Blue-emitting Iridium Complexes Research Articles

Bulky Iridium NHC Complexes for Bright, Efficient Deep‐Blue OLEDs

AbstractFour new deep‐blue‐emitting iridium(III) NHC complexes containing sterically demanding ligands are synthesized. The four complexes show bright, deep‐blue emission, with emission maxima between 420 and 427 nm in both acetonitrile solution and 30 wt% doped films in TSPO1; the two meridional isomers showing photoluminescence quantum yields, ΦPL, in doped films of 80% and 89%. The two meridional isomers are used to assess the impact of emitters containing bulky, sterically demanding ligands on the performance of organic light‐emitting diodes (OLEDs). OLEDs employing a stepped doping profile with mer‐Ir(tfpi_tmBn)3 as the emitter produce the highest performing devices in this study, with these devices exhibiting deep‐blue [λEL = 429 nm, CIE = (0.16, 0.08)] emission and a maximum external quantum efficiency (EQEmax) of 14.9%, which decreases to 11.7% at 100 cd m−2. The performance of the OLEDs shows very good efficiencies and moderate efficiency roll‐offs in comparison to reported phosphorescent deep‐blue OLEDs with CIEy ≤ 0.08, as required for commercial displays. The promising results suggest that the design strategy of adding steric bulk to blue emitting iridium complexes containing NHC ligands is a useful strategy for reducing intermolecular interactions between emitters in OLEDs.

Unraveling the marked differences of the phosphorescence efficiencies of blue-emitting iridium complexes with isomerized phenyltriazole ligands

Quantum chemical insights into the marked quantum efficiencies of blue-emitting iridium complexes with isomerized ptz ligands.

Functionalized phenylimidazole-based facial-homoleptic iridium(iii) complexes and their excellent performance in blue phosphorescent organic light-emitting diodes

Novel phenylimidazole-based blue-emitting iridium complexes were prepared and they showed the excellent electroluminescent performance.

Solubilised bright blue-emitting iridium complexes for solution processed OLEDs.

Combining a sterically bulky, electron-deficient 2-(2,4-difluorophenyl)-4-(2,4,6-trimethylphenyl)pyridine (dFMesppy) cyclometalating C∧N ligand with an electron rich, highly rigidified 1,1'-(α,α'-o-xylylene)-2,2'-biimidazole (o-xylbiim) N∧N ligand gives an iridium complex, [Ir(dFMesppy)2(o-xylbiim)](PF6), that achieves extraordinarily bright blue emission (ΦPL = 90%; λmax = 459 nm in MeCN) for a cationic iridium complex. This complex is compared with two reference complexes bearing 4,4'-di-tert-butyl-2,2'-bipyridine, and solution-processed organic light emitting diodes (OLEDs) have been fabricated from these materials.

Highly efficient blue and white phosphorescent OLEDs based on an iridium complex

Abstract A new blue-emitting iridium complex, Iridium(III) bis [(2,3,4-trifluorophenyl) -pyridinato-N, C2'] picolinate (Ir(tfpd)2pic) has been designed, synthesized and used to fabricate blue phosphorescent organic light-emitting devices with a efficiency up to 41.4 lm/W (52.6 cd/A). Furthermore, by combining the blue phosphorescence of Ir(tfpd)2pic and the yellow emission of iridium(III) bis [2-(2-naphthyl)-pyridine] (acetylacetonate) (Ir(npy)2acac), highly efficient white emission with a maximum efficiency of 49.0 lm/W (54.5 cd/A) has been achieved. The results indicate that Ir(tfpd)2pic is a promising blue phosphor for the applications in highly efficient blue and white OLEDs.

The New Iridium Complexes Involving Pyridylpyridine Derivatives for the Saturated Blue Emission

To obtain a saturated blue phosphorescent material with a good color purity, we have synthesized the new blue emitting iridium complexes with 2, 6-difluoro-3-(4-methylpyridin-2-yl)pyridine (4-Me-dfpypy) as a main ligand. We expected that the LUMO energy levels of the complex might increase upon introduction of an electron donating group such as a methyl group to the pyridyl moieties of the ligand, leading to a wide energy gap of the complex to give the saturated blue emission. We have also introduced a variety of the ancillary ligands to the iridium center to compare the effect of the ancillary ligards on the emission of their complexes. The resulting iridium complexes, Ir(4-Me-dfpypy)3, Ir(4-Me-dfpypy)2(acac), Ir(4-Me-dfpypy)2(pic) and Ir(4-Me-dfpypy)2(trzl-CH3) where acac, pic, and trzl-CH3 represent acetylacetonate, picolinate, and 2-(5-methyl-2H-1,2,4-triazol-3-yl) pyridinate, respectively exhibited the blue emission at 451, 447, 440 and 425 nm in CH2Cl2 solution. The organic light emitting device (OLED) employing homoleptic Ir(4-Me-dfpypy), as the blue dopant was prepared and their electroluminescence was investigated. Ir(4-Me-dfpypy)3 exhibited the blue emission of CIE coordinates (0.22, 0.32).

Improving optoelectronic properties of the 2,7-polyfluorene derivatives with carbazole and oxadiazole pendants by incorporating the blue-emitting iridium complex pendants in C-9 position of fluorine unit

To study the influence of a blue-emitting iridium complex pendant on the optoelectronic properties of its 2,7-polyfluorene (PF) derivatives with the carbazole and oxadiazole pendants, a class of 2,7-PF derivatives containing carbazole, oxadiazole, and/without the cyclometalated iridium complex pendants in the C-9 positions of fluorene unit were synthesized. Their thermal, photophysical, electrochemical, and electroluminescent (EL) properties were investigated. Among these 2,7-PF derivatives (P1–P4), P2 and P3 exhibited higher photoluminescence efficiency in dichloromethane and better EL properties in the single-emissive-layer polymer light-emitting devices. The highest brightness of 3888 cd/m2 and the maximum current efficiency of 2.9 cd/A were obtained in the P2- and P3-based devices, respectively. The maximum brightness and efficiency levels were 1.7 and 2.1 times, respectively, higher than the corresponding levels from the parent 2,7-PF derivative (P1)-based devices. Our work indicated that EL properties of 2,7-PF derivatives can be improved by introducing the blue-emitting iridium complex into the alkyl side chain of fluorine unit as pendant. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Optimizing blue iridium complex and orange-red osmium complex doping concentrations to improve phosphorescent white organic light emitting diodes

The effect of the phosphor doping concentration on the optoelectronic characteristics of white phosphorescence organic light emitting diodes (PHOLEDs) was studied. A high efficiency white PHOLED was achieved using the unique efficiency properties of the red-emitting Osmium complex ([fptz = 3-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazole, PPh2Me = phosphineligand]; Os(fptz) 2(PPh 2Me) 2) combined with a well known efficient blue-emitting Iridium complex (bis[(4,6-difluorophenyl)-pyridinate-N,C2′; FIrpic]). The results demonstrate that with 2 wt% Os(fptz) 2(PPh 2Me) 2 co-doped into the blue-emitting layer (10 wt% FIrpic doped into the host), the device can achieve a luminance efficiency of up to 40.4 cd/A (25.6 lm/W) with a good corresponding CIE coordinate (0.39, 0.40). The doping concentrations of the red-emitting and blue-emitting phosphors were adjusted to obtain a pure white light with a CIE coordinate of (0.368, 0.366). A high efficiency white PHOLED was achieved by optimizing the doping concentration of the red-emitting and blue-emitting complexes.

Theoretical Studies of Blue-Emitting Iridium Complexes with Different Ancillary Ligands

The structural and electronic properties of two heteroleptic iridium complexes Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) have been investigated theoretically, where dfppy = 2-(2,4-difluorophenyl) pyridine, pic = picolinic acid, and acac = acetoylacetonate. The geometries of ground and excited states are optimized at PBE0/LANL2DZ and CIS/LANL2DZ levels, respectively. Time-dependent density functional theory (TDDFT) method is employed to explore the absorption and emission properties. In the ground state, the highest-occupied molecular orbital has a significant mixture of metal Ir(d) and dfppy(pi), the lowest-unoccupied orbital locates primarily on pi* of pic for FIrpic and pi* of dfppy for FIracac. The luminescence of each complex originates from the lowest triplet excited state, which is assigned to the mixing of metal-to-ligand charge transfer and intraligand charge transfer characters. The effects of ancillary ligands pic and acac on absorption and emission spectra are observed by analysis of TDDFT results. The connection between the nature of excited states and the behavior of the complexes with different ancillary ligands is elucidated.

Theoretical studies of the spectroscopic properties of blue emitting iridium complexes

Electronic structures, absorptions and emissions of a series of (ppy)2Ir(acac) derivatives (ppy = 2- phenylpyridine; acac = acetoylacetonate) with fluoro substituent on ppy ligands were investigated theoretically. The ground and excited states geometries were fully optimized at B3LYP/LANL2DZ and CIS/LANL2DZ level, respectively. The HOMO is composed of d(Ir) and π(C∧N), while the LUMO is localized on C∧N ligand. The absorptions and emissions in CH2Cl2 media were calculated under the TD–DFT level with PCM model. The lowest-lying absorption of these complexes is dominantly attributed to metal-to-ligand and intraligand charge transfer (MLCT/ILCT) transitions and the emission of them originates from 3MLCT/3ILCT excited states. The absorption and emission of these complexes are blue-shifted by increasing the number of fluoro on phenyl, but the spectra are red-shifted by adding fluoro on pyridyl. While a single fluoro of different substituted site on phenyl results in different extent blue-shift to the spectra.

Blue-Emitting Iridium Complexes with Substituted 1,2,4-Triazole Ligands: Synthesis, Photophysics, and Devices

Neutral heteroleptic mononuclear iridium(III) complexes with (2,4-difluoro)phenylpyridine and different pyridine-1,2,4-triazole ligands were synthesized and fully characterized. We investigated the effects of substituents in the 5-position of the triazole ring on the photophysical and electrochemical behavior. Increasing the electron-withdrawing capabilities generally leads to a lowering of the HOMO level with a consequent slight widening of the HOMO-LUMO gap and a blue shift in emission. The complexes reported exhibit high emission quantum yields and long luminescent lifetimes, typical of iridium(III) complexes, and most of them show reversible redox processes in solution. Also, many of the complexes reported here have been obtained as single crystals suitable for X-ray crystallography. Two of the complexes were further tested as phosphorescent dyes in OLED devices and showed high external quantum efficiencies (~7%) and color points better than the "standard" for blue iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2']picolinate (FIrpic). We also report the full electrochemical investigation of FIrpic in different solvents.

Blue emitting iridium complexes: synthesis, photophysics and phosphorescent devices

Homoleptic Ir(Fnppy)3 and heteroleptic (Fnppy)2Ir(acac) complexes (n = 3: F3ppy = 2-(3′,4′,6′-trifluorophenyl)pyridine; n = 4: F4ppy = 2-(3′,4′,5′,6′-tetrafluorophenyl)pyridine; acac = acetylacetonate) have been synthesized and their spectroscopic properties investigated. The homoleptic complexes exist as two stereoisomers, facial (fac) and meridional (mer), that have been isolated and fully characterized. Their electrochemical and photophysical properties have been studied both in solution and in the solid state and electroluminescent devices have been fabricated. The emissive layers in devices have been obtained mixing the iridium complexes with a PVK [poly(9-vinylcarbazole)] host matrix, in the presence of the electron carrier Bu-PBD [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole]. The application of a voltage (5.0–6.5 V) between the electrodes of devices leads to electro-generated blue luminescence which has similar energy to the solution emissions. Interestingly, the stability of the devices made with the homoleptic fluorinated iridium complexes strongly depends on the stereochemistry of these phosphors and high (up to 5.5%) external quantum efficiencies for the fac complexes are measured.

Efficient and bright blue-emitting phosphorescent materials

A new type of ancillary ligand for blue-emitting heteroleptic iridium complexes has been successfully developed. New ligands, 3-(trifluoromethyl)-5-(pyridin-2-yl)-1,2,4-triazolate and 5- (pyridin-2-yl)-tetrazolate, show stronger blue-shifting power than that of the picolate of FIrpic (iridium (III) bis(4,6-difluorophenylpyridinato)picolate). Organic light-emitting diodes (OLEDs) fabricated with a new complex, FIrtaz (iridium (III) bis(4,6-difluorophenylpyridinato)(5-(pyridine-2-yl)-1,2,4-triazo- late) or FIrN4 ((iridium (III) bis(4,6-difluorophenylpyridinato)(5-(pyridin-2-yl)-tetrazolate), as the blue dopant in the host of mCP (1,3-bis(9-carbazolyl)benzene), exhibit near-saturated blue electrophos- phorescence with Commision Internale de l'Eclairage (CIEx,y) coordinates of (0.14, 0.18) and (0.15, 0.24), respectively. Organic phosphorophores have recently achieved one of the major breakthroughs in the area of organic light-emitting diodes (OLEDs). These materials can enhance the effi- ciency of electroluminescence (EL) by three-fold, for which the external quantum efficiency (EQE) is usually limited to ~5% for devices based on fluorescence emitters. Organic materials are mostly hydrocarbons that do not usually emit phosphorescent light from the triplet state, which is often a forbidden state. Heavy atoms, such as Ir, Os, and Pt, provide the effective spin-orbital coupling that partially relieves the forbidden transition. Among these metal complexes, Ir(ppy)3, ppy = 2-phenylpyridinato, is probably one of the most useful materials for OLED applications. Ir(ppy)3 gives green emission with high efficiency, and several other structural modifications have been attempted in the last decade for the required emitting material suitable in the fabrication of full-color OLED displays. 1 Although many green- and red-emitting iridium complexes have been suc- cessfully synthesized, 2 authentic blue-emitting iridium complexes for phosphorescent OLEDs are notably sparse (Scheme 1).