Six-coordinate iridium(III) triarylcorrole derivatives, Ir[TpXpc)]L 2 , where tpXpc = tris(para-X-phenyl) corrole (X = cf 3 , H, Me, and ocH 3) 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 twofold 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[Tpcf 3 pc)]L 2 derivatives, which were tested for their ability to sensitize singlet oxygen formation, were found to do so efficiently with quantum yields Φ(1 o 2) = 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. 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. Although many of the complexes were initially synthesized as part of curiosity-driven exercises, their photophysical properties now promise a wide range of practical applications 2,3 , as near-IR emitters, as oxygen sensors, and as photosensitizers for photodynamic therapy, dye-sensitized solar cells, and triplet-triplet annihilation upconversion 4-9. 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. Subsequently, Au 11-14 , OsN 15 , ReO 16 , and Pt 17 corroles were synthesized and found to exhibit significantly stronger phosphorescence 6-9. 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.

Interaction of 3,4-diiodopyrrole with 4-trifluoromethylbenzaldeyde under carefully optimized, Lindsey-type conditions at -10 °C has led to the synthesis of the first β-octaiodoporphyrin, H2[I8TpCF3PP]. The free ligand readily yielded Ni, Cu, and Zn complexes, which all proved amenable to single-crystal X-ray structure analyses. The zinc complex Zn[I8TpCF3PP] exhibits the most saddled porphyrin core for any simple porphyrin known to date and a dramatically red-shifted optical spectrum with a Soret maximum at 495 nm.

Although a cis tautomer has long been invoked as an intermediate in porphyrin tautomerism, the first such species was only recently isolated and structurally characterized in the form of a β-heptakis­(trifluoromethyl)-meso-tetraarylporphyrin. Reported herein is the molecular structure of a β-octaiodo-meso-tetraarylporphyrin solvate, which also exhibits a cis tautomeric structure. Both structures implicate two factors as critical to the stabilization of the cis tautomeric forma free-base porphyrin that is naturally strongly saddled on steric grounds and a hydroxylic or amphiprotic solvent that can provide hydrogen-bonded N–H···X-H···N (X = O in both the above examples) straps connecting the central NH groups with the antipodal unprotonated nitrogens. The results raise the prospect that a rational strategy affording cis porphyrin tautomers in a predictable manner may be within reach.

Although rather delicate on account of their propensity to undergo deiodination, β-octaiodoporphyrinoids are of considerable interest as potential precursors to novel β-octasubstituted macrocycles. Presented herein are early results of our efforts to synthesize β-octaiodocorrole derivatives. Oxidative condensation of 3,4-diiodopyrrole and aromatic aldehydes failed to yield free-base octaiodocorroles. Treatment of copper meso -tris( p -cyanophenyl)corrole with N -iodosuccinimide and trifluoroacetic acid over several hours, however, yielded the desired β-octaiodinated product in ∼22% yield. Single-crystal X-ray structure determination of the product revealed a strongly saddled corrole macrocycle with metrical parameters very close to those of analogous Cu octabromocorrole complexes. The compound was also found to exhibit an exceptionally red-shifted Soret maximum (464 nm in dichloromethane), underscoring the remarkable electronic effect of β-octaiodo substitution.

UV−vis spectrophotometric titrations have been carried out on mesotris(o/m/p-aminophenyl)corrole (H 3 [o/m/p-TAPC]) and meso-triphenylcorrole (H 3 [TPC]) in dimethyl sulfoxide with methanesulfonic acid (MSA). Monoprotonation was found to result in hyperporphyrin spectra characterized by new, red-shifted, and intense Q bands. The effect was particularly dramatic for H 3 [p-TAPC] for which the Q band red-shifted from ∼637 nm for the neutral species to 764 nm in the near-IR for H 4 [p-TAPC] + . Upon further protonation, the Q band was found to blue-shift back to 687 nm. A simple explanation of the phenomena has been offered in terms of quinonoid resonance forms.