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 1π,π* transitions below 300 nm, and charge transfer (1CT) and/or 1π,π* transitions between 300 and 520 nm. In 1, 2, 4, and 6, the low-energy bands were mixed 1CT/1π,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 1π,π* transition at 300–380 nm and an intraligand charge transfer (1ILCT) transition at 380–520 nm. All complexes were emissive at room temperature in CH3CN, but the parentage of the emitting state varied depending on the R substituent(s). Complex 1 exhibited predominantly phen ligand-localized 3π,π* emission mixed with metal-to-ligand charge transfer (3MLCT) character, whereas the emission of 2, 4, and 6 was predominantly from the excited-state with 3π,π*/3ILCT/3MLCT character. The emission from 3 and 5 was dominated by pyrene-based 3π,π* states mixed with 3ILCT 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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