pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis, Photophysical Properties, Computational Results, and Bioimaging Application

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

Rhenium(I) Block Copolymers Based on Polyvinylpyrrolidone: A Successful Strategy to Water-Solubility and Biocompatibility

A series of diphosphine Re(I) complexes Re1–Re4 have been designed via decoration of the archetypal core {Re(CO)2(N^N)} through the installations of the phosphines P0 and P1 bearing the terminal double bond, where N^N = 2,2′-bipyridine (N^N1), 4,4′-di-tert-butyl-2,2′-bipyridine (N^N2) or 2,9-dimethyl-1,10-phenanthroline (N^N3) and P0 = diphenylvinylphosphine, and P1 = 4-(diphenylphosphino)styrene. These complexes were copolymerized with the corresponding N-vinylpyrrolidone-based Macro-RAFT agents of different polymer chain lengths to give water-soluble copolymers of low-molecular p(VP-l-Re) and high-molecular p(VP-h-Re) block-copolymers containing rhenium complexes. Compounds Re1–Re4, as well as the copolymers p(VP-l-Re) and p(VP-h-Re), demonstrate phosphorescence from a 3MLCT excited state typical for this type of chromophores. The copolymers p(VP-l-Re#) and p(VP-h-Re#) display weak sensitivity to molecular oxygen in aqueous and buffered media, which becomes almost negligible in the model physiological media. In cell experiments with CHO-K1 cell line, p(VP-l-Re2) and p(VP-h-Re2) displayed significantly reduced toxicity compared to the initial Re2 complex and internalized into cells presumably by endocytic pathways, being eventually accumulated in endosomes. The sensitivity of the copolymers to oxygen examined in CHO-K1 cells via phosphorescence lifetime imaging microscopy (PLIM) proved to be inessential

Aggregation-Induced Ignition of Near-Infrared Phosphorescence of Non-Symmetric [Pt(C^N*N’^C’)] Complex in Poly(caprolactone)-based Block Copolymer Micelles: Evaluating the Alternative Design of Near-Infrared Oxygen Biosensors

In the present work, we described the preparation and characterization of the micelles based on amphiphilic poly(ε-caprolactone-block-ethylene glycol) block copolymer (PCL-b-PEG) loaded with non-symmetric [Pt(C^N*N’^C’)] complex (Pt1) (where C^N*N’^C’: 6-(phenyl(6-(thiophene-2-yl)pyridin-2-yl)amino)-2-(tyophene-2-yl)nicotinate). The obtained nanospecies displayed the ignition of near-infrared (NIR) phosphorescence upon an increase in the content of the platinum complexes in the micelles, which acted as the major emission component at 12 wt.% of Pt1. Emergence of the NIR band at 780 nm was also accompanied by a 3-fold growth of the quantum yield and an increase in the two-photon absorption cross-section that reached the value of 450 GM. Both effects are believed to be the result of progressive platinum complex aggregation inside hydrophobic poly(caprolactone) cores of block copolymer micelles, which has been ascribed to aggregation induced emission (AIE). The resulting phosphorescent (Pt1@PCL-b-PEG) micelles demonstrated pronounced sensitivity towards molecular oxygen, the key intracellular bioanalyte. The detailed photophysical analysis of the AIE phenomena revealed that the NIR emission most probably occurred due to the excimeric excited state of the 3MMLCT character. Evaluation of the Pt1@PCL-b-PEG efficacy as a lifetime intracellular oxygen biosensor carried out in CHO-K1 live cells demonstrated the linear response of the probe emission lifetime towards this analyte accompanied by a pronounced influence of serum albumin on the lifetime response. Nevertheless, Pt1@PCL-b-PEG can serve as a semi-quantitative lifetime oxygen nanosensor. The key result of this study consists of the demonstration of an alternative approach for the preparation of NIR biosensors by taking advantage of in situ generation of NIR emission due to the nanoconfined aggregation of Pt (II) complexes inside the micellar nanocarriers

Polymeric Nanoparticles with Embedded Eu(III) Complexes as Molecular Probes for Temperature Sensing

Three novel luminescent Eu(III) complexes, Eu1–Eu3, have been synthesized and characterized with CHN analysis, mass-spectrometry and 1H NMR spectroscopy. The complexes display strong emission in dichloromethane solution upon excitation at 405 and 800 nm with a quantum yield from 18.3 to 31.6%, excited-state lifetimes in the range of 243–1016 ms at 20 °C, and lifetime temperature sensitivity of 0.9%/K (Eu1), 1.9%/K (Eu2), and 1.7%/K (Eu3). The chromophores were embedded into biocompatible latex nanoparticles (NPs_Eu1–NPs_Eu3) that prevented emission quenching and kept the photophysical characteristics of emitters unchanged with the highest temperature sensitivity of 1.3%/K (NPs_Eu2). For this probe cytotoxicity, internalization dynamics and localization in CHO-K1 cells were studied together with lifetime vs. temperature calibration in aqueous solution, phosphate buffer, and in a mixture of growth media and fetal bovine serum. The obtained data were then averaged to give the calibration curve, which was further used for temperature estimation in biological samples. The probe was stable in physiological media and displayed good reproducibility in cycling experiments between 20 and 40 °C. PLIM experiments with thermostated CHO-K1 cells incubated with NPs_Eu2 indicated that the probe could be used for temperature estimation in cells including the assessment of temperature variations upon chemical shock (sample treatment with mitochondrial uncoupling reagent)

A curious interplay in the films of N-heterocyclic carbene Pt-II complexes upon deposition of alkali metals

The recently synthesized series of Pt-II complexes containing cyclometallating (phenylpyridine or benzoquinoline) and N-heterocyclic carbene ligands possess intriguing structures, topologies, and light emitting properties. Here, we report curious physicochemical interactions between in situ PVD-grown films of a typical representative of the aforementioned Pt-II complex compounds and Li, Na, K and Cs atoms. Based on a combination of detailed core-level photoelectron spectroscopy and quantum-chemical calculations at the density functional theory level, we found that the deposition of alkali atoms onto the molecular film leads to unusual redistribution of electron density: essential modification of nitrogen sites, reduction of the coordination Pt-II centre to Pt-0 and decrease of electron density on the bromine atoms. A possible explanation for this is formation of a supramolecular system Pt complex-alkali metal ion; the latter is supported by restoration of the system to the initial state upon subsequent oxygen treatment. The discovered properties highlight a considerable potential of the Pt-II complexes for a variety of biomedical, sensing, chemical, and electronic applications

Amphiphilic Diblock Copolymers Bearing Poly(Ethylene Glycol) Block: Hydrodynamic Properties in Organic Solvents and Water Micellar Dispersions, Effect of Hydrophobic Block Chemistry on Dispersion Stability and Cytotoxicity

Despite the fact that amphiphilic block copolymers have been studied in detail by various methods both in common solvents and aqueous dispersions, their hydrodynamic description is still incomplete. In this paper, we present a detailed hydrodynamic study of six commercial diblock copolymers featuring the same hydrophilic block (poly(ethylene glycol), PEG; degree of polymerization is ca. 110 ± 25) and the following hydrophobic blocks: polystyrene, PS35-b-PEG115; poly(methyl methacrylate), PMMA55-b-PEG95; poly(1,4-butadyene), PBd90-b-PEG130; polyethylene PE40-b-PEG85; poly(dimethylsiloxane), PDMS15-b-PEG115; and poly(ɛ-caprolactone), PCL45-b-PEG115. The hydrodynamic properties of block copolymers are investigated in both an organic solvent (tetrahydrofuran) and in water micellar dispersions by the combination of static/dynamic light scattering, viscometry, and analytical ultracentrifugation. All the micellar dispersions demonstrate bimodal particle distributions: small compact (hydrodynamic redii, Rh ≤ 17 nm) spherical particles ascribed to “conventional” core–shell polymer micelles and larger particles ascribed to micellar clusters. Hydrodynamic invariants are (2.4 ± 0.4) × 10−10 g cm2 s−2 K−1 mol−1/3 for all types of micelles used in the study. For aqueous micellar dispersions, in view of their potential biomedical applications, their critical micelle concentration values and cytotoxicities are also reported. The investigated micelles are stable towards precipitation, possess low critical micelle concentration values (with the exception of PDMS15-b-PEG115), and demonstrate low toxicity towards Chinese Hamster Ovarian (CHO-K1) cells

Chromophore-Functionalized Phenanthro-diimine Ligands and Their Re(I) Complexes

A series of diimine ligands has been designed on the basis of 2-pyridyl-1<i>H</i>-phenanthro­[9,10-<i>d</i>]­imidazole (<b>L1</b>, <b>L2</b>). Coupling the basic motif of <b>L1</b> with anthracene-containing fragments affords the bichromophore compounds <b>L3</b>–<b>L5</b>, of which <b>L4</b> and <b>L5</b> adopt a donor–acceptor architecture. The latter allows intramolecular charge transfer with intense absorption bands in the visible spectrum (lowest λ_abs 464 nm (ε = 1.2 × 10⁴ M^–1 cm^–1) and 490 nm (ε = 5.2 × 10⁴ M^–1 cm^–1) in CH₂Cl₂ for <b>L4</b> and <b>L5</b>, respectively). <b>L1</b>–<b>L5</b> show strong fluorescence in a fluid medium (Φ_em = 22–92%, λ_em 370–602 nm in CH₂Cl₂); discernible emission solvatochromism is observed for <b>L4</b> and <b>L5</b>. In addition, the presence of pyridyl (<b>L1</b>–<b>L5</b>) and dimethylaminophenyl (<b>L5</b>) groups enables reversible alteration of their optical properties by means of protonation. Ligands <b>L1</b>–<b>L5</b> were used to synthesize the corresponding [Re­(CO)₃X­(diimine)] (X = Cl, <b>1</b>–<b>5</b>; X = CN, <b>1</b>-<b>CN</b>) complexes. <b>1</b> and <b>2</b> exhibit unusual dual emission of singlet and triplet parentage, which originate from independently populated ¹ππ* and ³MLCT excited states. In contrast to the majority of the reported Re­(I) carbonyl luminophores, complexes <b>3</b>–<b>5</b> display moderately intense ligand-based fluorescence from an anthracene-containing secondary chromophore and complete quenching of emission from the ³MLCT state presumably due to the triplet–triplet energy transfer (³MLCT → ³ILCT)

Coordination to Imidazole Ring Switches on Phosphorescence of Platinum Cyclometalated Complexes: The Route to Selective Labeling of Peptides and Proteins via Histidine Residues

In this study, we have shown that substitution of chloride ligand for imidazole (Im) ring in the cyclometalated platinum complex Pt­(phpy)­(PPh₃)Cl (<b>1</b>; phpy, 2-phenylpyridine; PPh₃, triphenylphosphine), which is nonemissive in solution, switches on phosphorescence of the resulting compound. Crystallographic and nuclear magnetic resonance (NMR) spectroscopic studies of the substitution product showed that the luminescence ignition is a result of Im coordination to give the [Pt­(phpy)­(Im)­(PPh₃)]Cl complex. The other imidazole-containing biomolecules, such as histidine and histidine-containing peptides and proteins, also trigger luminescence of the substitution products. The complex <b>1</b> proved to be highly selective toward the imidazole ring coordination that allows site-specific labeling of peptides and proteins with <b>1</b> using the route, which is orthogonal to the common bioconjugation schemes via lysine, aspartic and glutamic acids, or cysteine and does not require any preliminary modification of a biomolecule. The utility of this approach was demonstrated on (i) site-specific modification of the ubiquitin, a small protein that contains only one His residue in its sequence, and (ii) preparation of nonaggregated HSA-based Pt phosphorescent probe. The latter particles easily internalize into the live HeLa cells and display a high potential for live-cell phosphorescence lifetime imaging (PLIM) as well as for advanced correlation PLIM and FLIM experiments