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.     

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.     

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.     

Color tuning of red phosphorescence: New iridium complexes containing fluorinated 2,3-diphenylquinoxaline ligands

The new iridium complexes containing a derivative of 2,3-diphenylquinoxaline as an emitting ligand were synthesized, and their photophysical properties were investigated. The complexes prepared in this study were bis(6-F-2,3-di(p-fluorophenyl) quinoxalinato)iridium acetylacetonate (Ir(6-F-2,3-dpqx-F₂)₂(acac)) and bis(6-F-2,3-di(p-fluorophenyl)quinoxalinato)iridium pyrazolonates (Ir(6-F-2,3-dpqx-F₂)₂(przl1) and Ir(6-F-2,3-dpqx-F₂)₂(przl2)). Their photoluminescence maxima appeared at 638, 632 and 630 nm, respectively. The electroluminescence study revealed that the CIE coordinates of the devices containing Ir(6-F-2,3-dpqx-F₂)₂(acac) and Ir(6-F-2,3-dpqx-F₂)₂(przl2) as a light-emitting dopant were (0.675, 0.313) and (0.685, 0.310), respectively, very close to those of the saturated standard red emission.     

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.     

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.     

Synthesis and characterization of phosphorescent platinum complexes containing phenylpyridazine

Synthesis and characterization of a series of square planar Pt(II)-phenylpyridazine complexes are reported. The complexes have the general structure of (C–N)Pt(O–O), where HC–N is 3-phenyl-pyridazine (ppdz), 3-(3′-trifluoromethylphenyl)pyridazine (3′tfmppdz), 3-(3′-methoxyphenyl)-pyridazine (3′meoppdz), 3-(4′-methoxyphenyl)pyridazine (4′meoppdz), or 3-phenyl-6-chloro-pyridazine (6Clppdz) and HO–O is acetylacetone (Hacac). Reaction of K₂PtCl₄ with a HC–N ligand forms the dimer, (C–N)Pt(μ-Cl)₂Pt(C–N), which is cleaved with Hacac to give the corresponding monomer, (C–N)Pt(O–O). The emission characteristics of these complexes are governed by the substituents of the cyclometalating ligands, showing emission λ_max values from 508 to 610 nm. Strong spin-orbit coupling of the platinum atom allows for the formally forbidden mixing of the ¹MLCT with the ³MCLT and ³(π–π*) states.     

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.     

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.     

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.     

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.     

The synthesis and characterization of Zn–Tryptophan complexes and their luminescence properties

In this paper, flake-like Zn–Tryptophan complexes were successfully synthesized by a refluxing method. We confirm that the reaction time influences Zn–Tryptophan complexes growth during the whole process. The intensity of the peaks of Zn–Tryptophan in photoluminescence spectrum increased twice or more compared with that of pure Tryptophan. This synthesis approach is expected to become an appropriate method for preparation of other M-organism complexes.     

Freestanding silicon quantum dots: origin of red and blue luminescence

In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface.     

In situ synthesis and luminescence characteristics of complexes of europium(III) with 4,6-diacetylresorcinol

The complexes of europium(III) with 4,6-diacetylresorcinol (H₂DAR) and a co-ligand (phen, bpy or 2,2′-bipyridine N,N′-dioxide (2,2′-bpyO₂)) were in situ synthesized in silica matrix via a two-step gel process. The formation of complexes in silica gel was confirmed by the luminescence excitation spectra. The silica gels that contain in situ synthesized europium complexes exhibit the characteristic emission bands of the Eu(III). The results show that there are two ways to enhance the emission intensity of the Eu(III): (i) synthesize the complex in silica matrix and (ii) synthesize the complex with a co-ligand, which coordinates with Eu(III) in the composite system and can efficiently transfer the energy from 4,6-diacetylresorcinol to the Eu(III). The order of the luminescence intensities of the complexes is: Eu₂(DAR)₃(phen)₂-(sol–gel) > Eu₂(DAR)₃(2,2′-bpyO₂)₂-(sol–gel) > Eu₂(DAR)₃ (bpy)₂-(sol–gel) > Eu₂(DAR)₃-(sol–gel) > pure Eu₂(DAR)₃·4H₂O.     

Controllable synthesis and luminescence property of CePO4:Tb nanorods

Highly luminescent Tb³⁺-doped CePO₄ with 1D nanostructures were prepared by a simple hydrothermal method. The obtained CePO₄:Tb has a hexagonal or monoclinic structure under different synthetical process. Uniform 1D nanorods with diameters between 40 and 500 nm, and the length ranging from several hundred nanometres to several micrometres were obtained. It is easy to increase the sizes of the samples by adding some CTAB. Results of the XPS show that there is no Ce⁴⁺ in the samples because of the absence of the signal around 917 eV, which is characteristic of Ce⁴⁺. The study of the photoluminescence of Tb³⁺-doped CePO₄ indicates that the luminescent properties of these nanophosphors are strongly dependent on their structures and morphologies.     

Water-soluble Ln-doped calcium fluoride nanocrystals: Controlled synthesis and luminescence properties

Water-soluble CaF₂ nanocrystals doped with different lanthanide ions (Eu³⁺, Tb³⁺) have been synthesized via a novel method using methanol as solvent. These nanocrystals can well disperse in water, forming a stable and transparent colloidal solution. The colloids of the Eu³⁺ and Tb³⁺ doped nanocrystals display intense red and green luminescence under ultraviolet excitation, respectively. The phase structures, morphology, surface structure and luminescence properties of CaF₂:Ln³⁺ nanocrystals were explored in detail. All of the results showed that these nanocrystals own powerful potentials as Down-conversion fluorescent label materials.