to circumvent this problem. The incorporation of a rhodamine unit endowed them with effective bioimaging and considerable reactive oxygen species (ROS) sensitization capabilities. Time-resolved transient absorption spectroscopy indicated a long-lived dark triplet state of rhodamine responsible for ROS photosensitization. An energy cascade pathway was proposed for the complexes, involving energy transfer from rhodamine singlet excited state (S₁) to rhodium-based triplet excited state (T₁’), and ultimately transfer to the lowest-lying rhodamine-based triplet excited state (T₁). Through a judicious choice of cyclometalating ligands, the energy cascade efficiency can be modulated to achieve an excellent balance between fluorescence and ROS photosensitization. Furthermore, the complexes specifically accumulated in the mitochondria and effectively induced pyroptosis, indicating their potential as theranostic agents.