Blue-emitting Iridium Research Articles

Investigation into characteristics of blue iridium (III) pyrimidine complexes with suppressed shoulder peak emission

Three novel blue-emitting iridium (III) complexes (Ir1, Ir2 and Ir3), embedding pyrimidine group have been synthesized. Density functional theory (DFT) calculations demonstrate that pyrimidine complex series occupy less component of triplet ligand centered (3LC) could restrain the vibronic shoulders comparing to the pyridine series. As for Ir1, the relative intensity of shoulder in electroluminescence (EL) spectrum is ∼0.6 of the intensity for dominate peak, relevant full width at half maximum (FWHM) is merely 47 nm. Such parameter gradually zooms out from trifluoro-to monofluoro-substitution in EL spectra. Furthermore, three complexes could obtain efficient EL performance, in which, devices based on Ir3 achieve the peak external quantum efficiency (EQE) of 28.9 %. In addition, the photoelectric properties of the three complexes display a certain regularity and this work indeed provides a strategy to modulate shoulder peak emission for blue phosphorescence.

Tetrafluorinated phenylpyridine based heteroleptic iridium(iii) complexes for efficient sky blue phosphorescent organic light-emitting diodes

Tetrafluorinated phenylpyridine has been used to achieve sky blue-emitting iridium(iii) phosphors and their organic light-emitting diodes exhibit high external quantum efficiencies (∼20%) with cyan emission colours.

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.

Density functional theory investigation on iridium(iii) complexes for efficient blue electrophosphorescence.

The geometrical structures, electronic structures, optoelectronic properties and phosphorescence efficiencies of four blue-emitting phosphors [Ir(fpmi)2(pyim)] (1), [Ir(pyim)2(fpmi)] (2), [Ir(fpmi)2(fptz)] (3), [Ir(tfmppz)2(pyim)] (4), [fpmi = 1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C2′; pyim = 2-(1H-imidazol-2-yl)pyridinato; fptz = 5-(trifluoromethyl-2H-1,2,4-triazol-3-yl)pyridine; tfmppz = 1-(4-trifluoromethylphenyl)pyrazolyl] were investigated by DFT and TDDFT methods. We first optimized geometrical structures in the ground and lowest triplet states, and computed the absorption and emission spectra of 1 and 5[Ir(fpmi)2(pypz)] [pypz = 2-(1H-pyrazol-5-yl)pyridinato], which have been synthesized and characterized in a laboratory, using three functionals, B3LYP, CAM-B3LYP, and M062X. The calculation results were compared with relevant experimental data to assess the performance of the functionals. The suitable methods and functionals were then applied to study properties of the three other complexes. The HOMOs of 1–3 are composed of d(Ir) and π(cyclometalated ligands), however, the HOMO of 4 resides on the pyim ligand, while the LUMOs of all four complexes are dominantly localized on the chelating ligands. The calculated absorption results show that the corresponding absorption peaks for the four mainly studied complexes are almost at the same positions, however, the absorption intensities of the bands differ largely from each other. The lowest energy emissions of the four complexes are localized at 507, 512, 468, and 513 nm, respectively. In order to estimate their efficiencies, we carried out simplified radiative rate constant calculations. It turns out that complex 3, which possesses the shortest emission wavelength and the largest radiative rate constant (kr) value, can be considered as a highly efficient blue-emitting iridium(iii) complex.

Sky Blue-Emitting Iridium(III) Complexes Bearing Nonplanar Tetradentate Chromophore and Bidentate Ancillary.

Tetradentate chelates bearing tripodal arranged terpyridine and a functional pyrazole unit (i.e., L1-H and L2-H) were employed in preparation of Ir(III) complexes [Ir(L1)Cl2] (1) and [Ir(L2)Cl2] (2); subsequent chloride-to-bipyrazolate substitution gave [Ir(L1)(bipz)] (3) and [Ir(L2)(bipz)] (4). Single-crystal X-ray structural studies on 1 and 3 showed the possession of a tetradentate chelate, whereas the remaining cis-sites are occupied by either dual chlorides or the bipz chelate, respectively. Sky blue organic light-emitting diode with peak efficiencies (10.1%, 19.8 cd·A-1, and 20.4 lm·W-1) was successfully fabricated using 3 (or 4) as dopant emitter, highlighting the potential application of this class of Ir(III) phosphor.

Tuning the Geometrical Structures and Optical Properties of Blue-Emitting Iridium(III) Complexes through Dimethylamine Substitutions: A Theoretical Study.

The geometrical structures and photophysical properties of Ir(4,6-dFppy)2(pic) (FIrpic) and its derivative (o-FIr, m-FIr, p-FIr) with dimethylamine substituted at the picolinic acid (N∧O) ligand were fully investigated by density functional theory and time-dependent density functional theory. The simulated electronic structure, as well as absorption and emission spectra of FIrpic are in good agreement with the experimental observations. The introduction of dimethylamine at the N∧O ligand at different positions is beneficial to extend the π-electron delocalization, increase HOMO energy levels, and hence improve the hole injection and transfer ability compared with those of FIrpic. Furthermore, o-FIr, m-FIr, and p-FIr have large absorption intensity and participation of metal-to-ligand charge transfer (MLCT) contribution in the main absorption spectra, which would be useful to improve the intersystem crossing (ISC) from the singlet to triplet excited state. More importantly, the high quantum yield of o-FIr (which is explained based on the detailed analysis of triplet energy, ET1), participation of 3MLCT contribution in the phosphorescent spectra, and energy difference between 3MLCT and triplet metal centered (3MC) d-d excited state compared with m-FIr and p-FIr indicate that o-FIr is expected to be an excellent blue phosphorescence emitter with high efficiency.

Self-host blue-emitting iridium dendrimer for solution-processed non-doped phosphorescent organic light-emitting diodes with flat efficiency roll-off and less phase segregation

A new blue-emitting dendrimer DCzFIrpic, has been successfully designed and synthesized with an iridium-cored as triplet emitter, which is covalently attached to a carbazole dendron through a non-conjugated linkage to form a self-host system. This phosphor possesses high thermal stability (Tg = 197 °C), suitable HOMO/LUMO levels (−5.42 eV/−2.53 eV), relatively high PLQY (79%) in neat films. As a result, this dendrimer was used to form a self-host system for solution-processed non-doped PhOLEDs. The blue-emitting devices achieved a superior CEmax of 15.6 cd/A and an EQEmax of 6.4%. Moreover, it is worth noting that there is a relatively flat efficiency roll-off from the peak efficiencies to the efficiencies at the brightness of 1000 cd/m2 (decreased 1.2% and 3.0%, respectively). This is mainly caused by the less phase segregation in such self-host system, and the molecular dynamics simulations have demonstrated it. At the same time, we noticed that the CE of this non-doped device is 1.6–2.7 fold than that of doped devices at the brightness of 1000 cd/m2, which indicated that this non-doped device is much more suitable for display.

Pyridylpyrazole N^N ligands combined with sulfonyl-functionalised cyclometalating ligands for blue-emitting iridium(iii) complexes and solution-processable PhOLEDs.

A series of blue iridium(iii) complexes (12-15) comprising sulfonyl-functionalised phenylpyridyl cyclometalating ligands and pyridylpyrazole N^N ligands are reported, with an X-ray crystal structure obtained for 12. The complexes are highly emissive with photoluminescence quantum yields of 0.52-0.70 in dichloromethane solutions: two of the complexes (12 and 14) show emissions at λ 457 nm which is considerably blue-shifted compared to the archetypal blue emitter FIrpic (λmax 468 nm). The short excited state lifetimes (1.8-3.3 μs) and spectral profiles are consistent with phosphorescence from a mixture of ligand-centred and MLCT excited states. Density functional (DFT) and time dependent DFT (TD-DFT) calculations are in agreement with the electrochemical properties and the blue phosphorescence of the complexes. The additional mesityl substituent on the pyridylpyrazole ligand of 12 and 13 enhances the solubility of the complexes facilitating thin film formation by solution processing. Phosphorescent organic light-emitting diodes (PhOLEDs) have been fabricated using 12 or 13 in a solution-processed single-emitting layer using either poly(vinylcarbazole) (PVK) or 1,3-bis(N-carbazolyl)benzene (mCP) as host. The most blue-shifted electroluminescence (λ 460 nm, CIEx,y 0.15, 0.21) is obtained for an OLED containing complex 12 and mCP, with a brightness of 5400 cd m-2 at 10 V which is high for PhOLEDs with similar blue CIE coordinates using a solution-processed emitter layer.

Self-Host Blue-Emitting Iridium Dendrimer Containing Bipolar Dendrons for Nondoped Electrophosphorescent Devices with Superior High-Brightness Performance.

A novel self-host blue-emitting iridium dendrimer, namely, B-CzPO, has been designed and synthesized via a postdendronization route, where a bipolar carbazole/triphenylphosphine oxide hybrid is selected as the peripheral dendron instead of the p-type oligocarbazole used in unipolar analogue B-CzG2. This structural modification can render B-CzPO with more balanced charge transportation relative to that of B-CzG2. As a result of the significantly reduced efficiency roll-off, the nondoped phosphorescent organic light-emitting diodes (PhOLEDs) of B-CzPO show a superior high-brightness performance, revealing a luminous efficiency of 21.2, 16.1, and 10.5 cd/A at 1000, 5000, and 10 000 cd/m2, respectively. Compared with that of B-CzG2 (i.e., 7.8 cd/A @5000 cd/m2), more than doubled high-brightness performance is achieved for B-CzPO. The results indicate that the design of self-host phosphorescent dendrimers with a bipolar feature will be a promising strategy to develop efficient nondoped PhOLEDs suitable for high-brightness applications including general illumination and micro displays.

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.

Single molecular tuning of the charge balance in blue-emitting iridium dendrimers for efficient nondoped solution-processed phosphorescent OLEDs

The single molecular tuning of charge balance has been demonstrated here by integrating a p-type dendron, an n-type dendron and a blue emissive Ir core into one dendritic platform. Compared with the commonly used physical blending, not only can the charge balance be well tailored, but also the intrinsic phase separation can be successfully eliminated in such developed single molecular systems (B-TCz2TPO1 and B-TCz1TPO2). As a consequence, their corresponding nondoped solution-processed PhOLEDs achieve more than doubled external quantum efficiencies accompanied by a negligible efficiency roll-off.

The effect of substituted moiety on the optoelectronic and photophysical properties of tris (phenylbenzimidazolinato) Ir (III) carbene complexes and the OLED performance: a theoretical study

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are used to analyse theoretically the optoelectronic, photophysical properties and organic light-emitting diode performance of a series of fac–mer blue-emitting Iridium (III) carbene complexes. Swain–Lupton constant is used to discuss the substituents effect. 5d-orbital splitting and d–d* transitions are calculated to assess the efficiency of the studied complexes. The reorganisation energies (λ), transfer integrals, mobilities, radiative decay rate (kr), and triplet exciton generation fraction (χT) are also calculated. Due to the higher χT of these complexes, the formation of triplet exciton will be more and it will cause a faster intersystem crossing. Two host materials are proposed and host–guest match (Dexter–Förster energy) is also discussed. We hope that this unified work will surely help to design new blue-emitting phosphorescent materials in future.

Tuning the electronic and phosphorescence properties of blue-emitting iridium(III) complexes through different cyclometalated ligand substituents: a theoretical investigation.

The geometric and electronic structures, phosphorescence properties and the organic light-emitting diode (OLED) performance of a series of Ir(III) complexes based on bis[(4,6-di-fluorophenyl)-pyridinato-N,C2']picolinate (FIrpic) were investigated by using density functional theory/time-dependent density functional theory (DFT/TD-DFT), including Ir(III)bis[2-(2,4-difluorophenyl)-4-(tert-butyl)pyridinato-N,C2']picolinate (1a), Ir(III)bis[2-(2,4-difluorophenyl)-4-(n-heptyl)pyridinato-N,C2']picolinate (2a), Ir(III)bis[2-(2,4-difluorophenyl)-4-(3-ethylheptyl)pyridinato-N,C2']picolinate (3a), Ir(III)bis[2-(2,4-difluorophenyl)-4-(2,4,6-trimethylphenyl)pyridinato-N,C2']picolinate (5a), and Ir(III)bis[2-(2,4-difluoro-3-(2,4,6-trimethylphenyl)phenyl)-pyridinato-N,C2'] picolinate (5b). To explore the influence of the substituted positions on the optical and electronic properties of the Ir(III) complexes, seven other new complexes were designed by introducing the substituted groups on the difluorophenyl rings or pyridine rings. After introducing the phenyl substituted groups on the pyridine or difluorophenyl rings of cyclometalated ligands, the HOMO-LUMO energy gap is decreased. Thus, the absorption spectra of 4a and 4b undergo a red-shifting, especially for 4a, and have a stronger absorption strength that will be beneficial to improving their quantum yields, which is proved by the further evaluation of the radiative (k(r)) and non-radiative (k(nr)) rate constants. The combinations of a larger (3)MLCT-(3)MC energy gap, smaller ΔE(S1-T1), and higher contribution of (3)MLCT in the emission process result in the higher quantum yields for complexes 4a and 4b. Besides, the designed complexes 4a and 4b are considered to be potential candidates as blue-emitting materials with better equilibrium between the hole transport (λ(hole)) and electron transport (λ(electron)).

Theoretical investigations on the structural, spectroscopic and photophysical properties of iridium (III) complexes with non-conjugated ligands toward blue phosphor in OLEDs

To achieve true blue-emitting iridium(III) complexes for organic light-emitting diodes, we theoretically designed a complex (dfbpy)2Ir(PˆN) (2, PˆN = 5-(diphenylphosphinomethyl)-3-(trifluoromethyl)pyrazole), dfbpy = 2-(2,4-difluorobenzyl)-pyridine), which derived from its prototype complex (dfppy)2Ir(PˆN) (1, dfppy: 2-(4,6-difluorophenyl)pyridine) by the insertion of methylene group between the pyridyl and phenyl segments. Then, their geometrical and electronic structure, absorption and emission were systematically and comparatively investigated using density functional theory calculations. It is found that the highest occupied molecular orbital (HOMO) (π component) of the functionalized complex 2 resides mainly on metal iridium atom and dfbpy ligand, whereas the lowest unoccupied molecular orbital (LUMO) is predominantly centered on π* orbital of cyclometalated dfbpy chelates. Our calculated results reveal the absorption at the 359 nm for 2 can be attributed to the mixture of MdπLπ*(dfbpy)CT (metal-ligand charge transition, MLCT) and ILπ(dfbpy)→π*(dfbpy)CT (intra-ligand charge transition, ILCT) transitions. In particular, the maxima emission for 2 is at 450 nm, which presents a blue shift up to 80 nm as compared to prototype complex 1, indicating the introduction of methylene group is a feasible strategy to archive true blue-emitting materials.

Self‐Host Blue‐Emitting Iridium Dendrimer with Carbazole Dendrons: Nondoped Phosphorescent Organic Light‐Emitting Diodes

A blue-emitting iridium dendrimer, namely B-G2, has been successfully designed and synthesized with a secondgeneration oligocarbazole as the dendron, which is covalently attached to the emissive tris[2-(2,4-difluorophenyl)-pyridyl]iridium(III) core through a nonconjugated link to form an efficient self-host system in one dendrimer. Unlike small molecular phosphors and other phosphorescent dendrimers, B-G2 shows a continuous enhancement in the device efficiency with increasing doping concentration. When using neat B-G2 as the emitting layer, the nondoped device is achieved without loss in efficiency, thus giving a state-of-art EQE as high as 15.3% (31.3 cdA1, 28.9 lmW1) along with CIE coordinates of (0.16, 0.29).

Highly efficient orange and white electrophosphorescence from solution-processible iridium(III) complexes

The luminescent properties of two cyclometalated iridium complexes containing 2-phenylbenzothiazole ligand were investigated. With diphenylamino groups in the ligands, these iridium complexes can form neat films by spin coating their solutions. It is also possible to prepare their doped films in small molecular host matrix N,N ′-dicarbazolyl-4,4′-biphenyl by spin coating. Partially solution-processed organic light-emitting diodes (OLEDs) were fabricated with a luminance efficiency of 20.18 cd A −1 (7.81%), which is even higher than the vacuum-deposited device data for these two complexes, and also better than those for other solution-processed iridium dopant/small molecular host systems. The two-element white OLEDs containing these orange phosphors and traditional blue-emitting iridium in N,N ′-dicarbazolyl-4,4′-biphenyl host also exhibited high efficiency of 12.39 cd A −1 . It is suggested that the incorporation of the diphenylamino substituents and thus the good hole-injecting and solution processing ability of these iridium complexes are responsible for the good performance. Two iridium complexes decorated by diphenylamino substituents in cyclometalating ligands are suitable for solution processing to fabricate monochromic and white organic light-emitting diodes, which exhibit even better performance than their vacuum-deposited devices or other solution-processed iridium phosphors/small molecular host systems. • The electrophosphorescence of two iridium complexes containing 2-phenylbenzothiazole ligands are studied. • The incorporation of arylamino groups in ligands makes these complexes solution-processible. • High-performance orange and white organic light-emitting diodes were realized with these iridium complexes as doped emitters.

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.

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.