Cyclometalated Iridium Complexes Research Articles

Cyclometalated Iridium(III) Complexes Containing Benzoxazole Derivatives and Different Ancillary Ligands for Catalytic Oxidation of Toluene

A series of cyclometalated iridium(III) complexes that have the general formula [(C^N)2Ir(NR)(X)] (C^N = monoanionic bidentate cyclometalating ligands; NR = pyridine derivatives; X = Cl− or I−) are designed, prepared, and applied for the transformation of toluene to benzaldehyde using a clean, highly efficient, and environmentally-friendly process. The activation energies that are needed for the catalytic oxidation of toluene when using these complexes as catalysts are quite low: between 22.9 and 30.8 kcal mol−1. The catalytic frequencies (TOF) are fairly high (up to 7.0 × 102 h−1) with excellent reliability, and the turnover number (TON) can reach 4.2 × 103 after 6 h of processing time. Catalytic tests, X-ray absorption near-edge structure (XANES), and kinetic modeling are used to derive detailed insights into the characteristics of the catalysts and their effects on the reactions that are featured in the catalytic oxidation of toluene.

Cyclometalated Ir(III) Complexes Involving Functionalized Terpyridine-Based Ligands Exhibiting Aggregation-Induced Emission and Their Potential Applications in CO2 Detection

Synthesis of three novel terpyridine-based donor–acceptor (D-A) ligands (TP1, TP2, TP3) and cyclometalated iridium complexes [Ir(ppy)2TP1]+PF6– (C1), [Ir(ppy)2TP2]+PF6– (C2), and [Ir(ppy)2TP3]+PF6– (C3) [ppy = 2−phenyl pyridine] involving these were described. The ligands and complexes were characterized by spectroscopic studies (1H, 13C, 19F, 31P, ESI-MS, UV-vis, and fluorescence). Crucial role of intermolecular interactions in aggregation-caused quenching (ACQ; C2) and aggregation-induced emission (AIE; C3) was rationalized by X-ray single-crystal analyses. Vital role of restricted intramolecular rotation (RIR) in inducing AIE upon aggregation via π–π interactions in these complexes was scrutinized by various studies. Because of strong intramolecular charge transfer these D−A based AIEgens exhibited solvatochromism. Further, AIE property of the complexes C1 and C3 was exploited toward detection of CO2.

Bichromophoric anticancer drug: Targeting lysosome with rhodamine modified cyclometalated Iridium(III) complexes

Four bichromophoric cyclometalated iridium complexes were synthesized and fully characterized. Their antiproliferative capacity against A549, HepG2 cells and HeLa cells as well as two human normal cells was studied by MTT assay. Rhodamine B modified complexes 2 and 4 were not cytotoxic to A549 cells while both rhodamine 6 g modified complexes 1 and 3 showed greater cytotoxicity than cisplatin. In particular, the antiproliferative activity of complex 3 was about 4.6 times than that of cisplatin. Thus, complex 3 was used for stability and pH sensitivity studies. The results indicated that the complex not only had rich fluorescence properties under acidic conditions, but also showed good stability. Further, interaction of complex with bovine serum albumin (BSA) has been investigated by UV−vis, fluorescence, synchronous fluorescence spectroscopy. The complexes have a certain ability to bind the BSA. Interestingly, these complexes can catalyze the reaction of the coenzyme NADH to NAD+, which is consistent with the apparent growth of ROS in cells. In addition, complex 3 can cause S phase arrest in the cell cycle, induce apoptosis, and affect mitochondrial membrane potential changes. Localization experiments of intracellular complexes by confocal microscopy suggested that these complexes enter cancer cells through energy dependence and specifically target the lysosomes, thus resulting in the damage of lysosomes.

Structure-activity relationship studies of antiplasmodial cyclometallated ruthenium(II), rhodium(III) and iridium(III) complexes of 2-phenylbenzimidazoles

Benzimidazoles, such as albendazole, thiabendazole and omeprazole have antiplasmodial activity against Plasmodium falciparum and are widely used as scaffolds for metal-based drug research. Incorporating substituents with various lipophilic and electronic properties can influence trans-membrane interactions and concomitantly improve the biological activity. To study structure-activity relationships, a series of 2-phenylbenzimidazoles and their corresponding Ru(II), Ir(III) and Rh(III) cyclometallated complexes were synthesised and evaluated for antiplasmodial activity against the chloroquine-sensitive (NF54) strain of the human malaria parasite Plasmodium falciparum. Selected metal complexes were further screened against the multidrug-resistant (K1) strain. In general, the 2-phenylbenzimidazole ligands showed weak antiplasmodial activities (IC50 ∼ 17.66–22.32 μM) while an enhancement of antiplasmodial activity was observed upon coordination of the ligands with either ruthenium, iridium or rhodium. The new cyclometallated complexes were found to be active against both parasite strains, with IC50 values in the low to submicromolar range (0.12–5.17 μM). In addition, the metal complexes have relatively low cytotoxicity against mammalian Chinese Hamster Ovarian (CHO) cells.

Homoleptic cyclometalated iridium(III) complex nanowires electrogenerated chemiluminescence sensors for high-performance discrimination of proline enantiomers based on the difference of electron-transfer capability

Chiral molecules can generally exhibit different physiological and biological activities, thus triggering wide interest in chiral discrimination. Herein, we demonstrate a sensitive homoleptic cyclometalated iridium(III) complexes nanowires (Ir(piq)3 NWs) electrogenerated chemiluminescence (ECL) sensor for high-performance discrimination of chiral molecules based on the difference in electron-transfer ability between different radicals. The developed Ir(piq)3 NWs/ITO sensor exhibits high ECL signal and satisfying stability without the presence of co-reactant based on hot electron-induced ECL, which would spur its further application in detecting proline enantiomers. Obvious difference on the ECL intensity towards L-Pro and D-Pro is observed under the presence of high concentration of proline enantiomers, which attributes to the different electron-transfer capability between Ir+(piq)3 and L-pro•/D-pro•. In order to attain high-performance discrimination, we construct the Ir(piq)3 NWs/TPrA sensor to achieve chiral discrimination for trace amounts of proline enantiomers (1.0 × 10−9 M), which attributes to the strong molecular interactions enhanced by introduction of active radical TPrA•.

Intramolecular π-π Interactions with a Chiral Auxiliary Ligand Control Diastereoselectivity in a Cyclometalated Ir(III) Complex.

The application of a chiral auxiliary ligand to control the diastereoselectivity in the synthesis of a cyclometalated iridium(III) complex is presented. The diastereomeric iridium(III) complexes 1a and 1b are reported, in which a phenoxyoxazoline auxiliary ligand incorporates a chiral center functionalized with a pendant pentafluorophenyl group. The diastereomers were readily separated, and their structural, electrochemical and photophysical properties are discussed. Solution-state NMR data and X-ray crystal structures establish that the pentafluorophenyl group engages in intramolecular π-π interactions. The X-ray analysis reveals that the two diastereomers display very different modes of intramolecular stacking. The variable-temperature 19F NMR data indicate that rotation of the pendant pentafluorophenyl rings in 1b and 1a is a temperature-dependent process and that there is a smaller energy barrier to rotation in 1b in comparison to 1a. This correlates with variable-temperature photoluminescence data, which show that upon heating the integrated emission intensity is reduced substantially more for 1b than for 1a, which is ascribed to the enhanced rotation in 1b, providing a more easily populated nonradiative pathway in comparison to 1a. These experimental data are supported by computational calculations. Phosphorescent organic light-emitting devices (PhOLEDs) using 1a as the dopant complex give blue-green emission with a high maximum external quantum efficiency (EQEmax) of 25.8% (at ca. 270 cd m-2) and with a low efficiency roll-off to 24.9% at 1000 cd m-2. Our results extend the scope of ligand design for cyclometalated iridium complexes which possess interesting structural and emission properties.

Photoluminescence of Cyclometalated Iridium Complexes in Poly(methyl methacrylate) Films

In this work, we describe the emission properties of 12 heteroleptic bis-cyclometalated iridium complexes immobilized in poly(methyl methacrylate) (PMMA) thin films. The compounds chosen for this study span a range of emission colors covering the visible spectrum and fall into three categories with electronically diverse ancillary ligands—four cationic bis(aryl isocyanide) complexes, four with chelating bis(acyclic diaminocarbene) (bis-ADC) ancillary ligands which can exist in two protonation states, and four neutral compounds with monoanionic, strongly π donating β-ketoiminate (acNac) and β-diketiminate (NacNac) ancillary ligands. Included in this suite of compounds are five new compounds, one with two aryl isocyanide ancillary ligands, two with bis-ADC ancillary ligands, and one each with acNac and NacNac; their syntheses are described, and crystal structures of three are presented. The bulk of the work presented here describes the emission spectra of all 12 compounds, comparing spectral features in sol...

Synthesis, photoluminescence and electroluminescence of triphenylphosphine functionalized cyclometalated iridium(III) complexes

Three greenish-blue iridium (III) phosphorescent complexes with the molecular structure of [Ir(pppy)2(PPh3)L] (Hpppy = 2-(4-phenoxyphenyl)pyridine; L = Cl−, NCS−, NCO−) were synthesized and characterized. Their photoluminescence spectra are significantly blue-shifted after introducing the strong-field ancillary ligand triphenylphosphine. These compounds exhibit emission peaks in the range of 474–479 nm in solutions at 298 K with quantum yields up to 0.33. The organic light-emitting diodes (OLEDs) were fabricated by thermal evaporation with complex 3. The best OLED D3 was obtained with turn-on voltage of 4.3 V, maximum luminance of 11931 cd m−2 under 15.7 V, highest luminance efficiency (ηL) as 16.6 cd A−1, power efficiency (ηP) as 12.2 lm W−1 and external quantum efficiency (EQE) as 5.6%.

Rapid and selective luminescent sensing of allergenic gluten by highly phosphorescent switch-on probe

First time the luminescent switch-on probe using novel water-soluble cyclometallated iridium complex (Ir-dc) has been developed for sensitive and selective detection of gluten in the presence of several interfering elements. Linear concentration range of gluten is obtained from 5 to 200 µg/mL with a limit of detection 2.6 µg/mL. The Ir-dc complex responded to the broad pH range which is advantageous for the detection of gluten in various food samples. Additionally, It has been successfully employed for the detection of gluten in commercial food samples of wheat flour and oats with highest recovery values, indicating applicability of Ir-dc for practical usage.

Luminescent methacrylic copolymers with side-chain cyclometalated iridium(III) complexes

A series of eight copolymers made of a poly (methyl methacrylate) backbone with a luminescent iridium(III)-based pendant is presented. These materials are obtained by radical copolymerization of different ratios of methyl methacrylate (MMA) with the reactive Ir-based MMA monomer [Ir(ppy)2(ptmma)]Cl (Hppy = 2-phenylpyridine, ptmma = 11-(4-pyridin-2-yltriazol-1-yl)undecyl 2-methylprop-2-enoate). The products are characterized by NMR, IR and DSC methods. The specific ratio among the two monomers, dictates the concentration of the emissive iridium-based centers along the polymer backbone, which affects the overall luminescence behavior of the material. The position of the absorption and emission maxima of the iridium-based monomer and of the related polymeric compounds are virtually identical, pinpointing the optically innocent character of the polymer matrix. Photophysical investigations show that the emission quantum yields (PLQYs) of the iridium-based emitters improve upon increasing the fraction of the inert MMA co-monomer, due to a progressive decrease in self-absorption and self-quenching, approaching the infinite-dilution limit at the relatively low [MMA]:[Ir] molar ratio of 500, with PLQY = 0.51 ± 0.05.The concentration-quenching rate process follows a power-law proportional to R–(2.6±0.9), where R is the calculated average distance between the emitting units of the copolymers. Such a deviation from the expected dipole-dipole interaction (i.e., R−6 dependence) suggests the partial formation of aggregates leading to the occurrence of static self-quenching in the solid matrix, as the concentration of the iridium centers increases. These results offer valuable hints for the formulation of strongly emitting hybrid materials in which a high concentration of triplet luminophores is embedded in organic polymers solely serving as solid transparent scaffolds.

Overcoming the Inertness of Iridium(III) in a Facile Single-Crystal to Single-Crystal Reaction of Iodine Vapor with a Cyclometalated Chloride Monomer.

Iridium(III) is often considered to be one of the most inert octahedral metal ions. Herein we present a phenanthroimidazole-based bis-cyclometalated iridium(III) chloride undergoing a facile chloro ligand exchange with iodine, in a gas-solid reaction under ambient conditions. Monitoring the progress of the reaction by X-ray diffraction analysis reveals the crystal-packing-induced exceptional stereoselectivity of this topochemical transformation. The results provide excellent opportunities to modulate the geometry and kinetic properties of cyclometalated iridium(III) complexes that may be very promising in catalysis and design of anticancer agents.

Emission Tuning of Ir(N∧C)2(pic)-Based Complexes via Torsional Twisting of Picolinate Substituents

Pyridine-2-carboxylate (pic) has been employed extensively as a blue-shifting ancillary ligand in the production of cyclometalated iridium complexes used in OLEDs, but surprisingly, further elaboration of this ligand has largely been unexplored. In this work we demonstrate a simple and versatile route for modifying picolinate ligands coordinated to iridium. Reacting a μ-chloro iridium(C∧N) dimer (where C∧N is a phenylpyridine-based ligand) with 4-bromopicolinic acid (HpicBr) yields the corresponding iridium(C∧N)2(picBr) complexes, which were readily modified by a Suzuki–Miyaura reaction to give the corresponding aryl-substituted picolinate complexes. The luminescent behavior of these complexes shows that by restricting the torsional angle between the substituent and pic the emission can be shifted by up to 77 nm.

Cyclometallated imidazo-phenanthroline iridium complexes and their anticancer activity

Abstract Cyclometallated mononuclear iridium complexes (1–3) were synthesized using dimeric precursor [\((\hbox {ppy})_{2}\hbox {Ir}^{\mathrm{III}}(\upmu \hbox {-Cl})]_{2}\) and imidazo-phenanthroline-based ligands (\(\mathbf{L}_{1}\)–\(\mathbf{L}_{3})\). The complexes were characterised using various analytical techniques. The complexes exhibited spin forbidden metal-to-ligand charge transfer (MLCT) transitions at wavelengths \({>}300 \hbox { nm}\) and high energy ligand-based transition below 300 nm. The complexes were found to be emissive and displayed emission bands at 580 nm upon excitation at 380 nm with quantum yields 0.07 with respect to standard [\(\hbox {Ru}(\hbox {bpy})_{3}]\hbox {Cl}_{2}\). The complexes 1–3 showed dose-dependent suppression of cell viability (\(\hbox {IC}_{50}\) values \({\sim }2.5 \,\upmu \hbox {M}\)) towards human breast (MCF7) cancer cell line. The hydrophobicity measurements and flow cytometry analysis suggest that cellular uptake is primarily responsible for observed cytotoxicity.GRAPHICAL ABSTRACT Synopsis Mononuclear anticancer-active cyclometallated iridium complexes (1–3) using imidazo-phenanthroline-based ligands (\(\mathbf{L}_{1}\)–\(\mathbf{L}_{3})\) were synthesized.

A phosphorescent cyclometalated iridium(III) complex as mitochondria-targeted theranostic anticancer agent

In the present study, a cyclometalated iridium(III) complex, [Ir(ppy)2(HPIP)]Cl (IrT, ppy = 2-phenylpyridine, HPIP = 2-(2-hydroxyphenyl)imidazo[4,5-f]1,10-phenanthroline), was synthesized and characterized. IrT exhibited more cytotoxicity than cisplatin did against several screened cancer cell lines. In particular, the cytotoxicity of IrT against the cisplatin-resistant cell line A549-CP/R is approximately 10 times higher than that of cisplatin. In addition, this complex could perform theranostic functions by simultaneously imaging and tracking mitochondrial morphological changes. Further study suggested that IrT induced changes in the mitochondrial membrane potential and triggered apoptosis via an intrinsic mitochondria-mediated pathway. Thus, IrT exhibited both antitumor and imaging properties, indicating that IrT may be a viable drug candidate as a mitochondria-targeted theranostic anticancer agent.

Insights into the luminescent properties of anionic cyclometalated iridium(III) complexes with ligands derived from natural products

AbstractIn the search of remarkable anionic electroluminescent semiconductors to be applied in energy conversion devices such as Light Emitting Electrochemical Cells, we report the electronic, photophysical, and charge injection/transfer properties of a series of cyclometalated iridium(III) complexes through a DFT/TD‐DFT procedure. The proposed semiconductors involve bidentated ligands based on natural products (salicylic acid and boldine), and phenylpyridine and phenylpyrazole as the cyclometalating units. The proposed compounds emit in the range of 446 to 571 nm, where the boldine based compounds have red‐shifted emissions compared to their analogs with salicylic acid. Blue phosphors were obtained by the use of phenylpyrazole units; however, the ligand field is weak in these cases compared to the ligand field exerted by the phenylpyridine ligands. The latter allows the accessibility to the radiationless states for emitters below 495 nm as a result of the increased stability of the metal centered excited states; consequently, the luminescent quantum yield could be decreased. Conversely, the semiconductors with phenylpyridine units show a restricted accessibility to radiationless processes, which could result in emitters with a high luminescent quantum yield and low non‐radiative constants. Finally, the proposed anionic semiconductors show a better balance between hole/electron transfer rate compared to related cationic Ir(III) complexes; while, the easier hole‐electron injection is favored for semiconductors with salicylic acid and phenylpyridine units.

Highly Luminescent Cyclometalated Iridium Complexes Generated by Nucleophilic Addition to Coordinated Isocyanides.

In this work, we report a new class of blue-emitting cyclometalated iridium complexes supported by acyclic diaminocarbene (ADC) ancillary ligands. These neutral, tris-chelated complexes are not obtainable via traditional synthesis routes and instead are generated through metal-mediated nucleophilic addition to a metal-bound isocyanide, which is followed by orthometalation of the ADC under mild conditions. Importantly, four of the variants exhibit efficient phosphorescence when immobilized in PMMA matrix, achieving quantum yields of 79% for blue emitters with a 2-(2,4-difluorophenyl)pyridine (F2ppy) C^N ligand and 30-37% for orange emitters with a 2-phenylbenzothiazole (bt) C^N ligand. Electrochemical studies demonstrate significantly higher-lying HOMO levels in the ADC complexes relative to the NHC analogues, a phenomenon that results in enhanced charge-transfer character in the excited states of the ADC complexes. This study demonstrates that ADC ancillary ligands not only give rise to new structures for Ir(III)-based phosphorescent emitters but also are promising targets for use in light-emitting devices and other thin-film optical applications.

Aggregation-Induced Electrochemiluminescence from a Cyclometalated Iridium(III) Complex.

Aggregation-induced emission has been extensively found in organic compounds and metal complexes. In contrast, aggregation-induced electrochemiluminescence (AI-ECL) is rarely observed. Here, we employ two tridentate ligands [2,2':6',2″-terpyridine (tpy) and 1,3-bis(1 H-benzimidazol-2-yl)benzene (bbbiH3)] to construct a cyclometalated iridium(III) complex, [Ir(tpy)(bbbi)] (1), showing strong AI-ECL. Its crystal structure indicates that neighboring [Ir(tpy)(bbbi)] molecules are connected through both π-π-stacking interactions and hydrogen bonds. These supramolecular interactions can facilitate the self-assembly of complex 1 into nanoparticles in an aqueous solution. The efficient restriction of molecular vibration in these nanoparticles leads to strong AI-ECL emission of complex 1. In a dimethyl sulfoxide-water (H2O) mixture with a gradual increase in the H2O fraction from 20% to 98%, complex 1 showed a ∼39-fold increase in the electrochemiluminescence (ECL) intensity, which was ∼4.04 times as high as that of [Ru(bpy)3]2+ under the same experimental conditions. Moreover, the binding of bovine serum albumin to the nanoparticles of complex 1 can improve the ECL emission of this complex, facilitating the understanding of the mechanism of AI-ECL for future applications.

Efficient near-infrared emission of π-extended cyclometalated iridium complexes based on pyrene in solution-processed polymer light-emitting diode

A novel iridium complex grafting hole-transporting triphenylamine (TPA) unit onto cyclometalated ligand, namely t-BuPyrPyTPA)2Ir(acac), was successfully synthesized and characterized. The photophysical, electrochemical and DFT/TD-DFT calculation, as well as electroluminescence properties of this iridium complex were fully investigated. Meanwhile, the PLEDs employing (t-BuPyrPyTPA)2Ir(acac) as dopant presented stable NIR emission peaked at 697 nm and a shoulder at 764 nm with a highest external quantum efficiency (EQE) of 0.56% at 4 wt% dopant concentration. These results demonstrate that expanding the conjugation length of the ligand is an effective way to achieve NIR emission.

Solution concentration-dependent tunable emission in cyclometalated iridium complex bearing perylene diimide (PDI) ligand: From visible to near-infrared emission

To explore the multicolor photoluminescence switching in perylene diimide (PDI) derivatives, a PDI-based iridium complex of (ppy)2IrPDI was prepared and characterized by NMR, MS and elemental analysis. This iridium complex showed clearly concentration-dependent emission spectra in solution, which is generated from different excited states (singlet and triplet), as evidenced by experimental and theoretical methods. Conversely, the iridium complex presented phosphorescent near-infrared emission (∼750 nm) both in neat film and in a solid-state polymethylmethacrylate (PMMA) matrix. This research provides another way to access the radiative emission from triplet states of PDI derivatives, which has an important significance for further understanding the photophysics of the PDI based phosphorescent complex.

DUPLICATE: Electrochemiluminescence of cyclometalated iridium (III) complexes

Summary The advent of highly emissive cyclometalated iridium complexes has injected new vigour into the field of electrochemiluminescence (ECL), with these materials now beginning to challenge the dominance of ruthenium diimine complexes for ECL applications. The promise of iridium-based electrochemiluminophores lies not only in their enhanced photophysical properties but in the relative ease with which their emission wavelengths and redox potentials can be tailored. However, innovative approaches and better understanding are required to tackle the challenges which sometimes arise associated with their susceptibility to quenching, especially in aqueous media.