Biocompatible Ir(III) Complexes as Oxygen Sensors for Phosphorescence Lifetime Imaging

Abstract

Synthesis of biocompatible near infrared phosphorescent complexes and their application in bioimaging as triplet oxygen sensors in live systems are still challenging areas of organometallic chemistry. We have designed and synthetized four novel iridium [Ir(N^C)2(N^N)]+ complexes (N^C–benzothienyl-phenanthridine based cyclometalated ligand; N^N–pyridin-phenanthroimidazol diimine chelate), decorated with oligo(ethylene glycol) groups to impart these emitters’ solubility in aqueous media, biocompatibility, and to shield them from interaction with bio-environment. These substances were fully characterized using NMR spectroscopy and ESI mass-spectrometry. The complexes exhibited excitation close to the biological “window of transparency”, NIR emission at 730 nm, and quantum yields up to 12% in water. The compounds with higher degree of the chromophore shielding possess low toxicity, bleaching stability, absence of sensitivity to variations of pH, serum, and complex concentrations. The properties of these probes as oxygen sensors for biological systems have been studied by using phosphorescence lifetime imaging experiments in different cell cultures. The results showed essential lifetime response onto variations in oxygen concentration (2.0–2.3 μs under normoxia and 2.8–3.0 μs under hypoxia conditions) in complete agreement with the calibration curves obtained “in cuvette”. The data obtained indicate that these emitters can be used as semi-quantitative oxygen sensors in biological systems.

Novel NIR-Phosphorescent Ir(III) Complexes: Synthesis, Characterization and Their Exploration as Lifetime-Based O2 Sensors in Living Cells

“…-COOH and -OMe) inserted into the benzothienyl-phenanthridine fragment of the N^C ligands did not perturb the emission energy, evidently due to a large “capacity” of this electronic reservoir. The phosphorescence quantum yields were rather high for the NIR-emitting phosphors (10.3–20.5% in degassed solution) and fit well with the other data obtained earlier for complexes of this type [ 10 , 26 , 27 , 28 , 30 , 31 , 32 , 33 , 34 , 35 ]. Interestingly, the emission efficiency (QY) was nearly the same for complexes 1–6 and sharply different for 7 , increasing for ca.…”

“…However, we were not able to obtain single crystals suitable for X-ray crystallographic analysis. The obtained analytical and spectroscopic data (see Figures S1–S14 , where the prefix S denotes tables and figures presented in the electronic supporting information file) were in complete agreement with the coordination environment typical for octahedral iridium complexes of this type [ 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. In these structural patterns, two coordinated N^C ligands give trans- and cis- dispositions of their N and C functions, respectively, whereas the chelate diimine ligand occupies the two sites at the coordination octahedron, which are trans to the C functions of the metalating ligands.…”

“…Two groups of [Ir( N^C ) 2 ( N^N )] + NIR-emitting orthometalated complexes were prepared using the standard synthetic procedure [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ] shown in Scheme 1 . In the first group of complexes ( 1–4 ), the orthometalating N^C ligand remained unchanged, whereas the nature of the diimine N^N ligand was varied considerably to investigate the effect of its aromatic system on the emission characteristics of the final products.…”

“…From the viewpoint of photophysical measurements, these media also simulated well the sensor microenvironment in cell cultures [ 27 , 28 , 38 , 39 , 40 ], and we carried out Stern-Volmer calibration for 1 and 7 using DMEM-FBS-PEG(200) solutions, as shown in Figure 3 and Table 2 . Quite expectedly, because of the essentially similar nature of the chromophore, both complexes showed nearly equal magnitudes of K SV , which, however, were very different from those observed in methanol solution (cf.…”

“…the lifetime difference for degassed and aerated solutions in aqueous media and methanol, Table 1 and Table 2 ). The reason for such different behavior was the interaction of rather hydrophobic sensor molecules with the components of DMEM/FBS mixtures, of which the bovine serum albumin (BSA) molecules were the most probable candidates for the sensors’ absorption in their hydrophobic pockets, thereby changing the photophysics of the iridium emitters [ 27 , 28 , 41 , 42 , 43 , 44 , 45 , 46 ]. To verify this hypothesis, we performed lifetime measurements for 1 and 7 in aerated and degassed aqueous solutions containing BSA as the only biological molecule (see Table 2 ).…”