The design, synthesis, and investigation of new molecular oxygen probes for bioimaging, based on phosphorescent transition metal complexes are among the topical problems of modern chemistry and advanced bioimaging. Three new iridium [Ir(N^C)2(N^N)]+ complexes with cyclometallating 4-(pyridin-2-yl)-benzoic acid derivatives and different di-imine chelate ligands have been synthesized and characterized by mass spectrometry and NMR spectroscopy. The periphery of these complexes is decorated with three relatively small “double-tail” oligo(ethylene glycol) fragments. All these complexes exhibit phosphorescence; their photophysical properties have been thoroughly studied, and quantum chemical calculations of their photophysical properties were also performed. It turned out that the changes in the nature of the di-imine ligand greatly affected the character of the electronic transitions responsible for their emission. Two complexes in this series show the desired photophysical characteristics; they demonstrate appreciable quantum yield (14–15% in degassed aqueous solutions) and a strong response to the changes in oxygen concentration, ca. three-fold increase in emission intensity, and an excited state lifetime upon deaeration of the aqueous solution. The study of their photophysical properties in model biological systems (buffer solutions containing fetal bovine serum—FBS) and cytotoxicity assays (MTT) showed that these complexes satisfy the requirements for application in bioimaging experiments. It was found that these molecular probes are internalized into cultured cancer cells and localized mainly in mitochondria and lysosomes. Phosphorescent lifetime imaging (PLIM) experiments showed that under hypoxic conditions in cells, a 1.5-fold increase in the excitation state lifetime was observed compared to aerated cells, suggesting the applicability of these complexes for the analysis of hypoxia in biological objects.

In this work, we obtained three new phosphorescent iridium complexes (Ir1–Ir3) of general stoichiometry [Ir(N^C)2(N^N)]Cl decorated with oligo(ethylene glycol) fragments to make them water-soluble and biocompatible, as well as to protect them from aggregation with biomolecules such as albumin. The major photophysical characteristics of these phosphorescent complexes are determined by the nature of two cyclometallating ligands (N^C) based on 2-pyridine-benzothiophene, since quantum chemical calculations revealed that the electronic transitions responsible for the excitation and emission are localized mainly at these fragments. However, the use of various diimine ligands (N^N) proved to affect the quantum yield of phosphorescence and allowed for changing the complexes’ sensitivity to oxygen, due to the variations in the steric accessibility of the chromophore center for O2 molecules. It was also found that the N^N ligands made it possible to tune the biocompatibility of the resulting compounds. The wavelengths of the Ir1–Ir3 emission maxima fell in the range of 630–650 nm, the quantum yields reached 17% (Ir1) in a deaerated solution, and sensitivity to molecular oxygen, estimated as the ratio of emission lifetime in deaerated and aerated water solutions, displayed the highest value, 8.2, for Ir1. The obtained complexes featured low toxicity, good water solubility and the absence of a significant effect of biological environment components on the parameters of their emission. Of the studied compounds, Ir1 and Ir2 were chosen for in vitro and in vivo biological experiments to estimate oxygen concentration in cell lines and tumors. These sensors have demonstrated their effectiveness for mapping the distribution of oxygen and for monitoring hypoxia in the biological objects studied.

In this work, we obtained three new phosphorescent iridium complexes (Ir1-Ir3) of general stoichiometry [Ir(N^C)2(N^N)]Cl decorated with oligo(ethylene glycol) fragments to make them water soluble and biocompatible, as well as to protect them from aggregation with biomolecules such as albumin. The major photophysical characteristics of these phosphorescent complexes are determined by the nature of two cyclometallating ligands (N^C) based on 2-pyridine-benzothiophene, since quantum chemical calculations revealed that the electronic transitions responsible for the excitation and emission are localized mainly at these fragments. However, the use of various diimine ligands (N^N) proved to affect the quantum yield of phosphorescence and allows for changing the complexes sensitivity to oxygen, due to the variations in the steric accessibility of the chromophore center for O2 molecules. It was also found that the N^N ligands made possible to tune the biocompatibility of the resulting compounds. The wavelengths of the Ir1-Ir3 emission maxima fall in the range of 630-650 nm, the quantum yields reach 17% (Ir1) in deaerated solution and sensitivity to molecular oxygen, estimated as ratio of emission lifetime in deaerated and aerated water solution, displays the highest value 8.2 for Ir1. The obtained complexes feature low toxicity, good water solubility and the absence of a significant effect of biological environment components on the parameters of their emission. Of the studied compounds Ir1 and Ir2 have been chosen for in vitro and in vivo biological experiments aimed at estimation of oxygen concentration in cell lines and tumors. These sensors have demonstrated their effectiveness for mapping the distribution of oxygen and for monitoring hypoxia in biological objects studied.