Heteroleptic Ir(III)N₆ Complexes with Long-Lived Triplet Excited States and in Vitro Photobiological Activities

A series of cationic heteroleptic iridium(III) complexes bearing tris-diimine ligands [Ir(phen) 2 (R-phen)] 3+ (R-phen = phenanthroline ( 1 ), 3,8-diphenylphenanthroline ( 2 ), 3,8-dipyrenylphenanthroline ( 3 ), 3-phenylphenanthroline ( 4 ), 3-pyrenylphenanthroline ( 5 ), and 3,8-diphenylethynylphenanthroline ( 6 )) were synthesized and characterized. These complexes possessed phen ligand-localized 1 π,π* transitions below 300 nm, and charge transfer ( 1 CT) and/or 1 π,π* transitions between 300 and 520 nm. In 1, 2, 4, and 6, the low-energy bands were mixed 1 CT/ 1 π,π*. However, the increased π-donating ability of the pyrenyl substituent(s) in 3 and 5 split the low-energy bands into a pyrene-based 1 π,π* transition at 300–380 nm and an intraligand charge transfer ( 1 ILCT) transition at 380–520 nm. All complexes were emissive at room temperature in CH 3 CN, but the parentage of the emitting state varied depending on the R substituent(s). Complex 1 exhibited predominantly phen ligand-localized 3 π,π* emission mixed with metal-to-ligand charge transfer ( 3 MLCT) character, whereas the emission of 2, 4, and 6 was predominantly from the excited-state with 3 π,π*/ 3 ILCT/ 3 MLCT character. The emission from 3 and 5 was dominated by pyrene-based 3 π,π* states mixed with 3 ILCT character. The different natures of the lowest triplet excited states were also reflected by the different spectral features and lifetimes of the triplet transient absorption of these complexes. Complexes 3 and 5 had singlet oxygen quantum yields as high as 81 and 72%, respectively. Both gave submicromolar phototoxicities toward cancer cells (SK-MEL-28 human melanoma) and bacteria ( S. aureus and S. mutans ) with visible-light activation (and marginal to no photobiological activity with red light). Their visible-light phototherapeutic indices (PIs) toward SK-MEL-28 cells were 248 for 3 and >435 for 5; PIs were lower in bacteria (≤62) because of their inherent antimicrobial activities. Both complexes were shown to produce substantial amounts of intracellular reactive oxygen species (ROS), which may account for their photobiological activities.

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