Phosphorescent Iridium Complexes Research Articles

Synthesis and Electroluminescent Characteristics of Red Phosphorescent Iridium(III) Complexes for Highly Efficient Organic Light-Emitting Diodes

Synthesis and Electroluminescent Characteristics of Red Phosphorescent Iridium(III) Complexes for Highly Efficient Organic Light-Emitting Diodes

Novel phosphorescent neutral iridium(III) complex with the steric hindrance for highly efficient red organic light-emitting diodes

A novel and highly efficient thiophenquinolone-based red iridium(III) complex bearing a bulky fluorophenyl moiety is designed and synthesized. The complex shows intensive red phosphorescence (596nm with shoulder at 642nm), high photoluminescence efficiency (0.62) and broad full width at half maximum (81nm). The bulky fluorophenyl moiety introduced into the complex could improve the efficiency of electroluminescence with the maximum current efficiency, power efficiency and the external quantum efficiency up to 29.0cd/A, 30.4lm/W and 17.6% due to the effective steric hindrance in solid states.

Blue Phosphorescent Iridium Complexes with Fluorine‐free Main Ligands for Efficient Organic Light‐emitting Diodes

Two heteroleptic iridium(III) complexes with fluorine‐free pyridylpyrimidine main ligands were designed and synthesized for use as blue phosphorescent emitters in organic light‐emitting diodes (OLEDs). Complexes 1 and 2 have a molecular formula of Ir(C∧N)2(L∧X), where C∧N = 2‐tert‐butyl‐5‐(pyridin‐2‐yl)pyrimidine (b5ppmH) is the main ligand and L∧X = picolinic acid (pic) and 2‐(1H‐tetrazol‐5‐yl)pyridine (ptz) are ancillary ligands. The thermal, photophysical, and electrochemical properties of these complexes were investigated. Even without electron‐withdrawing fluorine substituents, the complexes emitted blue light at 463 and 455 nm with moderate quantum efficiencies (ΦPL = 0.54 and 0.43), respectively. Furthermore, the replacement of the pyridine ring with a pyrimidine ring allowed blue emission without fluorine substituents. An organic light‐emitting device fabricated using complex 2 as a dopant and 1,3‐bis(N‐carbazolyl)benzene (mCP) as a host exhibited a maximum external quantum efficiency of 11.5% and color coordinates of (0.17, 0.26) at 100 cd/m2.

Wide range phosphorescent iridium complexes chelated with 2-naphtyl-thienopyridyl ligands

ABSTRACTPhotophysical luminescent and electrochemical properties of the phosphorescent iridium complexes chelated with 2-naphtylthienopyridyl (2-NTP) ligands were investigated. The use of highly conjugated 2-NTP as an ancillary ligand of the iridium complexes is expected to show bathochromic shift in phosphorescence and lead to wide emission range in wavelength. Thus, we introduced 4-(2-Naphthyl)thieno[3,2-c]pyridine (2-NTP) to the iridium center along with 2-phenylpyridine (ppy) and 2-phenylquinoline (pq) as main ligands. The resulting iridium complexes, (2-NTP)Ir(ppy)2 and (2-NTP)Ir(pq)2, exhibited the photoluminescence at 531 and 611 nm, respectively. They also showed rather large full width at half maximum (FWHM) over 100 nm, covering wide wavelength range. For the application to white organic light-emitting diodes (WOLEDs), such wide range phosphorescent iridium complexes are promising candidates as emitting materials.

Phosphorescent Cationic Iridium(III) Complexes with 1,3,4-Oxadiazole Cyclometalating Ligands: Solvent-Dependent Excited-State Dynamics

To elucidate the nature of low-lying triplet states and the effect of ligand modifications on the excited-state properties of functional cationic iridium complexes, the solvent-dependent excited-state dynamics of two phosphorescent cationic iridium(III) complexes, namely [Ir(dph-oxd)2(bpy)]PF6 (1) and [Ir(dph-oxd)2(pzpy)]PF6 (2), were investigated by femtosecond and nanosecond transient absorption spectroscopy. Upon photoexcitation to the metal-to-ligand charge-transfer (MLCT) states, the excited-state dynamics shows a rapid process (τ=0.7−3 ps) for the formation of solvent stabilized 3MLCT states, which significantly depends on the solvent polarity for both 1 and 2. Sequentially, a relatively slow process assigned to the vibrational cooling/geometrical relaxation and a long-lived phosphorescent emissive state is identified. Due to the different excited-state electronic structures regulated by ancillary ligands, the solvation-induced stabilization of the 3MLCT state in 1 is faster than that in 2. The present results provide a better sight of excited-state relaxation dynamics of ligand-related iridium(III) complexes and solvation effects on triplet manifolds.

Synthesis, structure and DFT calculation of a facial tris-cyclometalated phosphorescent iridium(III) complex containing substituted phthalazine ligands

The facial tris-cyclometalated iridium(III) complex fac-Ir(dmpopopz)3 (dmpopopzH=1-(2,6-dimethylphenyloxy)-4-(4-phenyloxyphenyl)-phthalazine) was synthesized from the reaction of iridium(III) chloride hydrate and ligand dmpopopzH in refluxing ethoxyethanol. The conformation of complex fac-Ir(dmpopopz)3 is described on the basis of the 1H NMR, 13C NMR, FT-IR and mass spectroscopies, the crystal structure of fac-Ir(dmpopopz)3 was determined by X-ray crystallography, and the theoretical calculation of complex fac-Ir(dmpopopz)3 was also investigated by the DFT method. The absorption and emission spectral results exhibit that complex fac-[Ir(dmpopopz)3] may be used as a red-emitting phosphorescent material. Efficient OLEDs have been achieved using the complex fac-[Ir(dmpopopz)3] in the red-emitting region.

Orange–Yellow Phosphorescent Iridium(III) Complex for Solution‐processed Organic Light‐Emitting Diodes: Structural, Optical and Electroluminescent Properties of Bis(2‐phenylbenzothiazole)[2‐(2‐hydroxyphenyl)benzothiazole]iridium(III)

New orange–yellow phosphorescent cyclometalated iridium(III) complex, (bt)2Ir(btz), where btH = 2‐phenylbenzothiazole and btzH = 2‐(2‐hydroxyphenyl)benzothiazole, was synthesized, and its structural, photophysical and electroluminescence properties were investigated to apply for organic light‐emitting diodes (OLEDs) applications. The coordination geometry around iridium metal ion was distorted octahedral, exhibiting cis‐C, C′ and trans‐N, N′ chelate dispositions with narrow ligand bite angles ranging from 79.7° (2) to 84.5° (1). The optical properties revealed that the π‐conjugated nature of bt ligand caused a bathochromic shift in emission and btz ligand played a role in fine color tuning. The synthesized (bt)2Ir(btz) was soluble in common organic solvents and the emitting layer was readily spin‐coated in chlorobenzene solution onto substrate. The OLED fabricated with the configuration of ITO/PEDOT:PSS/PVK:TCTA:OXD‐7: (bt)2Ir(btz) (32:32:28:8)/Bphen/LiF/Al exhibited a maximum external quantum efficiency (EQE) of 4.67% and a maximum luminance efficient (LE) of 8.61 cd/A with CIE (x, y) coordinates of (0.54,0.45).

Effect of n-propyl substituents on the emission properties of blue phosphorescent iridium(iii) complexes.

Ligand substitution is often used for tuning the emission color of phosphorescent iridium(iii) complexes that are used in organic light-emitting diodes. However, in addition to tuning the emission color, the substituents can also affect the radiative and non-radiative decay rates of the excited state and hence the photoluminescence quantum yield. Understanding the substituent effect is therefore important for the design of new iridium(iii) complexes with specific emission properties. Using (time dependent) density functional methods, we investigate the substituent effect of n-propyl groups on the structure, emission color, and emission efficiency of fac-tris(1-methyl-5-phenyl-[1,2,4]triazolyl)iridium(iii) based phosphorescent complexes by comparing the calculated results for structural models with and without the n-propyl substituents. We find that attachment of the n-propyl groups increases the length of three Ir-N bonds, and although the emission color does not change significantly, the radiative and non-radiative rates do, leading to a prediction of enhanced blue phosphorescence emission efficiency. Furthermore, the calculations show that the attachment of the n-propyl groups leads to a larger activation energy to degradation and the formation of dark states.

Synthesis of efficient blue phosphorescent heteroleptic Ir(III) complexes containing 4-alkoxy- or 4-alkylaminopicolinates as ancillary ligands

New blue phosphorescent iridium(III) complexes containing 2-phenylpyridine derivatives as the main ligands and 4-substituted picolinates as the ancillary ligands were synthesized and their photophysical properties were investigated. In addition, the structures of three of the iridium(III) complexes were determined by X-ray crystallography. The photophysical data show that the introduction of electron-donating long-alkoxy or dialkylamino chain (–OCH2CH2OCH2CH3, –OC5H11, –OC6H13, –OC7H13, –N(C3H7)2, or –N(C4H9)2) on the 4-position of picolinate improved the phosphorescence quantum efficiencies (Φp=0.57–0.77) of synthesized iridium(III) complexes with the maximum emission range of 461–464nm at 298K in ethanol solution. Interestingly, the substitution of the methyl group at the 4-position of the pyridine ring of the 2-phenylpyridine main ligands in the complexes with a more strongly electron-donating methoxy or dimethylamino group results in a significant bathochromic shift (69nm) rather than a hypsochromic shift of the phosphorescence emission maxima. Density field theory (DFT) calculations were performed to gain deeper understanding of these emission behaviors. The thermally most stable Ir7 among the synthesized blue phosphorescent iridium(III) complexes was tested as an emitting material in a simple OLED device.

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...

Novel Long-Lifetime Iridium Complex as Lab-on-a-Molecule for Hg2+ and pH-Activatable Probes

Novel phosphorescent iridium complex (pq)2Ir(cys) (where pq = 2-phenylquinoline, cys = l-cysteine) was synthesized and characterized. Bearing a pH-activatable carboxyl group, the complex can act as lab-on-a-molecule for determination of a trace amount of Hg2+, based on the fact that its phosphorescence can be sensitively and selectively quenched by Hg2+. Furthermore, its phosphorescence is reversibly responsive to pH changes in a wide pH range 7–13. Thus, the molecule was developed as highly selective orthogonal signaling protocol for Hg2+ and pH detection by one channel.

Light-Up Mitophagy in Live Cells with Dual-Functional Theranostic Phosphorescent Iridium(III) Complexes.

Phosphorescent Ir(III) complexes are expected to be new multifunctional theranostic platforms that enable the integration of imaging capabilities and anticancer properties. Mitophagy is an important selective autophagic process that degrades dysfunctional mitochondria. Until now, the regulation of mitophagy is still poorly understood. Herein, we present two phosphorescent cyclometalated iridium(III) complexes (Ir1 and Ir2) that can accumulate in mitochondria and induce mitophagy. Because of their intrinsic phosphorescence, they can specially image mitochondria and track mitochondrial morphological alterations. Mechanism studies show that Ir1 and Ir2 induce mitophagy by depolarization of mitochondrial membrane potential, depletion of cellular ATP, perturbation in mitochondrial metabolic status, and induction of oxidative stress. Moreover, no sign of apoptosis is observed in Ir1- and Ir2-treated cells under the same conditions that an obvious mitophagic response is initiated. We demonstrate that Ir1 is a promising theranostic agent that can induce mitophagy and visualize changes in mitochondrial morphology simultaneously.

Highly efficient thienylquinoline-based phosphorescent iridium(III) complexes for red and white organic light-emitting diodes

Highly efficient 2-(thiophen-2-yl)quinoline-based phosphorescent iridium(III) complexes bearing 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine or picolinic acid as ancillary ligands are designed and synthetised. The variation of ancillary ligands is attempted to finely tune the photophysical properties of these complexes, especially the solution phosphorescent quantum yields (ΦPL), full width at half maximum (FWHM), etc. The picolinic acid-based complex displays the slightly red-shifted dual-peak emission compared to triazolpyridine-based one. The complexes show bright emission with broad FWHM up to 83 nm, and the emissions are in red region with the very high absolute ΦPL up to 0.76 in solution. Moreover, high-performance red and three-color-based white organic light-emitting diodes (OLEDs) with excellent color stability have been fabricated. The maximum external quantum efficiencies of red and white OLEDs can reach 16.2% and 15.1%, respectively. The maximum current efficiency and power efficiency of white OLED are as high as 35.5 cd A−1 and 34.0 lm W−1, respectively. Especially, the designed white OLED exhibits excellent spectral stability under wide operating voltage range, and the 1931 Commission Internationale de L'Eclairage of white OLED only changes from (0.43, 0.42) to (0.44, 0.44), the color rendering index is in a narrow range of 75–77.

The new phosphorescent Iridium complexes containing naphthylisoquinoline ligands for WOLEDs

ABSTRACTFor the application to white organic light-emitting diodes (WOLEDs), new orange phosphorescent iridium complexes were designed and prepared. We introduced 1-(8-naphthyl)-isoquinoline ligands (8-NAIQ) to the iridium center along with the ancillary ligands, 2-phenylpyridine (ppy) and 2-phenylquinoline (pq). The resulting iridium complexes synthesized herein are (8-NAIQ)Ir(ppy)2 and (8-NAIQ)Ir(pq)2.The photoabsorption, photoluminescence (PL), and electrochemical properties of the irdium complexes synthesized in this study were investigated. The PL spectra of (8-NAIQ)Ir(ppy)2 and (8-NAIQ)Ir(pq)2 exhibited the luminescence peaks at 536 and 583 nm with the full-width at half-Maximum of 115 and 90 nm, respectively. These complexes exhibited yellow to orange phosphorescence, covering rather wide luminescence wavelength range, which are desirable as orange emitters for two emitting component WOLEDs.

Lessons learned in tuning the optoelectronic properties of phosphorescent iridium(iii) complexes.

This perspective illustrates our approach in the design of heteroleptic cationic iridium(iii) complexes for optoelectronic applications, especially as emitters in electroluminescent devices. We discuss changes in the photophysical properties of the complexes as a consequence of modification of the electronics of either the cyclometalating (C^N) or the ancillary (N^N) ligands. We then broach the impact on these properties as a function of modification of the structure of both types of ligands. We explain trends in the optoelectronic behaviour of the complexes using a combination of rationally designed structure-property relationship studies and theoretical modelling that serves to inform subsequent ligand design. However, we have found cases where the design paradigms do not always hold true. Nevertheless, all these studies contribute to the lessons we have learned in the design of heteroleptic cationic phosphorescent iridium(iii) complexes.

A near-infrared phosphorescent iridium(iii) complex for imaging of cysteine and homocysteine in living cells and in vivo

A novel NIR-emitting iridium(iii) complex was developed to detect Cys/Hcy levels in vitro and in vivo.

Mixed-ligand iridium(iii) complexes as photodynamic anticancer agents.

Many phosphorescent iridium complexes are potent candidates as photodynamic therapeutic agents. In this work, a series of mixed-ligand phosphorescent iridium complexes (Ir1: [Ir(L1)(bpy)Cl](PF6)2; Ir2: [Ir(L1)(ppy)Cl](PF6); Ir3: [Ir(L2)(bpy)Cl](PF6)2; Ir4: [Ir(L2)(ppy)Cl](PF6). L1 = 2,6-bis(2-benzimidazolyl)pyridine; bpy = 2,2'-bipyridine; L2 = 2,6-bis(1-methyl-benzimidazol-2-yl)pyridine; ppy = 2-phenylpyridine) have been synthesized and characterized. These complexes display high luminescence quantum yields and long phosphorescence lifetimes. All the complexes are resistant to hydrolysis in aqueous solutions, and can produce singlet oxygen (1O2) effectively upon irradiation. Ir1 and Ir2 show pH-sensitive emission properties. Interestingly, higher cellular uptake efficiency is observed for Ir2 and Ir4 with the cyclometalated ppy ligand in human lung adenocarcinoma A549 cells. Ir2 with pH-sensitive emission properties can selectively image lysosomes, and Ir4 can specifically target mitochondria. Both Ir2 and Ir4 exhibit potent photodynamic therapy (PDT) effects, with Ir2 displaying a higher phototoxicity index (PI) especially in A549 cells (PI > 54). Mechanism studies indicate that Ir2 and Ir4 can induce apoptosis through reactive oxygen species (ROS) generation and caspase activation upon visible light (425 nm) irradiation. As expected, Ir2 can damage lysosomes more effectively with a pH-sensitive singlet oxygen (1O2) yield, while Ir4 tends to impair mitochondrial function. Nevertheless, the practical application of Ir2 and Ir4 for PDT may be limited to superficial tumors due to the short excitation wavelength (425 nm). Our study gives insights into the design and anticancer mechanisms of new metal-based PDT anticancer agents.

A novel phosphorescent iridium(iii) complex bearing a donor-acceptor-type o-carboranylated ligand for endocellular hypoxia imaging.

The structure-property relationship of carborane-embedded cationic iridium(iii) complexes was investigated in this work, especially with donor-acceptor-type ligands. Firstly, an efficient synthetic approach for the new donor-acceptor-type ligands (2 and 4) was developed. By using these ligands, novel iridium(iii) complexes II and IV were prepared. Complex IV shows about a 92 nm red-shift in solution (ΦP = 0.13 in ethanol), and the emission color has an obvious change from green to red when compared with the Model complex. In addition, the photophysical characteristics of complex IV are quite sensitive to the oxygen level in living cells, based on which endocellular hypoxia imaging of complex IV has been realized by the phosphorescence lifetime imaging microscopy (PLIM) technology. Besides the experimental studies, density functional theory (DFT) and time dependent DFT (TD-DFT) calculations have been successfully applied to investigate the excited-state electronic structures of carborane-modified iridium complexes.

Novel phosphorescent cationic iridium(iii) complexes with o-carboranylation on the ancillary N^N ligand

A novel series of heteroleptic iridium complexes with 2-phenyl-pyridine as a main ligand and carborane-functionalized 2,2'-bipyridine as an ancillary ligand were synthesized, and characterized as [Ir(ppy)2(By)]PF6 (where ppy is 2-phenyl-pyridine, By is 5-(2-R-Cb)-2,2'-bipyridine, R = H (2a), CH3 (2b), Ph (2c), iPr (2d) and iBu (2e), or By is 4-(2-R-Cb)-2,2'-bipyridine while R = H (3a), CH3 (3b), Ph (3c), iPr (3d) and iBu (3e), Cb = o-carboran-1-yl). The R groups and the substitution sites of carborane on the pyridine ring have caused differences in the emission properties of these complexes. In addition, the quantum efficiency of [Ir(ppy)2(By)]PF6 complexes has been tuned as well through the introduction of various 2-R-substituted o-carboranes into the ancillary ligand 2,2'-bipyridine, no matter in the solid state (from 0.12 to 0.25) or in solution (from 0.04 to 0.25). The emission color was tuned from yellow to red by the o-carboranyl unit because of its inductive effect. Density functional theory (DFT) and time dependent DFT (TD-DFT) calculations have been applied to investigate excited-state electronic structures of the newly synthesized complexes, which are consistent with the observed red-shift emissions.

Targeting cancer cell metabolism with mitochondria-immobilized phosphorescent cyclometalated iridium(iii) complexes.

Cancer cell metabolism is reprogrammed to sustain the high metabolic demands of cell proliferation. Recently, emerging studies have shown that mitochondrial metabolism is a potential target for cancer therapy. Herein, four mitochondria-targeted phosphorescent cyclometalated iridium(iii) complexes have been designed and synthesized. Complexes 2 and 4, containing reactive chloromethyl groups for mitochondrial fixation, show much higher cytotoxicity than complexes 1 and 3 without mitochondria-immobilization properties against the cancer cells screened. Further studies show that complexes 2 and 4 induce caspase-dependent apoptosis through mitochondrial damage, cellular ATP depletion, mitochondrial respiration inhibition and reactive oxygen species (ROS) elevation. The phosphorescence of complexes 2 and 4 can be utilized to monitor the perinuclear clustering of mitochondria in real time, which provides a reliable and convenient method for in situ monitoring of the therapeutic effect and gives hints for the investigation of anticancer mechanisms. Genome-wide transcriptional analysis shows that complex 2 exerts its anticancer activity through metabolism repression and multiple cell death signalling pathways. Our work provides a strategy for the construction of highly effective anticancer agents targeting mitochondrial metabolism through rational modification of phosphorescent iridium complexes.