Unlocking the Potential of Ru(II) Dual‐Action Compounds with the Power of the Heavy Atom Effect

We report the synthesis, photochemical and biological characterization of two new Ru(II) photo-activated complexes based on [Ru(tpy)(Me2bpy)(L)]2+ (tpy = 2,2':6',2''-terpyridine, Me2bpy = 6,6'-dimethyl-2,2'-bipyridine), where L= pyridyl-BODIPY (pyBOD). Two pyBOD ligands were prepared bearing flanking hydrogen or iodine atoms. Ru(II)-bound BODIPY dyes show a red-shift of absorption maxima relative to the free dyes and undergo photodissociation of BODIPY ligands with green light irradiation. Addition of iodine into the BODIPY ligand facilitates intersystem crossing (ISC), which leads to efficient singlet oxygen production in the free dye, but also enhances quantum yield of release of the BODIPY ligand from Ru(II). This represents the first report of a strategy to enhance photodissociation quantum yields through the heavy atom effect in Ru(II) complexes. Furthermore, Ru(II)-bound BODIPY dyes display fluorescence turn-on once released, with a lead analog showing nanomolar EC50 values against triple negative breast cancer cells, >100-fold phototherapeutic indexes under green light irradiation, and higher selectivity towards cancer cells as compared to normal cells than the corresponding free BODIPY photosensitizer. Conventional Ru(II) photoactivated complexes require non-biorthogonal blue light for activation and rarely show sub-micromolar potency to achieve cell death. Our study represents an avenue for the improved photochemistry and potency of future Ru(II) complexes.

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