Abstract

Herein we report four [Ir(N^C)2(L^L)]n+, n = 0,1 complexes (1–4) containing cyclometallated N^C ligand (N^CH = 1-phenyl-2-(4-(pyridin-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole) and various bidentate L^L ligands (picolinic acid (1), 2,2′-bipyridine (2), [2,2′-bipyridine]-4,4′-dicarboxylic acid (3), and sodium 4,4′,4″,4‴-(1,2-phenylenebis(phosphanetriyl))tetrabenzenesulfonate (4). The N^CH ligand precursor and iridium complexes 1–4 were synthesized in good yield and characterized using chemical analysis, ESI mass spectrometry, and NMR spectroscopy. The solid-state structure of 2 was also determined by XRD analysis. The complexes display moderate to strong phosphorescence in the 550–670 nm range with the quantum yields up to 30% and lifetimes of the excited state up to 60 µs in deoxygenated solution. Emission properties of 1–4 and N^CH are strongly pH-dependent to give considerable variations in excitation and emission profiles accompanied by changes in emission efficiency and dynamics of the excited state. Density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations made it possible to assign the nature of emissive excited states in both deprotonated and protonated forms of these molecules. The complexes 3 and 4 internalize into living CHO-K1 cells, localize in cytoplasmic vesicles, primarily in lysosomes and acidified endosomes, and demonstrate relatively low toxicity, showing more than 80% cells viability up to the concentration of 10 µM after 24 h incubation. Phosphorescence lifetime imaging microscopy (PLIM) experiments in these cells display lifetime distribution, the conversion of which into pH values using calibration curves gives the magnitudes of this parameter compatible with the physiologically relevant interval of the cell compartments pH.

Highlights

  • Monitoring of pH in living cells is a rapidly growing field of optical sensor applications [1,2,3,4]

  • Acidity homeostasis is a crucial function of living organisms, the deviations of which from “normal” magnitudes is an indicator of pathology [5] or malignancy [6,7]

  • We focused on synthesis and investigation of the [Ir(NC)2 (LL)] complexes bearing phenantroimidazole-based NC ligand that contains nucleophilic nitrogen center potentially prone to protonation, giving a sensory response onto pH variations

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Summary

Introduction

Monitoring of pH in living cells is a rapidly growing field of optical sensor applications [1,2,3,4]. Transition metal complexes usually show high photostability, long-lived emission with large Stokes shift The latter two characteristics allow cutting off the sample autofluorescence, which helps to increase sensor sensitivity and the precision of the measurements [17]. The most studied phosphorescent sensors are platinum-group metal complexes, in particular cyclometalated Ir(III) compounds [18,19,20,21,22] These emitters usually demonstrate high quantum yields and large lifetime values, modifiable excitation profile, and emission wavelength, which makes it possible to design the phosphorescent dyes with required photophysical characteristics. Concentration sufficient for visualization and for obtaining a reliable lifetime map in the samples under study

Synthesis of Ligand and Complexes
Synthesis
(Figures
Tables orbitals in thisorbitals transition
LC character and to the tribution emissive
A substantial change of pH the interval lifetime is observed
Living
Co-staining
Synthesis of the Ligand and Complexes
X-ray Diffraction Analysis
Photophysical Measurements
Computational Details
Cell Culturing
MTT Assay
Lysosome Staining
Confocal Luminescence Microscopy and PLIM Experiment
Conclusions
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