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.


■ INTRODUCTION
There is great interest in iridium(III) phosphorescent emitters because they show a fast S 0 −T 1 intersystem crossing. Such ability allows them to harvest singlet and triplet excitons and to achieve internal quantum efficiencies close to 100%. 1 The highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap in these compounds depends upon the ligands, and therefore, it in principle appears to be possible to design compounds to obtain properties in accordance with the requirements of a given application. 2 Thus, complexes bearing different ligands mobilize special attention since they facilitate a better fine tuning of the features of the emitter. 3 Octahedral complexes coordinating three 3e-donor bidentate ligands (3b) are the most usual. Among them, species bearing two different types, [3b + 3b + 3b′], are particularly valued because they do not present the serious issues associated with the ligand distribution equilibria, 4 which are observed for the heteroleptic emitters [3b + 3b′ + 3b″] containing three different ligands. 5 The [3b + 3b + 3b′]-type emitters commonly contain two orthometalated phenyl-heterocycles (3b) and another ligand (3b′). Dimers [Ir(μ-Cl)(3b) 2 ] 2 are usually the starting point for the preparation of these compounds. In most of the cases, the synthesis procedure involves the replacement of the bridge chlorides by the own 3b′ ligand. 6 Selective postfunctionalization of some coordinated ligands is an alternative procedure, which can be also successfully employed. It takes place in two steps, which include a C−H bromination and subsequently a palladium-catalyzed Suzuki−Miyaura cross-coupling. 7 A third method scarcely used is the building of a new ligand on the metal coordination sphere by multicomponent reactions involving the coupling of several coordinated ligands or coordinated ligands and external molecules. 8 At first glance, it is more challenging and requires the use of starting compounds other than the dimers [Ir(μ-Cl)(3b) 2 ] 2 or derivatives thereof.
A common structural feature of the emitters obtained by these procedures is the mutually trans disposition of the heterocyclic rings, with some rare exception observed with fluorinated phenyl-pyridines. 9 This lack of structural diversity is a consequence of the retention of the stereochemistry of the mononuclear fragments of the dimers [Ir(μ-Cl)(3b) 2 ] 2 during the preparation reactions of the emitters. In the search for emitters with a cis disposition of the heterocycles, some linkers have been designed to tie them, but the rigidity of the resulting organic molecules greatly complicates the reactions usually employed for this type of synthesis. 10 Thus, the stabilization of dimers [Ir(μ-X)(3b) 2 ] 2 with a cis disposition of the heterocycles of the 3b ligands is a target of prominent importance for the field.
A promising alternative to the chloride bridge is the alkynyltype ligands, as chloride behave as monodentate 1e-donors in mononuclear compounds and bridge 3e-donors in bimetallic species. 11 However, the metal−alkynyl bond is significantly more versatile than the metal−chloride. In contrast to chloride, the π-system of the C−C triple bond in principle provides a pathway for electron density delocalization. Thus, the alkynyl anions, isoelectronic with the carbonyl ligand, display moderate π-acceptor ability, which allows them to participate in metal-to-ligand back-bonding. Furthermore, the substituent of the C−C triple bond can govern the contribution of the σligand-to-metal, π-metal-to-ligand, and π-ligand-to-metal bonding components to the metal−alkynyl bonding overall situation. 12 Because the metal−heterocycle and metal−aryl bonds of the chelate chromophores provide an asymmetric bonding situation, such modifications in the metal−alkynyl interaction could be relevant to stabilize a particular disposition of the chelating chromophore. Moreover, an increase in the substituent size should destabilize the bimetallic unit, affording five-coordinate transitory fragments, which could provide pathways to change the mutual disposition of the rings and prevent the retention of the stereochemistry during the reactions of the dimers. An additional advantage of the alkynyl ligands is their potential use in organometallic synthesis as building blocks. 13 We are interested in finishing with the monotonous structures imposed by the dimers [Ir(μ-Cl)(3b) 2 ] 2 . Thus, in the search for alternative starting materials, which would allow the preparation of emitters of the class [3b + 3b + 3b′] coordinating the 3b chromophores with their heterocycles cisdisposed, we have replaced the chloride bridges with acetylides. This paper demonstrates that in contrast to chloride, acetylide anions stabilize dimers [Ir(μ 2 -η 2 -C CR)(3b) 2 ] 2 coordinating the 3b ligands with the corresponding heterocycles in position cis, and such dimers allow to generate emitters [3b + 3b + 3b′], which retain the disposition, using the acetylide bridges as building blocks (Scheme 1).

■ RESULTS AND DISCUSSION
[Ir(μ 2 -η 2 -CCR)(3b) 2 ] 2 Complexes Bearing Cis-Heterocycles. The acetylenic C(sp)−H bond is generally much more reactive than the C(sp 3 )−H and even C(sp 2 )−H bonds. Thus, it affords hydride−metal−alkynyl derivatives by oxidative addition to unsaturated transition metal complexes 14 and generates metal−alkynyl species by heterolytic activation with saturated and unsaturated hydroxide compounds, where the OH group acts as an internal base. 15 The C−H bond reactivity of the terminal alkynes and the ability of the hydroxide ligand to promote the abstraction of the acetylenic hydrogen atom, giving water as a unique subproduct, inspired us to use terminal alkynes and the dihydroxo-bridged dimers trans-[Ir(μ-OH){κ 2 -C,N-(C 6 H 4 -Isoqui)} 2 ] 2 (1) and trans-[Ir(μ-OH){κ 2 -C,N-(MeC 6 H 3 -py)} 2 ] 2 (2) as the precursor molecules to prepare the respective target acetylide dimers. Furthermore, the preparation of these dimers is very easy, 8e and their stability is comparable to that of the respective Cl dimers. The selected orthometalated 1-phenylisoquinoline ligand of dimer 1 should generate emitters in the low-energy region; it is well-known that the increase in the conjugation in the heterocycle by fused aromatic groups produces a red shift in the emission. 16 In contrast, the orthometalated 2-(p-Scheme 1. Contextualization of the Work Inorganic Chemistry pubs.acs.org/IC Article tolyl)pyridine chromophore would afford emitters in the zone of moderate−high energies. Treatment of toluene suspensions of dimer 1 with 5.0 equiv of phenylacetylene and dimer 2 with 5.0 equiv of phenylacetylene and tert-butylacetylene, at room temperature, for 48 h leads to the dimers trans-[Ir(μ 2 -η 2 -CCPh){κ 2 -C,N-(C 6 H 4 -Isoqui)} 2 ] 2 (3) and trans-[Ir(μ 2 -η 2 -CCR){κ 2 -C,N-(MeC 6 H 3 -py)} 2 ] 2 (R = Ph (4), t Bu (5)), respectively, as a result of the OH-promoted heterolytic C(sp)−H bond activation of the respective terminal alkynes (Scheme 2). Complex 3 was obtained as a red solid in 69% yield, after Al 2 O 3 column chromatography purification, whereas the ptolylpyridine counterparts 4 and 5 were isolated as analytically pure-yellow solids in 96 and 73% yields, respectively, without the need for additional purification. In this context, we note that Lalinde and co-workers have prepared in moderate−good yields the related 2-phenylpyridine dimers trans-[Ir(μ 2 -η 2 -C CR){κ 2 -C,N-(C 6 H 4 -py)} 2 ] 2 (R = p-MeC 6 H 4 , p-MeOC 6 H 4 , 1-Np, t Bu, SiMe 3 ), by alkynylation of the chloride precursor trans-[Ir(μ-Cl){κ 2 -C,N-(C 6 H 4 -py)} 2 ] 2 with the corresponding LiCCR or by displacement of acetonitrile from the mononuclear solvento cation [Ir{κ 2 -C,N-(MeC 6 H 3py)} 2 (CH 3 CN) 2 ] + with the acetylide. 17 Complexes 3 and 4 were characterized by X-ray diffraction analysis. Both structures demonstrate the success of the C(sp)−H bond heterolytic activation, which takes place with total retention of the stereochemistry of the dimer precursors; the metal centers retain the cis disposition of the metalated phenyl groups and the trans disposition of the heterocycles, keeping the perpendicular chelate ligands in two groups of parallel planes. Previous density functional theory (DFT) calculations on the dihydroxo-bridged precursor 1 have revealed that this enantiomeric disposition is slightly more stable than a meso form. 8e Figure 1 shows the structure of 3, whereas Figure 2 gives a view of 4. The polyhedron around each metal center is the typical octahedron for a six-coordinate d 6 -ion, with the alkynyl bridge ligands bonded through the terminal carbon atom to a metal center and by the C−C triple bond to the other. The metal−alkynyl distances are in the usual range and compare well with those reported for Lalinde's compounds, 17 whereas the metal−phenyl bond lengths point out a marked difference in trans-influence between the terminal carbon atom of the alkynyl ligand and its triple bond. Thus, in both structures, the Ir−C distances trans to the triple bond are about 0.04 Å shorter than the Ir−C bond lengths trans to the terminal carbon atom. In agreement with the presence of the alkynyl ligands in these complexes, their 13 C{H} NMR spectra, at room temperature, in dichloromethane-d 2 contain two singlets, one of them between 102 and 115 ppm and the other between 70 and 80 ppm, due to the C α Scheme 2. Preparation of Complexes 3−8 Inorganic Chemistry pubs.acs.org/IC Article and C β sp-atoms, respectively. It should be also mentioned that the 1 H and 13 C{ 1 H} spectra of 4 and 5 furthermore reveal that the iridium centers exchange the C β atoms of the alkynyl ligands. Thus, they display only one resonance for the two inequivalent pairs of methyl groups of the orthometalated ptolyl substituents, at around 1.9 ppm in the 1 H and at about 22 ppm in the 13 C{ 1 H}. There are noticeable differences in behavior between the acetylide dimers 3−5 and their precursors 1 and 2 and the chloride counterparts. In contrast to 1 and 2 and the chloride dimers, the mononuclear fragments of 3−5 isomerize in toluene, at 120°C, changing the relative positions of one of the chelates. The isomerization gives rise to the strongly desired dimers cis-[Ir(μ 2 -η 2 -CCPh){κ 2 -C,N-(C 6 H 4 -Isoqui)} 2 ] 2 (6) and cis-[Ir(μ 2 -η 2 -CCR){κ 2 -C,N-(MeC 6 H 3 -py)} 2 ] 2 (R = Ph (7), t Bu (8)), bearing cis-heterocycles (Scheme 2). After 72 h, the transformation is quantitative. As a consequence, complexes 6−8 were isolated as analytically pure orange (6) or yellow (7 and 8) solids in high yields (53−87%). The X-ray diffraction analysis structures of 6 and 7 without a shadow of doubt demonstrate the isomerization and therefore the existence of dimers [Ir(μ-X)(3b) 2 ] 2 , with a cis disposition of the heterocycles of the 3b ligands, when the bridge ligand X is an acetylide group. Figure 3 shows the structure of the isoquinoline derivative 6, whereas Figure 4 shows the structure of the pyridine counterpart. In 3 and 4, the orthometalated ligands lie in two groups of parallel planes. In addition to the heterocycle-phenyl trans disposition in both mononuclear fragments, the most noticeable feature of the structures is the disposition of the acetylide bridges. Located in a perpendicular plane to the N−Ir−C phenyl directions, they dispose the terminal carbon atom trans to the remaining heterocycles, whereas the triple bond lies trans to the phenyl groups. The iridium− alkynyl distances and the iridium−phenyl bond lengths compare well with those of the isomeric precursors. In contrast to 3−5, the structures of the dimers 6−8 are rigid in solution. Consistent with Figure 4, the NMR spectra of 7 and 8, at room temperature, in dichloromethane-d 2 display two singlets assigned to the methyl groups of the p-tolyl substituents at about 1.9 and 2.3 ppm in the 1 H and between 21 and 22 ppm in the 13 C{ 1 H}. The 13 C{ 1 H} spectra also contain the signals due to the C α and C β sp-atoms of the alkynyl bridges, which are observed between 103 and 92 ppm and at about 72 ppm, respectively.   Inorganic Chemistry pubs.acs.org/IC Article Alkynyl Bridges as Building Blocks for the Preparation of New Chelating C,N-ligands. We reasoned that dimers 6−8 should be the entry to novel families of emitter compounds, bearing the heterocycles of the chromophores mutually cis-disposed, since the coordination of the acetylide anions to the iridium centers would produce an increase in the reactivity of the alkynyl triple bond, as a consequence of the nucleophilicity transfer from C α to C β . Thus, the C−C triple bond should be susceptible to add electrophiles to C β and nucleophiles to C α . As a concept validation proof, we decided to study the reactions of dimers 6−8 with 2-aminopyridine that has 2(1H)-pyridinimine as an imino tautomer. 18 Addition of 1.5 equiv of the amine to solutions of 6 and 7 in toluene at 120°C leads to the mononuclear derivatives Ir{κ 2 -C,N-[C(CHPh)-py-NH]}{κ 2 -C,N-(C 6 H 4 -Isoqui)} 2 (9) and Ir{κ 2 -C,N-[C(CHPh)-py-NH]}{κ 2 -C,N-(MeC 6 H 3 -py)} 2 (10), after 24 h, as a result of the cleavage of the bridges of the dimer precursors, the addition of the N−H bond of the heterocycle of the imino tautomer of the N-reagent to the C− C triple bond of the acetylide ligands, and the coordination of the exocyclic imino group to the iridium centers. Complexes 9 and 10 were obtained as red and orange solids in 68 and 76% yields, respectively (Scheme 3).
Complexes 9 and 10 were characterized by X-ray diffraction analysis. Figure 5 gives a view of the structure of the isoquinoline derivative 9, whereas Figure 6 shows the structure of the pyridine complex 10. Both structures prove the addition of the 2(1H)-pyridinimine tautomer to the triple bonds of the dimer precursors. The reactions give rise to a 3e-donor C,Nchelating styrylpyridinimine ligand. Thus, the polyhedron around the metal centers can be idealized as octahedrons defined by three C,N-chelating ligands with fac dispositions of carbons and heteroatoms. The most remarkable characteristic of the generated ligand is the E-stereochemistry of the styryl moiety, with the hydrogen atom pointing out the electron cloud of the orthometalated substituent of one of the heterocycles and the metal fragment and the phenyl group trans-disposed with regard to the C−C double bond. The 1 H and 13 C{ 1 H} NMR spectra, at room temperature, in dichloromethane-d 2 reveal that in solution, these compounds exist as a mixture of E-and Z-styryl isomers, in about 3:2 molar ratio. Thus, the 1 H spectra display two broad singlets at about 5.8 and 5.4 ppm due to the NH-hydrogen atom of the imine moiety, whereas the signals due to the CHPh-hydrogen atom are observed at 6.43 (9) and 6.67 (10) ppm for an isomer and around 4.9 ppm for the other. We assume that isomer E is the major one in both cases since it has lower steric hindrance and its styryl CHPh resonance appears at higher field as a Scheme 3. Preparation of Complexes 9−12 Inorganic Chemistry pubs.acs.org/IC Article consequence of the ring current effect. In the 13 C{ 1 H} spectra, the resonances corresponding to the endocyclic carbon atom of the styryl moiety appear close to 150 ppm for both isomers of both complexes. The styrylpyridinimine ligand of 9 and 10 rearranges to give an iridaimidazo [1,2-a]pyridine bicycle, in toluene, at 120°C. The transformation is slow and partial. Thus, under the abovementioned conditions, complexes 9 and 10 evolve to the iridaimidazopyridine derivatives Ir{κ 2 -C,N-[C(CH 2 Ph)Npy]}-{κ 2 -C,N-(C 6 H 4 -Isoqui)} 2 (11) and Ir{κ 2 -C,N-[C(CH 2 Ph)-Npy]}{κ 2 -C,N-(MeC 6 H 3 -py)} 2 (12), to afford a mixture of both classes of constitutional isomers, in about 7:3 molar ratio, after a week (Scheme 3). Complexes 11 and 12 were separated from the mixture by silica column chromatography and isolated as orange and yellow solids, respectively, in about 10% yield.
The isoquinoline derivative 11 was characterized by X-ray diffraction analysis. The structure, which contains two chemically equivalent but crystallographically independent molecules in the asymmetrical unit, demonstrates the formation of the iridaimidazo[1,2-a]pyridine bicycle. It formally results from the addition of the NH 2 group of the amino tautomer of 2-aminopyridine to the triple bonds of the dimeric precursors. As shown for one of the molecules in Figure 7, the donor atoms of the ligands define an octahedron around the iridium atom, displaying fac dispositions of carbons and heteroatoms, in a similar manner to its styrylpyridinimine isomer. The most noticeable features of the structure are the bond lengths in the five-member metallaimidazo ring. The distances Ir−C(1) of 1.992(10) and 1.998(9) Å, C(1)−N(2) of 1.326(11) and 1.294(11) Å, and N(2)−C(9) of 1.336 (12) and 1.389(11) Å, which are intermediate between single and double bonds, suggest that there is electron delocalization in the bond sequence Ir (1) 19 However, the values of the nuclear independent chemical shift (NICS) computed at the center of the five-member ring and out of plane at 1 Å above and below the center (−1.7, −1.2, and −1.4 ppm) are scarcely negative, pointing out very poor aromaticity. The 1 H and 13 C{ 1 H} NMR spectra of 11 and 12, at room temperature, in dichloromethane-d 2 are congruous with Figure 7. In the 1 H spectra, the most remarkable details are the absence of any NH and CHPh resonances and the presence of an AB spin system centered at about 4.0 ppm and defined by Δν ≈ 44 Hz and J A−B ≈ 13 Hz, due to the CH 2 Ph substituent of the generated five-member ring. In agreement with a significant double character for the Ir−C bond in the latter, the resonance corresponding to such a carbon atom appears at notable low field, about 228 ppm, in the 13 C{ 1 H} spectra.
The tert-butyl group destabilizes the styrylpyridinimine isomer, while it decreases the activation energy for the formation of the iridaimidazopyridine derivative. Thus, in contrast to 6 and 7, the treatment of suspensions of the dimer 8, in toluene, with 1.5 equiv of 2-aminopyridine, at 120°C, for 24 h directly leads to Ir{κ 2 -C,N-[C(CH 2 t Bu)Npy]}{κ 2 -C,N-(MeC 6 H 3 -py)} 2 (13) with no observation of any styrylpyridinimine isomer (Scheme 4). Complex 13 was isolated as a yellow solid in 55% yield. In accordance with 11 and 12, its 1 H NMR spectrum, in dichloromethane-d 2 , at room temperature, shows an AB spin system at 2.66 ppm and defined by Δν = 36 Hz and J A−B = 14.8 Hz, whereas the 13 C{ 1 H} contains the expected singlet at 234.5 ppm, two characteristic resonances supporting the formation of the iridaimidazo [1,2-a]pyridine bicycle also in this case.
Photophysical and Electrochemical Properties of the Iridaimidazopyridine Derivatives. Table 1 gathers selected absorptions from the UV−vis spectra of 10 −5 M solutions of   Figures S4−S6 give views of the frontier orbitals. The spectra can be divided into three energy regions: <350, 350−450, and >450 nm. The absorptions observed at energies higher than 350 nm result from 1 π−π* intra-and interligand transitions. Bands in the range of 350−450 nm correspond to metal-to-ligand combined with ligand-to-ligand or intraligand spin-allowed charge transfers. Weak absorption tails after 450 nm were attributed to formally spin-forbidden transitions, mainly HOMO-to-LUMO, produced by a large spin−orbit coupling resulting from the iridium presence. The HOMO is disposed on the metal center (41−47%) and the 3b (49−51%) and 3b′ (6− 8%) ligands, while the LUMO is mainly situated on the 3b ligands (91−96%). The electrochemical behavior of 11−13 was analyzed to obtain additional information about their frontier orbitals. The cyclic voltammetry measurements were carried out in dichloromethane under argon, using [Bu 4 N]PF 6 as a supporting electrolyte (0.1 M). Figure S8 shows the voltammograms. Table 2 gathers the potentials versus Fc/ Fc + . It also includes the HOMO energy levels, obtained from the oxidation potentials, and HOMO and LUMO energy levels were DFT-calculated. Complex 11 exhibits two irreversible oxidations at 0.51 and 1.02 V, whereas two quasi-reversible oxidations are observed for the pyridine counterparts 12 and 13 between 0.30 and 0.95 V. As expected, the HOMO− LUMO gap is significantly smaller for the isoquinoline derivative 11 than for the p-tolylpyridine species 12 and 13.
Complexes 11−13 are the first members of the iridaimidazopyridine family of phosphorescent iridium(III) emitters. They are emissive upon photoexcitation in a doped poly-(methyl methacrylate) (PMMA) film at 5 wt %, at room temperature, and 2-MeTHF at room temperature and at 77 K. Table 3 collects the main photophysical features. The estimated values, from the difference in energy between the optimized triplet states T 1 and the singlet states S 0 in tetrahydrofuran, are almost equal to those experimentally obtained, as expected for emissions corresponding to T 1 excited states.
Isoquinoline complex 11 is an orange emitter (572−632 nm), which displays lifetimes in the range of 7.4−1.8 μs and moderate quantum yields of about 0.13. In contrast, the ptolylpyridine counterparts 12 and 13 are very efficient green emitters (473−517 nm) as expected from a higher HOMO− LUMO gap. They exhibit shorter lifetimes, 4.2−1.3 μs, and quantum yields higher than 0.75. Worthy of note is the quantum yield of 13, which reaches the unity in both the PMMA film and 2-MeTHF at room temperature. Another noticeable feature of 12 and 13 with regard to 11 is their narrower emissions. This, which is evident in the emission spectra (Figure 8), points out a lower difference between the structure of the excited state and the ground state for the ptolylpyridine case. 3a The spectra of the three compounds also show broad structureless bands at room temperature, which split into vibronic fine structures in 2-MeTHF at 77 K in a   Inorganic Chemistry pubs.acs.org/IC Article congruent manner with a significant participation of ligandcentered 3 π−π* transitions in the excited state. 20 Electroluminescence (EL) Properties of an Organic Light-Emitting Diode (OLED) Device. To support the applicability of the developed synthetic methodology in the fabrication of OLED devices, complex 13 as an example of saturated green phosphorescent emitters has been tested in bottom-emission OLED structures. Figure 9 shows a scheme of the devices, including energy levels, layer thickness, and materials.
The devices were made by high-vacuum (<10 −7 Torr) thermal evaporation. The anode electrode was 750 Å of indium tin oxide (ITO). The cathode was composed by 10 Å of LiF and 1000 Å of Al. All devices were encapsulated with an epoxysealed glass lid glovebox (<1 ppm H 2 O and O 2 ) immediately after building, and a moisture scavenger was incorporated within the package. Following the anode-to-cathode sequence, the device organic stack consisted of 100 Å of HATCN as the hole injection layer (HIL); 400 Å of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl) 4,4′-diamine (NPD) as a holetransporting layer (HTL); 300 Å of an emissive layer (EML) containing the host (H1) doped with complex 13 as a green emitter at the investigated concentration; 50 Å of hole blocker material (BL); and 400 Å of Alq 3 as an electron-transporting layer (ETL). Concentrations of 6, 9, and 12% of emitter were compared side by side in the same structure. The device performance is summarized in Table 4. Electroluminescence (EL) spectra are shown in Figure 10, whereas Figure 11 displays external quantum efficiency (EQE) versus luminance and plots of current density versus voltage (see the inset).
Electroluminescence spectra of the fabricated devices revealed that complex 13 provided very saturated green emission with maximum wavelength at 500 nm, full width at half maximum (FWHM) of 68 nm, and emission offset about 470 nm (Figure 10). It corresponds to over 2.6 eV triplet emission energy of the emitter. On the other hand, maximum EQE slightly over 12% was observed, which is low for phosphorescent devices of this class displaying high efficiency. The reason appears to be related to the low triplet of the NPD hole-transporting layer since higher triplet material layers are required to efficiently confine the high triplet excitons of the emitter. In this context, the presence of a clear emission shoulder around 430−440 nm in the EL spectrum of the device containing 6% of emitter 13 should be pointed out (see the expansion in Figure 10). It originates from the NPD layer and strongly supports exciton leakage from the emissive layer and quenching by the low triplet of NPD.
One way to improve the device performance is to increase the emitter concentration. This increase should move the recombination zone away from the low triplet NPD HTL interface, minimizing interface quenching and thus improving the device efficiency. This is exactly what is observed from the device performance. Increasing the emitter concentration from 6 to 9 to 12% significantly improves device EQE, especially at higher luminance levels (see Table 4 and Figure 11), and reduces the amount of the undesirable NPD emission shoulder in the device EL spectrum (see the expansion in Figure 8). A further increase in the emitter concentration over 12%, however, causes concentration emission quenching, which results in the reduction of device efficiency.

■ CONCLUDING REMARKS
Acetylide anions have received considerable attention as ancillary ligands in connection with the design of transition metal phosphorescent emitters; 21 their strong field character creates a strong interaction through a p π −d π overlap, which contributes to raise the metal-centered d−d energy states. This study reveals that they are much more. In addition to improve the photophysical properties of the emitters, they have now demonstrated an extraordinary synthetic usefulness. Acetylide anions stabilize structures that are elusive for other 3e-donor ligands. Thus, the use of such ability allows us to design alternative synthetic precursors to those currently employed for the preparation of phosphorescent emitters. As a consequence, emitters with unusual stereochemistries can be easily prepared with their properties studied. Furthermore, the coordination of the acetylide to a metal center modifies and enhances the reactivity of the carbon atoms of the triple bond, converting it into an interesting building block, which on the metal coordination sphere generates new types of ligands characteristic of novel families of emitters.
Dimers 6−8, with a cis disposition of the heterocycles, and their transformation first into styrylpyridinimine derivatives and later into the iridaimidazo[1,2-a]pyridine emitters 11−13, of an octahedral structure with a fac disposition of carbon and nitrogen atoms, are clear concept validation proofs of what we say. The quantum yields of 100% displayed by the green emitter 13, in both the PMMA film and 2-MeTHF at room temperature, should be furthermore highlighted from the point of view of the photophysical properties. The designed synthetic pathway goes beyond a conceptual improvement; it has practical applicability as demonstrated by the fabrication of OLED devices based on complex 13. In this context, it should be mentioned that such an emitter demonstrated in the device very saturated green emission at a peak wavelength of 500 nm, with an external quantum efficiency of over 12% or 30.7 cd/A luminous efficacy. Such deep-green color saturation phosphorescent emitters can find application in future OLED displays with BT.2020 specification.
General information for the experimental section, Eyring plots, structural analysis, and NMR spectra (PDF) xyz coordinates (XYZ)