Iridium Corroles Exhibit Weak Near-Infrared Phosphorescence but Efficiently Sensitize Singlet Oxygen Formation

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.

www.nature.com/scientificreports www.nature.com/scientificreports/ near-iR phosphorescence. For the final set of measurements, we chose to focus on the TpCF 3 PC derivatives but with an expanded set of axial ligands (L) including py, tma, isoquinoline (isoq), 4-dimethylaminopyridine (dmap), and 4-picolinic acid (4pa). All the complexes were found to exhibit weak phosphorescence in the near-infrared part of the spectrum in anoxic solutions (Fig. 3a). The phosphorescence was almost completely quenched in the presence of molecular oxygen. The excitation spectra (Fig. 3b) matched the absorption spectra very well. As expected, the emission spectra were much narrower at 77 K (Fig. 3c), which enabled more precise determination of the triplet state energies from the edge of the emission spectra. Disappointingly, the  phosphorescence quantum yields for all the complexes turned out to be very low, which made their precise estimation difficult. Quantum yields of around 0.02 -0.04% were estimated at ambient temperature with about a two-fold increase at 77 K. The phosphorescence of the bis-pyridine and bis-isoquinoline complexes was found to be stronger than that of bis-tma and bis-dmap complexes. Furthermore, the emission spectra for the pyridine, isoquinoline, and 4-picolinic acid complexes turned out almost identical (Fig. 3a, Table 3). The decay time profiles for these complexes are mono-exponential (Fig. 4). The fits provide very similar values of ~5 µs at 295 K and ~10 µs at 77 K. Interestingly, in the case of the bis-dmap complex, a relatively long-decaying component was observed both at ambient temperature and at 77 K.
Singlet oxygen sensitization. Many metalloporphyrins and related compounds are known to be powerful sensitizers of singlet oxygen owing to efficient intersystem crossing and long triplet state lifetimes [3][4][5][6][7][8][9] . The fact that Ir corroles exhibit room temperature phosphorescence indicates that the triplet state is populated to at least some degree. In this study, the Ir-TpCF 3 PC complexes with py and 4pa axial ligands were evaluated for their 1 O 2 sensitization capabilities. The assay relied on 9,10-dimethylanthracene as a singlet oxygen acceptor 18 . Methylene blue, which exhibits a quantum yield for 1 O 2 formation [Φ( 1 O 2 )] of 0.48 and is spectrally compatible with the corroles, was used as the reference 7 . Fig. 5 shows that Ir(III) corroles efficiently sensitize the formation of singlet oxygen. In fact, Φ( 1 O 2 ) was found to be 0.71 and 0.38 for L = py and 4pa, respectively. The lower value for 4pa correlates with the lower phosphorescence quantum yield for the same complex (Table 3). These Φ( 1 O 2 ) values are several fold higher than those reported for Ir(III) tris(4-cyanophenyl)corrole derivatives, which might reflect more efficient radiationless deactivation of the triplet states of the latter (which exhibit phosphorescence quantum yields of 0.01%) 5 . Efficient sensitization of 1 O 2 by Ir(III) complexes is of great interest from the standpoint of photodynamic therapy [19][20][21] owing to the co-occurrence of two valuable properties: (i) a long excitation wavelength that enables deeper light penetration and (ii) high flexibility in the choice of axial ligands that should facilitate attachment of tumor markers in the axial positions. Covalent attachment via 4-picolinic acid represents a simple, potential synthetic approach for the latter.  www.nature.com/scientificreports www.nature.com/scientificreports/ conclusion Although 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. Several additional complexes were accordingly synthesized and examined herein, with a view to identifying axial ligands most conducive to NIR phosphorescence. Unfortunately, regardless of the triarylcorrole and the axial ligands (which varied over pyridine, trimethylamine, isoquinoline, 4-dimethylaminopyridine, and 4-picolinic acid), the phosphorescence quantum yield turned out to be very low, estimated at around 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[TpCF 3 PC)]L 2 derivatives were found to efficiently sensitize singlet oxygen formation, 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. The possibility of varying the axial ligand and of attaching tumor-targeting groups at the axial positions makes iridium corroles particularly exciting as drug candidates.

experimental Section
Materials. Unless otherwise mentioned, all chemicals were obtained from Merck. Silica gel 60 (0.04-0.063 mm particle size, 230-400 mesh) was employed for flash chromatography. Silica gel 60 preparative thin-layer chromatographic plates (20 cm ×20 cm, 0.5 mm thick, Merck) and aluminum oxide 60 preparative thin-layer chromatographic plates (20 cm × 20 cm, 1.5 mm thick, Merck) were used for final purification of all complexes. Free-base corroles were prepared according to previously reported procedures 22,23 . instrumental methods. The methods used were essentially the same as in our earlier work 11,13,15,16 .
UV − visible spectra were recorded on an HP 8453 spectrophotometer. 1 H NMR spectra were recorded on a 400 MHz Bruker Avance III HD spectrometer equipped with a 5 mm BB/1H SmartProbe in CDCl 3 and referenced to residual CHCl 3 7.26 ppm, C 6 D 6 and referenced to residual C 6 H 6 7.16 ppm, C 3 D 6 O and referenced to residual C 3 H 6 O 2.05 ppm or CD 3 OD and referenced to residual CH 3 OH 3.31 ppm. High-resolution electrospray-ionization (HR-ESI) mass spectra were recorded from methanolic solution on an LTQ Orbitrap XL spectrometer.
Cyclic voltammetry was performed at 298 K using an EG&G model 263 A potentiostat with a three-electrode system, comprising a glassy carbon working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE), in CH 2 Cl 2 (distilled from CaH 2 ) as solvent (as in earlier work 11,13,15,16 ). The reference electrode was separated from the bulk solution by a fritted-glass bridge filled with the electrolyte solution. The electrolyte solution was purged with argon for several minutes, and electrochemical measurements were conducted under an argon blanket. All potentials are referenced to the SCE.
The phosphorescence of the Ir(III) corroles was studied on a Fluorolog 3 fluorescence spectrometer from Horiba (Japan) equipped with an NIR-sensitive photomultiplier R2658 from Hamamatsu (Japan). The spectra were corrected for the sensitivity of the photomultiplier and smoothing processing (adjusting averaging function) was applied to eliminate noise due to low signals. For measurements at room temperature, dye solutions in a sealable quartz cell (Hellma Analytics, Mülheim, Germany) were deoxygenated by bubbling nitrogen (purity 99.9999%, Linde gas, Austria) for 15 min. Measurements at 77 K were conducted in toluene:tetrahydrofuran (4:6 v/v) or ethanol/methanol (4:1 v/v) frozen glass using accessories for deep-temperature measurements from Horiba. Luminescence decay times were measured on the Fluorolog 3 spectrometer equipped with a DeltaHub module (Horiba Scientific) controlling a SpectraLED-460 light source and using DAS-6 Analysis software for data analysis. www.nature.com/scientificreports www.nature.com/scientificreports/ Singlet oxygen generation was studied as previously described 7 . Briefly, a stirred solution containing 9,10-dimethylanthracene (0.2 mM) as a singlet oxygen acceptor and a sensitizer (concentration adjusted to achieve identical absorption for all the sensitizers at λ ex ) was irradiated with light from the xenon lamp of the Fluorolog spectrometer (λ ex 575 nm). The degradation of the acceptor was assessed via measurement of the UV-vis spectra. , cH 3 , H, cf 3 , L = tma, py, dmap,  4pa, isoq). The iridium complexes were prepared according to a previously reported procedure 10 with slight modifications. Bis(1,5-cyclooctadiene)diiridium(I) dichloride (Merck, 2 eq) and potassium carbonate (10 eq.) were dissolved in a solution of the free-base corrole (~0.1 mmol, 1 eq) in anhydrous tetrahydrofuran (150 mL). After degassing with argon for a few minutes, the solution was brought to reflux under argon. After 1.5 h, a reagent corresponding to the axial ligand (15 eq) was added all at once and the solution was left to reach room temperature (1 h). For L = tma, the reagent used was trimethylamine N-oxide; in the other cases, the unmodifed ligand was used. The reaction mixture was then rotary-evaporated to dryness. Unless otherwise mentioned, the residue was dissolved in a small amount of dichloromethane and subjected to column chromatography (silica, 1:1 CH 2 Cl 2 :hexanes) followed by preparative thin-layer chromatography (PTLC, silica, 1:1 CH 2 Cl 2 :pentane). Additional details of purification and characterization for each new compound are given below. As in earlier studies of Ir corroles 5,10 , accurate elemental analyses could not be consistently obtained; proof of composition and purity was accordingly obtained via thin-layer chromatography, clean high-resolution mass spectra that matched theoretical simulations, and, in three cases, single-crystal X-ray structure determinations.