Synthesis and properties of electrophosphorescent chelating polymers with iridium complexes in the conjugated backbone

The synthesis of electrophosphorescent chelating polymers by Suzuki polycondensation of A-A- and B-B-type monomers is described, in which the fluorene-alt-carbazole (PFCz) segment is used as polymer backbone. By using alkyl-substituted ligands of iridium complex monomers, chelating copolymers with higher contents of iridium complex can be synthesized. Chemical and photophysical characterization confirm that the Ir complex is incorporated into the polymer backbone as one of the monomer repeat units by means of two 5-bromotolylpyridine ligands. Chelating polymers with Ir complexes in the conjugated polymer backbone show highly efficient energy transfer of excitons from the PFCz host segment to the Ir complex by an intramolecular trapping mechanism. The external quantum and luminous efficiencies of a device made with PFCzMppyIrhm4 copolymer reach 4.1 % ph/el (photons/electron) and 5.4 cd A(-1), respectively, at a current density of 32.2 mA cm(-2), an emission peak of 577 nm, and a luminance of 1730 cd cm(-2). Most important, the devices made from the chelating copolymers show no notable efficiency decay with increasing current density due to reduced concentration quenching and triplet-triplet (T-T) annihilation. This indicates that incorporation of the phosphorescent complex into the rigid conjugated polymer main chain is a new way to simultaneously realize high efficiency, long-term stability, and simple processing of phosphorescent polymer light-emitting diodes.     

基于不同辅助配体螯合电磷光聚合物的合成与性能研究

通过A-A, B-B型的Suzuki缩聚反应将三个具有不同辅助配体的2-(2'-苯并[b]噻吩)吡啶(btp)双环铱金属配合物引入芴和咔唑交替共聚物(PFCz)的主链上, 合成一系列最大发光波长在660 nm的红光螯合电磷光聚合物, 并研究了它们的电致发光性能. 实验证明了在这类电磷光聚合物中配合物单元辅助配体合理的分子设计, 可以提高配合物单体的溶解度, 提高聚合物的分子量和发光性能. 通过优化器件结构, 聚合物PFBtpIrf5单层器件(ITO/PEDOT:PSS/polymer＋PBD(30 wt%)/Ba/Al)的性能最好: 在电流密度为14.3 mA/cm²时, 最大外量子效率为1.93%.     

High-efficiency red-light emission from polyfluorenes grafted with cyclometalated iridium complexes and charge transport moiety

We report a new route for the design of electroluminescent polymers by grafting high-efficiency phosphorescent organometallic complexes as dopants and charge transport moieties onto alky side chains of fully conjugated polymers for polymer light-emitting diodes (PLED) with single layer/single polymers. The polymer system studied involves polyfluorene (PF) as the base conjugated polymer, carbazole (Cz) as the charge transport moiety and a source for green emission by forming an electroplex with the PF main chain, and cyclometalated iridium (Ir) complexes as the phosphorescent dopant. Energy transfer from the green Ir complex or an electroplex formed between the fluorene main chain and side-chain carbazole moieties, in addition to that from the PF main chain, to the red Ir complex can significantly enhance the device performance, and a red light-emitting device with the high efficiency 2.8 cd/A at 7 V and 65 cd/m2, comparable to that of the same Ir complex-based OLED, and a broad-band light-emitting device containing blue, green, and red peaks (2.16 cd/A at 9 V) are obtained.     

Solution-processible conjugated electrophosphorescent polymers

We report the synthesis and photophysical study of a series of solution-processible phosphorescent iridium complexes. These comprise bis-cyclometalated iridium units [Ir(ppy)(2)(acac)] or [Ir(btp)(2)(acac)] where ppy is 2-phenylpyridinato, btp is 2-(2'-benzo[b]thienyl)pyridinato, and acac is acetylacetonate. The iridium units are covalently attached to and in conjugation with oligo(9,9-dioctylfluorenyl-2,7-diyl) [(FO)(n)] to form complexes [Ir(ppy-(FO)(n))(2)(acac)] or [Ir(btp-(FO)(n))(2)(acac)], where the number of fluorene units, n, is 1, 2, 3, approximately 10, approximately 20, approximately 30, or approximately 40. All the complexes exhibit emission from a mixed triplet state in both photoluminescence and electroluminescence, with efficient quenching of the fluorene singlet emission. Short-chain complexes, 11-13, [Ir(ppy-(FO)(n)-FH)(2)(acac)] where n = 0, 1, or 2, show green light emission, red-shifted through the FO attachment by about 70 meV, but for longer chains there is quenching because of the lower energy triplet state associated with polyfluorene. In contrast, polymer complexes 18-21 [Ir(btp-(FO)(n))(2)(acac)] where n is 5-40 have better triplet energy level matching and can be used to provide efficient red phosphorescent polymer light-emitting diodes, with a red shift due to the fluorene attachment of about 50 meV. We contrast this small (50-70 meV) and short-range modification of the triplet energies through extended conjugation, with the much more substantial evolution of the pi-pi* singlet transitions, which saturate at about n = 10. These covalently bound materials show improvements in efficiency over simple blends and will form the basis of future investigations into energy-transfer processes occurring in light-emitting diodes.     

New preparative procedure for photoluminescent metallopolymers having a biphenyl‐2,2′‐diyl iridium(III) unit bound to a phosphine copolymer ligand

We first prepared polymer‐bound photoluminescent iridium complexes bearing a cyclometalated 2,2′‐biphenylene ligand via an easy procedure in which the metallopolymer was synthesized by the reaction of a metal precursor with a polymer ligand. The iridium compound, [Ir(cod)(biph)Cl]₂ (where cod and biph are 1,5‐cyclooctadiene and biphenyl‐2,2′‐diyl, respectively), was used as the iridium material, and a copolymer built by the radical copolymerization of 4‐styryldiphenylphosphine and methyl methacrylate was employed as the polymer ligand. The obtained metallopolymers were highly crosslinked by iridium atoms forming P? Ir? P bonds. The content of the iridium was experimentally clarified to be in the range of 0.06–0.6 mmol/g of the polymer. Photoluminescence of the iridium polymer in the solid state was observed at 597 nm when the polymer was irradiated at 350 nm. As the Ir content in the copolymer increased to 0.2 mmol/g, the intensity of the luminescence also increased, but more iridium content decreased the intensity. Furthermore, the intensity of the photoluminescence in these photoluminescent polymers depended on the molecular weight of the copolymer ligands. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4204–4213, 2006     

Phosphorescent organic light‐emitting diodes using an iridium complex polymer as the solution‐processible host material

We prepared an iridium polymer complex having 2‐phenylpyridine as a η²‐cyclometallated ligand, a new OLED containing a solution‐processible iridium polymer as a host, and a phosphorescent iridium complex, [Ir(piq‐^tBu)₃] as a guest. This is the first example to apply a phosphorescent iridium complex polymer to a host material in a phosphorescent OLED. A phosphine copolymer ligand made from methyl methacrylate (MMA) and 4‐styryldiphenylphosphine can be used as an anchor polymer, which coordinates to luminescent iridium units to form a host metallopolymer easily. The OLED containing the host iridium‐complex polymer film, in which the guest, 2 wt % Ir(piq‐^tBu)₃, was doped, showed red electroluminescence as a result of efficient energy transfer from the iridium polymer host to the iridium guest. The maximum current efficiency of the device was 1.00, suggesting that a soluble iridium complex polymer can be used as a solution‐processible polymer host in EL devices. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4358–4365, 2009     

Triplet energy back transfer in conjugated polymers with pendant phosphorescent iridium complexes

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)(2)(acac)] fragment (where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH(2))(8)- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 A between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH(2))(8)- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.     

Simple Conjugated Polymers with On‐Chain Phosphorescent Iridium(III) Complexes: Toward Ratiometric Chemodosimeters for Detecting Trace Amounts of Mercury(II)

For the development of excellent optical probes for mercury(II), a series of simple conjugated polymers that contain phosphorescent iridium(III) complexes as receptors for mercury(II) were designed and synthesized. These conjugated polymers showed energy transfer from the polymer host to iridium(III) complex guest in both solution and the solid state. Unexpectedly, they can work as excellent polymer chemodosimeters for mercury(II) by utilizing the mercury(II)‐induced decomposition of iridium(III) complex. They exhibit a pronounced optical signal change with switchable phosphorescence and fluorescence, even when the concentration of a solution of mercury(II) in THF was as low as 0.5 ppb. With the addition of mercury(II), the phosphorescent emission intensity of iridium(III) complexes was quenched completely. As the emission from polymer backbones increased, the emission wavelength was redshifted simultaneously, thereby realizing ratiometric detection. Excellent selectivity toward mercury(II) over other potentially interfering cations was also realized. In addition, an obvious emission color change of polymer solution from red to yellow‐green was observed, thus realizing a “naked‐eye” detection of mercury(II). More importantly, the solid films of these polymer chemodosimeters also exhibited high sensitivity and rapid response to mercury(II), thereby demonstrating the possibility of the fabrication of sensing devices with fast and convenient detection of mercury(II). The sensing mechanism was also investigated in detail. This is the first report on chemodosimeters based on conjugated polymers with phosphorescent iridium(III) complexes.     

Structural,Electronic and Optical Properties of Multifunctional Iridium(III) and Platinum(II) Metallophosphors for Organic Light‐Emitting Diodes

An elaborated theoretical investigation on the optical and electronic properties of three fluorene‐based platinum(II) and iridium(III) cyclometalated complexes Pt‐a , Ir‐a and Ir‐b is reported. The geometric and electronic structures of the complexes in the ground state are studied with density functional theory and Hartree Fock approaches, while the lowest triplet excited states are optimized by singles configuration interaction (CIS) methods. At the time‐dependent density functional theory (TD‐DFT) level, molecular absorption and emission properties were calculated on the basis of optimized ground‐ and excited‐state geometries, respectively. The computational results show that the appearance of triphenylamino (TPA) moiety at the 9‐position of fluorene ring favors the hole‐creation and leads to red‐shifts of absorption and emission spectra. Moreover, Pt‐a and Ir‐b are nice hole‐transporting materials whereas Ir‐a has good charge‐transfer balance, which render them useful for the realization of efficient OLEDs (Organic Light‐Emitting Diodes).     

Facile Synthesis of Luminescent Ir–Pt–Ir Trimetallic Complexes

A new class of luminescent, heterotrimetallic supramolecular constructs partnering two bis-cyclometalated iridium centers with a diimine platinum acetylide center is introduced. Whereas most supramolecular constructs featuring cyclometalated iridium involve elaborate bridging ligands and are prepared under forcing conditions with low to moderate yields, the three Ir–Pt–Ir complexes described here are prepared at room temperature from simple precursors and isolated in near-quantitative yields. ESI-MS, NMR spectroscopy, and diffusion ordered spectroscopy confirm the identity and homogeneity of the trimetallic products. In comparison with monometallic model complexes, analysis of UV/Vis absorption, steady-state photoluminescence and time-resolved emission reveals the impacts of supramolecular assembly on the photophysical properties. UV/Vis absorption and cyclic voltammetry suggest perturbation of some frontier orbital energies as a result of assembly, and the emission spectra and lifetimes reveal efficient excited-state energy transfer via a Dexter mechanism, and show that the site of luminescence (platinum or iridium) depends on the identity of the cyclometalating ligand bound to iridium.     

Heterogeneous Catalysis for Water Oxidation by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

Heterogenization of metal‐complex catalysts for water oxidation without loss of their catalytic activity is important for the development of devices simulating photosynthesis. In this study, efficient heterogeneous iridium complexes for water oxidation were prepared using bipyridine‐bridged periodic mesoporous organosilica (BPy‐PMO) as a solid chelating ligand. The BPy‐PMO‐based iridium catalysts (Ir‐BPy‐PMO) were prepared by postsynthetic metalation of BPy‐PMO and characterized through physicochemical analyses. The Ir‐BPy‐PMOs showed high catalytic activity for water oxidation. The turnover frequency (TOF) values for Ir‐BPy‐PMOs were one order of magnitude higher than those of conventional heterogeneous iridium catalysts. The reusability and stability of Ir‐BPy‐PMO were also examined, and detailed characterization was conducted using powder X‐ray diffraction, nitrogen adsorption, ¹³C DD MAS NMR spectroscopy, TEM, and XAFS methods.     

d → f energy transfer in a series of Ir(III)/Eu(III) dyads: energy-transfer mechanisms and white-light emission

An extensive series of blue-luminescent iridium(III) complexes has been prepared containing two phenylpyridine-type ligands and one ligand containing two pyrazolylpyridine units, of which one is bound to Ir(III) and the second is pendant. Attachment of {Ln(hfac)(3)} (Ln = Eu, Gd; hfac = anion of 1,1,1,5,5,5,-hexafluoropentanedione) to the second coordination site affords Ir(III)/Ln(III) dyads. Crystallographic analysis of several mononuclear iridium(III) complexes and one Ir(III)/Eu(III) dyad reveals that in most cases the complexes can adopt a folded conformation involving aromatic π stacking between a phenylpyridine ligand and the bis(pyrazolylpyridine) ligand, but in one series, based on CF(3)-substituted phenylpyridine ligands coordinated to Ir(III), the steric bulk of the CF(3) group prevents this and a quite different and more open conformation arises. Quantum mechanical calculations well reproduce these two types of "folded" and "open" conformations. In the Ir(III)/Eu(III) dyads, Ir → Eu energy transfer occurs with varying degrees of efficiency, resulting in partial quenching of the Ir(III)-based blue emission and the appearance of a sensitized red emission from Eu(III). Calculations based on consideration of spectroscopic overlap integrals rule out any significant contribution from F?rster (dipole-dipole) energy transfer over the distances involved but indicate that Dexter-type (exchange) energy transfer is possible if there is a small electronic coupling that would arise, in part, through π stacking between components. In some cases, an initial photoinduced electron-transfer step could also contribute to Ir → Eu energy transfer, as shown by studies on isostructural iridium/gadolinium model complexes. A balance between the blue (Ir-based) and red (Eu-based) emission components can generate white light.     

Energy transfer and triplet exciton confinement in polymeric electrophosphorescent devices

Energy transfer and triplet exciton confinement in polymer/phosphorescent dopant systems have been investigated. Various combinations of host‐guest systems have been studied, consisting of two host polymers, poly(vinylcarbazole) (PVK) and poly[9,9‐bis(octyl)‐fluorene‐2,7‐diyl] (PF), blended with five different phosphorescent iridium complexes with different triplet energy levels. These combinations of hosts and dopants provide an ideal situation for studying the movement of triplet excitons between the host polymers and dopants. The excitons either can be confined at the dopant sites or can flow to the host polymers, subject to the relative position of the triplet energy levels of the material. For PF, because of its low triplet energy level, the exciton can flow back from the dopants to PF when the dopant has a higher triplet energy and subsequently quench the device efficiency. In contrast, efficient electrophosphorescence has been observed in doped PVK films because of the high triplet energy level of PVK. Better energy transfer from PVK to the dopants, as well as triplet exciton confinement on the dopants, leads to higher device performance than found in PF devices. Efficiencies as high as 16, 8.0, and 2.6 cd/A for green, yellow, and red emissions, respectively, can be achieved when PVK is selected as the host polymer. The results in this study show that the energy transfer and triplet exciton confinement have a pronounced influence on the device performance. In addition, this study also provides material design and selection rules for the efficient phosphorescent polymer light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2681–2690, 2003     

A cross-coupling strategy for the synthesis of dimetallic assemblies containing mixed bipyridine-terpyridine bridging ligands: luminescence and energy transfer properties

Tris-bidentate complexes of the form [Ir(N[symbol: see text]C)2(N[symbol: see text]N)]+ and [Ru(N[symbol: see text]N)3]2+ incorporating a boronic acid substituent (N[symbol: see text]C=2-phenylpyridyl, N[symbol: see text]N=a 2,2'-bipyridyl ligand), are cross-coupled with bromo-substituted bis-terpyridyl ruthenium and iridium complexes to generate heterometallic assemblies comprising bpy-phi(n)-tpy bridges; they display efficient energy transfer from Ir(III) to Ru(II).     

以铱配合物为核、聚(芴-咔唑)为臂的白光星型磷光聚合物

设计合成了以苯基苯并咪唑和吡啶三唑为配体的高效的黄绿光铱配合物(M1),并通过Suzuki缩聚反应制备了以磷光铱配合物客体为中心核、蓝光荧光聚(芴-咔唑)主体为臂的星型磷光聚合物(P2.5、P5.0和P10),着重对M1和聚合物的发光性能、电化学性能及热稳定性能进行研究。 结果表明,M1具有较高的荧光量子效率(32.06%),其荧光寿命为1.09 μs,聚合物荧光寿命为2.223.93 μs,均表现为磷光;通过调节主客体的比例,利用主客体的部分能量转移机制,来实现聚合物的不同光色,发光颜色可从蓝光向黄光变化;当M1摩尔分数为2.5%时,获得的白光聚合物(P2.5)具有较好的发光性能和热稳定性能,色坐标为(0.30,0.32),位于白光区域,其最高占有轨道(HOMO)能级和最低未占有轨道(LUMO)能级分别为5.49和2.43 eV,荧光量子产率为14.3%,荧光寿命为2.22 μs。     

The influence of triplet energy levels of bridging ligands on energy transfer processes in Ir(III)/Eu(III) dyads

A series of N^N,O^O-bridging ligands based on substituted 1-(pyridin-2-yl)-3-methyl-5-pyrazolone and their corresponding heteroleptic iridium(III) complexes as well as Ir-Eu bimetallic complexes were synthesized and fully characterized. The influence of the triplet energy levels of the bridging ligands on the energy transfer (ET) process from the Ir(III) complexes to Eu(III) ions in solution was investigated at 77 K in Ir(III)/Eu(III) dyads. Photophysical experiment results show the bridging ligands play an important role in the ET process. Only when the triplet energy level of the bridging ligand was lower than the triplet metal-to-ligand charge transfer ((3)MLCT) energy level of the Ir moiety, was pure emission from the Eu(III) ion observed, implying complete ET took place from the Ir moiety to the Eu(III) ion.     

Iridium(III) Complexes with [−2, −1, 0] Charged Ligand Realized Deep-Red/Near-Infrared Phosphorescent Emission

A series of [−2, −1, 0] charged-ligand based iridium(III) complexes of [Ir(bph)(bpy)(acac)] ( 1 ), [Ir(bph)(2MeO-bpy)(acac)] ( 2 ), [Ir(bph)(2CF₃-bpy)(acac)] ( 3 ), [Ir(bph)(bpy)(2^tBu-acac)] ( 4 ) and [Ir(bph)(bpy)(CF₃-acac)] ( 5 ), which using biphenyl as dianionic ligand [−2], acetylacetone (or its derivatives) as monoanionic ligand [−1], and 2,2′-bipyridine (or its derivatives) as neutral ligand [0] were designed and synthesized. The chemical structures were well characterized. All of the ligands have simple chemical structures, thus further making the complexes have excellent thermal stability and are easy to sublimate and purify. Phosphorescent characteristics with short emission lifetime were demonstrated for these emitters. Notably, all of the complexes exhibit remarkable deep red/near infrared emission, which is quite different from the reported [−1, −1, −1] charged-ligand based iridium(III) complexes. The photophysical properties of these complexes are regularly improved by introducing electron-donating or -withdrawing groups into [−1] or [0] charged-ligand. The related organic light-emitting diodes exhibited deep red/near infrared emission with acceptable external quantum efficiency and low turn-on voltage (<2.6 V). This work provides a new idea for the construction of new type phosphorescent iridium(III) emitters with different valence states of [−2, −1, 0] charged ligands, thus offering new opportunities and challenges for their optoelectronic applications.     

Cyclometalated iridium and platinum complexes as singlet oxygen photosensitizers: quantum yields, quenching rates and correlation with electronic structures

Photophysical properties are reported for a series of cyclometalated platinum and iridium complexes that can serve as photosensitizers for singlet oxygen. The complexes have the formula (C;N)(2)Ir(O;O) or (C;N)Pt(O;O) where C;N is a monoanionic cyclometalating ligand such as 2-(phenyl)pyridyl and 2-(phenyl)quinolyl, and O;O is the ancillary ligand acetylacetonate (acac) or dipivaloylmethane (dpm). Also examined were a series of (N;N)PtMe(2) complexes where N;N is a diimine such as 2,2'-bipyridyl. In general, the cyclometalated complexes are excellent photosensitizers for the production of singlet oxygen, while the (N;N)PtMe(2) complexes were ineffective at this reaction. Quantum yields of singlet oxygen production range from 0.9-1.0 for the cyclometalated Pt complexes and 0.5-0.9 for Ir complexes. Luminescence quenching and singlet oxygen formation of the Ir complexes occurs from a combination of electron and energy transfer processes, whereas the Pt complexes only react by energy transfer. For Ir complexes with low emission energy, physical deactivation of the triplet excited state becomes competitive with energy transfer to ground state dioxygen. The rates of singlet oxygen quenching for the complexes presented here are in the range 6 x 10(6)-2 x 10(7) M(-1) s(-1) for Pt complexes and 2 x 10(5)-2 x 10(7) M(-1) s(-1) for Ir complexes, respectively. Differences in the efficiency of both forming and quenching singlet oxygen between the Ir and Pt cyclometalates are believed to come about from the more exposed coordination geometry in the latter species.     

Novel intramolecular C-H bond activation in an iridium dppm complex

Reaction of TpIr(C(2)H(4))(2) (Tp = tris-pyrazolylborate) with various chelating phosphine ligands has been explored. Reaction with bis-diphenylphosphinoethane leads to complete displacement of the Tp ligand. With bis-diphenylphosphinomethane, an intramolecular proton transfer from the methylene bridge to the iridium center occurs to give an iridium hydride complex formally resulting from oxidative C-H bond activation. Reaction with 2,2-bis(diphenylphosphino)propane (dppip) affords an Ir(I) complex formulated as kappa(2)-TpIr(dppip). Protonation of this Ir(I) complex gives a six coordinate Ir(III) hydride species.     

Highly phosphorescent bis-cyclometalated iridium complexes: synthesis, photophysical characterization, and use in organic light emitting diodes.

The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C(**)N) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., C(**)N2Ir(LX). The C(**)N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all beta-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C(**)N2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1-0.6), and microsecond lifetimes (e.g., 1-14 micros). The strongly allowed phosphorescence in these complexes is the result of significant spin-orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C(**)N2Ir(acac) complexes is a mixture of (3)MLCT and (3)(pi-pi) states. By choosing the appropriate C(**)N ligand, C(**)N2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C(**)N ligands, which lead to bathochromic shifts of the free ligands, lead to similar bathochromic shifts in the Ir complexes of the same ligands, consistent with "C(**)N2Ir"-centered emission. Three of the C(**)N2Ir(acac) complexes were used as dopants for organic light emitting diodes (OLEDs). The three Ir complexes, i.e., bis(2-phenylpyridinato-N,C2')iridium(acetylacetonate) [ppy2Ir(acac)], bis(2-phenyl benzothiozolato-N,C2')iridium(acetylacetonate) [bt2Ir(acac)], and bis(2-(2'-benzothienyl)pyridinato-N,C3')iridium(acetylacetonate) [btp2Ir(acac)], were doped into the emissive region of multilayer, vapor-deposited OLEDs. The ppy2Ir(acac)-, bt2Ir(acac)-, and btp2Ir(acac)-based OLEDs give green, yellow, and red electroluminescence, respectively, with very similar current-voltage characteristics. The OLEDs give high external quantum efficiencies, ranging from 6 to 12.3%, with the ppy2Ir(acac) giving the highest efficiency (12.3%, 38 lm/W, >50 Cd/A). The btp2Ir(acac)-based device gives saturated red emission with a quantum efficiency of 6.5% and a luminance efficiency of 2.2 lm/W. These C(**)N2Ir(acac)-doped OLEDs show some of the highest efficiencies reported for organic light emitting diodes. The high efficiencies result from efficient trapping and radiative relaxation of the singlet and triplet excitons formed in the electroluminescent process.