Abstract
Six-coordinate iridium(III) triarylcorrole derivatives, Ir[TpXPC)]L2, where TpXPC = 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[TpCF3PC)]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.
Highlights
The 5d transition metal corroles represent an unusual class of size-mismatched metal-ligand assemblies, which combine a large 5d ion and a sterically constrained, macrocyclic corrole ligand[1]
A reduction potential could not be observed within the available potential window for the solvent, except for Ir[TpCF3PC]py[2], which showed a reduction potential of -1.71 V, a reflection of the highly electron-rich character of the macrocycle in Ir corroles (Table 1)
The photophysical studies of six-coordinate Ir(III) corroles have been previously reported, only a handful of complexes have been examined to date, all exhibiting weak NIR phosphorescence at room temperature
Summary
The 5d transition metal corroles represent an unusual class of size-mismatched metal-ligand assemblies, which combine a large 5d ion and a sterically constrained, macrocyclic corrole ligand[1]. Iridium corroles were among the first 5d metallocorroles to be synthesized[10] and reported as exhibiting as near-IR phosphorescence at room temperature[4,5]. Au11–14, OsN15, ReO16, and Pt17 corroles were synthesized and found to exhibit significantly stronger phosphorescence[6,7,8,9]. In this reexamination of six-coordinate Ir corroles (structures depicted below), we attempted to determine whether different axial ligands, including pyridine (py), trimethylamine (tma), isoquinoline (isoq), 4-dimethylaminopyridine (dmap), and 4-picolinic acid (4pa), might be exploited to enhance the phosphorescent behavior
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