Cyclometalated iridium-coumarin ratiometric oxygen sensors: improved signal resolution and tunable dynamic ranges

In this work we introduce a new series of ratiometric oxygen sensors based on phosphorescent cyclometalated iridium centers partnered with organic coumarin fluorophores. Three different cyclometalating ligands and two different pyridyl-containing coumarin types were used to prepare six target complexes with tunable excited-state energies. Three of the complexes display dual emission, with fluorescence arising from the coumarin ligand, and phosphorescence from either the cyclometalated iridium center or the coumarin. These dual-emitting complexes function as ratiometric oxygen sensors, with the phosphorescence quenched under O2 while fluorescence is unaffected. The use of blue-fluorescent coumarins results in good signal resolution between fluorescence and phosphorescence. Moreover, the sensitivity and dynamic range, measured with Stern–Volmer analysis, can be tuned two orders of magnitude by virtue of our ability to synthetically control the triplet excited-state ordering. The complex with cyclometalated iridium 3MLCT phosphorescence operates under hyperoxic conditions, whereas the two complexes with coumarin-centered phosphorescence are sensitive to very low levels of O2 and function as hypoxic sensors.

S3 iHR 320 spectrometer equipped with a Symphony liquid nitrogen cooled InGaAs NIR linear array detector was used to obtain singlet oxygen emission spectra. To get sufficient signal-to-noise, an integration time of 240 seconds (4 minutes) was used and 2 scans were completed for each sample for a total of 480 seconds (8 minutes). A 1000-nm long pass filter was used, and the same path length of 1 cm was also used for every sample. To calculate the singlet oxygen quantum yield (Φ Δ ), the samples were compared to the [Ru(bpy) 3 ] 2+ standard which has a known Φ Δ of 0.57 in DMF. 5 A modified quantum yield expression 6 (Equation S1 below) was used, where ΦΔ S is the quantum yield of the sample, ΦΔr is the quantum yield of the reference, I r and I s are the fractions of incident light absorbed by the reference and sample, respectively, given by 1−10 −A at the excitation wavelength of 365 nm, I Δr and I ΔS are the integrated singlet oxygen emission intensities of the reference and sample, respectively, and τ r and τ s are the singlet oxygen phosphorescence lifetimes in the reference and sample solvents, respectively. The value of in DMF is known to be 12.1 µs 7 , while the value of in CH 2 Cl 2 is known to be 91 µs. 8 Photostability experiments. Iridium-coumarin complexes 3a, 3c, and 5a were dissolved in dichloromethane under aerated conditions. Stock solutions of each complex were further diluted in quartz cuvettes to concentrations of 2.0×10 −6 M. To test the photostability of each complex under UV light, the complexes were irradiated in the fluorimeter directly using 310 nm irradiation in 15-minute increments for a total of 2 hours. In each UV irradiation experiment the monochromator slit widths were opened to 10 nm during irradiation and were set back to 5 nm to collect emission spectra. Since 3c also has absorption in the visible range, photostability tests were likewise performed by irradiation using a glass bowl wrapped with blue LED strips, purchased from Creative Lighting Solutions (Model: Sapphire Blue LED Tape −12vdc), and wrapped on the outside with aluminum foil. To maintain irradiation temperature, the vessel was filled with water. During the experiments involving irradiation with blue LEDs the water bath reached a maximum temperature of 30 ºC. For all experiments, emission spectra were collected by exciting at 310 nm and recording the emission spectrum from 350-700 nm. The same path length of 1 cm was used in each case.
X-ray crystallography details. Single crystals were grown by vapor diffusion or liquid-liquid layering. Crystals were mounted on a Bruker Apex II three-circle diffractometer using MoKα radiation (λ = 0.71073 Å). The data was collected at 123(2) K and was processed and refined within the APEXII software. Structures were solved by intrinsic phasing in SHELXT and refined by standard difference Fourier techniques in the program SHELXL. 9 Hydrogen atoms were placed in calculated positions using the standard riding model and refined isotropically; all non-hydrogen atoms were refined anisotropically. The crystal of 3a was a non-merohedral twin, so for this crystal two unit cell domains were identified in the program CELL_NOW and the data was integrated against both components. The program TWINABS was used to perform the absorption correction, and HKLF5 refinement was performed to refine the structure against both domains. The structure of 4a included a disordered benzene solvent molecule, which was modeled as a two-part disorder. The bond distances and angles in all disordered parts were restrained using SADI commands, and the ellipsoid parameters were restrained with the rigid-bond restraints SIMU and DELU. The structure of 5a included heavily disordered solvent electron density that could not be satisfactorily refined, necessitating the use of the SQUEEZE function in PLATON. 10 Crystallographic details are summarized in Table S1.

Syntheses
Synthesis of coumarin C-1. This compound was prepared as previously described. 11  Synthesis of coumarin C-2. This product was prepared as previously described. 12 Inside the glovebox, a solution containing coumarin-3-carboxylic acid (100 mg, 0.53 mmol), 4-hydroxypyridine (53 mg, 0.56 mmol) and dimethylaminopyridine (6 mg) in anhydrous DCM (100 mL) was combined with EDC•HCl (110 mg, 0.57 mmol) and allowed to stir at room temperature overnight. Afterwards, the mixture was washed with DI water and the organic layer was collected and dried over MgSO 4 , which was then filtered, and the filtrate was dried over vacuum. Inside the glovebox, complex 2a (100 mg, 0.14 mmol) was dissolved in 10 mL of CH 2 Cl 2 and combined with AgPF 6 (34 mg, 0.13 mmol) and 3-(3-pyridyl)coumarin (C-1) (31 mg, 0.14 mmol), which turned into a cloudy yellow suspension. The mixture was stirred overnight at room temperature. Then the reaction mixture was filtered, and the solvent was removed under vacuum to obtain an oily yellow material. The final product was obtained after silica gel column chromatography eluting with CH 2 Cl 2 /Ethyl acetate ( Inside the glovebox, complex 2b (100 mg, 0.13 mmol) was dissolved in 10 mL of CH 2 Cl 2 and combined with AgPF 6 (33 mg, 0.13 mmol) and 3-(3-pyridyl)coumarin (C-1) (30 mg, 0.13 mmol), which turned into a cloudy orange suspension. The mixture was stirred overnight at room temperature, and then filtered and the solvent was removed under vacuum. The crude product was purified by alumina column chromatography using CH 2 Cl 2 /ethyl acetate (1:1) and crystallization from CH 2 Cl 2 /Et 2 O to obtain the final product as orange powder. (Yield: 88 mg, 62%). 1   Inside the glovebox, complex 2a (100 mg, 0.14 mmol) was dissolved in CH 2 Cl 2 (10 mL) and AgPF 6 (34 mg, 0.14 mmol) and C-2 (36 mg, 0.14 mmol) were added. The mixture was stirred at room temperature for two days. The reaction was then filtered, and CH 2 Cl 2 was removed under vacuum. The final product was obtained after silica gel column chromatography using CH 2 Cl 2 /Ethyl acetate (7:3) S41. Photoluminescence spectra of 3c with repeated cycling of N 2 and aerobic atmospheres. 1) The initial N 2saturated sample was prepared in a nitrogen-filled glovebox using deoxygenated CH 2 Cl 2 .
2) The sample was then exposed to air to obtain the aerated spectrum.
3) The aerated sample was deareated with three freeze-pump-thaw cycles on a Schlenk line, and then added back into the cuvette in the glovebox. This process was repeated over a total of three cycles.