Phosphorescent Heteroleptic Iridium Research Articles

Phosphorescent heteroleptic iridium(III) cyclometallates: Improved syntheses of acetylacetonate complexes and quantum chemical studies of their excited state properties

We have investigated methods to prepare cyclometallated iridium(III) complexes with efficient photoluminescence spanning a broad color palette. In particular, we find that addition of ancillary ligands to chloro-bridged iridium dimers proceeds cleanly in refluxing 1,2-dimethoxyethane (DME) without the need for additional product purification. This represents an improvement over the conventional use of 2-ethoxyethanol which requires column chromatographic separation. Our efforts in this work have focused on acetylacetonate complexes such as (F2ppy)2Ir(acac), where F2ppy = 2-(4′,6′-diflurophenyl)pyridinato. We have prepared fifteen compounds by the route, eight of which are newly reported; in four cases we were able to prepare complexes which were inaccessible via the conventional route. Nine of the complexes were characterized by single crystal x-ray diffraction and possess the same distorted octahedral geometry around the iridium with two bidentate phenylpyridine ligands and one bidentate acetylacetonate ligand. Seven of the complexes exhibited efficient photoluminescence with colors ranging from yellow to blue and quantum yields of 0.51–0.74. All of the compounds with trifluoromethyl or phenyl substituents on the acetylacetone displayed emission in the orange with low quantum efficiency. The use of TD-DFT calculations, along with natural transition orbitals (NTOs), permitted a detailed interpretation of the electronic structures for the complexes. The nature of the acceptor orbitals for the low energy triplet state NTOs proved to be an important predictor for the emission spectra of the compounds.

Theoretical insight into the photophysical properties of five phosphorescent heteroleptic iridium(iii) complexes bearing oxadiazol-substituted amide ligands.

A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra and the phosphorescent emission properties of five heteroleptic iridium(iii) complexes have been performed to obtain a better understanding of structure-property relationships. The key aim was to investigate the effect of π-conjugation on the photophysical properties of these studied complexes. The lowest energy absorption wavelengths are located at 461 nm for 1, 418 nm for 2, 422 nm for 3, 426 nm for 4 and 417 nm for 5. The lowest energy emissions of complexes 1-5 are localized at 572, 463, 583, 707 and 553 nm, respectively, simulated in acetonitrile medium at M052X level. These findings could be useful to provide good guidance for the further design of new potential phosphorescent organic light-emitting diode (OLED) materials.

Theoretical insight into the photophysical properties of six phosphorescent heteroleptic iridium(III) complexes with different monodentate ligands

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the geometrical structures, electronic structures and photophysical properties of six heteroleptic iridium(III) complexes. The key aim is to investigate the effect of substituent groups with different conjugation length and different electron-donating/withdrawing ability on photophysical properties for these studied complexes. The lowest energy absorption wavelengths are located at 416 nm for 1, 426 nm for 2, 458 nm for 3, 470 nm for 4, 459 nm for 5 and 439 nm for 6, respectively. The lowest energy emissions of complexes 1–6 are localized at 559, 722, 890, 914, 648 and 768 nm, respectively, simulated in tetrahydrofuran medium at VSXC level. Furthermore, ionization potential (IP), electron affinities (EA) and reorganization energy (λhole/electron) have also been studied to evaluate the charge transfer and balance properties between hole and electron. It could be useful to design and synthesize potential phosphorescent organic light-emitting diodes (OLEDs) materials.

Tuning the photophysical properties of heteroleptic Ir(III) complexes through ancillary ligand substitution: Experimental and theoretical investigation

A series of new phosphorescent heteroleptic Iridium(III) complexes with 2-phenylpyridine, acetylacetone and 2-picolinic acid as ancillary ligands have been designed and synthesized. The photophysical and electrochemical properties have been investigated experimentally and theoretically. The UV–vis absorption and photoluminescence (PL) emission spectra of the complexes were carried out in solution as well as thin film. The PL study indicates that the Iridium complexes show broadband emission ranging from green to yellowish-green in solution and greenish-yellow to yellow emission in PMMA films with appropriate CIE color gamut. The PLQY of the complexes were found to be in the range of 0.12–0.29. Singlet and triplet energy levels were calculated by using time dependent DFT (TD-DFT) calculations. The excited triplet energy was estimated and correlated to structural and energetic characteristics of the reported host materials. Further, investigations have been performed to explore the optical, electronic, charge transport, and stability properties of Ir(III) complexes as charge transport and emissive materials for organic light emitting devices (OLEDs). Furthermore, all the complexes are promising luminescent and its hole transporting performance is more favorable than electron transport performances. According to theoretical calculations, it is clearly indicating that, Ben-Ir-acac complex is a more potential emitter for OLED applications.

Selective Synthesis and Photophysical Properties of Phosphorescent Heteroleptic Iridium(III) Complexes with Two Different Bidentate Groups and Two Different Monodentate Ligands

Rational synthetic procedures for the preparation of phosphorescent heteroleptic iridium(III) complexes, with controlled stereochemistry, of general formula IrX{κ2-N,C-(qui-C6H4)}(acac)(PiPr3) and their photophysical properties are reported. The d4 pentahydride IrH5(PiPr3)2 (1) promotes the chelate-supported C–H bond activation of 2-phenylquinoline to give IrH2{κ2-N,C-(qui-C6H4)}(PiPr3)2 (2), which affords the five-coordinate monohydride [IrH{κ2-N,C-(qui-C6H4)}(PiPr3)2]BF4 (3) by protonation with HBF4. Treatment of 3 with potassium acetylacetonate (Kacac) yields the hydrido target compound IrH{κ2-N,C-(qui-C6H4)}(acac)(PiPr3) (4). Phenylacetylene and acetylene insert into the Ir–H bond of 3 to form the five-coordinate alkenyl derivatives [Ir((E)-CH═CHR){κ2-N,C-(qui-C6H4)}(PiPr3)2]BF4 (R = Ph (5), H (6)). Similarly to 3, complexes 5 and 6 react with K(acac) to give Ir((E)-CH═CHR){κ2-N,C-(qui-C6H4)}(acac)(PiPr3) (R = Ph (7), H (8)). The chloride counterpart of 4, 7, and 8 has been obtained by starting from I...

Red emitting cyclometallated iridium(III) complexes: Synthesis, characterization and evaluation of biological activities

A series of phosphorescent heteroleptic iridium(III) complexes containing two cyclometallating 6-chloro-2-(4-methoxyphenyl)-3-phenylquinoline (p-OMepq) ligands and one chromophoric ancillary ligand were synthesized and characterized: [(p-OMepq)2Ir(prz)] (1), [(p-OMepq)2Ir(biim)]Cl (2), [(p-OMepq)2Ir(bpy)]Cl (3) and [(p-OMepq)2Ir(dppe)]Cl (4) (where prz=pyrazine-2-carboxylic acid, biim=biimidazole, bpy=2,2′-bipyridine, dppe=1,2-bis(diphenylphosphino)ethane). The absorption, emission, lifetime measurements, cyclic voltammetry, molecular docking studies and thermal analysis of these complexes were systematically investigated. Binding of complexes 1–4 with calf thymus DNA was investigated by UV–Vis, fluorescence, circular dichroic spectroscopy and viscosity measurements. The intrinsic binding constants Kb of complexes 1–4 with CT-DNA, obtained from UV–Vis absorption studies, were 2.3×104, 4.7×104, 4.3×104 and 5.0×104M−1, respectively. The results indicated that the four complexes are able to bind to DNA with different binding affinities in the order 4>2>3>1. Complexes 1–4 exhibit a good binding propensity to bovine serum albumin (BSA) proteins, having relatively high binding constant values. A gel electrophoresis assay demonstrated that complexes 1–4 cannot promote the pBR322 plasmid DNA, plasmid pBluescript II KS+ DNA or pUC19 DNA cleavage in the presence of the reducing agent 3-mercaptopropionic acid. The cytotoxic activity of iridium(III) complexes 1–4 were tested with the human cervical cancer cell line (HeLa) and complex 4 was found to be more active compared to the other complexes.

Synthesis and Characterization of New Phosphorescent Heteroleptic Iridium (III) Complex by Using Tetrazole as an Ancillary Ligand for Organic Light-Emitting Diodes

The yellow emitting tetrazole based heteroleptic iridium(III) complex, bis(diphenylquinoline)iridium(pyridyltetrazole) [(DPQ)2Ir(PyTz)], was synthesized and conformed by 1H-,13C NMR spectral techniques. The purity was also confirmed by HPLC. The thermal, electrochemical, photophysical and electroluminescent properties were intrinsically investigated. The Ir(III) complex is thermally more stable having thermal decomposition temperature (Td, 5% weight loss) more than 350°C and it shows very high glass transition temperature Tg-233°C. We have followed the easy and cost effective solution process for (DPQ)2Ir(PyTz) device fabrication and achieved better performance yellow phosphorescent organic light-emitting diodes (PhOLEDs), maximum external quantum efficiency (EQE) of 5.68%, luminance efficiency 12.63 cd/A, and CIE coordinate of (0.56, 0.43).

Highly Efficient Organic Light‐Emitting Devices Prepared with a Phosphorescent Heteroleptic Iridium (III) Complex Containing 7,8‐Benzoquinoline as the Cyclometalated Ligand

The performance of a heteroleptic bis‐cyclometalated complex based on 7,8‐benzoquinoline ligands is investigated as a new emitter in phosphorescent OLEDs. The devices show very high efficiency at various dopant concentrations and maximum efficiency is achieved at 8% dopant concentration. A comparison of the performance of PHOLEDs with the new emitter versus those with Ir(ppy)3 emitter reveals an improvement of efficiency at all dopant concentrations. These preliminary results obtained with the first bzq‐based emitter indicate that this family of cyclometalated proligands is a promising compound for the development of highly emissive complexes.

Effective Alkoxylation of Phosphorescent Heteroleptic Iridium(III) Compounds Bearing Fluorinated Bipyridine Ligands

Facile one-pot synthetic methods for new alkoxo-functionalized heteroleptic cyclometalated Ir(III) compounds have been developed. Nucleophilic substitution of fluorine atoms in [(dfpypy)2Ir(μ-Cl)]2 (dfpypy = 2′,6′-difluoro-2,3′-bipyridine) by a methoxyethanol in the presence of an ancillary ligand led to the formation of the new alkoxo-functionalized heteroleptic Ir(III) compounds Ir[(RO)2pypy]2(N∧O) in good yields. These compounds have a distorted octahedral geometry around the Ir(III) center with an N,N-trans-meridional configuration. They exhibit intense blue to yellow phosphorescence (λmax = 453–558 nm) with moderate to excellent quautum efficiencies (0.22 to 0.96). Comparative studies on the fluoro analogues, Ir(dfpypy)2(N∧O), were also carried out. Our investigation established that the substitution of a fluorine atom by an alkoxy chain can greatly improve the solubility of the compounds in common organic solvents without significantly altering the emission color and efficiencies, making alkoxy-func...

Highly Efficient Red Emissive Heteroleptic Cyclometalated Iridium(III) Complexes Bearing Two Substituted 2-Phenylquinoxaline and One 2-Pyrazinecarboxylic Acid

Department of Chemistry, Chungnam National University, Daejeon 305-764, KoreaReceived September 20, 2012, Accepted October 25, 2012A series of highly efficient red phosphorescent heteroleptic iridium(III) complexes 1-6 containing twocyclometalating 2-(2,4-substitued phenyl)quinoxaline ligands and one chromophoric ancillary ligand weresynthesized: (pqx)

A promising phosphorescent heteroleptic iridium complex with carbazole-functionalized substituent: Synthesis, photophysical and electroluminescent performances

A promising heteroleptic iridium complex with carbazole-functionalized substituent, i.e., (cbbt)2Ir(acac), in which cbbt was 2-(4-(4-(9H-carbazol-9-yl)butoxy)phenyl)benzo[d]thiazole and acac acetylacetonate, was designed, synthesized and structurally characterized. The complex emitted intensely orange phosphorescence upon photoexcitation with high luminescent quantum efficiency (0.30), short triplet lifetime (0.15μs) and high decomposition temperature (310°C). The electronic properties of (cbbt)2Ir(acac) were also examined according to the density functional theory calculations. Organic light-emitting diodes using (cbbt)2Ir(acac) as doped emitter exhibited orange electrophosphorescence with prominent performances. An extremely high brightness of 17,910cdm−2 at a high voltage (17V) and a maximum luminance efficiency of 25.5cdA−1 were achieved. More importantly, those devices exhibited low efficiency roll-off of 30–35% from 10 to 100mAcm−2, which was probably attributed to the short triplet lifetime of (cbbt)2Ir(acac).

A Robust Yellow‐Emitting Metallophosphor with Electron‐Injection/‐Transporting Traits for Highly Efficient White Organic Light‐Emitting Diodes

With the aim of endowing triplet emitters in the development of organic light-emitting devices (OLEDs) with electron-injection/-transporting (EI/ET) features, the phenylsulfonyl moiety was introduced into the phenyl ring of a 2-phenylpyridine (Hppy) ligand and the yellow phosphorescent heteroleptic iridium(III) complex 1 was developed. It was shown that the SO(2)Ph unit could provide EI/ET character to 1, as indicated from both electrochemical and computational data. Complex 1 is a promising yellow-emitting material for both monochromatic OLEDs and white OLEDs (WOLEDs). The outstanding electronic traits associated with 1, coupled with careful device design, afforded very attractive electroluminescent performances for two-element WOLEDs, including a low turn-on voltage of less than 3.7 V, a maximum brightness of 48,000 cd m(-2), an external quantum efficiency of 13.0%, a luminance efficiency of 34.7 cd A(-1), and a power efficiency of 24.3 Lm W(-1). In addition, a good color rendering index (CRI) of about 74, a stable white color with a Commission Internationale de L'Eclairage (CIE(x,y)) variation of Δ(x, y) < ±(0.02, 0.02), and a correlated color temperature higher than 5130 K were obtained. These encouraging results indicate the potential of these WOLEDs as good candidates for warm indoor lighting sources, as well as the critical contribution of such key EI/ET properties to triplet emitters to advance new OLED research.

Green and blue-green phosphorescent heteroleptic iridium complexes containing carbazole-functionalized β-diketonate for non-doped organic light-emitting diodes

Two novel iridium complexes both containing carbazole-functionalized β-diketonate, Ir(ppy)2(CBDK) [bis(2-phenylpyridinato-N,C2)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)], Ir(dfppy)2(CBDK) [bis(2-(2,4-difluorophenyl)pyridinato-N,C2)iridium(1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate)] and two reported complexes, Ir(ppy)2(acac) (acac=acetylacetonate), Ir(dfppy)2(acac) were synthesized and characterized. The electrophosphorescent properties of non-doped device using the four complexes as emitter, respectively, with a configuration of ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) (20nm)/iridium complex (20nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (5nm)/tris(8-hydroxyquinoline)aluminum (AlQ) (45nm)/Mg0.9Ag0.1 (200nm)/Ag (80nm) were examined. In addition, a most simplest device, ITO/Ir(ppy)2(CBDK) (80nm)/Mg0.9Ag0.1 (200nm)/Ag (80nm), and two double-layer devices with configurations of ITO/NPB (30nm)/Ir(ppy)2(CBDK) (30nm)/Mg0.9Ag0.1 (200nm)/Ag (80nm) and ITO/Ir(ppy)2(CBDK) (30nm)/AlQ (30nm)/Mg0.9Ag0.1 (200nm)/Ag (80nm) were also fabricated and examined. The results show that the non-doped four-layer device for Ir(ppy)2(CBDK) achieves maximum lumen efficiency of 4.54lm/W and which is far higher than that of Ir(ppy)2(acac), 0.53lm/W, the device for Ir(dfppy)2(CBDK) achieves maximum lumen efficiency of 0.51lm/W and which is also far higher than that of Ir(dfppy)2(acac), 0.06lm/W. The results of simple devices involved Ir(ppy)2(CBDK) show that the designed complex not only has a good hole transporting ability, but also has a good electron transporting ability. The improved performance of Ir(ppy)2(CBDK) and Ir(dfppy)2(CBDK) can be attributed to that the bulky carbazole-functionalized β-diketonate was introduced, therefore the carrier transporting property was improved and the triplet–triplet annihilation was reduced.