Cyclometalated Iridium Complexes Research Articles

Effects of Ancillary Ligands on Deep Red to Near-Infrared Cyclometalated Iridium Complexes.

The design of organometallic compounds with efficient phosphorescence in the deep red to near-infrared portions of the spectrum is a long-standing fundamental challenge. Here we describe a series of heteroleptic bis-cyclometalated iridium complexes with phosphorescence in these low-energy regions of the spectrum. The cyclometalating ligands in this study feature a metalated benzothiophene aryl group substituted with a quinoline, isoquinoline, or phenanthridine heterocycle. Increasing the conjugation on the heterocycle stabilizes the ligand-centered LUMO, decreases the HOMO-LUMO gap, and enables phosphorescence to occur at long wavelengths. These cyclometalating ligands are paired with a variety of electron-rich ancillary ligands, such as dithiocarbamate (dipdtc), β-ketoiminate (acNac), β-diketiminate (NacNac), amidinate (dipba), and hexahydropyrimidopyrimidine (hpp), some of which have significant influences on the phosphorescence wavelength and excited-state dynamics. The syntheses of seven compounds in this series are described, three of which are structurally validated by single-crystal X-ray diffraction. Cyclic voltammetry reveals the effects of ligand modification on the frontier orbital energies. The photophysical properties of all compounds are thoroughly characterized by UV-vis absorption spectroscopy and steady-state photoluminescence at room-temperature and 77 K. Photoluminescence quantum yields and lifetimes of all compounds are reported.

From green to near-infrared emission of cyclometalated Iridium (III) complexes modified with flavonoids: a theoretical insight

We report a theoretical investigation of four cyclometalated Ir (III) complexes constructed with bioactive ligands such as flavonol (1), chrysin (2), kaemferol (3) and luteolin (4). The geometry and electronic structure of the ground and excited states of each complex are studied using DFT and TD-DFT approaches. All Ir (III) complexes absorb UV–Vis light and emit in a wide range of the spectrum: green (4, 537 nm), orange (2, 622 nm), red (3, 641 nm) and near-infrared (1, 928 nm) region. The estimation of phosphorescent quantum yield demonstrates possible higher efficiency for complexes 3 than for others.

External Magnetic Field Driven, Ambidextrous Circularly Polarized Electroluminescence from Organic Light Emitting Diodes Containing Racemic Cyclometalated Iridium(III) Complexes

AbstractThis work reports the development of external magnetic field (EMF) driven, ambidextrous, circularly polarized electroluminescence (CPEL) devices, by embedding racemic mixtures of phosphorescent organoiridium(III) emitters in the active emitting layer. Homoleptic tris‐cyclometalated Ir(III)(ppy)3 and heteroleptic bis‐cyclometalated Ir(III)(ppy)2(acac) (ppy=2‐phenylpyridinate, acac=acetylacetonate) were used as the representative organoiridium(III) compounds. Chiroptical inversion of CPEL was influenced by the ligand environment in the Ir(III) complexes (homoleptic or heteroleptic), as well as the Faraday geometry.

Tuning the emission maxima of iridium systems using benzimidazole-based cyclometallating framework

Herein, we describe the development of four mononuclear cyclometallated iridium complexes [1](PF6) - [4](PF6) using alkyl chain appended benzimidazole-based cyclometallting ligands (L1 and L2). The complexes were characterized using different analytical techniques. The crystal structures of [1](PF6), and [2](PF6) reveal the distorted octahedral geometry around iridium center, and the nitrogen donors of the imidazole unit are coordinated with expected trans orientation. The experimental bond parameters are matching with the DFT calculated data. The theoretical investigation further reveals that the HOMO of the complexes is localized on iridium and cyclometallating ligands, and LUMO is formed by bpy/phen unit. All the complexes are emissive with moderate quantum yield at room temperature. Both the absorption and emission maxima are found to be highly dependent on the substituent present on the ligand framework. A significant shift (∼80 nm) of emission maxima is achieved by employing electron withdrawing fluorophenyl unit to π-conjugated thiophene unit. Tuning emission maxima by simple variation of substitution is exciting and would be of great importance in developing luminescent materials.

Advancing Near-Infrared Phosphorescence with Heteroleptic Iridium Complexes Bearing a Single Emitting Ligand: Properties and Organic Light-Emitting Diode Applications

Cyclometalated iridium complexes, emitters of choice in organic light-emitting diodes (OLEDs), hold great potential for near-infrared (NIR) applications. Upon increasing the conjugation size and chemical complexity of the ligands, as required to push the emission toward the NIR), overall high-molecular-weight complexes (both homoleptic and heteroleptic β-diketonate ones) are obtained, posing related issues in OLED processing. One, so far barely explored, question arises: “Why endow Ir(III) with three or two emissive ligands when one might work just as well?” Herein, as proof of concept for OLED technology, we disclose three novel deep-red to NIR emitters of formula Ir(C^N)2(iqbt), with a single iqbt (1-(benzo[b]thiophen-2-yl)-isoquinolinate) ligand responsible for the emission in the NIR range. (C^N) are cyclometalated ligands with higher triplet energy than that of iqbt. We demonstrate that the presence of a single iqbt ligand is sufficient to enable efficient phosphorescence matching that of homoleptic Ir(iqbt)3; moreover, the Ir(C^N)2(iqbt) based OLEDs display efficiency exceeding the one of Ir(iqbt)3. These compounds offer several important benefits: (i) advantageous synthetic protocols limiting to the last step of the use of novel and synthetic costly NIR ligands (implying a lower amount of ligand required), (ii) commercially available (C^N) to prepare the starting Ir chloro-dimers, and (iii) lower molecular weight of the complexes compared to that of the homoleptic parent ones fruitful for easier vacuum thermal processing of the emitters.

Brominated cyclometalated iridium(III) complexes for mitochondrial immobilization as potential anticancer agents.

Mitochondria-targeted iridium complexes for anticancer studies have received increasing attention in recent years. Herein, three cyclometalated iridium(III) complexes Ir1-Ir3 [Ir(N^C)2(N^N)](PF6) (N^N = 2,2'-bipyridine (bpy)) or 2-(5-bromopyridin-2-yl)benzo[d]thiazole (bpybt); [N^C = 2-phenylpyridine (ppy) or 2-phenylquinoline (pq) or 2-(4-bromophenyl)benzo[d]thiazole (bpbt)] had been explored as potential mitochondria-targeted anticancer agents. All of the complexes mainly localized in the mitochondria and could be fixed on the mitochondria through a nucleophilic reaction with reactive mitochondrial proteins. Further studies revealed that these complexes showed high anticancer activity, induced mitochondrial depolarization, elevated intracellular reactive oxygen species (ROS) levels, restrained thioredoxin reductase (TrxR) activity, and inhibited the formation of tumor cell colonies and angiogenesis. Further mechanistic studies showed that complex Ir2 could markedly stimulate the activation of caspase-3, regulate the expression of Bax and KI67, and trigger apoptosis.

Synthesis and Photophysical Studies on Cyclometalated Iridium Complexes Containing Various Monodentate Phenyl-Azole Ancillary Ligands

Synthesis and Photophysical Studies on Cyclometalated Iridium Complexes Containing Various Monodentate Phenyl-Azole Ancillary Ligands

A cyclometallated iridium(III) complex with multi-photon absorption properties as an imaging-guided photosensitizer.

Conventional photosensitizers (PSs) often have shorter excitation wavelengths and poor cancer cell targeting, resulting in a limited tissue penetration depth and increased biotoxicity, which are significant barriers to ensuring effective photodynamic therapy (PDT) in vivo. In this work, a cyclometallated iridium(III) complex (Ir-Biotin) with a long excitation wavelength and effective cancer cell targeting was designed and synthesized. The initial in vitro assessment indicated that Ir-Biotin shows excellent PDT activity with a high singlet-oxygen (1O2) generation yield (0.19) due to the facilitated intersystem crossing process. Further study shows that Ir-Biotin shows good biocompatibility, has specific selectivity for cancer cells, and can induce apoptosis under laser irradiation. Furthermore, Ir-Biotin can be applied for imaging-guided PDT using an in vivo imaging system, and showed significant anti-tumour effects (tumour growth inhibition value: 87.66%). These results reveal the importance of long excitation wavelengths of photosensitizers for efficient PDT and suggest a promising strategy for developing effective photosensitizers.

Photofunctional cyclometallated iridium(III) polypyridine methylsulfone complexes as sulfhydryl-specific reagents for bioconjugation, bioimaging and photocytotoxic applications.

We report herein near-infrared (NIR)-emitting cyclometallated iridium(III) complexes bearing a heteroaromatic methylsulfone moiety as sulfhydryl-specific reagents; one of the complexes was conjugated to cysteine and cysteine-containing peptides and proteins for bioimaging and photocytotoxic applications.

Enhanced deep red to near-infrared (DR-NIR) phosphorescence in cyclometalated iridium(iii) complexes

Bis-cyclometalated iridium complexes with highly conjugated cyclometalating ligands and electron-rich ancillary ligands have exceptional quantum yields for deep-red to near-infrared phosphorescence.

Enantiopure, luminescent, cyclometalated Ir(III) complexes with N-heterocyclic carbene-naphthalimide chromophore: design, vibrational circular dichroism and TD-DFT calculations.

A series of chiral cyclometalated iridium complexes of the type [Ir(C^N)2(C^C:)], {(C^N) = ppy (2); dfppy (3)} featuring a naphthalimide N-heterocyclic carbene ligand (C^C:) = (Naphthalimide-NHC) are described and fully characterized. The racemic complexes rac-2 and rac-3 were resolved via chiral column chromatography techniques into their corresponding enantiopure species Δ-2, Λ-2, Δ-3, Λ-3 as confirmed by their CD curves. This unique class of molecules containing organic and inorganic chromophores might be used as a platform to probe the stereochemical effect on the photophysical properties. Vibrational circular dichroism (VCD) was used as an important tool to assign successfully the stereochemistry of the enantiomers. TD-DFT calculations are also advanced to support the experimental studies and to rationalize the observed results.

Cyclometalated iridium complexes based on monodentate aminophosphanes.

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh2] and SiMe3NP [SiMe3N(4-tolyl)PPh2] react with [Ir(μ-Cl)(cod)]2 affording tetra- or pentacoordinate complexes of formula [IrCl(L)n(cod)] (L = HNP, n = 1, 2; L = SiMe3NP, n = 1). The reaction of [IrCl(SiMe3NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)3(SiMe3NP)2][IrCl2(CO)2]. The reaction of HNP or SiMe3NP with [Ir(CH3CN)2(cod)][PF6] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ2C,P-NR(4-C6H3CH3)PPh2}(cod)(CH3CN)][PF6] (R = H, SiMe3) as a result of the intramolecular oxidative addition of the tolyl C2-H bond to iridium. The straighforward formation of [IrH{κ2C,P-SiMe3N(4-C6H3CH3)PPh2}(cod)(CH3CN)]+ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(cod)(CH3CN)]+, including the formation of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(HNP)(cod)]+ and [Ir(cod)(HNP)2]+. The "mixed" complex [IrH{κ2C,P-SiMe3N(4-C6H3CH3)PPh2}(HNP)(cod)]+ was obtained upon reaction of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(cod)(CH3CN)][PF6] with SiMe3NP at 233 K. Finally, the reaction of [Ir(CH3CN)2(coe)2][PF6] with SiMe3NP or HNP resulted in the formation of [Ir(CH3CN)2(SiMe3NP)2][PF6] and [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(HNP)2(CH3CN)][PF6], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ2C,P-NH(4-C6H3CH3)PPh2}(HNP)2(CH3CN)]+ - featuring facial and meridional dispositions of the phosphorus atoms, respectively - were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)2(cod)] being the most active, with TOF1 min of about 2300 h-1 (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).

Synthesis, structural characterization and biological properties of cyclometalated iridium(III) complexes containing 2-methyl-3-{(E)-[(quinolin-2-yl)methylidene]amino}quinazolin-4(3H)-one

Two Iridium(III) complexes [Ir(ppy)2((mqmaq)]+ (1) and [Ir(bhq)2((mqmaq)]+ (2) {ppy = 2-phenylpyridine, bhq = benzo[h]quinoline, mqmaq = 2-methyl-3-{(E)-[(quinolin-2-yl)methylidene]amino}quinazolin-4(3H)-one} have been designed, synthesized and structurally characterized. Complex 2 has been structurally characterized by single-crystal X-ray structure determination. The experimentally obtained results of photophysical properties of complexes 1 and 2 well agree with the theoretically calculated results acquired by density functional theory (DFT) studies. The HOMO of complexes 1 and 2 are delimited on the iridium and cyclometalated aromatic ligands, while LUMO, is localized on quinoline-based mqmaq ligand whereas LUMO + 1, and LUMO + 2 are primarily restricted on cyclometalated ligands. The interaction of complexes with calf thymus DNA (CT-DNA) was investigated by absorption titration and emission titration experiments. Furthermore, the cytotoxicity and cellular localization properties of these complexes towards HeLa cells have been investigated.

Fluorescence Imaging and Photodynamic Inactivation of Bacteria Based on Cationic Cyclometalated Iridium(III) Complexes with Aggregation‐Induced Emission Properties (Adv. Healthcare Mater. 24/2021)

Aggregation-Induced Emission In article number 2100706 by Wai-Yeung Wong, Sijie Chen, and co-workers, cationic Ir(III) complexes with aggregation-induced emission properties are reported. These red phosphors can stain both Gram-positive and Gram-negative bacteria and demonstrate various photocytotoxicities against the bacteria. The results imply that phosphorescent metal complexes are potential candidates for bacterial imaging and efficient photo-based antibacterial agents.

Dipyridylmethane Ethers as Ligands for Luminescent Ir Complexes.

This work reports two new cationic heteroleptic cyclometalated iridium complexes, containing ether derivatives of di(pyridin-2-yl)methanol. The new ligands are based on dipyridin-2-ylmethane and are designed to obtain ether-based intermediates with extended electronic conjugation by insertion of π system such as phenyl, allyl and ethynyl. Different synthetic strategies were employed to introduce these units, as molecular wires, between the dipyridin-2-ylmethane chelating portion and the terminal N-containing functional group, such as amine and carbamide. The corresponding complexes show luminescence in the blue region of the spectrum, lifetimes between 0.6 and 2.1 μs, high quantum yield and good electrochemical behavior. The computational description (DFT) of the electronic structure highlights the key role of the conjugated π systems on optical and electrochemical properties of the final products.

Investigation into dual emission of a cyclometalated iridium complex: The role of ion-pairing

Time-resolved emission of [Ir(ppy)2dcbpy]Cl (1a, ppy = 2-phenylpyridine, dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) dissolved in DMF revealed a biexponential decay of the excited state. Prior studies on cyclometalated iridium complexes indicated that such dual emission behavior can be attributed to an impurity, compounds that have two LUMO states that are close in energy, or the presence of ion-pairing. To probe each of these effects, a series of homoleptic, Ir(ppy)3 (2), and heteroleptic complexes, [Ir(ppy)2dcbpy]X [X = PF6−(1b), BPh4−(1c)] and [Ir(ppy)2bpy]X [bpy = 2,2′-bipyridine, X = Cl− (3a) PF6−(3b)], were synthesized and photophysically characterized. Among these complexes, only 1a exhibited dual emission with the emission lifetimes of 430 ± 5 ns (80%) and 125 ± 6 ns (20%) in DMF. These lifetimes were further confirmed with the nanosecond transient absorption kinetics, indicating that the dual emission likely did not originate from an impurity. When probing the emission and absorption kinetics of the salts 1b and 1c only one lifetime of 550 ± 10 ns was observed, indicating that the short lifetime of the Cl− salt (1a) comes from a putative ion-paired complex. Density-Functional Theory (DFT) calculations on 1 and 2 fragments in DMF helped uncover the reason behind the ion-pair formation, with carboxyl substituents in 1a causing a loss in electron density on the 3,3′-position of dcbpy. Additionally, the loss in electron density was experimentally validated with 1H NMR, which showed a noticeable downfield shift of the 3,3′-protons in relative to the shifts in 1b and 1c. The data supports that the dual emission observed in 1a is not from two separately emissive states, rather from the dissociated and ion-paired complexes.

Long-lived cyclometallated iridium complexes: Synthesis, structure, DFT and photocatalytic aspects

Herein, we present synthesis, photophysical and photocatalytic aspects of four mononuclear luminescent iridium complexes ([1]+ - [4]+) incorporating cyclometallating benzimidazole-based (L1 and L2) and phenanthroline pyrazine-based (L3 – L5) ligands. Several analytical techniques were used for the characterization of the complexes. The structural elucidation of [2]+ and [3]+ using x-ray crystallography show a distorted Oh structure with nitrogen donors of cyclometallating ligands are in trans-direction. All the complexes are emitting in the orange-red region and displayed moderately good quantum yield. The complexes have also shown moderately good luminescence lifetime (τ = 80–1900 ns) in different solvents. Importantly, the complex [3]+ exhibits comparative longer emission lifetime (τ = 1800 ns) in water at ambient condition. The complexes have shown moderately good photocatalytic efficacy towards azo dyes degradation in aqueous solution. The mechanistic investigation suggests that in the presence of light in aqueous medium the complexes induce the generation of (HO•/(O2•−) species which is responsible for the photodecomposition of azo dyes.

Theoretical insight on electronic structure and photophysical properties of a series of cyclometalated iridium(III) complexes bearing the substituted phenylpyrazole with different electron-donating or electron-accepting groups.

By using the density functional theory (DFT) and time-dependent density functional theory (TDDFT), the electronic structure and photophysical properties of a series of cyclometalated iridium(III) complexes bearing the substituted phenylpyrazole have been theoretically investigated. All studied iridium(III) complexes have the distorted octahedral geometry with cis-C,C, cis-O,O, and trans-N,N chelate disposition. The lowest lying singlet → singlet absorptions of all studied iridium(III) complexes are respectively located at 405nm, 387nm, 382nm, 370nm, and 387nm. The calculated emission wavelengths for all studied iridium(III) complexes are 654nm, 513nm, 506nm, 505nm and 499nm, respectively. The calculated emission wavelength for complex 4 at the CAM-B3LYP level is in good agreement with the experimental value. From the theoretical results, it can be seen that the electron-donating substituent groups have the important effect on the electronic structure and photophysical properties of all studied iridium(III) complexes. We hope that this study can provide valuable guidance for the design of new phosphorescent organic light-emitting diodes (OLEDs) materials.

Mitochondria-targeted phosphorescent cyclometalated iridium(III) complex for bioimaging of H2S

The selective visualization of H2S in mitochondria is still a challenge, but it correlates closely with mitochondrial damage and some related diseases. In this work, a cyclometalated iridium complex Ir-DNB, [Ir(ppy)2(N^N)](PF6) (ppy = 2-phenylpyridine, N^N = (4′-methyl-[2,2′-bipyridin]-4-yl)methyl 2-((2,4-dinitrophenyl) thio)benzoate) has been explored for the detection of mitochondrial H2S. Adding H2S to a solution of complex Ir-DNB results in a clearly luminescence enhancement, and displays high selectivity and sensitivity. Moreover, this complex displays negligible toxicity and good mitochondrial localization to HeLa cells, and has also been successfully used for endogenous and exogenous H2S imaging in vitro and in vivo.

Mono-/Bimetallic Neutral Iridium(III) Complexes Bearing Diketopyrrolopyrrole-Substituted N-Heterocyclic Carbene Ligands: Synthesis and Photophysics.

The synthesis and photophysics (UV-vis absorption, emission, and transient absorption) of four neutral heteroleptic cyclometalated iridium(III) complexes (Ir-1-Ir-4) incorporating thiophene/selenophene-diketopyrrolopyrrole (DPP)-substituted N-heterocyclic carbene (NHC) ancillary ligands are reported. The effects of thiophene versus selenophene substitution on DPP and bis- versus monoiridium(III) complexation on the photophysics of these complexes were systematically investigated via spectroscopic techniques and density functional theory calculations. All complexes exhibited strong vibronically resolved absorption in the regions of 500-700 nm and fluorescence at 600-770 nm, and both are predominantly originated from the DPP-NHC ligand. Complexation induced a pronounced red shift of this low-energy absorption band and the fluorescence band with respect to their corresponding ligands due to the improved planarity and extended π-conjugation in the DPP-NHC ligand. Replacing the thiophene units by selenophenes and/or biscomplexation led to the red-shifted absorption and fluorescence spectra, accompanied by the reduced fluorescence lifetime and quantum yield and enhanced population of the triplet excited states, as reflected by the stronger triplet excited-state absorption and singlet oxygen generation.