Strong ligand field effects of blue phosphorescent Ir(III) complexes with phenylpyrazole and phosphines

In the paper, we describe new Ir complexes for achieving efficient blue phosphorescence. New blue-emitting mixed-ligand Ir complexes comprising one cyclometalating, two phosphines trans to each other such as Ir(dppz)(PPh3)2(H)(L) (Ll= Cl, NCMe+, CN), [dppz = 3,5-Diphenylpyrazole] were synthesized and studied to tune the phosphorescence wavelength to the deep blue region and to enhance the luminescence efficiencies. To gain insight into the factors responsible for the emission color change and the variation of luminescence efficiency, we investigate the electron-withdrawing capabilities of ancillary ligands using DFT and TD-DFT calculations on the ground and excited states of the complexes. To achieve deep blue emission and increase the emission efficiency, (1) we substitute the phenyl group on the 3-position of the pyrazole ring that lowers the triplet energy enough that the quenching channel is not thermally accessible and (2) change the ancillary ligands coordinated to iridium atom to phosphine and cyano groups known as very strong field ligands. Their inclusion in the coordination sphere can increase the HOMO-LUMO gap to achieve the hypsochromic shift in emission color and lower the HOMO and LUMO energy level, which causes a large d-orbital energy splitting and avoids the quenching effect to improve the luminescence efficiency. The maximum emission spectra of Ir(dppz)(PPh3)2(H)(CI) and Ir(dppz)(PPh3)2(H)(CN) were in the ranges of 439, 432 nm, respectively.     

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.     

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-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.     

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.     

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.     

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.     

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.     

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.     

Heteroleptic tris-cyclometalated iridium(III) complexes with phenylpridine and diphenylquinoline derivative ligands

The synthesis and photophysical study of efficient phosphorescent heteroleptic tris-cyclometalated iridium(III) complexes having two different (C^N) ligands are reported. In order to improve the luminescence efficiency by avoiding triplet-triplet (T-T) annihilation, new heteroleptic tris-cyclometalated iridium complexes, Ir(ppy)₂(dpq), Ir(ppy)₂(dpq-3-F) and Ir(ppy)₂(dpq-CF₃), are designed and prepared where ppy, dpq, dpq-3-F and dpq-CF₃ represent 2-phenylpyridine, 2,4-diphenylquinoline, 2-(3-fluorophenyl)-4-phenylquinoline, and 4-phenyl-2-(4-(trifluoromethyl)phenyl)quinoline, respectively. Ppy ligands and dpq derivatives can act as a source of energy supply. When new heteroleptic tris-cyclometalated iridium complex, Ir(ppy)₂(dpq-3-F) is placed in the lowest excited state, the excitation energy is neither quenched nor deactivated but quickly intermolecularly transferred from two ppy ligands to one luminescent dpq-3-F ligand. Such transfer can occur because the triplet energy level of Ir(ppy)₃ is higher than that of Ir(dpq-3-F)₃ and because Ir(dpq-3-F)₃ was known to have a shorter lifetime than that of Ir(ppy)₃. As a result, Ir(ppy)₂(dpq-3-F) shows strong emission band at 620 nm from dpq-3-F ligand in the end. Thus it allows more reddish luminescent color and improves the luminescence by the decrease of quenching or energy deactivation by decreasing the number of the luminescent ligand. To analyze luminescent mechanism, we calculated these complexes theoretically by using computational method.     

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.     

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.     

Highly efficient phosphorescent organic light emitting diodes based on iridium(III) complex with bulky substituent spacers

We developed highly efficient phosphorescent organic light emitting diodes (PHOLEDs) using iridium(III) complex, fac-tris[4-methyl-2-2(4'-trimethylsilylphenyl)pyridine] [Ir(msippy)3]. PHOLEDs based on Ir(msippy)3 complex exhibit the yellowish-green emission with CIE color coordinates of (0.31,0.64). These device performances were compared with those of the green emitting Ir(ppy)3-based devices. The higher external quantum efficiency (EQE) of 25.6% and the current efficiency of 84.4 cd/A were achieved for Ir(msippy)3-based device. The results show that the complete energy and/or charge transfer from the host to Ir(msippy)3 dopant in the emitting layer (EML) of the device resulted in the higher device efficiencies compared with those of Ir(ppy)3-based devices.     

Extraction of iridium(III) and iridium(IV) chloride complexes by a new sorbent containing sulfur and nitrogen

Sorption of iridium(III) and iridium(IV) chloride complexes from hydrochloric acid and chloride solutions by a sorbent MITKhAT containing sulfur and nitrogen is studied. Kinetic and sorption capacity of the sorbent depending on the composition of a solution and the oxidation state of iridium are determined. The most probable mechanism of sorption and the composition of an iridium complex formed are proposed.     

Ultrabright oxygen optodes based on cyclometalated iridium(III) coumarin complexes

Cyclometalated iridium(III) coumarin complexes represent new types of probes for optical oxygen sensing. In comparison to the most commonly used ruthenium(II) polypyridyl dyes and porphyrin complexes with platinum group metals, they possess much more efficient visible absorption and higher quantum yields, which results in much higher brightnesses. Spectral properties and oxygen sensitivity can be fine-tuned by varying the nature of the coumarin ligand and using respective monomeric or dimeric complexes. When incorporated in a model polystyrene film the probes show optimal dynamics of luminescence decay time for oxygen monitoring in the range from 0% to 100% air saturation. Cross-sensitivity to temperature is significantly lower than for the commonly used ruthenium(II)-tris-4,7-diphenyl-1,10-phenanthroline oxygen probe. The probes, however, exhibit significantly lower photostability, which restricts their application. If long-term measurements are not required, the probes can be successfully used for reliable monitoring of oxygen concentration. High brightness of the complexes makes them particularly attractive for application in thin films (for monitoring of fast processes) and various types of nano- and microparticles, including magnetic beads. If temperature compensation is not applied, the novel optodes result in the lower errors in determination of oxygen content.     

Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes

The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F₂-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)₃ has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F₂-ppy)₃ was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂ were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F₂-ppy)₂ was 3.63% at J = 10 mA/cm².     

Phosphorescence color tuning of oxadiazole-based iridium(III) complexes for organic light emitting diode

The new heteroleptic iridium complexes bearing 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenolate (ODZ), were synthesized and characterized for application to organic light-emitting diodes (OLEDs). As main ligands (C^N), the anions of 2-phenylpyridine (ppy), 2-phenylquinoline (pq) and 2-(2,4-difluorophenyl)pyridine (F2-ppy) were chelated to the iridium center and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenolate (ODZ) was introduced as an ancillary ligand for luminescence modulation of their iridium complexes. We expected that the relative energy levels of the main and ancillary ligands in the complexes could lead to emission color tuning and luminous efficiency improvement by possible inter-ligand energy transfer (ILET). The photoabsorption, photoluminescence and electroluminescence of the complexes were studied. Ir(F2-ppy)2(ODZ), Ir(ppy)2(ODZ) and Ir(pq)2(ODZ) exhibited the photoluminescence maxima between 505-610 nm at room temperature in CH2Cl2, depending on both main and ancillary ligands. The longer pi conjugation in the cyclometallating pq ligands leads to the bathochromic shift in luminescence of their iridium complexes. The electroluminescent properties of the complexes were influenced by ILET.     

Red-phosphorescent OLEDs employing iridium (III) complexes based on 5-benzoyl-2-phenylpyridine derivatives

A series of red-phosphorescent iridium (III) complexes 1-4 based on 5-benzoyl-2-phenylpyridine derivatives was synthesized. Their photophysical and electrophosphorescent properties were investigated. Multilayered OLEDs were fabricated with a device structure ITO/4,4′,4″-tris(N-(naphtalen-2-yl)-N-phenyl-amino)triphenylamine (60 nm)/4,4′-bis(N-naphtylphenylamino)biphenyl (20 nm)/Ir(III) complexes (8%) doped in 4,4′-N,N′-dicarbazolebiphenyl (30 nm)/2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (10 nm)/tris(8-hydroxyquinolinyl)aluminum(III) (20 nm)/Liq (2 nm)/Al (100 nm). All devices exhibited efficient red emissions. Among those, in a device containing iridium complex 1 dopant, the maximum luminance was 14200 cd/m² at 14.0 V. Also, its luminous, power, and quantum efficiency were 10.40 cd/A, 3.44 lm/W and 9.21% at 20 mA/cm², respectively. The peak wavelength of the electroluminescence was 607 nm, with CIE coordinates of (0.615, 0.383) at 12.0 V, and the device also showed a stable color chromaticity with various voltages.