Organic electroluminescent materials and devices

Abstract

A compound comprising a first ligand L.sup.A of Formula I: ##STR00001##
is described. In Formula I, Z is selected from O, S, and Se; at least one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen, while the remainder are carbon; Y.sup.1 and Y.sup.2 are independently selected from NR, O, and S; R, R.sup.A, R.sup.B, and R.sup.C are each independently selected from a variety of substituents, and adjacent substitutions in R, R.sup.A, R.sup.B, and R.sup.C are optionally joined to form a fused ring; ligand L.sup.A is at least a tridentate ligand coordinated to a metal M; M is coordinated to ring A and to ring B through metal-carbene bonds; and the ligand L.sup.A is optionally linked with other ligands to comprise a tetradentate, pentadentate or hexadentate ligand. Formulations and devices, such as an OLEDs, that include the compound with a ligand L.sub.A of Formula I are also described.

Claims

1. A compound comprising a first ligand L.sup.A of Formula I: ##STR00220## Formula I; wherein Z is selected from the group consisting of O, S, and Se; wherein X.sup.1, X.sup.2, X.sup.3, and X.sup.4 are carbon or nitrogen; wherein at least one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen; wherein Y.sup.1 and Y.sup.2 are each independently selected from the group consisting of NR, O, and S; wherein R.sup.A and R.sup.B each independently represent mono, or di substitution, or no substitution; wherein R.sup.C represents mono, di, or tri substitution, or no substitution; wherein R, R.sup.A, R.sup.B, and R.sup.C are each independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; wherein any adjacent substitutions in R, R.sup.A, R.sup.B, and R.sup.C are optionally linked together to form a fused ring; wherein the ligand L.sup.A is at least a tridentate ligand coordinated to a metal M; wherein M is coordinated to ring A and to ring B through metal-carbene bonds; and wherein the ligand L.sup.A is optionally linked with other ligands to comprise a tetradentate, pentadentate or hexadentate ligand.

2. The compound of claim 1, wherein M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, and Cu.

3. The compound of claim 1, wherein M is Os.

4. The compound of claim 1, wherein Z is O.

5. The compound of claim 1, wherein exactly one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen.

6. The compound of claim 1, wherein Y.sup.1 and Y.sup.2 are each independently NR.

7. The compound of claim 1, wherein X.sup.1 is nitrogen, X.sup.2, X.sup.3, and X.sup.4 are carbon, and X.sup.2 is substituted by R.sup.C.

8. The compound of claim 1, wherein X.sup.1 and X.sup.3 are nitrogen, X.sup.2 and X.sup.4 are carbon, and X.sup.2 is substituted by R.sup.C.

9. The compound of claim 1, wherein adjacent R.sup.A substituents are linked together to form an aromatic ring, and adjacent R.sup.B substituents are linked together to form an aromatic ring.

10. The compound of claim 1, wherein L.sup.A is selected from the group consisting of: ##STR00221## ##STR00222## ##STR00223## ##STR00224## ##STR00225## ##STR00226## ##STR00227## ##STR00228## ##STR00229## ##STR00230## ##STR00231##

11. The compound of claim 1, wherein the compound further comprises a second ligand L.sup.B selected from the group consisting of: ##STR00232## ##STR00233## ##STR00234## ##STR00235## ##STR00236## ##STR00237## ##STR00238## ##STR00239## ##STR00240## ##STR00241## ##STR00242##

12. The compound of claim 10, wherein the compound is Compound Ax having the formula of Os(L.sup.i).sub.2, wherein x=i, i is an integer from 1 to 40.

13. The compound of claim 10, wherein the compound is Compound By having the formula of Os(L.sup.i)(L.sup.Bj), wherein y=i+40j−40, i is an integer from 1 to 40, j is an integer from 1 to 42; and wherein L.sup.B1 to L.sup.B42 have the following structures: ##STR00243## ##STR00244## ##STR00245## ##STR00246## ##STR00247## ##STR00248## ##STR00249## ##STR00250## ##STR00251## ##STR00252## ##STR00253##

14. A device comprising one or more organic fight emitting devices, at least one of the one or more organic light emitting devices comprising: an anode; a cathode; and an organic layer, disposed between the anode and the cathode, comprising a compound comprising a first ligand L.sup.A of Formula I: ##STR00254## Formula I; wherein Z is selected from the group consisting of O, S, and Se; wherein X.sup.1, X.sup.2, X.sup.3, and X.sup.4 are carbon or nitrogen; wherein at least one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen; wherein Y.sup.1 and Y.sup.2 are each independently selected from the group consisting of NR, O, and S; wherein R.sup.A and R.sup.B each independently represent mono, or di substitution, or no substitution; wherein R.sup.C represents mono, di, or tri substitution, or no substitution; wherein R, R.sup.A, R.sup.B, and R.sup.C are each independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; wherein any adjacent substitutions in R, R.sup.A, R.sup.B, and R.sup.C are optionally linked together to form a fused ring; wherein the ligand L.sup.A is at least a tridentate ligand coordinated to a metal M; wherein M is coordinated to ring A and to ring B through metal-carbene bonds; and wherein the ligand L.sup.A is optionally linked with other ligands to comprise a tetradentate, pentadentate or hexadentate ligand.

15. The device of claim 14, wherein the device is selected from the group consisting of a consumer product, an electronic component module, an organic light-emitting device, and a lighting panel.

16. The device of claim 14, wherein the organic layer is an emissive layer and the compound is an emissive dopant or a non-emissive dopant.

17. The device of claim 14, wherein the organic layer further comprises a host, wherein the host comprises a triphenylene containing benzo-fused thiophene or benzo-fused furan; wherein any substituent in the host is an unfused substituent independently selected from the group consisting of C.sub.nH.sub.2n+1, OC.sub.nH.sub.2n+1, OAr.sub.1, N(C.sub.NH.sub.2n+1).sub.2, N(Ar.sub.1)(Ar.sub.2), CH=C.sub.HH.sub.2n+1, C≡CC.sub.nH.sub.2n+1, Ar.sub.1, Ar.sub.1—Ar.sub.2, C.sub.nH.sub.2n—Ar.sub.1, or no substitution; wherein n is from 1 to 10; and wherein Ar.sub.1 and Ar.sub.2 are independently selected from the group consisting of benzene, biphenyl, naphthalene, triphenylene, carbazole, and heteroaromatic analogs thereof.

18. The device of claim 14, wherein the organic layer further comprises a host, wherein the host comprises at least one chemical group selected from the group consisting of triphenylene, carbazole, dibenzothiphene, dibenzofuran, dibenzoselenophene, azatriphenylene, azacarbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene.

19. The device of claim 14, wherein the organic layer further comprises a host and the host is selected from the group consisting of: ##STR00255## ##STR00256## ##STR00257## ##STR00258## ##STR00259## and combinations thereof.

20. A formulation comprising a compound comprising a first ligand L.sup.A of Formula I: ##STR00260## Formula I; wherein Z is selected from the group consisting of O, S, and Se; wherein X.sup.1, X.sup.2, X.sup.3, and X.sup.4 are carbon or nitrogen; wherein at least one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen; wherein Y.sup.1 and Y.sup.2 are each independently selected from the group consisting of NR, O, and S; wherein R.sup.A and R.sup.B each independently represent mono, or di substitution, or no substitution; wherein R.sup.C represents mono, di, or tri substitution, or no substitution; wherein R, R.sup.A, R.sup.B, and R.sup.C are each independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; wherein any adjacent substitutions in R, R.sup.A, R.sup.B, and R.sup.C are optionally linked together to form a fused ring; wherein the ligand L.sup.A is at least a tridentate ligand coordinated to a metal M; wherein M is coordinated to ring A and to ring B through metal-carbene bonds; and wherein the ligand L.sup.A is optionally linked with other ligands to comprise a tetradentate, pentadentate or hexadentate ligand.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an organic light emitting device.

(2) FIG. 2 shows an inverted organic light emitting device that does not have a separate electron transport layer.

(3) FIG. 3 shows a first ligand L.sup.A of Formula I as disclosed herein.

DETAILED DESCRIPTION

(4) Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.

(5) The initial OLEDs used emissive molecules that emitted light from their singlet states (“fluorescence”) as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds.

(6) More recently, OLEDs having emissive materials that emit light from triplet states (“phosphorescence”) have been demonstrated. Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,” Nature, vol. 395, 151-154, 1998; (“Baldo-I”) and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference.

(7) FIG. 1 shows an organic light emitting device 100. The figures are not necessarily drawn to scale. Device 100 may include a substrate 110, an anode 115, a hole injection layer 120, a hole transport layer 125, an electron blocking layer 130, an emissive layer 135, a hole blocking layer 140, an electron transport layer 145, an electron injection layer 150, a protective layer 155, a cathode 160, and a barrier layer 170. Cathode 160 is a compound cathode having a first conductive layer 162 and a second conductive layer 164. Device 100 may be fabricated by depositing the layers described, in order. The properties and functions of these various layers, as well as example materials, are described in more detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which are incorporated by reference.

(8) More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m-MTDATA doped with F.sub.4-TCNQ at a molar ratio of 50:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety.

(9) FIG. 2 shows an inverted OLED 200. The device includes a substrate 210, a cathode 215, an emissive layer 220, a hole transport layer 225, and an anode 230. Device 200 may be fabricated by depositing the layers described, in order. Because the most common OLED configuration has a cathode disposed over the anode, and device 200 has cathode 215 disposed under anode 230, device 200 may be referred to as an “inverted” OLED. Materials similar to those described with respect to device 100 may be used in the corresponding layers of device 200. FIG. 2 provides one example of how some layers may be omitted from the structure of device 100.

(10) The simple layered structure illustrated in FIGS. 1 and 2 is provided by way of non-limiting example, and it is understood that embodiments of the invention may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional OLEDs may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting. For example, in device 200, hole transport layer 225 transports holes and injects holes into emissive layer 220, and may be described as a hole transport layer or a hole injection layer. In one embodiment, an OLED may be described as having an “organic layer” disposed between a cathode and an anode. This organic layer may comprise a single layer, or may further comprise multiple layers of different organic materials as described, for example, with respect to FIGS. 1 and 2.

(11) Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in FIGS. 1 and 2. For example, the substrate may include an angled reflective surface to improve out-coupling, such as a mesa structure as described in U.S. Pat. No. 6,091,195 to Forrest et al., and/or a pit structure as described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated by reference in their entireties.

(12) Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. Pat. No. 7,431,968, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing.

(13) Devices fabricated in accordance with embodiments of the present invention may further optionally comprise a barrier layer. One purpose of the barrier layer is to protect the electrodes and organic layers from damaging exposure to harmful species in the environment including moisture, vapor and/or gases, etc. The barrier layer may be deposited over, under or next to a substrate, an electrode, or over any other parts of a device including an edge. The barrier layer may comprise a single layer, or multiple layers. The barrier layer may be formed by various known chemical vapor deposition techniques and may include compositions having a single phase as well as compositions having multiple phases. Any suitable material or combination of materials may be used for the barrier layer. The barrier layer may incorporate an inorganic or an organic compound or both. The preferred barrier layer comprises a mixture of a polymeric material and a non-polymeric material as described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos. PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporated by reference in their entireties. To be considered a “mixture”, the aforesaid polymeric and non-polymeric materials comprising the barrier layer should be deposited under the same reaction conditions and/or at the same time. The weight ratio of polymeric to non-polymeric material may be in the range of 95:5 to 5:95. The polymeric material and the non-polymeric material may be created from the same precursor material. In one example, the mixture of a polymeric material and a non-polymeric material consists essentially of polymeric silicon and inorganic silicon.

(14) Devices fabricated in accordance with embodiments of the invention can be incorporated into a wide variety of electronic component modules (or units) that can be incorporated into a variety of electronic products or intermediate components. Examples of such electronic products or intermediate components include display screens, lighting devices such as discrete light source devices or lighting panels, etc. that can be utilized by the end-user product manufacturers. Such electronic component modules can optionally include the driving electronics and/or power source(s). Devices fabricated in accordance with embodiments of the invention can be incorporated into a wide variety of consumer products that have one or more of the electronic component modules (or units) incorporated therein. Such consumer products would include any kind of products that include one or more light source(s) and/or one or more of some type of visual displays. Some examples of such consumer products include flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads-up displays, fully or partially transparent displays, flexible displays, laser printers, telephones, cell phones, tablets, phablets, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, 3-D displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18 degrees C. to 30 degrees C., and more preferably at room temperature (20-25 degrees C.), but could be used outside this temperature range, for example, from −40 degree C. to +80 degree C.

(15) The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures.

(16) The term “halo,” “halogen,” or “halide” as used herein includes fluorine, chlorine, bromine, and iodine.

(17) The term “alkyl” as used herein contemplates both straight and branched chain alkyl radicals. Preferred alkyl groups are those containing from one to fifteen carbon atoms and includes methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, and the like. Additionally, the alkyl group may be optionally substituted.

(18) The term “cycloalkyl” as used herein contemplates cyclic alkyl radicals. Preferred cycloalkyl groups are those containing 3 to 7 carbon atoms and includes cyclopropyl, cyclopentyl, cyclohexyl, and the like. Additionally, the cycloalkyl group may be optionally substituted.

(19) The term “alkenyl” as used herein contemplates both straight and branched chain alkene radicals. Preferred alkenyl groups are those containing two to fifteen carbon atoms. Additionally, the alkenyl group may be optionally substituted.

(20) The term “alkynyl” as used herein contemplates both straight and branched chain alkyne radicals. Preferred alkynyl groups are those containing two to fifteen carbon atoms. Additionally, the alkynyl group may be optionally substituted.

(21) The terms “aralkyl” or “arylalkyl” as used herein are used interchangeably and contemplate an alkyl group that has as a substituent an aromatic group. Additionally, the aralkyl group may be optionally substituted.

(22) The term “heterocyclic group” as used herein contemplates aromatic and non-aromatic cyclic radicals. Hetero-aromatic cyclic radicals also means heteroaryl. Preferred hetero-non-aromatic cyclic groups are those containing 3 or 7 ring atoms which includes at least one hetero atom, and includes cyclic amines such as morpholino, piperdino, pyrrolidino, and the like, and cyclic ethers, such as tetrahydrofuran, tetrahydropyran, and the like. Additionally, the heterocyclic group may be optionally substituted.

(23) The term “aryl” or “aromatic group” as used herein contemplates single-ring groups and polycyclic ring systems. The polycyclic rings may have two or more rings in which two carbons are common to two adjoining rings (the rings are “fused”) wherein at least one of the rings is aromatic, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl, heterocycles, and/or heteroaryls. Additionally, the aryl group may be optionally substituted.

(24) The term “heteroaryl” as used herein contemplates single-ring hetero-aromatic groups that may include from one to three heteroatoms, for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine and pyrimidine, and the like. The term heteroaryl also includes polycyclic hetero-aromatic systems having two or more rings in which two atoms are common to two adjoining rings (the rings are “fused”) wherein at least one of the rings is a heteroaryl, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl, heterocycles, and/or heteroaryls. Additionally, the heteroaryl group may be optionally substituted.

(25) The alkyl, cycloalkyl, alkenyl, alkynyl, aralkyl, heterocyclic group, aryl, and heteroaryl may be optionally substituted with one or more substituents selected from the group consisting of hydrogen, deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, cyclic amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof.

(26) As used herein, “substituted” indicates that a substituent other than H is bonded to the relevant position, such as carbon. Thus, for example, where R.sup.1 is mono-substituted, then one R.sup.1 must be other than H. Similarly, where R.sup.1 is di-substituted, then two of R.sup.1 must be other than H. Similarly, where R.sup.1 is unsubstituted, R.sup.1 is hydrogen for all available positions.

(27) The “aza” designation in the fragments described herein, i.e. aza-dibenzofuran, aza-dibenzothiophene, etc. means that one or more of the C—H groups in the respective fragment can be replaced by a nitrogen atom, for example, and without any limitation, azatriphenylene encompasses both dibenzo[f,h]quinoxaline and dibenzo[f,h]quinoline. One of ordinary skill in the art can readily envision other nitrogen analogs of the aza-derivatives described above, and all such analogs are intended to be encompassed by the terms as set forth herein.

(28) It is to be understood that when a molecular fragment is described as being a substituent or otherwise attached to another moiety, its name may be written as if it were a fragment (e.g. phenyl, phenylene, naphthyl, dibenzofuryl) or as if it were the whole molecule (e.g. benzene, naphthalene, dibenzofuran). As used herein, these different ways of designating a substituent or attached fragment are considered to be equivalent.

(29) Osmium (II) complexes has been investigated for OLED applications, see, e.g., review: Eur. J. Inorg. Chem. 2006, 3319-3332. The octahedral ligand arrangement of the Os(II) complexes resembles that of Ir(III) complexes. Bistridentate Os(II) carbene complexes offer unique advantages for OLED applications. The rigid nature of the tridentate ligands generally results in narrow emission line widths and short excited state lifetimes, which can result in better color purity and longer device lifetime, making them suitable for display applications. However, Os(II) complexes generally exhibit low oxidation potential, i.e., shallower HOMO energy level than Ir(III) complexes. It is desirable to bring down the HOMO and LUMO of Os(II) complexes to suit the energy levels of common host materials. International Application WO2009046266 disclosed bistridentate osmium complexes with an electron deficient heterocycle bound to osmium such as pyridine, or electron withdrawing groups such as fluorine and cyano. These strategies often result in less stable devices. In this invention, we incorporate aza-DBX (dibenzofuran, dibenzoselenophene, and dibenzothiophene) moieties to the ligand. The introduction of the aza-DBX moiety makes the HOMO and LUMO deeper and make the compounds more suitable as emitters for PHOLEDs.

(30) According to one embodiment, a compound comprising a first ligand L.sup.A of Formula I:

(31) ##STR00004##
is described. In Formula I:

(32) Z is selected from the group consisting of O, S, and Se;

(33) X.sup.1, X.sup.2, X.sup.3, and X.sup.4 are carbon or nitrogen;

(34) at least one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen;

(35) Y.sup.1 and Y.sup.2 are each independently selected from the group consisting of NR, O, and S;

(36) R.sup.A and R.sup.B each independently represent mono, or di substitution, or no substitution;

(37) R.sup.C represents mono, di, or tri substitution, or no substitution;

(38) R, R.sup.A, R.sup.B, and R.sup.C are each independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof;

(39) any adjacent substitutions in R, R.sup.A, R.sup.B, and R.sup.C are optionally linked together to form a fused ring;

(40) the ligand L.sup.A is at least a tridentate ligand coordinated to a metal M;

(41) M is coordinated to ring A and to ring B through metal-carbene bonds; and

(42) the ligand L.sup.A is optionally linked with other ligands to comprise a tetradentate, pentadentate or hexadentate ligand.

(43) In some embodiments, M is a metal having an atomic number greater than 40. In some embodiments, M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, and Cu. In some embodiments, metal M is Cu. In some embodiments, M is Os.

(44) In some embodiments, Z is O. In some embodiments, Y.sup.1 and Y.sup.2 are each independently NR.

(45) In some embodiments, exactly one of X.sup.1, X.sup.2, X.sup.3, and X.sup.4 is nitrogen. In some embodiments, X.sup.1 is nitrogen, X.sup.2, X.sup.3, and X.sup.4 are carbon, and X.sup.2 is substituted by R.sup.C. In some embodiments, X.sup.1 and X.sup.3 are nitrogen, X.sup.2 and X.sup.4 are carbon, and X.sup.2 is substituted by R.sup.C.

(46) In some embodiments, adjacent R.sup.A substituents are linked together to form an aromatic ring, and adjacent R.sup.B substituents are linked together to form an aromatic ring.

(47) In some embodiments, each R and R.sup.C is independently selected from the group consisting of alkyl, cycloalkyl, aryl and substituted aryl. In some embodiments, each R and R.sup.C is independently selected from the group consisting of methyl, ethyl, propyl, 1-methylethyl, butyl, 1-methylpropyl, 2-methylpropyl, pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, cyclopentyl, and cyclohexyl, wherein each group is optionally partially or fully deuterated.

(48) In some embodiments, L.sup.A is selected from the group consisting of:

(49) ##STR00005## ##STR00006## ##STR00007## ##STR00008## ##STR00009## ##STR00010## ##STR00011## ##STR00012## ##STR00013## ##STR00014## ##STR00015##

(50) In some embodiments, the compound is homoleptic. In some embodiments, the compound is heteroleptic.

(51) In some embodiments, the compound further comprises a second ligand L.sup.B selected from the group consisting of:

(52) ##STR00016## ##STR00017## ##STR00018## ##STR00019## ##STR00020## ##STR00021## ##STR00022## ##STR00023## ##STR00024## ##STR00025## ##STR00026##

(53) In some embodiments, the compound is Compound Ax having the formula of Os(L.sup.i).sub.2, wherein x=i, i is an integer from 1 to 40.

(54) In some embodiments, the compound is Compound By having the formula of Os(L.sup.i)(L.sup.Bj), wherein y=i+40j−40, i is an integer from 1 to 40, j is an integer from 1 to 42; and wherein L.sup.B1 to L.sup.B42 have the structures recited herein.

(55) In some embodiments, the compound can be an emissive dopant. In some embodiments, the compound can produce emissions via phosphorescence, fluorescence, thermally activated delayed fluorescence, i.e., TADF (also referred to as E-type delayed fluorescence), triplet-triplet annihilation, or combinations of these processes.

(56) According to another aspect of the present disclosure, a device that includes one or more organic light emitting devices is also provided. At least one of the one or more organic light emitting devices can include an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer may include a host and a phosphorescent dopant. The emissive layer can include a compound comprising a first ligand L.sup.A of Formula I, and its variations as described herein.

(57) The device can be one or more of a consumer product, an electronic component module, an organic light-emitting device and a lighting panel. The organic layer can be an emissive layer and the compound can be an emissive dopant in some embodiments, while the compound can be a non-emissive dopant in other embodiments.

(58) The organic layer can also include a host. In some embodiments, the host can include a metal complex. The host can be a triphenylene containing benzo-fused thiophene or benzo-fused furan. Any substituent in the host can be an unfused substituent independently selected from the group consisting of C.sub.nH.sub.2n+1, OC.sub.nH.sub.2n+1, OAr.sub.1, N(C.sub.nH.sub.2n+1).sub.2, N(Ar.sub.1)(Ar.sub.2), CH═CH.sub.n—CH.sub.2n+1, C≡C—C.sub.nH.sub.2n+1, Ar.sub.1, Ar.sub.1—Ar.sub.2, and C.sub.nH.sub.2n—Ar.sub.1, or no substitution. In the preceding substituents n can range from 1 to 10; and Ar.sub.1 and Ar.sub.2 can be independently selected from the group consisting of benzene, biphenyl, naphthalene, triphenylene, carbazole, and heteroaromatic analogs thereof.

(59) The host can be a compound comprising at least one chemical group selected from the group consisting of triphenylene, carbazole, dibenzothiphene, dibenzofuran, dibenzoselenophene, azatriphenylene, azacarbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene. The host can include a metal complex. The host can be a specific compound selected from the group consisting of:

(60) ##STR00027## ##STR00028## ##STR00029## ##STR00030## ##STR00031##
and combinations thereof.

(61) In yet another aspect of the present disclosure, a formulation that comprises a compound comprising a first ligand L.sup.A of Formula I. The formulation can include one or more components selected from the group consisting of a solvent, a host, a hole injection material, hole transport material, and an electron transport layer material, disclosed herein.

(62) Combination with Other Materials

(63) The materials described herein as useful for a particular layer in an organic light emitting device may be used in combination with a wide variety of other materials present in the device. For example, emissive dopants disclosed herein may be used in conjunction with a wide variety of hosts, transport layers, blocking layers, injection layers, electrodes and other layers that may be present. The materials described or referred to below are non-limiting examples of materials that may be useful in combination with the compounds disclosed herein, and one of skill in the art can readily consult the literature to identify other materials that may be useful in combination.

(64) HIL/HTL:

(65) A hole injecting/transporting material to be used in the present invention is not particularly limited, and any compound may be used as long as the compound is typically used as a hole injecting/transporting material. Examples of the material include, but are not limited to: a phthalocyanine or porphyrin derivative; an aromatic amine derivative; an indolocarbazole derivative; a polymer containing fluorohydrocarbon; a polymer with conductivity dopants; a conducting polymer, such as PEDOT/PSS; a self-assembly monomer derived from compounds such as phosphonic acid and silane derivatives; a metal oxide derivative, such as MoO.sub.x; a p-type semiconducting organic compound, such as 1,4,5,8,9,12-Hexaazatriphenylenehexacarbonitrile; a metal complex, and a cross-linkable compound.

(66) Examples of aromatic amine derivatives used in HIL or HTL include, but are not limited to the following general structures:

(67) ##STR00032##

(68) Each of Ar.sup.1 to Ar.sup.9 is selected from the group consisting of aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, and azulene; the group consisting of aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and the group consisting of 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each Ar is further substituted by a substituent selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof.

(69) In one aspect, Ar.sup.1 to Ar.sup.9 is independently selected from the group consisting of:

(70) ##STR00033##
wherein k is an integer from 1 to 20; X.sup.101 to X.sup.108 is C (including CH) or N; Z.sup.101 is NAr.sup.1, O, or S; Ar.sup.1 has the same group defined above.

(71) Examples of metal complexes used in HIL or HTL include, but are not limited to the following general formula:

(72) ##STR00034##
wherein Met is a metal, which can have an atomic weight greater than 40; (Y.sup.101-Y.sup.102) is a bidentate ligand, Y.sup.101 and Y.sup.102 are independently selected from C, N, O, P, and S; L.sup.101 is an ancillary ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and k′+k″ is the maximum number of ligands that may be attached to the metal.

(73) In one aspect, (Y.sup.101-Y.sup.102) is a 2-phenylpyridine derivative. In another aspect, (Y.sup.101-Y.sup.102) is a carbene ligand. In another aspect, Met is selected from Ir, Pt, Os, and Zn. In a further aspect, the metal complex has a smallest oxidation potential in solution vs. Fc.sup.+/Fc couple less than about 0.6 V.

(74) Host:

(75) The light emitting layer of the organic EL device of the present invention preferably contains at least a metal complex as light emitting material, and may contain a host material using the metal complex as a dopant material. Examples of the host material are not particularly limited, and any metal complexes or organic compounds may be used as long as the triplet energy of the host is larger than that of the dopant. While the Table below categorizes host materials as preferred for devices that emit various colors, any host material may be used with any dopant so long as the triplet criteria is satisfied.

(76) Examples of metal complexes used as host are preferred to have the following general formula:

(77) ##STR00035##
wherein Met is a metal; (Y.sup.103-Y.sup.104) is a bidentate ligand, Y.sup.103 and Y.sup.104 are independently selected from C, N, O, P, and S; L.sup.101 is an another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and k′+k″ is the maximum number of ligands that may be attached to the metal.

(78) In one aspect, the metal complexes are:

(79) ##STR00036##
wherein (O—N) is a bidentate ligand, having metal coordinated to atoms O and N.

(80) In another aspect, Met is selected from Ir and Pt. In a further aspect, (Y.sup.103-Y.sup.104) is a carbene ligand.

(81) Examples of organic compounds used as host are selected from the group consisting of aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, and azulene; the group consisting of aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and the group consisting of 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each group is further substituted by a substituent selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof.

(82) In one aspect, the host compound contains at least one of the following groups in the molecule:

(83) ##STR00037## ##STR00038##
wherein R.sup.101 to R.sup.107 is independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar's mentioned above. k is an integer from 0 to 20 or 1 to 20; k′″ is an integer from 0 to 20. X.sup.101 to X.sup.108 is selected from C (including CH) or N. Z.sup.101 and Z.sup.102 is selected from NR.sup.101, O, or S.
HBL:

(84) A hole blocking layer (HBL) may be used to reduce the number of holes and/or excitons that leave the emissive layer. The presence of such a blocking layer in a device may result in substantially higher efficiencies as compared to a similar device lacking a blocking layer. Also, a blocking layer may be used to confine emission to a desired region of an OLED.

(85) In one aspect, compound used in HBL contains the same molecule or the same functional groups used as host described above.

(86) In another aspect, compound used in HBL contains at least one of the following groups in the molecule:

(87) ##STR00039##
wherein k is an integer from 1 to 20; L.sup.101 is an another ligand, k′ is an integer from 1 to 3.
ETL:

(88) Electron transport layer (ETL) may include a material capable of transporting electrons. Electron transport layer may be intrinsic (undoped), or doped. Doping may be used to enhance conductivity. Examples of the ETL material are not particularly limited, and any metal complexes or organic compounds may be used as long as they are typically used to transport electrons.

(89) In one aspect, compound used in ETL contains at least one of the following groups in the molecule:

(90) ##STR00040##
wherein R.sup.101 is selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar's mentioned above. Ar.sup.1 to Ar.sup.3 has the similar definition as Ar's mentioned above. k is an integer from 1 to 20. X.sup.101 to X.sup.108 is selected from C (including CH) or N.

(91) In another aspect, the metal complexes used in ETL include, but are not limited to the following general formula:

(92) ##STR00041##
wherein (O—N) or (N—N) is a bidentate ligand, having metal coordinated to atoms O, N or N, N; L.sup.101 is another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal.

(93) In any above-mentioned compounds used in each layer of the OLED device, the hydrogen atoms can be partially or fully deuterated. Thus, any specifically listed substituent, such as, without limitation, methyl, phenyl, pyridyl, etc. encompasses undeuterated, partially deuterated, and fully deuterated versions thereof. Similarly, classes of substituents such as, without limitation, alkyl, aryl, cycloalkyl, heteroaryl, etc. also encompass undeuterated, partially deuterated, and fully deuterated versions thereof.

(94) In addition to and/or in combination with the materials disclosed herein, many hole injection materials, hole transporting materials, host materials, dopant materials, exciton/hole blocking layer materials, electron transporting and electron injecting materials may be used in an OLED. Non-limiting examples of the materials that may be used in an OLED in combination with materials disclosed herein are listed in Table A below. Table A lists non-limiting classes of materials, non-limiting examples of compounds for each class, and references that disclose the materials.

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No. 5,061,569 EP650955 J. Mater. Chem. 3, 319 (1993) Appl. Phys. Lett. 90, 183503 (2007) 0 Appl, Phys. Lett. 90, 183503 (2007) Triarylamine on spirofluorene core Synth. Met. 91, 209 (1997) Arylamine carbazole compounds Adv. Mater. 6, 677 (1994), US20080124572 Triarylamine with (di)benzothiophene/ (di)benzofuran US20070278938, US20080106190 US20110163302 Indolocarbazoles Synth. Met. 111, 421 (2000) Isoindole compounds Chem. Mater. 15, 3148 (2003) Metal carbene complexes US20080018221 Phosphorescent OLED host materials Red hosts Arylcarbazoles Appl. Phys. Lett. 78, 1622 (2001) Metal 8-hydroxyquinolates (e.g. Alq.sub.3, BAlq) Nature 395, 151 (1998) US20060202194 0 WO2005014551 WO2006072002 Metal phenoxybenzothiazole compounds Appl. Phys. Lett. 90, 123509 (2007) Conjugated oligomers and polymers (e.g., polyfluorene) Org, Electron. 1, 15 (2000) Aromatic fused rings WO2009066779, WO2009066778, WO2009063833, US20090045731, US20090045730, WO2009008311, US20090008605, US20090009065 Zinc complexes WO2010056066 Chrysene based compounds WO2011086863 Green hosts Arylcarbazoles Appl. Phys, Lett. 78, 1622 (2001) US20030175553 WO2001039234 Aryltriphenylene compounds 0 US20060280965 US20060280965 WO2009021126 Poly-fused heteroaryl compounds US20090309488 US20090302743 US20100012931 Donor acceptor type molecules WO2008056746 Aza-carbazole/ DBT/DBF JP2008074939 US20100187984 Polymers (e.g., PVK) Appl. Phys. Lett. 77, 2280 (2000) Spirofluorene compounds WO2004093207 Metal phenoxybenzooxazole compounds WO2005089025 0 WO2006132173 JP200511610 Spirofluorene- carbazole compounds JP2007254297 JP2007254297 Indolocarbazoles WO2007063796 WO2007063754 5-member ring electron deficient heterocycles (e.g., triazole, oxadiazole) J. Appl. Phys. 90, 5048 (2001) WO2004107822 Tetraphenylene complexes US20050112407 Metal phenoxypyridine compounds WO2005030900 Metal coordination complexes (e.g., Zn, Al with N{circumflex over ( )}N ligands) 00 US20040137268, US20040137267 Blue hosts Arylcarbazoles 01 Appl. Phys. Lett, 82, 2422 (2003) 02 US20070190359 Dibenzothiophene/ Dibenzofuran- carbazole compounds 03 WO2006114966, US20090167162 04 US20090167162 05 WO2009086028 06 US20090030202, US20090017330 07 US20100084966 Silicon aryl compounds 08 US20050238919 09 WO2009003898 Silicon/Germanium aryl compounds 0 EP2034538A Aryl benzoyl ester WO2006100298 Carbazole linked by non-conjugated groups US20040115476 Aza-carbazoles US20060121308 High triplet metal organometallic complex U.S. Pat. No. 7,154,114 Phosphorescent dopants Red dopants Heavy metal porphyrins (e.g., PtOEP) Nature 395, 151 (1998) Iridium(III) organometallic complexes Appl. Phys. Lett. 78, 1622 (2001) US20030072964 US20030072964 US20060202194 0 US20060202194 US20070087321 US20080261076 US20100090591 US20070087321 Adv. Mater. 19, 739 (2007) WO2009100991 WO2008101842 U.S. Pat. No. 7,232,618 Platinum(II) organometallic complexes WO2003040257 US20070103060 Osmium(III) complexes 0 Chem. Mater. 17, 3532 (2005) Ruthenium(II) complexes Adv. Mater. 17, 1059 (2005) Rhenium (I), (II), and (III) complexes US20050244673 Green dopants Iridium(III) organometallic complexes Inorg. Chem. 40, 1704 (2001) US20020034656 US. Pat. No. 7,332,232 US20090108737 WO2010028151 EP1841834B US20060127696 0 US20090039776 U.S. Pat. No. 6,921,915 US20100244004 U.S. Pat. No. 6,687,266 Chem. Mater. 16, 2480 (2004) US20070190359 US 20060008670 JP2007123392 WO2010086089, WO2011044988 Adv. Mater. 16, 2003 (2004) Angew. Chem. Int. Ed. 2006, 45, 7800 0 WO2009050290 US20090165846 US20080015355 US20010015432 US20100295032 Monomer for polymeric metal organometallic compounds U.S. Pat. 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EXPERIMENTAL

(96) Compound B481, Os(L.sup.1)(L.sup.B13), was synthesized from the scheme below following a the procedure previously reported in U.S. Patent Application Publication No. 2015-0028290.

(97) ##STR00218##

(98) Compound B481 has an emission peak wavelength of 570 nm in toluene at room temperature (˜25° C.). The oxidation potention of Compound B481 is −0.57 V vs. Fc.sup.+/Fc and its reduction potential is −3 V vs. Fc.sup.+/Fc. On the other hand, Compound A, a comparative compound that has no azadibenzofuran moiety in the structure, has an oxidation potential of −0.5 V vs. Fc.sup.+/Fc and its reduction potential was outside the detection limit of the solvent system.

(99) ##STR00219##

(100) The introduction of the azadibenzofuran moiety to the ligand in the current invention stabilizes the LUMO of the complex and does not alter the HOMO energy level, making it more stable to electrons. Therefore, the device performance can be improved.

(101) It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why the invention works are not intended to be limiting.