Ligand Design and Preparation, Photophysical Properties, and Device Performance of an Encapsulated-Type Pseudo-Tris(heteroleptic) Iridium(III) Emitter.

The organic molecule 2-(1-phenyl-1-(pyridin-2-yl)ethyl)-6-(3-(1-phenyl-1-(pyridin-2-yl)ethyl)phenyl)pyridine (H3L) has been designed, prepared, and employed to synthesize the encapsulated-type pseudo-tris(heteroleptic) iridium(III) derivative Ir(κ6-fac-C,C',C″-fac-N,N',N″-L). Its formation takes place as a result of the coordination of the heterocycles to the iridium center and the ortho-CH bond activation of the phenyl groups. Dimer [Ir(µ-Cl)(η4-COD)]2 is suitable for the preparation of this compound of class [Ir(9h)] (9h = 9-electron donor hexadentate ligand), but Ir(acac)3 is a more appropriate starting material. Reactions were carried out in 1-phenylethanol. In contrast to the latter, 2-ethoxyethanol promotes the metal carbonylation, inhibiting the full coordination of H3L. Complex Ir(κ6-fac-C,C',C″-fac-N,N',N″-L) is a phosphorescent emitter upon photoexcitation, which has been employed to fabricate four yellow emitting devices with 1931 CIE (x:y) ∼ (0.52:0.48) and a maximum wavelength at 576 nm. These devices display luminous efficacies, external quantum efficiencies, and power efficacies at 600 cd m-2, which lie in the ranges 21.4-31.3 cd A-1, 7.8-11.3%, and 10.2-14.1 lm W1-, respectively, depending on the device configuration.

Alkynyl Ligands as Building Blocks for the Preparation of Phosphorescent Iridium(III) Emitters: Alternative Synthetic Precursors and Procedures.

Alkynyl ligands stabilize dimers [Ir(µ-X)(3b)2]2 with a cis disposition of the heterocycles of the 3b ligands, in contrast to chloride. Thus, the complexes of this class─cis-[Ir(µ2-η2-C≡CPh){κ2-C,N-(C6H4-Isoqui)}2]2 (Isoqui = isoquinoline) and cis-[Ir(µ2-η2-C≡CR){κ2-C,N-(MeC6H3-py)}2]2 (R = Ph, tBu)─have been prepared in high yields, starting from the dihydroxo-bridged dimers trans-[Ir(µ-OH){κ2-C,N-(C6H4-Isoqui)}2]2 and trans-[Ir(µ-OH){κ2-C,N-(MeC6H3-py)}2]2 and terminal alkynes. Subsequently, the acetylide ligands have been employed as building blocks to prepare the orange and green iridium(III) phosphorescent emitters, Ir{κ2-C,N-[C(CH2Ph)Npy]}{κ2-C,N-(C6H4-Isoqui)}2 and Ir{κ2-C,N-[C(CH2R)Npy]}{κ2-C,N-(MeC6H3-py)}2 (R = Ph, tBu), respectively, with an octahedral structure of fac carbon and nitrogen atoms. The green emitter Ir{κ2-C,N-[C(CH2tBu)Npy]}{κ2-C,N-(MeC6H3-py)}2 reaches 100% of quantum yield in both the poly(methyl methacrylate) (PMMA) film and 2-MeTHF at room temperature. In organic light-emitting diode (OLED) devices, it demonstrates very saturated green emission at a peak wavelength of 500 nm, with an external quantum efficiency (EQE) of over 12% or luminous efficacy of 30.7 cd/A.

Pseudo-Tris(heteroleptic) Red Phosphorescent Iridium(III) Complexes Bearing a Dianionic C,N,C',N'-Tetradentate Ligand.

1-Phenyl-3-(1-phenyl-1-(pyridin-2-yl)ethyl)isoquinoline (H2MeL) has been prepared by Pd(N-XantPhos)-catalyzed "deprotonative cross-coupling processes" to synthesize new phosphorescent red iridium(III) emitters (601-732 nm), including the carbonyl derivative Ir(κ4-cis-C,C'-cis-N,N'-MeL)Cl(CO) and the acetylacetonate compound Ir(κ4-cis-C,C'-cis-N,N'-MeL)(acac). The tetradentate 6e-donor ligand (6tt') of these complexes is formed by two different bidentate units, namely, an orthometalated 2-phenylisoquinoline and an orthometalated 2-benzylpyridine. The link between the bidentate units reduces the number of possible stereoisomers of the structures [6tt' + 3b] (3b = bidentate 3e-donor ligand), with respect to a [3b + 3b' + 3b″] emitter containing three free bidentate units, and it permits a noticeable stereocontrol. Thus, the isomers fac-Ir(κ4-cis-C,C'-cis-N,N'-MeL){κ2-C,N-(C6H4-py)}, mer-Ir(κ4-cis-C,C'-cis-N,N'-MeL){κ2-C,N-(C6H3R-py)}, and mer-Ir(κ4-trans-C,C'-cis-N,N'-MeL){κ2-C,N-(C6HR-py)} (R = H, Me) have also been selectively obtained. The new emitters display short lifetimes (0.7-4.6 µs) and quantum yields in a doped poly(methyl methacrylate) film at 5 wt % and 2-methyltetrahydrofuran at room temperature between 0.08 and 0.58. The acetylacetonate complex Ir(κ4-cis-C,C'-cis-N,N'-MeL)(acac) has been used as a dopant for a red PhOLED device with an electroluminescence λmax of 672 nm and an external quantum efficiency of 3.4% at 10 mA/cm2.

Preparation of Tris-Heteroleptic Iridium(III) Complexes Containing a Cyclometalated Aryl-N-Heterocyclic Carbene Ligand.

A new class of phosphorescent tris-heteroleptic iridium(III) complexes has been discovered. The addition of PhMeImAgI (PhMeIm = 1-phenyl-3-methylimidazolylidene) to the dimer [Ir(µ-Cl)(COD)]2 (1; COD = 1,5-cyclooctadiene) affords IrCl(COD)(PhMeIm) (2), which reacts with 1-phenylisoquinoline, 2-phenylpyridine, and 2-(2,4-difluorophenyl)pyridine to give the respective dimers [Ir(µ-Cl){κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6H4-isoqui)}]2 (3), [Ir(µ-Cl){κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6H4-py)}]2 (4), and [Ir(µ-Cl){κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6F2H2-py)}]2 (5), as a result of the N-heterocyclic carbene (NHC)- and N-heterocycle-supported o-CH bond activation of the aryl substituents and the hydrogenation of a C-C double bond of the coordinated diene. In solution, these dimers exist as a mixture of isomers a (Im trans to N) and b (Im trans to Cl), which lie in a dynamic equilibrium. The treatment of 3-5 with Kacac (acac = acetylacetonate) yields isomers a (Im trans to N) and b (Im trans to O) of Ir{κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6H4-isoqui)}(κ2- O, O-acac) (6a and 6b), Ir{κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6H4-py)}(κ2- O, O-acac) (7a and 7b), and Ir{κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6F2H4-py)}(κ2- O, O-acac) (8a and 8b), which were separated by column chromatography. The treatment of 6a with HX in acetone-water produces the protonation of the acac ligand and the formation of the bis(aquo) complex [Ir{κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6H4-isoqui)}(H2O)2]X [X = BF4 (9a[BF4]), OTf (9a[OTf])]. The salt 9a[BF4] reacts with 2-(2-pinacolborylphenyl)-5-methylpyridine in the presence of 40 equiv of K3PO4 to afford Ir{κ2- C, C-(C6H4-ImMe)}{κ2- C, N-(C6H4-isoqui)}{κ2- C, N-(C6H4-Mepy)} (10a). Complexes 6a, 6b, 7a, 7b, 8a, 8b, and 10a are phosphorescent emitters (λem = 465-655 nm), which display short lifetimes in the range of 0.2-5.6 µs. They show high quantum yields both in doped poly(methyl methacrylate) films (0.34-0.87) and in 2-methyltetrahydrofuran at room temperature (0.40-0.93). From the point of view of their applicability to the fabrication of organic-light-emitting-diode devices, a notable improvement with regard to those containing two cyclometalated C,N ligands is achieved. The introduction of the cyclometalated aryl-NHC group allows one to reach a brightness of 1000 cd/m2 at a lower voltage and appears to give rise to higher luminous efficacy and power efficacy.

1,8-Naphthalimides in phosphorescent organic LEDs: the interplay between dopant, exciplex, and host emission.

Four different 1,8-naphthalimide derivatives were examined in phosphorescent organic light emitting diodes (OLEDs), i.e., 1,8-naphthalimide, N-phenyl-1,8-naphthalimide, N-2,6-dibromophenyl-1,8-naphthalimide (niBr), and bis-N,N-1,8-naphthalimide. Photoluminescence from all four naphthalimides have violet-blue fluorescence and phosphorescent bands between 550 and 650 nm (visible at 77 K). While all four compounds gave good glassy films when doped with a phosphorescent dopant, only the niBr films remained glassy for extended periods. OLED studies focused on niBr, with two different architectures. One OLED structure (type 1) had the niBr layer as a doped luminescent layer and an undoped niBr layer to act as a hole-blocking layer. The alternate structure (type 2) utilizes a doped CBP layer as the luminescent layer and the niBr layer is used as a hole-blocking layer only (CBP = 4,4'-N,N'-dicarbazolylbiphenyl). Type 1 and 2 OLEDs were prepared with green, yellow, and red emissive phosphorescent dopants (Irppy, btIr, and btpIr, respectively). The dopants were organometallic Ir complexes, previously shown to give highly efficient OLEDs. Of the three dopants, the btpIr-based OLEDs showed the best device performance in both structures (peak efficiencies for type 2: 3.2% and 2.3 lum/W at 6.3 V; type 1: 1.7% and 1.3 lm/W at 6.1 V). The green and yellow dopants gave very similar performance in both type 1 and 2 devices (peak efficiencies are 0.2-0.3%), which were significantly poorer than the btpIr-based OLEDs. The emission spectrum of the btIr- and btpIr-based devices (type 1 and 2) are the same as the solution photoluminescence spectrum of the dopant alone, while the Irppy device gives a broad red emission line (lambda(max) = 640 nm). The red Irppy.niBr emission line is assigned to an Irppy.niBr exciplex. The type 2 Irppy-based device gave a voltage-dependent spectrum, with the red emission observed at low bias (4-8 V), switching over to strong green emission as the bias was raised. All other devices showed bias-independent spectra. Estimates of HOMO, LUMO, and excited-state energies (dopant, niBr, and exciplex) were used to explain the observed spectral properties of these devices. btpIr-based devices emit efficiently from isolated dopant states (external efficiencies = 3.2 %, 2.3 lum/W). Irppy-based devices emit only from exciplex states, with low efficiency (external efficiency = 0.3%). btIr.niBr films have very similar energies for the dopant, exciplex, and niBr triplet states, such that relaxation can go through any of these states, leading to low device efficiency (external efficiency = 0.4%). High device efficiency is achieved only when dopant emission is the dominant pathway for relaxation, since exciplex and niBr triplet states give either weak or no electroluminescence.