A new family of ten dinuclear Ru(II) complexes based on the bis[pyrrolyl Ru(II)] triad scaffold, where two Ru(bpy)<sub>2</sub> centers are separated by a variety of organic linkers, was prepared to evaluate the influence of the organic chromophore on the spectroscopic and in vitro photodynamic therapy (PDT) properties of the compounds. The bis[pyrrolyl Ru(II)] triads absorbed strongly throughout the visible region, with several members having molar extinction coefficients (e) ≥10<sup>4</sup> at 600–620 nm and longer. Phosphorescence quantum yields were generally less than 0.1% and in some cases undetectable. The singlet oxygen quantum yields ranged from 5% to 77% and generally correlated with their photocytotoxicities toward human leukemia (HL-60) cells regardless of the wavelength of light used. Dark cytotoxicities varied ten-fold, with EC<sub>50</sub> values in the range of 10–100 µM and phototherapeutic indices (PIs) as large as 5,400 and 260 with broadband visible (28 J cm<sup>-2</sup>, 7.8 mW cm<sup>-2</sup>) and 625-nm red (100 J cm<sup>-2</sup>, 42 mW cm<sup>-2</sup>) light, respectively. The bis[pyrrolyl Ru(II)] triad with a pyrenyl linker (5h) was especially potent, with an EC50 value of 1 nM and PI >27,000 with visible light and subnanomolar activity with 625-nm light (100 J cm<sup>-2</sup>, 28 mW cm<sup>-2</sup>). The lead compound 5h was also tested in a tumor spheroid assay using the HL60 cell line and exhibited greater photocytotoxcicity in this more resistant model (EC<sub>50</sub>=60 nM and PI>1,200 with 625-nm light) despite a lower dark cytotoxicity. The in vitro PDT effects of 5h extended to bacteria, where submicromolar EC<sub>50</sub> values and PIs >300 against <i>S. mutans</i> and <i>S. aureus </i>were obtained with visible light. This activity was attenuated with 625-nm red light, but PIs were still near 50. The ligand-localized <sup>3</sup>ππ* state contributed by the pyrenyl linker of 5h likely plays a key role in its phototoxic effects toward cancer cells and bacteria.<br><br>
Growth and Acclimation of In Vitro-Propagated M9 Apple Rootstock Plantlets under Various Visible Light Spectrums
