Rhodamine-functionalised rhodium( <scp>iii</scp> ) complexes: dual role as bioimaging agents and controllable reactive oxygen species photosensitisers for photocytotoxicity applications

Compared to well-developed iridium(iii) complexes, cyclometallated rhodium(iii) complexes are underexplored as bioimaging reagents and photosensitisers, primarily due to the presence of non-emissive low-lying d-d excited states that limit their photophysical properties. In this work, a series of rhodamine-containing rhodium(iii) complexes [Rh(N^C)₂(bpy-Rho)](PF₆)₂ was designed and synthesised to circumvent this problem. The incorporation of a rhodamine unit into cyclometallated rhodium(iii) complexes endowed them with effective bioimaging and considerable reactive oxygen species (ROS) sensitisation capabilities upon low-energy photoexcitation. Time-resolved transient absorption spectroscopy of the complexes revealed a long-lived dark triplet state of rhodamine that was responsible for the enhanced ROS photosensitisation. An energy cascade pathway was proposed for the complexes, involving energy transfer from a rhodamine singlet excited state (S₁) to a Rh(N^C)₂-based triplet excited state (T₁'), and ultimately to the lowest-lying rhodamine-based triplet excited state (T₁). Through a judicious choice of cyclometallating ligands, the energy cascade efficiency can be modulated to achieve a delicate balance between fluorescence and ROS photosensitisation. Furthermore, the complexes specifically accumulated in the mitochondria and showed excellent photocytotoxicity by inducing pyroptosis, highlighting their potential as theranostic agents for bioimaging and photodynamic therapy.