Theoretical studies of blue phosphorescent iridium(III) complexes with phenylpyrazole and phosphines

New blue emitting mixed ligand iridium(III) complexes comprising one cyclometalating, two phosphines trans to each other such as Ir(dppz)(PPhMe2)2(H)(C1) and Ir(dppz)(PPhMe2)2(H)(CN), [dppz = 3,5-Diphenylpyrazole] were studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. To achieve deep blue emission and increase the emission efficiency, the following must occur: (1) substitution of phenyl group on the 3-position of the pyrazole ring that lower the triplet energy enough that the quenching channel is not thermally accessible, (2) changing ancillary ligands coordinated to iridium atom to phosphine and cyano groups known as very strong field ligands. Using the density functional theory (DFT) and time-dependent DFT method calculations on the ground and excited states of the complexes, we have studied how the ancillary ligand influences the emission peak as well as MLCT transition efficiency. It is showed that the strong-field ancillary ligand such as CN, PPhMe2 alters the energy gap mainly by changing the highest occupied molecular orbitals (HOMO) energy level. Their inclusion in the coordination sphere can increase the energy gap to achieve the hypsochromic shift in emission color and also lower the triplet energy level to avoid the thermal quenching.     

Strong ligand field effects of blue phosphorescent mono-cyclometalated iridium(III) complexes

A series of mono-cyclometalated blue phosphorescent iridium(III) complexes with two phosphines trans to each other and two cis-ancillary ligands, such as Ir(F₂Meppy)(PPhMe₂)₂(H)(Cl), [Ir(F₂Meppy)(PPhMe₂)₂(H)(NCMe)]⁺ and Ir(F₂Meppy)(PPhMe₂)₂-(H)(CN), [F₂Meppy = 2-(2′,4′-difluorophenyl)-4-methyl-pyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. We investigate the electron-withdrawing capabilities of ancillary ligands using the DFT and TD-DFT calculations on the ground and excited states of the three complexes to gain insight into the factors responsible for the emission color change and the different luminescence efficiency. Reducing the molecular weight of phosphine ligand with PPhMe₂ leads to a strategy of the efficient deep blue organic light-emitting devices (OLED) by thermal processing instead of the solution processing. The electron-withdrawing difluoro group substituted on the phenyl ring and the cyano strong field ancillary ligand in the trans position to the carbon atom of phenyl ring increased HOMO-LUMO gap and achieved the hypsochromic shift in emission color. As a result, the maximum emission spectra of Ir(F₂Meppy)(PPhMe₂)₂(H)(Cl), [Ir(F₂Meppy)(PPhMe₂)₂(H)-(NCMe)]⁺ and Ir(F₂Meppy)(PPh-Me₂)₂ (H)(CN) were in the ranges of 446, 440, 439 nm, respectively.     

Synthesis and photophysical studies of blue phosphorescent Ir(III) complexes with dimethylphenylphospine

New blue emitting mixed ligand iridium(III) complexes comprising one cyclometalating, two phosphines trans to each other such as Ir{(CF3)2Meppy}(PPhMe3)2(H)(L) [L = CI, NCMe, CN] [(CF3)2Meppy = 2-(3', 5'-bis-trifluoromethylphenyl)-4-methylpyridine] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. To achieve deep blue emission, the trifluoromethyl group substituted on the phenyl ring and the methyl group substituted on the pyridyl ring increased HOMO-LUMO gap and achieved the hypsochromic shift. To gain insight into the factors responsible for the emission color change and the different luminescence efficiency, we investigate the electron-withdrawing capabilities of ancillary ligands using the DFT and TD-DFT calculations on the ground and excited states of the complexes. From these results, we discuss how the ancillary ligand influences the emission peak as well as the metal to ligand charge transfer (MLCT) transition efficiency. The maximum emission spectra of Ir{(CF3)2Meppy}(PPhMe3)2(H)(Cl), [Ir{(CF3),Meppy)(PPhMe3),(H)(NCMe)]+ and Ir{(CF3)2Meppy}(PPhMe3)2(H)(CN) were in the ranges of 441, 435, 434 nm, respectively.     

Solution-processed high-efficiency organic phosphorescent devices utilizing a blue Ir(III) complex

We report high-efficiency blue phosphorescence organic light-emitting devices by solution process utilizing a blue Ir(III) complex [(F2ppy)2Ir(ph-imz)CN] (F2ppy = 2',6' -difluoro-2-phenyl pyridine and ph-imz = N-phenyl imidazole) blended with the host mCP (N, N'-dicarbazolyl-3,5-benzene), and the inert polymers polystyrene (PS) and polymethylmethacrylate (PMMA). The effects of the dopant confinement in the PS and PMMA matrix on the device performance are studied by field emission transmission electron microscopy (FE-TEM) and atomic force microscopy (AFM). The complex shows photoluminescence peaked at 458 nm with CIE color coordinates (0.14, 0.21) in solution and (0.14, 0.18) in doped PMMA film. The PS based device showed better device performance than the PMMA based device with a maximum luminous efficiency of (etaL) 5.11 cd/A with CIE color coordinates (0.17, 0.29) (at 10 mA/cm2) and a maximum luminance of 9765 cd/m2.     

Improved photoluminescence performance from polymer nanofibers doped with phosphorescent Ir(III) complex

In the present paper, an Ir(III) complex of Ir(PD)(PBT)₂ was synthesized, where PD and PBT stand for pentane-2,4-dione and 2-phenylbenzo[d]thiazole, respectively. The molecular identity of Ir(PD)(PBT)₂ was confirmed by its single crystal. It was found that Ir(PD)(PBT)₂ belonged to monoclinic system with two molecules in each unit cell. Theoretical calculation on Ir(PD)(PBT)₂ suggested that the onset electronic transitions owned a mixed character of metal-to-ligand-charge-transfer and ligand-to-ligand-charge-transfer. By doping this complex into a polymer matrix of poly(vinylpyrrolidone) (PVP), the photophysical features of the resulting composite fibers were discussed and compared with those of Ir(PD)(PBT)₂ solution. It was found that the immobilization in PVP host could improve the photoluminescence performance by restraining the excited state geometric relaxation, showing higher emission energy, longer excited state lifetime and better photostability.     

Efficient red-emitting phosphorescent iridium(III) complexes of fluorinated 2,4-diphenylquinolines

Ir(β) complexes of fluorinated dpqs(dpq-3-F, dpq-4-CF₃) as a cyclometallated ligand were prepared and their photonic properties were investigated, where dpq-3-F and dpq-4-CF₃ represent 2-(3-fluoro-phenyl)-4-phenylquinoline and 4-phenyl-2-(4-trifluoromethylphenyl)quinoline, respectively. Fluorinated dpq derivatives were introduced to the iridium complexes to increase the efficiency compared to Ir(dpq)₂(acac) which was recently reported to have emission wavelength of 614 nm with quantum efficiency of 0.14. These fluorinated ligands and their Ir(III) complexes were computationally calculated by ab initio methods to support our experimental results. It was found that the Ir complex containing dpq-3-F ligands exhibits the largest emission efficiency with maximum emission peak at 593.5 nm. The result of ab initio calculation using the time-dependent density functional theory (TD-DFT) showed that the strong ³MLCT transition of the complex occurs due to the strong coupling between the 5d orbital of the Ir atom and the highest occupied molecular orbitals (HOMOs) of these ligands.     

Highly efficient red phosphorescent Ir(III) complexes for organic light- emitting diodes based on aryl(6-arylpyridin-3-yl)methanone ligands

A series of phosphorescent Ir(III) complexes 1-4 were synthesized based on aryl(6-arylpyridin-3-yl)methanone ligands, and their photophysical and electroluminescent properties were characterized. Multilayer devices with the configuration, Indium tin oxide/4,4′,4″-tris(N-(naphthalene-2-yl)-N-phenyl-amino)triphenylamine (60 nm)/4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (20 nm)/Ir(III) complexes doped in N,N′-dicarbazolyl-4,4′-biphenyl (30 nm, 8%)/2,9-dimethyl-4,7-diphenyl-phenathroline (10 nm)/tris(8-hydroxyquinoline)-aluminum (20 nm)/lithium quinolate (2 nm)/ Al (100 nm), were fabricated. Among these, the device employing complex 2 as a dopant exhibited efficient red emission with a maximum luminance, luminous efficiency, power efficiency and quantum efficiency of 16200 cd/m² at 14.0 V, 12.20 cd/A at 20 mA/cm², 4.26 lm/W and 9.26% at 20 mA/cm², respectively, with Commission Internationale de l'Énclairage coordinates of (0.63, 0.37) at 12.0 V.     

Orange-red phosphorescent OLEDs using iridium(III) complexes with benzoylphenylpyridine ligands containing fluorine and trifluoromethyl groups

We have designed and synthesized four orange-red phosphorescent Ir(III) complexes based on the benzoylphenylpyridine ligand with fluorine and trifluoromethyl substitution. Multilayered OLEDs were fabricated using these complexes as dopant materials. Particularly, by using 1 as a dopant in the emitting layer, a highly efficient orange-red OLED was fabricated, showing a maximum luminance of 10410 cd/m2 at 10 V, a luminous efficiency of 17.47 cd/A, a power efficiency of 7.19 Im/W, an external quantum efficiency of 6.27% at 20 mA/cm2, respectively, and CIE(x,y) coordinates of (0.51, 0.48) at 10 V. Furthermore, a red OLED using dopant 2, with CIE(x,y) coordinates of (0.61, 0.39), exhibited a maximum luminance of 5797 cd/m2 at 10 V, a luminous efficiency of 11.43 cd/A at, a power efficiency of 4.12 Im/W, and an external quantum efficiency of 6.62% at 20 mA/cm2, respectively.     

Microwave synthesis of novel Ir(III) complexes and their application to an electroluminescence device

A microwave synthetic method has been developed for novel electroluminescent Ir(III)-polypyridine complexes. The novel Ir(III)-polypyridine complexes give intensively multi-colored luminescence whose emission peaks are occurred ranging from 450 to 630 nm. The chemical properties and electroluminescence character of these Ir(III) complexes are studied. These Ir complexes exhibit red electrophosphorescence and they might be promising red emitting materials for EL devices.     

Theoretical studies of boron(III) complexes for the new blue luminescent material

Boron(III) complexes, BPh₂(2-py-aza) and Bph₂(2-py-in), are known as blue emitting materials. In this paper, we have studied various ligand effects of boron complex on the absorption (UV) and electroluminescence (EL) peaks computationally. To obtain optical properties, TD-DFT(B3LYP) methods are used with 6-31+G(d) basis set. It was found that EL peaks of those materials are calculated at 454 and 510 nm, which are considerably consistent with experimental data. From the results, we newly proposed two materials, BPh₂(PBI-Me) and BPh₂(PBI-Ph), as blue luminescent materials, whose calculated EL peaks are at 456 and 480 nm, respectively. Through the calculation results, newly designed compounds showed possibility as efficient and promising emitters in EL device.     

New blue phosphorescent iridium complexes containing phenylpyridine and triazole ligands: synthesis and luminescence studies

The synthesis and luminescence of iridium(III) complexes containing new phenylpyridine (C(see test for symbol)N) ligands, 4-Me-4'-F-ppy, 4-Me-4'-CF3-ppy and 4-OMe-4'-CF3-ppy, were studied. These ligands were designed for development of the blue light-emitting iridium complexes by introducing the electron-withdrawing group (F, CF3) and the electron-donating group (Me, OMe) at the para positions of the phenyl and pyridine ligand rings, respectively. As an ancillary ligand, trzl-CMe3 was employed where trzl-CMe3 represents 2-(5-tert-butyl-2H-1,2,4-triazol-3-yl)pyridine. The resulting iridium complexes, Ir(4-Me-4'-F-ppy)2(trzl-CMe3), Ir(4-OMe-4'-CF3-ppy)2 (trzl-CMe3) and Ir(4-Me-4'-CF3-ppy)2(trzl-CMe3) exhibited the blue emission at 472, 484 and 494 nm in CH2Cl2 solution, respectively. Ir(4-Me-4'-F-ppy)2(trzl-CMe3) showed the most hypsochromic shift in photoluminescence (PL) among the complexes prepared herein. In the electroluminescence (EL) spectra, Ir(4-Me-4'-F-ppy)2(trzl-CMe3) and Ir(4-Me-4'-CF3-ppy)2(trzI-CMe3) exhibited the luminescence peak at 437 nm and 496 nm, respectively. In the aspect of blue emission color purity, Ir(4-Me-4'-F-ppy)2(trzl-CMe3) had the CIE coordinates of (0.176, 0.143), very close to the saturated standard blue emission.     

Synthesis and luminescence of a new phosphorescent iridium(III) pyrazine complex

The synthesis and luminescent study of a new iridium pyrazine complex are reported. The iridium complex [Ir(MDPP)2(acac)] (MDPP=5-methyl-2,3-diphenylpyrazine, acac=acetylacetone) shows strong ¹MLCT (singlet metal-to-ligand charge-transfer) and ³MLCT (triplet metal to ligand charge-transfer) absorption at 386 and 507 nm, respectively. Organic light emitting device (OLED) with a configuration of ITO / NPB (30 nm) / NPB: 7% (wt.) Ir(MDPP)₂(acac) (25 nm) / BCP (10 nm) / Alq₃(30 nm) / Mg : Ag (mass ratio 10 : 1)120 nm / Ag(10 nm) exhibits an external quantum efficiency of 6.02% (power efficiency 9.89 lm W^− 1 ) and a maximum brightness of 78,924 cd m^− 2. The device also shows high color purity with a maximum peak at 576 nm without any shoulder.     

Theoretical study of a new phosphorescent iridium(III) quinazoline complex

In this study, Ir(III) complex with 4,6-diphenylquinazoline (DPQN) was designed and characterized theoretically. The Hartree-Fock (HF) method with the 3-21G(d) basis set and density functional theory (DFT) utilizing the B3LYP functional with the 6-31G(d) basis set were used for the geometry optimization and the energy level calculation of the ground state of these complexes, respectively. Excited triplet and singlet states are examined using the time-dependent density functional theory (TD-DFT). As a result, it was found that these complexes produced a deep red emission due to the elongated conjugation length. The Ir(III) complex with DPQN ligands exhibits the large emission efficiency and emits light of the deep red wavelength.     

Efficient red electrophosphorescent organic light-emitting diodes based on the new sensitized heteroleptic tris-cyclometalated Ir(III) complexes

Organic light-emitting device (OLED) was fabricated using the novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, bis(2-phenylpyridine)iridium(III)[2(5′-methylphenyl)-4-diphenylquinoline] [Ir(ppy)₂(dpq-5CH₃)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq-5CH₃) ligand. Generally, the ppy ligand in heteroleptic iridium complexes plays an important role as “sensitizer” in the efficient energy transfer from the host (CBP; 4,4,N,N′-dicarbazolebiphenyl) to the luminescent ligand (dpq-5CH₃). We demonstrated that high efficiency through the “sensitizer” can be obtained, when the T₁ of the emitting ligand is close to T₁ of the sensitizing ligand. The device containing Ir(ppy)₂(dpq-5CH₃) produced red light emission of 614 nm with maximum luminescence efficiency and power efficiency of 8.29 cd/A (at 0.09 mA/cm²) and 5.79 lm/W (at 0.09 mA/cm²), respectively.     

Highly efficient red phosphorescent organic light-emitting devices using iridium(III) complexes based on 2-(biphenyl-3-yl)quinoline derived ligands

Red phosphorescent emitters were synthesized based on Ir(III) phenylquinoline complexes for applications to OLEDs. Ir(III) complexes 1-3 were based on 2-(biphenyl-3-yl)-quinoline units connected to various phenyl groups such as 5-phenyl, 5-(4-fluorophenyl), and 6-phenyl groups. The EL efficiencies were quite sensitive to the structural features of the dopants in the emitting layers. In particular, a high-efficiency red OLED was fabricated using complex 1 as the dopant in the emitting layer. This OLED showed a maximum luminance, luminous efficiency, power efficiency, external quantum efficiency and CIE(x,y) coordinates of 21,600 cd/m2 at 16 V, 11.80 cd/A at 20 mA/cm2, 3.57 Im/W at 20 mA/cm2, 10.90% at 20 mA/cm2, and (x = 0.63, y = 0.32) at 12 V, respectively.     

An Ir(III) complex chemosensor for the detection of thiols

In this study, we report the use of a cyclometalated luminescent iridium(III) complex for the visualization of thiols. The detection of glutathione (GSH) by complex 1 is achieved through the reduction of its phendione N^N donor, which influences the metal-to-ligand charge-transfer (MLCT) of the complex. Complex 1 produced a maximum threefold luminescence enhancement at 587 nm in response to GSH. The linear detection range of 1 for GSH is between 0.2 and 2 M equivalents of GSH, with a detection limit of 1.67 μM. Complex 1 also displays good selectivity for thiols over other amino acids.     

Theoretical study of iridium complexes with phenylpyridine based ligands and phosphines

Recently, iridium complexes with phenylpyridine based ligands and phosphines, Ir(C(see text for symbol)N)2 (PPh3)(CN), [(C(see text for symbol)N) = dfppy, dfMeppy] are reported as blue phosphorescent OLED materials. These iridium complexes have novel blue color and emit light at 441 nm to 439 nm. However, these complexes have low external quantum efficiency because they exhibit less MLCT than iridium complexes with phenylpyridine, and some other ancillary ligands. To improve quantum efficiency of iridium complexes with phenylpyridine based ligands and phosphines, a time dependent density functional theory (TDDFT) study of these phosphors was performed. Using these results, this paper discusses how the ancillary ligand influences the emission peak, as well as the metal to ligand charge transfer (MLCT) transition efficiency.     

Highly efficient phosphorescent blue and white organic light-emitting devices with simplified architectures

Blue phosphorescent organic light-emitting devices (PhOLEDs) with quantum efficiency close to the theoretical maximum were achieved by utilizing a double-layer architecture. Two wide-triplet-gap materials, 1,3-bis(9-carbazolyl)benzene and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene, were employed in the emitting and electron-transport layers respectively. The opposite carrier-transport characteristics of these two materials were leveraged to define the exciton formation zone and thus increase the probability of recombination. The efficiency at practical luminance (100 cd/m²) was as high as 20.8%, 47.7 cd/A and 31.2 lm/W, respectively. Furthermore, based on the design concept of this simplified architecture, efficient warmish-white PhOLEDs were developed. Such two-component white organic light-emitting devices exhibited rather stable colors over a wide brightness range and yielded electroluminescence efficiencies of 15.3%, 33.3 cd/A, and 22.7 lm/W in the forward directions.     

Speciation of chromium(III) and cobalt(III) (amino)carboxylate complexes using capillary electrophoresis

Capillary electrophoresis (CE) can very efficiently resolve different dissolved metal ion species as long as rates of ligand exchange are slow relative to time scales required for electromigration. Here, we detail the separation of several Cr(III) and Co(III) complexes with the multidentate chelating agents iminodiacetic acid, nitrilotriacetic acid, trans-1,2-cyclohexanediaminetetracetic acid, N-(2-hydroxyethyl)ethylenediaminetriacetic acid, trimethylenediaminetetraacetic acid, and ethylenediaminetetraacetic acid. Successes in speciating some Ni(II) and Co(II) complexes are also reported. For Cr(III) and Co(III), subtle differences in metal ion-chelating agent stereochemistry, chelating agent denticity, and number of bridging ligands are discernible due to the high resolving power of CE. New peaks and heightened baselines were encountered when a pH buffer with strong complexing properties (orthophosphate) was employed in the background electrolyte. For this reason, we recommend using pH buffers with very weak or negligible complex properties (e.g., MES and MOPS).