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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.

Metallocorroles involving 5d transition metals are currently of interest as near-IR phosphors and as photosensitizers for oxygen sensing and photodynamic therapy. Their syntheses, however, are often bedeviled by capricious and low-yielding protocols. Against this backdrop, we describe rhenium-imido corroles, a new class of 5d metallocorroles, synthesized simply and in respectable (∼30%) yields via the interaction of a free-base corrole, Re2(CO)10, K2CO3, and aniline in 1,2,4-trichlorobenzene at ∼190 °C in a sealed vial under strict anaerobic conditions. The generality of the method was shown by the synthesis of six derivatives, including those derived from meso-tris(pentafluorophenyl)corrole, H3[TPFPC], and five different meso-tris(p-X-phenyl)corroles, H3[TpXPC], where X = CF3, F, H, CH3, OCH3. Single-crystal X-ray structures obtained for two of the complexes, Re[TpFPC](NPh) and Re[TpCF3PC](NPh), revealed relatively unstrained equatorial Re-N distances of ∼2.00 Å, a ∼ 0.7-Å displacement of the Re from the mean plane of the corrole nitrogens, and an Re-Nimido distance of ∼1.72 Å. Details of the corrole skeletal bond distances, diamagnetic 1H NMR spectra, relatively substituent-independent Soret maxima, and electrochemical HOMO-LUMO gaps of ∼2.2 V all indicated an innocent corrole macrocycle. Surprisingly, unlike several other classes of 5d metallocorroles, the Re-imido complexes proved nonemissive in solution at room temperature and also failed to sensitize singlet oxygen formation, indicating rapid radiationless deactivation of the triplet state, presumably via the rapidly rotating axial phenyl group. By analogy with other metal-oxo and -imido corroles, we remain hopeful that the Re-imido group will prove amenable to further elaboration and thereby contribute to the development of a somewhat challenging area of coordination chemistry.

A series of stable Pt(IV) corrole complexes with the general formula PtIV[TpXPC](m/p-C6H4CN)(py), where TpXPC3- is the trianion of a tris(p-X-phenyl)corrole and X = CF3, H, and CH3, has been synthesized, affording key physicochemical data on a rare and elusive class of metallocorroles. Single-crystal X-ray structures of two of the complexes revealed very short equatorial Pt-N distances of 1.94-1.97 Å, an axial Pt-C distance of ∼2.03 Å, and an axial Pt-N distance of ∼2.22 Å. The complexes exhibit Soret maxima at ∼430 nm, which are essentially independent of the meso-aryl para substituents, and strong Q bands with the most intense peak at 595-599 nm. The substituent-independent Soret maxima are consistent with an innocent PtIV-corrole3- description for the complexes. The low reduction potentials (-1.45 ± 0.08 V vs saturated calomel reference electrode) also support a highly stable Pt(IV) ground state as opposed to a noninnocent corrole•2- description. The reductions, however, are irreversible, which suggests that they involve concomitant cleavage of the Pt-aryl bond. Unlike Pt(IV) porphyrins, two of the complexes, PtIV[TpXPC](m-C6H4CN)(py) (X = CF3 and CH3), were found to exhibit room-temperature near-IR phosphorescence with emission maxima at 813 and 826 nm, respectively. The quantum yield of ∼0.3% is comparable to those observed for six-coordinate Ir(III) corroles.

Oxygen sensing capabilities of a series of luminescent zirconium-based porphyrinic metal–organic frameworks (MOFs) (PCN-222, PCN-223, and PCN-224) have been investigated. Design of experiments allowed identification of the most sensitive material, a PCN-224 MOF. In this material, luminescence in air is quenched by up to 7.3-fold with a bimolecular quenching constant kq of 56,000 Pa–1 s–1, which is significantly above the previously reported benchmark material of the same MOF type (kq of 34,000 Pa–1 s–1). The oxygen sensitivity increases in the order PCN-222 < PCN-223 < PCN-224 with effective Stern–Volmer (SV) constants in the range of 0.13 kPa–1 for PCN-222, 0.18 kPa–1 for PCN-223 and up to 0.45 kPa–1 for PCN-224-type materials. This effect is attributed to the increase in the porphyrin–porphyrin separation in these nanostructured materials that results in the increase in the fluorescence decay time and thus the probability of the quenching event. Defects in the nanostructure also are important and explain variations in the sensitivity within the MOFs of the same type. Among the investigated materials, those belonging to the PCN-223 type were found to be the most robust in terms of oxygen sensing capabilities with only little variation in the SV constants. The analogues of the PCN MOFs that utilize a Pt-(II) porphyrin as a building block were also prepared. It is shown that the same synthetic protocols established for the metal-free systems can be used for the metal complex. Analogous to the MOFs based on the metal-free porphyrin, the oxygen sensitivity increases from Pt-(II)-PCN-222 (KSV 30 kPa–1) via Pt-(II)-PCN-223 (KSV 40 kPa–1) to Pt-(II)-PCN-224 (KSV up to 87 kPa–1). [ABSTRACT FROM AUTHOR]