Patent Publication Number: US-2019189935-A1

Title: Organic electroluminescent materials and devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/598,585, filed Dec. 14, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to compounds for use as emitters, and devices, such as organic light emitting diodes, including the same. 
     BACKGROUND 
     Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting diodes/devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. 
     OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety. 
     One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Alternatively the OLED can be designed to emit white light. In conventional liquid crystal displays emission from a white backlight is filtered using absorption filters to produce red, green and blue emission. The same technique can also be used with OLEDs. The white OLED can be either a single EML device or a stack structure. Color may be measured using CIE coordinates, which are well known to the art. 
     One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy) 3 , which has the following structure: 
     
       
         
         
             
             
         
       
     
     As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between. 
     As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form. 
     A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand. 
     As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level. 
     As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions. 
     More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety. 
     SUMMARY 
     A compound that includes a ligand L A  of Formula I, the ligand L A  coordinated to a metal M 
     
       
         
         
             
             
         
       
     
     wherein 
     X 1  to X 11  are each independently selected from CR A  or N; 
     R A , R B , and R C  independently represent mono to the maximum possible substitution, or no substitution; 
     each R A , R B , and R C  are 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 two adjacent R A , R B , or R C  may join to form a ring; 
     wherein at least one of R A , R B , or R C  is selected from the group consisting of 
     
       
         
         
             
             
         
       
     
     wherein 
     X 12 , X 13 , X 14 , and X 15  are independently selected from CR D  or N, and two of X 12 , X 13 , X 14 , or X 15  are N; 
     R 1 , R 2 , R 3 , R 4  and each R D  are independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, and heteroaryl; wherein any two adjacent R 1 , R 2 , R 3 , R 4  and R D  may join to form a ring; 
     with the proviso that R 1  and R 2 , and R 3  and R 4 , do not join to form a fused benzene ring; and 
     wherein the metal M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, Ag, and Cu, and ligand L A  is optionally linked to another ligand to form a tridentate, tetradentate, pentadentate, or hexadentate ligand. 
     A compound comprising a ligand L A  of Formula VI, the ligand L A  coordinated to a metal M 
     
       
         
         
             
             
         
       
     
     wherein 
     R A , R B , and R C  independently represent no substitution to a maximum possible number of substitutions; 
     R 1 , and each R A , R B , and R C , are 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; or optionally, any two adjacent R A , R B , or R C  join to form to ring; and 
     wherein the metal M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, Ag, and Cu, and ligand L A  is optionally linked to another ligand to form a tridentate, tetradentate, pentadentate, or hexadentate ligand. 
     An organic light emitting device (OLED) that includes an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer includes a compound comprising a ligand L A  of Formula I or Formula VI above. 
     A consumer product that includes an organic light-emitting device (OLED) above. Again, the OLED will include an organic layer that comprises a compound comprising a ligand L A  of Formula I or Formula VI above. The consumer product is selected from the group consisting of a flat panel display, a computer monitor, a medical monitor, a television, a billboard, a light for interior or exterior illumination and/or signaling, a heads-up display, a fully or partially transparent display, a flexible display, a laser printer, a telephone, a cell phone, tablet, a phablet, a personal digital assistant (PDA), a wearable device, a laptop computer, a digital camera, a camcorder, a viewfinder, a micro-display that is less than 2 inches diagonal, a 3-D display, a virtual reality or augmented reality display, a vehicle, a video walls comprising multiple displays tiled together, a theater or stadium screen, a light therapy device, and a sign. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an organic light emitting device. 
         FIG. 2  shows an inverted organic light emitting device that does not have a separate electron transport layer. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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. 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”), 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. 
       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. 
     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 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. 
       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 . 
     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 . 
     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. 
     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 organic vapor jet printing (OVJP). 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. 
     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. 
     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. A consumer product comprising an OLED that includes the compound of the present disclosure in the organic layer in the OLED is disclosed. 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, curved 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, rollable displays, foldable displays, stretchable displays, laser printers, telephones, mobile phones, tablets, phablets, personal digital assistants (PDAs), wearable devices, laptop computers, digital cameras, camcorders, viewfinders, micro-displays (displays that are less than 2 inches diagonal), 3-D displays, virtual reality or augmented reality displays, vehicles, video walls comprising multiple displays tiled together, theater or stadium screen, a light therapy device, and 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. 
     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. 
     The terms “halo,” “halogen,” and “halide” are used interchangeably and refer to fluorine, chlorine, bromine, and iodine. 
     The term “acyl” refers to a substituted carbonyl radical (C(O)—R s ). 
     The term “ester” refers to a substituted oxycarbonyl (—O—C(O)—R s  or —C(O)—O—R s ) radical. 
     The term “ether” refers to an —OR s  radical. 
     The terms “sulfanyl” or “thio-ether” are used interchangeably and refer to a —SR s  radical. 
     The term “sulfinyl” refers to a —S(O)—R s  radical. 
     The term “sulfonyl” refers to a —SO 2 —R s  radical. 
     The term “phosphino” refers to a —P(R s ) 3  radical, wherein each R s  can be same or different. 
     The term “silyl” refers to a —Si(R s ) 3  radical, wherein each R s  can be same or different. 
     In each of the above, R s  can be hydrogen or a substituent selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, and combination thereof. Preferred R s  is selected from the group consisting of alkyl, cycloalkyl, aryl, heteroaryl, and combination thereof. 
     The term “alkyl” refers to and includes both straight and branched chain alkyl radicals. Preferred alkyl groups are those containing from one to fifteen carbon atoms and includes 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, and the like. Additionally, the alkyl group is optionally substituted. 
     The term “cycloalkyl” refers to and includes monocyclic, polycyclic, and spiro alkyl radicals. Preferred cycloalkyl groups are those containing 3 to 12 ring carbon atoms and includes cyclopropyl, cyclopentyl, cyclohexyl, bicyclo[3.1.1]heptyl, spiro[4.5]decyl, spiro[5.5]undecyl, adamantyl, and the like. Additionally, the cycloalkyl group is optionally substituted. 
     The terms “heteroalkyl” or “heterocycloalkyl” refer to an alkyl or a cycloalkyl radical, respectively, having at least one carbon atom replaced by a heteroatom. Optionally the at least one heteroatom is selected from O, S, N, P, B, Si and Se, preferably, O, S or N. Additionally, the heteroalkyl or heterocycloalkyl group is optionally substituted. 
     The term “alkenyl” refers to and includes both straight and branched chain alkene radicals. Alkenyl groups are essentially alkyl groups that include at least one carbon-carbon double bond in the alkyl chain. Cycloalkenyl groups are essentially cycloalkyl groups that include at least one carbon-carbon double bond in the cycloalkyl ring. The term “heteroalkenyl” as used herein refers to an alkenyl radical having at least one carbon atom replaced by a heteroatom. Optionally the at least one heteroatom is selected from O, S, N, P, B, Si, and Se, preferably, O, S, or N. Preferred alkenyl, cycloalkenyl, or heteroalkenyl groups are those containing two to fifteen carbon atoms. Additionally, the alkenyl, cycloalkenyl, or heteroalkenyl group is optionally substituted. 
     The term “alkynyl” refers to and includes both straight and branched chain alkyne radicals. Preferred alkynyl groups are those containing two to fifteen carbon atoms. Additionally, the alkynyl group is optionally substituted. 
     The terms “aralkyl” or “arylalkyl” are used interchangeably and refer to an alkyl group that is substituted with an aryl group. Additionally, the aralkyl group is optionally substituted. 
     The term “heterocyclic group” refers to and includes aromatic and non-aromatic cyclic radicals containing at least one heteroatom. Optionally the at least one heteroatom is selected from O, S, N, P, B, Si, and Se, preferably, O, S, or N. Hetero-aromatic cyclic radicals may be used interchangeably with heteroaryl. Preferred hetero-non-aromatic cyclic groups are those containing 3 to 7 ring atoms which includes at least one hetero atom, and includes cyclic amines such as morpholino, piperidino, pyrrolidino, and the like, and cyclic ethers/thio-ethers, such as tetrahydrofuran, tetrahydropyran, tetrahydrothiophene, and the like. Additionally, the heterocyclic group may be optionally substituted. 
     The term “aryl” refers to and includes both single-ring aromatic hydrocarbyl groups and polycyclic aromatic 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 an aromatic hydrocarbyl group, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl, heterocycles, and/or heteroaryls. Preferred aryl groups are those containing six to thirty carbon atoms, preferably six to twenty carbon atoms, more preferably six to twelve carbon atoms. Especially preferred is an aryl group having six carbons, ten carbons or twelve carbons. Suitable aryl groups include phenyl, biphenyl, triphenyl, triphenylene, tetraphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, and azulene, preferably phenyl, biphenyl, triphenyl, triphenylene, fluorene, and naphthalene. Additionally, the aryl group is optionally substituted. 
     The term “heteroaryl” refers to and includes both single-ring aromatic groups and polycyclic aromatic ring systems that include at least one heteroatom. The heteroatoms include, but are not limited to O, S, N, P, B, Si, and Se. In many instances, O, S, or N are the preferred heteroatoms. Hetero-single ring aromatic systems are preferably single rings with 5 or 6 ring atoms, and the ring can have from one to six heteroatoms. The hetero-polycyclic ring systems can have 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. The hetero-polycyclic aromatic ring systems can have from one to six heteroatoms per ring of the polycyclic aromatic ring system. Preferred heteroaryl groups are those containing three to thirty carbon atoms, preferably three to twenty carbon atoms, more preferably three to twelve carbon atoms. Suitable heteroaryl groups include 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, preferably dibenzothiophene, dibenzofuran, dibenzoselenophene, carbazole, indolocarbazole, imidazole, pyridine, triazine, benzimidazole, 1,2-azaborine, 1,3-azaborine, 1,4-azaborine, borazine, and aza-analogs thereof. Additionally, the heteroaryl group is optionally substituted. 
     Of the aryl and heteroaryl groups listed above, the groups of triphenylene, naphthalene, anthracene, dibenzothiophene, dibenzofuran, dibenzoselenophene, carbazole, indolocarbazole, imidazole, pyridine, pyrazine, pyrimidine, triazine, and benzimidazole, and the respective aza-analogs of each thereof are of particular interest. 
     The terms alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aralkyl, heterocyclic group, aryl, and heteroaryl, as used herein, are independently unsubstituted, or independently substituted, with one or more general substituents. 
     In many instances, the general substituents are selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof. 
     In some instances, the preferred general substituents are selected from the group consisting of deuterium, fluorine, alkyl, cycloalkyl, heteroalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, heteroaryl, nitrile, isonitrile, sulfanyl, and combinations thereof. 
     In some instances, the preferred general substituents are selected from the group consisting of deuterium, fluorine, alkyl, cycloalkyl, alkoxy, aryloxy, amino, silyl, aryl, heteroaryl, sulfanyl, and combinations thereof. 
     In yet other instances, the more preferred general substituents are selected from the group consisting of deuterium, fluorine, alkyl, cycloalkyl, aryl, heteroaryl, and combinations thereof. 
     The terms “substituted” and “substitution” refer to a substituent other than H that is bonded to the relevant position, e.g., a carbon or nitrogen. For example, when R 1  represents mono-substitution, then one R 1  must be other than H (i.e., a substitution). Similarly, when R 1  represents di-substitution, then two of R 1  must be other than H. Similarly, when R 1  represents no substitution, R 1 , for example, can be a hydrogen for available valencies of ring atoms, as in carbon atoms for benzene and the nitrogen atom in pyrrole, or simply represents nothing for ring atoms with fully filled valencies, e.g., the nitrogen atom in pyridine. The maximum number of substitutions possible in a ring structure will depend on the total number of available valencies in the ring atoms. 
     As used herein, “combinations thereof” indicates that one or more members of the applicable list are combined to form a known or chemically stable arrangement that one of ordinary skill in the art can envision from the applicable list. For example, an alkyl and deuterium can be combined to form a partial or fully deuterated alkyl group; a halogen and alkyl can be combined to form a halogenated alkyl substituent; and a halogen, alkyl, and aryl can be combined to form a halogenated arylalkyl. In one instance, the term substitution includes a combination of two to four of the listed groups. In another instance, the term substitution includes a combination of two to three groups. In yet another instance, the term substitution includes a combination of two groups. Preferred combinations of substituent groups are those that contain up to fifty atoms that are not hydrogen or deuterium, or those which include up to forty atoms that are not hydrogen or deuterium, or those that include up to thirty atoms that are not hydrogen or deuterium. In many instances, a preferred combination of substituent groups will include up to twenty atoms that are not hydrogen or deuterium. 
     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. 
     As used herein, “deuterium” refers to an isotope of hydrogen. Deuterated compounds can be readily prepared using methods known in the art. For example, U.S. Pat. No. 8,557,400, Patent Pub. No. WO 2006/095951, and U.S. Pat. Application Pub. No. US 2011/0037057, which are hereby incorporated by reference in their entireties, describe the making of deuterium-substituted organometallic complexes. Further reference is made to Ming Yan, et al.,  Tetrahedron  2015, 71, 1425-30 and Atzrodt et al.,  Angew. Chem. Int. Ed . ( Reviews ) 2007, 46, 7744-65, which are incorporated by reference in their entireties, describe the deuteration of the methylene hydrogens in benzyl amines and efficient pathways to replace aromatic ring hydrogens with deuterium, respectively. 
     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. 
     A compound that includes a ligand L A  of Formula I, the ligand L A  coordinated to a metal M 
     
       
         
         
             
             
         
       
     
     wherein 
     X 1  to X 11  are each independently selected from CR A  or N; 
     R A , R B , and R C  independently represent mono to the maximum possible substitution, or no substitution; 
     each R A , R B , and R C  are 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 two adjacent R A , R B , or R C  may join to form a ring; 
     wherein at least one of R A , R B , or R C  is selected from the group consisting of 
     
       
         
         
             
             
         
       
     
     wherein 
     X 12 , X 13 , X 14 , and X 15  are independently selected from CR D  or N, and two of X 12 , X 13 , X 14 , or X 15  are N; 
     R 1 , R 2 , R 3 , R 4  and each R D  are independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, and heteroaryl; wherein any two adjacent R 1 , R 2 , R 3 , R 4  and R D  may join to form a ring; with the proviso that R 1  and R 2 , and R 3  and R 4 , do not join to form a fused benzene ring; and wherein the metal M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, Ag, and Cu, and ligand L A  is optionally linked to another ligand to form a tridentate, tetradentate, pentadentate, or hexadentate ligand. 
     In one embodiment, the compounds that comprises a ligand L A  of Formula I, each R A , R B , and R C  are independently selected from hydrogen, or independently a substituent selected from the group consisting of deuterium, fluorine, alkyl, cycloalkyl, heteroalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, heteroaryl, nitrile, isonitrile, sulfanyl, and combinations thereof. 
     In another embodiment, the compounds that comprises a ligand L A  of Formula I, each R A , R B , and R C  are independently selected from hydrogen, or independently a substituent selected from the group consisting of hydrogen, deuterium, fluorine, alkyl, cycloalkyl, alkoxy, aryloxy, amino, silyl, aryl, heteroaryl, sulfanyl, and combinations thereof. In some instances, one or two of R A , R B , and R C  are selected from the group consisting of a C 1 -C 6  alkyl, a C 6 -C 14  aryl, a C 5 -C 14  heteroaryl, and combinations thereof, each of which is partially or completely deuterated. 
     In one embodiment, the compounds that comprises a ligand L A  of Formula I, R 1 , R 2 , R 3 , R 4  and each R D  are independently selected from the group consisting of hydrogen, deuterium, fluorine, alkyl, cycloalkyl, heteroalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, heteroaryl, nitrile, isonitrile, sulfanyl, and combinations thereof. In some instances, one or two of R 1 , R 2 , R 3 , R 4  or R D  are selected from the group consisting of a C 1 -C 6  alkyl, a C 6 -C 14  aryl, a C 5 -C 14  heteroaryl, and combinations thereof, each of which is partially or completely deuterated. 
     Of particular interest are compounds that comprise a ligand L A  of Formula I in which the ligand L A  is coordinated to iridium or platinum. Moreover, the compounds can be what is referred to in the art of OLED emitting dopants as homoleptic or heteroleptic. 
     In one embodiment, at least one of R A  is a structure of Formulae II or III. In another embodiment, at least one of R B  is a structure of Formulae II or III. In yet another embodiment, at least one of R C  is a structure of Formulae II or III. In many instances, at least one of R C  is a structure of Formulae II or III is preferred, and optionally, the pyrrole ring is substituted at the para or 4-position of ring C in relationship to ring C attachment to the imidazole ring nitrogen. 
     Of interest are compounds that comprise a ligand L A  of Formula I where two adjacent R A  form a fused ring. In many instances, the fused ring is selected from the group consisting of benzene, pyridine, pyrimidine, pyrazine, and pyrrole. Moreover, this embodiment can include select compounds where each of R 1 , R 2 , R 3 , and R 4  of Formula II are hydrogen. 
     Another embodiment of interest are compounds that comprise a ligand L A  of Formula I where each of R 1 , R 2 , R 3 , and R 4  of Formula II are hydrogen. 
     There is also interest in compounds that comprise a ligand L A  of Formula I where the aromatic rings can be linked or connected to form a ligand L A  that has a fused polycyclic structure. In one instance, X 6  and X 7  are carbon and are connected by a direct bond. In another instance, X 1  and X 11  are carbon and are connected by an organic linker. In many instances the organic linker is selected from phenylene, or an alkylene selected from —CRR′-G- or SiRR′-G-; wherein G is selected from the group consisting of CRR′, SiRR′, O, and NR, and R and R′ is selected form the group consisting of hydrogen, deuterium, alkyl, alkoxy, amino, aryl, heteroaryl, and a combination thereof. In one embodiment, the compounds comprising a ligand L A  of Formula I, each of X 1  to X 11  is carbon. 
     In one embodiment, X 1  and X 2  are each carbon, or one of X 1  or X 2  is nitrogen and the other carbon. 
     In one embodiment, the compounds comprising a ligand L A  of Formula I, the aromatic ring C, which is defined by ring members X 7 , X 8 , X 9 , X 10  and X 11 , is selected from the group consisting of pyridine, pyrimidine, and pyrazine. In other embodiments, at least one R A  is selected from the group consisting of C 1 -C 10  alkyl, C 6 -C 14  aryl, and C 5 -C 13  heteroaryl, each of which is optionally substituted, and in some instances, the C 1 -C 10 alkyl, C 6 -C 14  aryl, and C 5 -C 13  heteroaryl, are substituted at one or more positions with deuterium. Alternatively, two adjacent R A  will join to form a fused cycloalkyl, heteroalkyl, aryl, or heteroaryl ring, each of which is optionally substituted, and in some instances, the fused cycloalkyl, heteroalkyl, aryl, or heteroaryl ring, are substituted at one or more positions with deuterium. 
     In another embodiment, at least one R B  is selected from the group consisting of C 1 -C 10 alkyl, C 6 -C 14  aryl, and C 5 -C 13  heteroaryl, each of which is optionally substituted, and in some instances, the C 1 -C 10 alkyl, C 6 -C 14  aryl, and C 5 -C 13  heteroaryl, are substituted at one or more positions with deuterium. Alternatively, two adjacent R B  will join to form a fused cycloalkyl, heteroalkyl, aryl, or heteroaryl ring, each of which is optionally substituted, and in some instances, the fused cycloalkyl, heteroalkyl, aryl, or heteroaryl ring, are substituted at one or more positions with deuterium. 
     In another embodiment, the compounds comprising a ligand L A  of Formula I, will have a fused ring system in which ring B together with two adjacent R B  forms a group of Formula IV, wherein X is selected from NR N , O, S, or Se; 
     
       
         
         
             
             
         
       
     
     A 1 , A 2 , A 3 , and A 4  are independently CR A1 , CR A2 , CR A3 , and CR A4 , respectively, or N, and no more than two of A 1 , A 2 , A 3 , and A 4  are N; R A1 , R A2 , R A3 , and R A4  are independently defined by R A  above; and R N  is selected from the group consisting of hydrogen, deuterium, alkyl, cycloalkyl, aryl, heteroaryl, and combinations thereof. The * of Formula IV represents attachment of the B ring to the aromatic BN ring. 
     Compounds of Formula I of particular interest include a Ligand L A  selected from the group consisting of 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In one embodiment, the compounds comprising a ligand L A  of Formula I, Formula II, or Formula III, are of a formula M(L A ) x (L B ) y (L C ) z : wherein L B  and L C  are each a bidentate ligand; and x is 1, 2, or 3; y is 1 or 2; z is 0, 1, or 2; and x+y+z is the oxidation state of the metal M. The bidentate ligands L B  and L C  are selected from 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     For the above bidentate ligand structures, Y 1  to Y 13  are independently selected from the group consisting of carbon and nitrogen; Y 1  is selected from the group consisting of BR e , NR e , PR e , O, S, Se, C═O, S═O, SO 2 , CR e R f , SiR e R f , and GeR e R f ; wherein R e  and R f  optionally join to form a ring. R a , R b , R c , and R d  independently represent from mono substitution to the maximum possible number of substitution, or no substitution, and each R a , R b , R c , R d , R e  and R f  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 acid, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; or optionally, any two adjacent substituents of R a , R b , R c , and R d  join to form a ring or form a multidentate ligand. 
     In another embodiment, the compounds comprise a ligand L A  of Formula I, and are of a formula M(L A ) x (L B ) y (L C ) z : wherein L B  and L C  are each a bidentate ligand; and x is 1, 2, or 3; y is 1 or 2; z is 0, 1, or 2; and x+y+z is the oxidation state of the metal M. The bidentate ligands L B  and L C  are selected from 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     For the above bidentate ligand structures, R a , R b  and R c  represent mono to the maximum allowable substitution, or no substitution, and each R a , R b  and R c  is independently selected from the group consisting of hydrogen, deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, and combinations thereof; or optionally, any two adjacent R a , R b  and R c  can join to form a ring. 
     In one embodiment, compounds of interest will have a formula of Pt(L A )(L B ) or Pt(L A ) 2 ; wherein L A  can be same or different, and L B  is a ligand selected from select groups of L B  ligands identified above. 
     In one embodiment, compounds of interest are selected from the group consisting of: a Compound Ax having the formula Ir(L Ai ) 3 ; wherein x=i; i is an integer from 1 to 59; and a Compound By having the formula Ir(L Ai )(L Bk ) 2 ; wherein y=468i+k−468; i is an integer from 1 to 59, and k is an integer from 1 to 468; and wherein L Bk  has the following structures below, and ligand L A  is one of the ligand structures L A1  to L A59  above. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The invention is also directed to a compound comprising a ligand L A  of Formula VI, the ligand L A  coordinated to a metal M 
     
       
         
         
             
             
         
       
     
     wherein 
     R A , R B , and R C  independently represent no substitution to a maximum possible number of substitutions; 
     R 1 , and each R A , R B , and R C , are 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; or optionally, any two adjacent R A , R B , or R C  join to form to ring; and 
     wherein the metal M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, Ag, and Cu, and ligand L A  is optionally linked to another ligand to form a tridentate, tetradentate, pentadentate, or hexadentate ligand. 
     In one embodiment, the compounds of Formula VI will have R 1 , and each R A , R B , and R C , independently selected from the group consisting of hydrogen, deuterium, fluorine, alkyl, cycloalkyl, heteroalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, heteroaryl, nitrile, isonitrile, sulfanyl, and combinations thereof. 
     Of interest are compounds of Formula VI where R 1  is selected from alkyl, aryl, heteroaryl group, or a combination thereof. Of particular interest are compounds of Formula VI where R 1  is an aryl group containing at least one ortho alkyl group. 
     In one embodiment, the compounds of Formula VI will have R 1  at least one R C  that is an ortho alkyl group. 
     In another embodiment, the compounds of Formula VI will have each R A  being hydrogen, or one R A  will be an alkyl group and the remaining R A  are hydrogen. 
     In another embodiment, the compounds of Formula VI will have each R B  being hydrogen. 
     In another embodiment, the compounds of Formula VI will have the CN group of ring B present at the para ring position with respect to ring B coordination to M. 
     Compounds of Formula VI that are of selected interest will have a ligand L AX  selected from the group consisting of 
     
       
         
         
             
             
         
       
     
     wherein 
     R 2  is selected from the group consisting of hydrogen, deuterium, fluorine, alkyl, 
     cycloalkyl, heteroalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, heteroaryl, nitrile, isonitrile, and combinations thereof. Groups R 1 , R B , and R C  are defined as above for compounds of Formula VI. 
     In addition, as to the select compounds above, compounds with a ligand L AA  are selected from compounds of Formula VII or Formula VIII, identified below where the groups R 1 , R 2 , R B , and R C  are represented in the Tables below. 
     
       
         
           
               
            
               
                   
               
               
                 Formula VII 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 L AAX   
                 R C   
                 R 1   
                 R A   
                 R B   
               
               
                   
               
               
                 L AA1   
                 CH 3 ,CH 3   
                 Phenyl 
                 CH 3   
                 H 
               
               
                 L AA2   
                 CH 3 ,CH 3   
                 Phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA3   
                 CH 3 ,CH 3   
                 Phenyl 
                 H 
                 H 
               
               
                 L AA4   
                 CH 3 ,CH 3   
                 Phenyl 
                 H 
                 CH 3   
               
               
                 L AA5   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 H 
                 H 
               
               
                 L AA6   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 H 
                 CH 3   
               
               
                 L AA7   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 CH 3   
                 H 
               
               
                 L AA8   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 CH 3   
                 CH 3   
               
               
                 L AA9   
                 CH 3 ,CH 3   
                 i-Pr 
                 CH 3   
                 CH 3   
               
               
                 L AA10   
                 CH 3 ,CH 3   
                 i-Pr 
                 CH 3   
                 H 
               
               
                 L AA11   
                 CH 3 ,CH 3   
                 i-Pr 
                 H 
                 CH 3   
               
               
                 L AA12   
                 CH 3 ,CH 3   
                 i-Pr 
                 H 
                 H 
               
               
                 L AA13   
                 CH 3 ,CH 3   
                 CH 3   
                 CH 3   
                 CH 3   
               
               
                 L AA14   
                 CH 3 ,CH 3   
                 CH 3   
                 H 
                 CH 3   
               
               
                 L AA15   
                 CH 3 ,CH 3   
                 CH 3   
                 CH 3   
                 H 
               
               
                 L AA16   
                 CH 3 ,CH 3   
                 CH 3   
                 H 
                 H 
               
               
                 L AA17   
                 CH 3 ,i-Pr 
                 phenyl 
                 H 
                 H 
               
               
                 L AA18   
                 CH 3 ,i-Pr 
                 phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA19   
                 CH 3 ,i-Pr 
                 phenyl 
                 CH 3   
                 H 
               
               
                 L AA20   
                 CH 3 ,i-Pr 
                 phenyl 
                 H 
                 CH 3   
               
               
                 L AA21   
                 i-Pr,i-Pr 
                 phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA22   
                 i-Pr,i-Pr 
                 phenyl 
                 H 
                 H 
               
               
                 L AA23   
                 i-Pr,i-Pr 
                 phenyl 
                 CH 3   
                 H 
               
               
                 L AA24   
                 i-Pr,i-Pr 
                 phenyl 
                 H 
                 CH 3   
               
               
                 L AA25   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 H 
                 H 
               
               
                 L AA26   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 CH 3   
                 CH 3   
               
               
                 L AA27   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 CH 3   
                 H 
               
               
                 L AA28   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 H 
                 CH 3   
               
               
                 L AA29   
                 Cy-hex,H 
                 phenyl 
                 H 
                 H 
               
               
                 L AA30   
                 Cy-hex,H 
                 phenyl 
                 CH 3   
                 H 
               
               
                 L AA31   
                 Cy-hex,H 
                 phenyl 
                 H 
                 CH 3   
               
               
                 L AA32   
                 Cy-hex,H 
                 phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA33   
                 Cy-hex,H 
                 cyclopropyl 
                 CH 3   
                 CH 3   
               
               
                 L AA34   
                 Cy-hex,H 
                 cyclopropyl 
                 CH 3   
                 H 
               
               
                 L AA35   
                 Cy-hex,H 
                 cyclopropyl 
                 H 
                 CH 3   
               
               
                 L AA36   
                 Cy-hex,H 
                 cyclopropyl 
                 H 
                 H 
               
               
                 L AA37   
                 Cy-hex,H 
                 CH 3   
                 CH 3   
                 CH 3   
               
               
                 L AA38   
                 Cy-hex,H 
                 CH 3   
                 CH 3   
                 H 
               
               
                 L AA39   
                 Cy-hex,H 
                 CH 3   
                 H 
                 CH 3   
               
               
                 L AA40   
                 Cy-hex,H 
                 CH 3   
                 H 
                 H 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Formula VIII 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 L AAX   
                 R C   
                 R I   
                 R A   
                 R B   
               
               
                   
               
               
                 L AA41   
                 CH 3 ,CH 3   
                 Phenyl 
                 CH 3   
                 H 
               
               
                 L AA42   
                 CH 3 ,CH 3   
                 Phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA43   
                 CH 3 ,CH 3   
                 Phenyl 
                 H 
                 H 
               
               
                 L AA44   
                 CH 3 ,CH 3   
                 Phenyl 
                 H 
                 CH 3   
               
               
                 L AA45   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 H 
                 H 
               
               
                 L AA46   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 H 
                 CH 3   
               
               
                 L AA47   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 CH 3   
                 H 
               
               
                 L AA48   
                 CH 3 ,CH 3   
                 cyclopropyl 
                 CH 3   
                 CH 3   
               
               
                 L AA49   
                 CH 3 ,CH 3   
                 i-Pr 
                 CH 3   
                 CH 3   
               
               
                 L AA50   
                 CH 3 ,CH 3   
                 i-Pr 
                 CH 3   
                 H 
               
               
                 L AA51   
                 CH 3 ,CH 3   
                 i-Pr 
                 H 
                 CH 3   
               
               
                 L AA52   
                 CH 3 ,CH 3   
                 i-Pr 
                 H 
                 H 
               
               
                 L AA53   
                 CH 3 ,CH 3   
                 CH 3   
                 CH 3   
                 CH 3   
               
               
                 L AA54   
                 CH 3 ,CH 3   
                 CH 3   
                 H 
                 CH 3   
               
               
                 L AA55   
                 CH 3 ,CH 3   
                 CH 3   
                 CH 3   
                 H 
               
               
                 L AA56   
                 CH 3 ,CH 3   
                 CH 3   
                 H 
                 H 
               
               
                 L AA57   
                 CH 3 ,i-Pr 
                 Phenyl 
                 H 
                 H 
               
               
                 L AA58   
                 CH 3 ,i-Pr 
                 Phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA59   
                 CH 3 ,i-Pr 
                 Phenyl 
                 CH 3   
                 H 
               
               
                 L AA60   
                 CH 3 ,i-Pr 
                 Phenyl 
                 H 
                 CH 3   
               
               
                 L AA61   
                 i-Pr,i-Pr 
                 Phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA62   
                 i-Pr,i-Pr 
                 Phenyl 
                 H 
                 H 
               
               
                 L AA63   
                 i-Pr,i-Pr 
                 phenyl 
                 CH 3   
                 H 
               
               
                 L AA64   
                 i-Pr,i-Pr 
                 phenyl 
                 H 
                 CH 3   
               
               
                 L AA65   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 H 
                 H 
               
               
                 L AA66   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 CH 3   
                 CH 3   
               
               
                 L AA67   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 CH 3   
                 H 
               
               
                 L AA68   
                 i-Pr,i-Pr 
                 cyclopropyl 
                 H 
                 CH 3   
               
               
                 L AA69   
                 Cy-hex,H 
                 phenyl 
                 H 
                 H 
               
               
                 L AA70   
                 Cy-hex,H 
                 phenyl 
                 CH 3   
                 H 
               
               
                 L AA71   
                 Cy-hex,H 
                 phenyl 
                 H 
                 CH 3   
               
               
                 L AA72   
                 Cy-hex,H 
                 phenyl 
                 CH 3   
                 CH 3   
               
               
                 L AA73   
                 Cy-hex,H 
                 cyclopropyl 
                 CH 3   
                 CH 3   
               
               
                 L AA74   
                 Cy-hex,H 
                 cyclopropyl 
                 CH 3   
                 H 
               
               
                 L AA75   
                 Cy-hex,H 
                 cyclopropyl 
                 H 
                 CH 3   
               
               
                 L AA76   
                 Cy-hex,H 
                 cyclopropyl 
                 H 
                 H 
               
               
                 L AA7   
                 Cy-hex,H 
                 CH 3   
                 CH 3   
                 CH 3   
               
               
                 L AA78   
                 Cy-hex,H 
                 CH 3   
                 CH 3   
                 H 
               
               
                 L AA79   
                 Cy-hex,H 
                 CH 3   
                 H 
                 CH 3   
               
               
                 L AA80   
                 Cy-hex,H 
                 CH 3   
                 H 
                 H 
               
               
                   
               
            
           
         
       
     
     The invention is also directed to an organic light emitting device (OLED) that includes an anode, a cathode, and an organic layer disposed between the anode and the cathode, and the organic layer includes a compound comprising a ligand L A  of Formula I, where the ligand L A  is coordinated to a metal M. 
     
       
         
         
             
             
         
       
     
     wherein 
     X 1  to X 11  are each independently selected from CR A  or N; 
     R A , R B , and R C  independently represent mono to the maximum possible substitution, or no substitution; each R A , R B , and R C  are 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 two adjacent R A , R B , or R C  may join to form a ring; 
     wherein at least one of R A , R B , or R C  is selected from the group consisting of 
     
       
         
         
             
             
         
       
     
     wherein 
     X 12 , X 13 , X 14 , and X 15  are independently selected from CR D  or N, and two of X 12 , X 13 , X 14 , or X 15  are N; 
     R 1 , R 2 , R 3 , R 4  and each R D  are independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, silyl, alkenyl, cycloalkenyl, heteroalkenyl, aryl, and heteroaryl; wherein any two adjacent R 1 , R 2 , R 3 , R 4  and R D  may join to form a ring; 
     with the proviso that R 1  and R 2 , and R 3  and R 4 , do not join to form a fused benzene ring; and 
     wherein the metal M is selected from the group consisting of Ir, Rh, Re, Ru, Os, Pt, Au, Ag, and Cu, and ligand L A  is optionally linked to another ligand to form a tridentate, tetradentate, pentadentate, or hexadentate ligand. 
     The invention is also directed to an organic light emitting device (OLED) that includes an anode, a cathode, and an organic layer disposed between the anode and the cathode, and the organic layer includes a compound comprising a ligand L A  of Formula VI. 
     In some embodiments, the OLED has one or more characteristics selected from the group consisting of being flexible, being rollable, being foldable, being stretchable, and being curved. In some embodiments, the OLED is transparent or semi-transparent. In some embodiments, the OLED further comprises a layer comprising carbon nanotubes. 
     In some embodiments, the OLED further comprises a layer comprising a delayed fluorescent emitter. In some embodiments, the OLED comprises a RGB pixel arrangement or white plus color filter pixel arrangement. In some embodiments, the OLED is a mobile device, a hand held device, or a wearable device. In some embodiments, the OLED is a display panel having less than 10 inch diagonal or 50 square inch area. In some embodiments, the OLED is a display panel having at least 10 inch diagonal or 50 square inch area. In some embodiments, the OLED is a lighting panel. 
     The compounds of the invention when used as phosphorescent emitter dopants in an organic layer of an OLED offer those skilled in the art design flexibility in terms of color and emission lifetime. The color is governed to some extent by the coordinating aryl substituted imidazole ring, that is ring A. In this instance, emission wavelength of the compounds that comprise a ligand L A  of Formula I are generally in the blue to green portion of the visible spectrum. The compounds emit in a range from the blue (about 460 nm) to the green (about 540 nm) depending on the structure of the coordination ring B structures. Just as important, however, is that the compounds with the pyrrole groups of Formulae of II and III have a reduced lifetime emission if compared to compounds of similar structure, but without the pyrrole substitution. For some reason, which is not quite understood pyrrole substitution off the aryl ring can decrease emission lifetime by at least 70%, at least 60%, at least 50% and in some instances, by at least 40%. In general, the compounds of interest will have an emission lifetime from 0.1 μm to 3 μm, preferably from 0.1 μm to 1 μm, 
     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; see, e.g., U.S. application Ser. No. 15/700,352, which is hereby incorporated by reference in its entirety), triplet-triplet annihilation, or combinations of these processes. 
     According to another aspect, a formulation comprising the compound described herein is also disclosed. 
     The OLED disclosed herein can be incorporated into one or more of a consumer product, an electronic component module, 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. 
     The organic layer can also include a host. In some embodiments, two or more hosts are preferred. In some embodiments, the hosts used maybe a) bipolar, b) electron transporting, c) hole transporting or d) wide band gap materials that play little role in charge transport. 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 n H 2n+1 , OC n H 2n+1 , OAr 1 , N(C n H 2n+1 ) 2 , N(Ar 1 )(Ar 2 ), CH═CH—C n H 2n+1 , C≡C—C n H 2n+1 , Ar 1 , Ar 1 —Ar 2 , and C n H 2n —Ar 1 , or the host has no substitutions. In the preceding substituents n can range from 1 to 10; and Ar 1  and Ar 2  can be independently selected from the group consisting of benzene, biphenyl, naphthalene, triphenylene, carbazole, and heteroaromatic analogs thereof. The host can be an inorganic compound. For example a Zn containing inorganic material e.g. ZnS. 
     The host can be a compound comprising at least one chemical group selected from the group consisting of triphenylene, carbazole, dibenzothiophene, dibenzofuran, dibenzoselenophene, azatriphenylene, azacarbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene. The host can include a metal complex. The host can be, but is not limited to, a specific compound selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     and combinations thereof. 
     Additional information on possible hosts is provided below. 
     In yet another aspect of the present disclosure, a formulation that comprises the novel compound disclosed herein is described. 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, electron blocking material, hole blocking material, and an electron transport material, disclosed herein. 
     Combination with Other Materials 
     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. 
     Conductivity Dopants: 
     A charge transport layer can be doped with conductivity dopants to substantially alter its density of charge carriers, which will in turn alter its conductivity. The conductivity is increased by generating charge carriers in the matrix material, and depending on the type of dopant, a change in the Fermi level of the semiconductor may also be achieved. Hole-transporting layer can be doped by p-type conductivity dopants and n-type conductivity dopants are used in the electron-transporting layer. 
     Non-limiting examples of the conductivity dopants that may be used in an OLED in combination with materials disclosed herein are exemplified below together with references that disclose those materials: EP01617493, EP01968131, EP2020694, EP2684932, US20050139810, US20070160905, US20090167167, US2010288362, WO006081780, WO2009003455, WO2009008277, WO2009011327, WO2014009310, US2007252140, US2015060804, US20150123047, and US2012146012. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     HIL/HTL: 
     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 x ; a p-type semiconducting organic compound, such as 1,4,5,8,9,12-Hexaazatriphenylenehexacarbonitrile; a metal complex, and a cross-linkable compounds. 
     Examples of aromatic amine derivatives used in HIL or HTL include, but not limit to the following general structures: 
     
       
         
         
             
             
         
       
     
     Each of Ar 1  to Ar 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. Each Ar may be unsubstituted or may be substituted by a substituent selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acids, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof. 
     In one aspect, Ar 1  to Ar 9  is independently selected from the group consisting of: 
     
       
         
         
             
             
         
       
     
     wherein k is an integer from 1 to 20; X 101  to X 108  is C (including CH) or N; Z 101  is NAr 1 , O, or S; Ar 1  has the same group defined above. 
     Examples of metal complexes used in HIL or HTL include, but are not limited to the following general formula: 
     
       
         
         
             
             
         
       
     
     wherein Met is a metal, which can have an atomic weight greater than 40; (Y 101 —Y 102 ) is a bidentate ligand, Y 101  and Y 102  are independently selected from C, N, O, P, and S; L 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. 
     In one aspect, (Y 101 —Y 102 ) is a 2-phenylpyridine derivative. In another aspect, (Y 101 —Y 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 + /Fc couple less than about 0.6 V. 
     Non-limiting examples of the HIL and HTL materials that may be used in an OLED in combination with materials disclosed herein are exemplified below together with references that disclose those materials: CN102702075, DE102012005215, EP01624500, EP01698613, EP01806334, EP01930964, EP01972613, EP01997799, EP02011790, EP02055700, EP02055701, EP1725079, EP2085382, EP2660300, EP650955, JP07-073529, JP2005112765, JP2007091719, JP2008021687, JP2014-009196, KR20110088898, KR20130077473, TW201139402, U.S. Ser. No. 06/517,957, US20020158242, US20030162053, US20050123751, US20060182993, US20060240279, US20070145888, US20070181874, US20070278938, US20080014464, US20080091025, US20080106190, US20080124572, US20080145707, US20080220265, US20080233434, US20080303417, US2008107919, US20090115320, US20090167161, US2009066235, US2011007385, US20110163302, US2011240968, US2011278551, US2012205642, US2013241401, US20140117329, US2014183517, U.S. Pat. Nos. 5,061,569, 5,639,914, WO05075451, WO07125714, WO08023550, WO08023759, WO2009145016, WO2010061824, WO2011075644, WO2012177006, WO2013018530, WO2013039073, WO2013087142, WO2013118812, WO2013120577, WO2013157367, WO2013175747, WO2014002873, WO2014015935, WO2014015937, WO2014030872, WO2014030921, WO2014034791, WO2014104514, WO2014157018. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     EBL: 
     An electron blocking layer (EBL) may be used to reduce the number of electrons and/or excitons that leave the emissive layer. The presence of such a blocking layer in a device may result in substantially higher efficiencies, and/or longer lifetime, 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. In some embodiments, the EBL material has a higher LUMO (closer to the vacuum level) and/or higher triplet energy than the emitter closest to the EBL interface. In some embodiments, the EBL material has a higher LUMO (closer to the vacuum level) and/or higher triplet energy than one or more of the hosts closest to the EBL interface. In one aspect, the compound used in EBL contains the same molecule or the same functional groups used as one of the hosts described below. 
     Host: 
     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. Any host material may be used with any dopant so long as the triplet criteria is satisfied. 
     Examples of metal complexes used as host are preferred to have the following general formula: 
     
       
         
         
             
             
         
       
     
     wherein Met is a metal; (Y 103 —Y 104 ) is a bidentate ligand, Y 103  and Y 104  are independently selected from C, N, O, P, and S; L 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. 
     In one aspect, the metal complexes are: 
     
       
         
         
             
             
         
       
     
     wherein (O—N) is a bidentate ligand, having metal coordinated to atoms O and N. 
     In another aspect, Met is selected from Ir and Pt. In a further aspect, (Y 103 —Y 104 ) is a carbene ligand. 
     Examples of other organic compounds used as host are selected from the group consisting of aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, tetraphenylene, 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. Each option within each group may be unsubstituted or may be substituted by a substituent selected from the group consisting of deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acids, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof. 
     In one aspect, the host compound contains at least one of the following groups in the molecule: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein R 101  is selected from the group consisting of hydrogen, deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acids, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, and when it is aryl or heteroaryl, it has the similar definition as Ar&#39;s mentioned above. k is an integer from 0 to 20 or 1 to 20. X 101  to X 108  are independently selected from C (including CH) or N. Z 101  and Z 102  are independently selected from NR 101 , O, or S. 
     Non-limiting examples of the host materials that may be used in an OLED in combination with materials disclosed herein are exemplified below together with references that disclose those materials: EP2034538, EP2034538A, EP2757608, JP2007254297, KR20100079458, KR20120088644, KR20120129733, KR20130115564, TW201329200, US20030175553, US20050238919, US20060280965, US20090017330, US20090030202, US20090167162, US20090302743, US20090309488, US20100012931, US20100084966, US20100187984, US2010187984, US2012075273, US2012126221, US2013009543, US2013105787, US2013175519, US2014001446, US20140183503, US20140225088, US2014034914, U.S. Pat. No. 7,154,114, WO2001039234, WO2004093207, WO2005014551, WO2005089025, WO2006072002, WO2006114966, WO2007063754, WO2008056746, WO2009003898, WO2009021126, WO2009063833, WO2009066778, WO2009066779, WO2009086028, WO2010056066, WO2010107244, WO2011081423, WO2011081431, WO2011086863, WO2012128298, WO2012133644, WO2012133649, WO2013024872, WO2013035275, WO2013081315, WO2013191404, WO2014142472, US20170263869, US20160163995, U.S. Pat. No. 9,466,803, 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Additional Emitters: 
     One or more additional emitter dopants may be used in conjunction with the compound of the present disclosure. Examples of the additional emitter dopants are not particularly limited, and any compounds may be used as long as the compounds are typically used as emitter materials. Examples of suitable emitter materials include, but are not limited to, compounds which 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. 
     Non-limiting examples of the emitter materials that may be used in an OLED in combination with materials disclosed herein are exemplified below together with references that disclose those materials: CN103694277, CN1696137, EB01238981, EP01239526, EP01961743, EP1239526, EP1244155, EP1642951, EP1647554, EP1841834, EP1841834B, EP2062907, EP2730583, JP2012074444, JP2013110263, JP4478555, KR1020090133652, KR20120032054, KR20130043460, TW201332980, U.S. Ser. No. 06/699,599, U.S. Ser. No. 06/916,554, US20010019782, US20020034656, US20030068526, US20030072964, US20030138657, US20050123788, US20050244673, US2005123791, US2005260449, US20060008670, US20060065890, US20060127696, US20060134459, US20060134462, US20060202194, US20060251923, US20070034863, US20070087321, US20070103060, US20070111026, US20070190359, US20070231600, US2007034863, US2007104979, US2007104980, US2007138437, US2007224450, US2007278936, US20080020237, US20080233410, US20080261076, US20080297033, US200805851, US2008161567, US2008210930, US20090039776, US20090108737, US20090115322, US20090179555, US2009085476, US2009104472, US20100090591, US20100148663, US20100244004, US20100295032, US2010102716, US2010105902, US2010244004, US2010270916, US20110057559, US20110108822, US20110204333, US2011215710, US2011227049, US2011285275, US2012292601, US20130146848, US2013033172, US2013165653, US2013181190, US2013334521, US20140246656, US2014103305, U.S. Pat. Nos. 6,303,238, 6,413,656, 6,653,654, 6,670,645, 6,687,266, 6,835,469, 6,921,915, 7,279,704, 7,332,232, 7,378,162, 7,534,505, 7,675,228, 7,728,137, 7,740,957, 7,759,489, 7,951,947, 8,067,099, 8,592,586, 8,871,361, WO06081973, WO06121811, WO07018067, WO07108362, WO07115970, WO07115981, WO08035571, WO2002015645, WO2003040257, WO2005019373, WO2006056418, WO2008054584, WO2008078800, WO2008096609, WO2008101842, WO2009000673, WO2009050281, WO2009100991, WO2010028151, WO2010054731, WO2010086089, WO2010118029, WO2011044988, WO2011051404, WO2011107491, WO2012020327, WO2012163471, WO2013094620, WO2013107487, WO2013174471, WO2014007565, WO2014008982, WO2014023377, WO2014024131, WO2014031977 WO2014038456, WO2014112450. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     HBL: 
     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 and/or longer lifetime 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. In some embodiments, the HBL material has a lower HOMO (further from the vacuum level) and/or higher triplet energy than the emitter closest to the HBL interface. In some embodiments, the HBL material has a lower HOMO (further from the vacuum level) and/or higher triplet energy than one or more of the hosts closest to the HBL interface. 
     In one aspect, compound used in HBL contains the same molecule or the same functional groups used as host described above. 
     In another aspect, compound used in HBL contains at least one of the following groups in the molecule: 
     
       
         
         
             
             
         
       
     
     wherein k is an integer from 1 to 20; L 101  is an another ligand, k′ is an integer from 1 to 3. 
     ETL: 
     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. 
     In one aspect, compound used in ETL contains at least one of the following groups in the molecule: 
     
       
         
         
             
             
         
       
     
     wherein R 101  is selected from the group consisting of hydrogen, deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, heterocycloalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carboxylic acids, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar&#39;s mentioned above. Ar 1  to Ar 3  has the similar definition as Ar&#39;s mentioned above. k is an integer from 1 to 20. X 101  to X 108  is selected from C (including CH) or N. 
     In another aspect, the metal complexes used in ETL contains, but not limit to the following general formula: 
     
       
         
         
             
             
         
       
     
     wherein (O—N) or (N—N) is a bidentate ligand, having metal coordinated to atoms O, N or N, N; L 101  is another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal. 
     Non-limiting examples of the ETL materials that may be used in an OLED in combination with materials disclosed herein are exemplified below together with references that disclose those materials: CN103508940, EP01602648, EP01734038, EP01956007, JP2004-022334, JP2005149918, JP2005-268199, KR0117693, KR20130108183, US20040036077, US20070104977, US2007018155, US20090101870, US20090115316, US20090140637, US20090179554, US2009218940, US2010108990, US2011156017, US2011210320, US2012193612, US2012214993, US2014014925, US2014014927, US20140284580, U.S. Pat. Nos. 6,656,612, 8,415,031, WO2003060956, WO2007111263, WO2009148269, WO2010067894, WO2010072300, WO2011074770, WO2011105373, WO2013079217, WO2013145667, WO2013180376, WO2014104499, WO2014104535, 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Charge Generation Layer (CGL) 
     In tandem or stacked OLEDs, the CGL plays an essential role in the performance, which is composed of an n-doped layer and a p-doped layer for injection of electrons and holes, respectively. Electrons and holes are supplied from the CGL and electrodes. The consumed electrons and holes in the CGL are refilled by the electrons and holes injected from the cathode and anode, respectively; then, the bipolar currents reach a steady state gradually. Typical CGL materials include n and p conductivity dopants used in the transport layers. 
     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. may be undeuterated, partially deuterated, and fully deuterated versions thereof. Similarly, classes of substituents such as, without limitation, alkyl, aryl, cycloalkyl, heteroaryl, etc. also may be undeuterated, partially deuterated, and fully deuterated versions thereof. 
     EXPERIMENTAL 
     Preparation of 4-iodo-2,6-diisopropylaniline 
     
       
         
         
             
             
         
       
     
     Reaction (1): 2,6-Diisopropylaniline (1.73 g, 9.76 mmol) was dissolved in 30 mL of acetonitrile. 1-Iodopyrrolidine-2,5-dione (2.20 g, 9.76 mmol) was added and the reaction mixture stirred for 18 h. The reaction mixture was then concentrated to dryness under vacuum and dissolved in dichloromethane. The resulting solution was washed with aqueous sodium thiosulfate, followed by water. The organic layer was dried over sodium sulfate and concentrated under vacuum. The crude product was chromatographed on silica gel eluted with 6% ethyl acetate in heptane to afford 2.4 g 4-iodo-2,6-diisopropylaniline as a red oil. 
     
       
         
         
             
             
         
       
     
     Preparation of 2,6-diisopropyl-4-(1H-pyrrol-1-yl)aniline 
     Reaction (2): A 250 mL 2-necked flask was charged with CuI (0.503 g, 2.64 mmol), sodium-t-butoxide (3.80 g, 39.6 mmol) and 1-hydroxypyrrolidine-2,5-dione (0.607 g, 5.28 mmol). The flask was evacuated and backfilled with nitrogen, and then 40 mL of DMSO added. The resulting mixture was stirred at ambient temperature for 30 min before addition of a solution of 4-iodo-2,6-diisopropylaniline (8.0 g, 26.4 mmol) and pyrrole (1.95 g, 29.0 mmol) dissolved in 40 mL of DMSO. The reaction mixture was stirred under nitrogen at 120° C. for 20 h. After cooling to room temperature, the reaction mixture was diluted with ethyl acetate, filtered through Celite®, and extracted with water. The organic phase was concentrated and chromatographed on silica gel eluted with 5-10% ethyl acetate in heptane to afford 4.34 g of 2,6-diisopropyl-4-(1H-pyrrol-1-yl)aniline. 
     Preparation of N-(2,6-diisopropyl-4-(1H-pyrrol-1-yl)phenyl)benzimidamide 
     
       
         
         
             
             
         
       
     
     Reaction (3): 2,6-diisopropyl-4-(1H-pyrrol-1-yl)aniline (4.2 g, 17.3 mmol) was dissolved in 120 mL of anhydrous THF, and the solution cooled in a dry ice/acetone bath. n-Butyllithium (2.5 N, 6.9 mL) was added and the reaction mixture warmed to ambient temperature over a period of 30 min with stirring. The reaction mixture was again cooled using a dry ice/acetone bath, and benzonitrile (1.79 mL, 17.3 mmol) added. The resulting mixture was stirred under nitrogen for 18 h. The reaction mixture was then quenched with water, diluted with ethyl acetate, and washed with water. The organic layer was evaporated and triturated in heptane to afford 4.15 g of N-(2,6-diisopropyl-4-(1H-pyrrol-1-yl)phenyl)benzimidamide. 
     Preparation of 1-(2,6-diisopropyl-4-(1H-pyrrol-1-yl)phenyl)-2-phenyl-1H-imidazole 
     
       
         
         
             
             
         
       
     
     Reaction (4): N-(2,6-diisopropyl-4-(1H-pyrrol-1-yl)phenyl)benzimidamide (4.15 g, 12.0 mmol) was dissolved in 100 mL of 2-propanol. 2-Chloroacetaldehyde (3.0 mL, 24.0 mmol) and sodium hydrogen carbonate (2.02 g, 24.0 mmol) were added and the reaction mixture stirred at reflux for 4 h. The reaction mixture was then concentrated to dryness and partitioned between water and dichloromethane. The organic layer was chromatographed on silica gel eluted with 20% ethyl acetate in heptane to afford 4.2 g of 1-(2,6-diisopropyl-4-(1H-pyrrol-1-yl)phenyl)-2-phenyl-1H-imidazole as an orange foam.  1 H NMR (CD 2 Cl 2 ) □ 2.48 (m, 2H), 1.14 (d, 3H), 0.92 (t, 3H). 
     Preparation of Compound I 
     
       
         
         
             
             
         
       
     
     Reaction (5): A Schlenk tube was charged with 1-(2,6-diisopropyl-4-(1H-pyrrol-1-yl)phenyl)-2-phenyl-1H-imidazole (2.0 g, 5.4 mmol), iridium trisacetylacetonate (0.53 g, 1.1 mmol) and a stir bar. After evacuating and backfilling with nitrogen, pentadecane (2.0 mL) was added, and the reaction mixture was stirred while heating in a sand bath at reflux for 24 H. The product was purified by silica gel chromatography using a mobile phase of 1:1 dichloromethane-heptanes to afford 0.49 g of Compound I. 
     Comparative Example: Compound II 
     The excited state lifetime of Compound I was measured to be 2.5 microseconds at 77K. In comparison, the excited state lifetime of Compound II is 2.9 microseconds at 77K. As demonstrated, an attachment of a pyrrole group leads to a reduction of excited state lifetime, and lead to an improvement in device stability, particularly, for Compound I, which emits in the blue region of the visible spectrum. 
     
       
         
         
             
             
         
       
     
     Preparation of a Compound of Formula VI 
     Compounds of Formula VI can be synthesized in two steps, as shown in Scheme I below. As indicated, 2.00 g (9.95 mmol) of 2-bromo-N,N-dimethylpyridin-4-amine, 1.75 g (11.9 mmol) of (3-isocyanophenyl)boronic acid, 0.575 g (0.497 mmol) of Pd(PPh 3 ) 4 , and 2.11 g (19.9 mmol) of sodium carbonate were added to a 250 mL round bottom flask. Dioxane (75 mL) and water (25 mL) were added, and the reaction mixture was stirred at reflux for 24 h. The mixture was then diluted with ethyl acetate and filtered through Celite. The filtrate was washed with water, and the organic layer concentrated and chromatographed on silica gel eluted with 0-3% methanol in dichloromethane to afford 1.30 g (59%) of 2-(3-isocyanophenyl)-N,N-dimethylpyridin-4-amine as a yellow solid. 
     In the second step, 1.25 g (5.60 mmol) of 2-(3-isocyanophenyl)-N,N-dimethylpyridin-4-amine was dissolved in 15 mL of N-methylpyrrolidone. To this was added 0.693 g Ir(COD) 2 (BF 4 ) (1.40 mmol). The reaction mixture was stirred in an oil bath at 190° C. for 18 h. The solvent was removed in vacuo, and the crude product was chromatographed on silica gel eluted with 0-2% methanol in dichloromethane to afford Compound I of Formula VI. 
     
       
         
         
             
             
         
       
     
     A second compound of Formula VI was synthesized in three steps, as shown in Scheme II below. In the first step, 0.24 g (0.208 mmol) of Pd(PPh 3 ) 4 , 1.00 g (5.20 mmol) of 2-bromo-4-chloropyridine, 0.76 g (5.20 mmol) of (3-isocyanophenyl)boronic acid, and 1.10 g (10.4 mmol) of sodium carbonate were added to a 250 mL round bottom flask. Dioxane (50 mL) and water (10 mL) were added and the reaction mixture was stirred at reflux for 20 h. The crude product was concentrated and partitioned between ethyl acetate and brine. The organic layer was chromatographed on silica gel eluted with 10% ethyl acetate in heptane to afford 0.94 g (84% yield) of 4-chloro-2-(3-isocyanophenyl)pyridine. 
     In the second step, 0.35 g (1.77 mmol) of di-o-tolylamine, 0.324 g (1.51 mmol) of 4-chloro-2-(3-isocyanophenyl)pyridine, 0.041 g (0.089 mmol) of RuPhoa, 0.065 g (0.089 mmol) of “Catalyst A”, and 0.256 g (2.66 mmol) of sodium t-butoxide were added to a Schlenk tube. The Schlenk tube was evacuated and backfilled with nitrogen. O-xylene (17 mL) was added and the reaction mixture was stirred at reflux for 20 h. The crude product was diluted with ethyl acetate and filtered through Celite. The filtrate was concentrated and chromatographed on silica gel eluted with 0-10% ethyl acetate in dichloromethane to afford 0.24 g (36% yield) of 2-(3-isocyanophenyl)-N,N-di-o-tolylpyridin-4-amine. 
     In the third step, 0.61 g (1.62 mmol) of 2-(3-isocyanophenyl)-N,N-di-o-tolylpyridin-4-amine was dissolved in 10 mL of N-methylpyrrolidone. To this was added 0.20 g (0.41 mmol) of Ir(COD) 2 BF 4 . The reaction mixture was stirred in an oil bath at 190° C. for 20 h. The solvent was removed in vacuo and the crude was chromatographed on silica gel eluted with 0-2% ethyl acetate in dichloromethane to afford Compound II of Formula VI. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     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.