Near-Infrared [Ir(N^∧C)₂(N^∧N)]⁺ Emitters and Their Noncovalent Adducts with Human Serum Albumin: Synthesis and Photophysical and Computational Study

Abstract

Near-infrared (NIR) molecular emitters based on transition-metal complexes have attracted growing attention due to their potential application for in vivo and in vitro bioimaging experiments. Their photophysical characteristics (large Stokes shift and lifetime in the microsecond domain) offer some important advantages in comparison to organic fluorophores and may provide better imaging resolution and higher sensitivity: for example, in mapping the oxygen concentration in biological objects. We have synthesized a series of [Ir(N ∧ C) 2 (N ∧ N)] + complexes with emission in the NIR region (N ∧ C = (2-benzothienyl)phenanthridine and 6-(2-benzothienyl)phenanthridine-2-carboxylic acid; N ∧ N = functionalized pyridine-triazole chelates), which also display a considerable red shift of their excitation spectra to the edge of the window of transparency. The flexible protocol for the synthesis of the N ∧ N ligands makes possible wide variations in the peripheral ligand environment: e.g., insertion of hydrophilic carboxyl group and further attachment of the other biologically relevant functions. The compounds obtained were completely characterized using spectroscopic methods, and their ground-state structures and photophysical properties were studied by DFT and TD DFT methods. To analyze the behavior of these emitters in biological systems, we investigated their interaction with human serum albumin (HSA), as the most abundant serum protein. It was found that these complexes readily form noncovalent {HSA−complex} adducts by embedding into hydrophobic cavities of this protein that also induced its partial aggregation. The complexes demonstrated preferential redistribution toward aggregated forms of HSA; the complex:HSA molar ratio did not exceed 1:3 for nonaggregated species. It was also shown that interaction of the hydrophobic complexes with albumin and the resulting aggregation dramatically change their important photophysical parameters such as emitter lifetime and its sensor response onto molecular oxygen.

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

“…The complexes display large Stokes shift, lifetime in microsecond domain, and emission quenching by molecular oxygen that points to the emission origin from triplet excited state, i.e., phosphorescence. According to the data of DFT calculations for closely analogous iridium complexes [ 52 ], the chromophore in the emitters of this sort is localized onto the central fragment of the molecule and observed emission can be assigned to ligand centered (LC) transition with some admixture of the ligand to ligand (LLCT) and metal to ligand (MLCT) charge transfer. The experimental data are in complete agreement with this assignment and indicate that the OEG pendants and counterions have nearly no effect onto photophysical characteristics of these complexes because of their lateral disposition relative to the chromophore center.…”

“…Mass spectra were recorded on a Bruker maXis HRMS-ESI-QTOF in the ESI + mode. Boc-L-aspartic acid [ 61 ], 6-(benzo[b]thiophen-2-yl)phenanthridine-2-carboxylic [ 52 ] acid and 2,5,8,12,15,18-hexaoxanonadecan-10-amine [ 62 ] ( NH 2 -2OEG ) were obtained according to the published procedures. Other reagents (Merck KGaA, Darmstadt, Germany) and solvents (Vekton, St. Petersburg, Russia) were used as received without further purification.…”

Red‐to‐NIR Iridium(III) Emitters: Synthesis, Photophysical and Computational Study, the Effects of Cyclometallating and β‐Diketonate Ligands

“…[36] Dimeric and acetonitrile iridium complexes were obtained according to the published procedures with slight modifications. [21,[37][38][39][40] Acetonitrile complexes [Ir(N^C1)(CH 3 CN) 2 ]OTf and [Ir(N^C2)(CH 3 CN) 2 ]OTf have already been characterized, [37,40] therefore, their experimental characteristics are not presented in this work. 1,2-Dichloroethane was distilled for photophysical measurements, THF was distilled over sodium with the addition of benzophenone.…”

“…[50] The electronic density difference during the S 0 !S i transition was determined in the same way as in our previous work. [21] To obtain the electronic density difference during the T 1 !S 0 transition, single point TD-DFT calculations in the optimized triplet state were performed with 10 excited triplet states.…”

“…[17] The complexes of this type possess a few characteristic features such as high chemical stability, bright luminescence and wide range of emission colors, which encompasses the whole visible spectrum from blue [18][19][20] to deep red [8,19] extending further to near-infrared (NIR) region. [8,[20][21][22] It is well established [23][24][25] that two major processes: metal-to-ligand charge transfer (MLCT) from Ir(III) ion to N^C ligands orbitals and N^C ligand centered (LC) transition dominate in formation of emissive triplet excited state of these chromophores. These observations indicate that fine tuning of emission wavelength, which is of key importance for various applications, can be done by variations in the nature and properties of the metalating N^C ligands.…”