Bis-Cyclometallated Ir(III) Complexes as
New Singlet Oxygen Sensitizers 

In solution, singlet oxygen is usually prepared by using various sensitizers (a dye or pigment, a quinone or an aromatic compound, examples are Methylene Blue, Rose Bengal, C₆₀). The excited sensitizer, usually in the triplet state, reacts with triplet oxygen to give singlet oxygen.  Some organometallics are also photosensitizers for singlet dioxygen, but overall, there has been relatively little work in this area. We are examining the photophysical properties of the bis(cyclometallated) Ir(III) and Pt(II) complexes prepared by the group of Prof. Thompson at the University of Southern California. We have found that these complexes are excellent sensitizers for the production of singlet oxygen. The bis-cyclometallated Ir(III) complexes  (BT)₂Ir(acac), (BSN)₂Ir(acac), and (PQ)₂Ir(acac); BT = 2-phenylbenzothiazole, BSN = 2-(1-naphthyl)benzothiazole, PQ = 2-phenylquinoline, and acac = acetylacetonate) studied are depicted below.  Complexes with acetylacetonate ancillary ligands give singlet oxygen quantum yields near unity (F_D = (0.7 - 1.0) ± 0.1)., whether exciting the ligand-based state or the lowest energy excited state (MLCT + ³LC). Similar complexes were prepared with amino acids (glycine) or pyridine tethered to the Ir(III) center (i.e. BSN-G, and (BT-py; G =glycine, and py = pyridine).

One disadvantage for use of these complexes in photodynamic therapy is the fact that they generally absorb from the near UV range to about 520 – 560 nm. This is not an ideal range as far as application for photodynamic therapy is concerned. Human skin has an absorption minimum near 800 nm. There are no good sensitizers that absorb near 800 nm, despite decades of efforts to prepare such sensitizers. Addition of suitable substituents to the ligands may allow significant red-shifting of the absorption maxima of these sensitizers, thereby making them interesting candidates for photodynamic therapy.