Iridium corroles exhibit weak near-infrared phosphorescence but efficiently sensitize singlet oxygen formation

Abstract

Six-coordinate iridium(III) triarylcorrole derivatives, Ir[T<i>p</i>XPC)]L₂, where T<i>p</i>XPC = tris(para-X-phenyl)corrole (X = CF₃, H, Me, and OCH₃) and L = pyridine (py), trimethylamine (tma), isoquinoline (isoq), 4-dimethylaminopyridine (dmap), and 4-picolinic acid (4pa), have been examined, with a view to identifying axial ligands most conducive to near-infrared phosphorescence. Disappointingly, the phosphorescence quantum yield invariably turned out to be very low, about 0.02 – 0.04% at ambient temperature, with about a two-fold increase at 77 K. Phosphorescence decay times were found to be around ~5 µs at 295 K and ~10 µs at 77 K. Fortunately, two of the Ir[T<i>p</i>CF₃PC)]L₂ derivatives, which were tested for their ability to sensitize singlet oxygen formation, were found to do so efficiently with quantum yields Φ(¹O₂) = 0.71 and 0.38 for L = py and 4pa, respectively. Iridium corroles thus may hold promise as photosensitizers in photodynamic therapy (PDT). The possibility of varying the axial ligand and of attaching biotargeting groups at the axial positions makes iridium corroles particularly exciting as PDT drug candidates.