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

Abstract

A series of [Ir(N^C)2(N^N)]+ NIR-emitting orthometalated complexes (1–7) has been prepared and structurally characterized using elemental analysis, mass-spectrometry, and NMR spectroscopy. The complexes display intense phosphorescence with vibrationally structured emission bands exhibiting the maxima in the range 713–722 nm. The DFT and TD DFT calculations showed that the photophysical characteristics of these complexes are largely determined by the properties of the metalating N^C ligands, with their major contribution into formation of the lowest S1 and T1 excited states responsible for low energy absorption and emission, respectively. Emission lifetimes of 1–7 in degassed methanol solution vary from 1.76 to 5.39 µs and show strong quenching with molecular oxygen to provide an order of magnitude lifetime reduction in aerated solution. The photophysics of two complexes (1 and 7) were studied in model physiological media containing fetal bovine serum (FBS) and Dulbecco’s Modified Eagle Medium (DMEM) to give linear Stern-Volmer calibrations with substantially lower oxygen-quenching constants compared to those obtained in methanol solution. These observations were interpreted in terms of the sensors’ interaction with albumin, which is an abundant component of FBS and cell media. The studied complexes displayed acceptable cytotoxicity and preferential localization, either in mitochondria (1) or in lysosomes (7) of the CHO-K1 cell line. The results of the phosphorescence lifetime imaging (PLIM) experiments demonstrated considerable variations of the sensors’ lifetimes under normoxia and hypoxia conditions and indicated their applicability for semi-quantitative measurements of oxygen concentration in living cells. The complexes’ emission in the NIR domain and the excitation spectrum, extending down to ca. 600 nm, also showed that they are promising for use in in vivo studies.

Systematic Conversion of Photoinduced Intramolecular Electron Transfer in Trinuclear M-Ir-M (M = Re, Ir) Complexes to Intersystem Charge Transfer by Coupling to Gold Nanoclusters

“…Ir and Re coordination complexes are well known to show high triplet quantum yields because of the mixing of the singlet and the triplet excited states through spin−orbit coupling, leading to high phosphorescence efficiencies. 9,46,47 The PL from phosphorescence in all the compounds showed characteristic intensification on decreasing the temperature (Figure 4) both in the solution and solid states. Interestingly, room temperature phosphorescence (RTP) was common for all the four complexes in solid films (Figure 4b,d,f,h).…”

Investigation of the N^C Ligand Effects on Emission Characteristics in a Series of Bis-Metalated [Ir(N^C)2(N^N)]+ Complexes

“…Ligands in the coordination sphere of these complexes form a pseudo-octahedral environment at the iridium ion, with the nitrogen and carbon atoms of the N^C ligands disposed in trans - and cis -positions, respectively. This structural pattern is typical for complexes of this sort; bond angles and lengths in the coordination octahedron are not exceptional and fall in the range characteristic for the cationic [Ir(N^C) 2 (N^N)] + complexes [ 26 , 27 , 28 ]. The observed deviations from the ideal octahedral geometry around the iridium center are due to short bite angles of the N^C ligands, which are about 80° in these structures.…”

“…In this series, the Ir2-NO 2 gives a record QY value of 81% in degassed solution that is a result of the lowest k nr and highest k r ; the latter may be explained by a very high contribution of metal orbitals in the emissive T 1 →S 0 transition ( Table S13 ) and, consequently, higher magnitude of spin-orbital constant, which facilitates the spin-forbidden transition. The complexes with benzothienyl moiety coordinated to iridium center (Ir4-F and Ir4-NO 2 ) display a large bathochromic shift of emission band maxima to the NIR region, up to 792 nm in the case of Ir4-NO 2, that is typical for the ligands containing this metalated aromatic system [ 11 , 26 , 29 , 30 ]. Emission efficiency for both complexes demonstrated extremely strong reduction that did not allow reliable determination of the quantum yield values.…”

Intracellular pH Sensor Based on Heteroleptic Bis‐Cyclometalated Iridium(III) Complex Embedded into Block‐Copolymer Nanospecies: Application in Phosphorescence Lifetime Imaging Microscopy

“…[42,[53][54][55] [1,10] phenanthroline-1-yl) benzoic acid (N^N-Br) was synthesized according to literature methodology. [84] N-vinylpyrrolidone (VP), N-vinylformamide (VFA) (Sigma-Aldrich, Merck, Munich, Germany), glutaraldehyde (GA) (Sigma-Aldrich, Merck, Munich, Germany) and dioxane (Vekton, St.Petersburg, Russia) were distilled under vacuum before use. Azobis(isobutyronitrile) (AIBN) was recrystallized two times from ethanol at 60 °C and vacuum-dried at 20 °C.…”