Abstract:
Subject matter disclosed herein relates to a series of palladium based materials, their preparation method and their applications in an organic light-emitting diode (OLED).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional application Ser. No. 61/569,184, filed on Dec. 9, 2011, which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Subject matter disclosed herein relates to a class of tetradentate palladium(II) complexes, their preparation method and their application in organic light-emitting diodes (OLED). 
     BACKGROUND 
     Organic light-emitting diodes (OLEDs) are recognized as a next-generation display and/or lighting technology. Due to the 3:1 triplet to singlet exciton issue, development of emitting materials for OLED application maybe mainly focused on phosphorescent materials. Among phosphorescent materials, metal organic materials containing heavy transition metals may exhibit desirable performance in OLED application. Iridium is a commonly used heavy transition metal while platinum is an up-and-coming candidate. Devices fabricated from iridium- and platinum-based materials have good device performance for mass production. 
     However, prices for iridium- and platinum-based materials may be relatively high. There is a desire to reduce costs for emitting materials. Zinc-based materials may provide an advantage related to cost, for example. Nevertheless, efficiency and/or stability of devices fabricated from zinc-based materials may be questionable. 
     SUMMARY 
     In various embodiments, phosphorescent materials, which can be used for OLED applications, comprise palladium as a metal center. One advantage associated with palladium over other materials is the relatively low price. 
     In various embodiments, palladium-based light-emitting materials comprise a molecular structure of Structure I: 
                                
wherein R 1 -R 14  are independently selected from hydrogen, halogen, oxygen, nitrogen, sulphur, selenium, hydroxyl, an unsubstituted alkyl, a substituted alkyl, cycloalkyl, an unsubstituted aryl, a substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino, aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl, aryloxycarbonyl, phenoxycarbonyl, or an alkoxycarbonyl group, for example. Individual R 1 -R 14  can independently form 5 to 8 member ring(s) with adjacent R group(s). The notation “R 1 -R 14 ” means R 1 , R 2 , R 3 , R 4  . . . R 11 , R 12 , R 13 , R 14 , for example.
 
     Individual R 1 -R 14  can independently comprise the same atom(s) as an adjacent R group and form a 5 member ring with four X atoms to form a complex with a chemical structure of Structure II, for example, X 1 -X 20  can be independently selected from boron, carbon, nitrogen, oxygen, or silicon, for example. The notation “X 1 -X 20 ” means X 1 , X 2 , X 3 , X 4  . . . X 17 , X 18 , X 19 , X 20 , for example. Structure II is represented as: 
                                
wherein R 1 -R 14  are independently selected from hydrogen, halogen, oxygen, nitrogen, sulphur, selenium, hydroxyl, an unsubstituted alkyl, a substituted alkyl, cycloalkyl, an unsubstituted aryl, a substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino, aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl, aryloxycarbonyl, phenoxycarbonyl, or an alkoxycarbonyl group and X 1 -X 20  can be independently selected from boron, carbon, nitrogen, oxygen, or silicon.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
         FIG. 1  shows an x-ray crystal structure for illustrative complex 101, according to an embodiment. 
         FIG. 2  shows an x-ray crystal structure for illustrative complex 104, according to an embodiment. 
         FIG. 3  shows a UV-visible spectrum of illustrative complexes 101-103 in CH 2 Cl 2  solution, according to an embodiment. 
         FIG. 4  shows a UV-visible spectrum of illustrative complexes 104-106 in CH 2 Cl 2  solution, according to an embodiment. 
         FIG. 5  shows photoluminescent spectra of illustrative complexes 101-103 in CH 2 Cl 2  solution, according to an embodiment. 
         FIG. 6  shows photoluminescent spectra of illustrative complexes 104-106 in CH 2 Cl 2  solution, according to an embodiment. 
         FIG. 7  shows solid-state (298 K), solid-state (77 K) and glassy (77 K) photoluminescent spectra for Complex 101, according to an embodiment. 
         FIG. 8  shows solid-state (298 K), solid-state (77 K) and glassy (77 K) photoluminescent spectra for Complex 102, according to an embodiment. 
         FIG. 9  shows solid-state (298 K), solid-state (77 K) and glassy (77 K) photoluminescent spectra for Complex 10, according to an embodiment. 
         FIG. 10  shows solid-state (298 K), solid-state (77 K) and glassy (77 K) photoluminescent spectra for Complex 104, according to an embodiment. 
         FIG. 11  shows solid-state (298 K), solid-state (77 K) and glassy (77 K) photoluminescent spectra for Complex 105, according to an embodiment. 
         FIG. 12  shows solid-state (298 K), solid-state (77 K) and glassy (77 K) photoluminescent spectra for Complex 106, according to an embodiment. 
         FIG. 13  shows thermograms for illustrative complexes 104-106, according to an embodiment. 
         FIG. 14  shows EL spectra for OLEDs 601-604, according to an embodiment. 
         FIG. 15  shows current density-voltage-brightness (J-V-B) relationships for OLEDs 601-604, according to an embodiment. 
         FIG. 16  shows external quantum efficiency-current density relationships for OLEDs 601-604, according to an embodiment. 
         FIG. 17  shows current efficiency-current density relationships for OLEDs 601-604, according to an embodiment. 
         FIG. 18  shows power efficiency-current density relationships for OLEDs 601-604, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Palladium(II) typically has four coordinating sites. Consequently, five types of palladium(II) complexes are observable—PdL 1 L 2 L 3 L 4 ; PdL 1 L 2 L 5 ; PdL 5 L 6 ; PdL 1 L 7 ; and PdL 8 , where L 1 -L 4  comprise monodentate ligands, which comprise the same ligand; L 5  and L 6  comprise bidentate ligands, L 7  comprises a tridentate ligand, and L 8  comprises a tetradentate ligand, for example. PdL 8 -type complexes can have relatively strong binding between the ligand and the palladium center since four metal-ligand bonds are involved. Therefore, PdL 8 -type complexes can have relatively high stability, and an OLED fabricated from PdL 8 -type complexes can have relatively high stability and long lifetime. Since electronically neutral complexes can be more readily sublimated for thermal deposition OLED fabrication, a di-anionic ligand can be used for palladium(II) complexes for OLED applications, for example. 
     In an embodiment, an emissive palladium(II) complex system with a chemical structure of Structure I can be designed for an OLED application, Structure I represented as: 
                                
wherein R 1 -R 14 , if present, can be independently selected from hydrogen, halogen, oxygen, nitrogen, sulphur, selenium, hydroxyl, an unsubstituted alkyl having 1 to 14 carbon atoms, a substituted alkyl having 1 to 14 carbon atoms, cycloalkyl having 1 to 14 carbon atoms, an unsubstituted aryl having 1 to 14 carbon atoms, a substituted aryl having 1 to 14 carbon atoms, acyl having 1 to 14 carbon atoms, alkoxy having 1 to 14 carbon atoms, acyloxy having 1 to 14 carbon atoms, amino, nitro, acylamino having 1 to 14 carbon atoms, aralkyl having 1 to 14 carbon atoms, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl, aryloxycarbonyl, phenoxycarbonyl, or an alkoxycarbonyl group. Individual R 1 -R 14  can also independently form 5 to 8 member ring(s) with adjacent R group(s). Individual R 1 -R 14  can independently comprise the same atom(s) as the adjacent R group and form a 5 member ring with four X atoms to form a complex with a chemical structure of Structure II, for example. X 1 -X 20  can be independently selected from boron, carbon, nitrogen, oxygen, or silicon, for example.
 
     In another embodiment, in Structure I, each R 1 -R 14  can be independently selected from hydrogen, halogen (such as fluorine, chlorine bromine, and iodine), hydroxyl, an unsubstituted alkyl including from 1 to 10 carbon atoms, a substituted alkyl including from 1 to 20 carbon atoms, cycloalkyl including from 1 to 20 carbon atoms, an unsubstituted aryl including from 1 to 20 carbon atoms, a substituted aryl including from 1 to 20 carbon atoms, acyl including from 1 to 20 carbon atoms, alkoxy including from 1 to 20 carbon atoms, acyloxy including from 1 to 20 carbon atoms, amino, nitro, acylamino including from 1 to 20 carbon atoms, aralkyl including from 1 to 20 carbon atoms, cyano, carboxyl including from 1 to 20 carbon atoms, thio, styryl, aminocarbonyl including from 1 to 20 carbon atoms, carbamoyl including from 1 to 20 carbon atoms, aryloxycarbonyl including from 1 to 20 carbon atoms, phenoxycarbonyl including from 1 to 20 carbon atoms, or an alkoxycarbonyl group including from 1 to 20 carbon atoms. 
     In another embodiment, a total number of carbon atoms provided by R 1 -R 14  groups can be in a range from 1 to 40. In another embodiment, a total number of carbon atoms provided by R 1 -R 14  groups can be in a range from 2 to 30. 
     In another embodiment, R 4 =R 5 =carbon atom to form a five member ring with four X atoms to form a complex with a chemical structure of Structure II, 
                                
wherein R 1 -R 14  groups and X 1 -X 20  are as defined in Structure I and R 15 -R 16  can be independently selected from hydrogen, halogen, hydroxyl, an unsubstituted alkyl including from 1 to 20 carbon atoms, a substituted alkyl including from 1 to 20 carbon atoms, cycloalkyl including from 1 to 20 carbon atoms, an unsubstituted aryl including from 1 to 20 carbon atoms, a substituted aryl including from 1 to 20 carbon atoms, acyl including from 1 to 20 carbon atoms, alkoxy including from 1 to 20 carbon atoms, acyloxy including from 1 to 20 carbon atoms, amino, nitro, acylamino including from 1 to 20 carbon atoms, aralkyl including from 1 to 20 carbon atoms, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl, aryloxycarbonyl, phenoxycarbonyl, or an alkoxycarbonyl group.
 
     Some examples of palladium(II) complexes are shown below, though claimed subject matter is not so limited: 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
     
     In an implementation, complexes such as those shown above can be prepared by reacting a palladium(II) salt with a corresponding protonated ligand in the presence of one or more suitable solvents. Examples of palladium(II) salts include but are not limited to palladium acetate, palladium chloride, dichloro(1,5-cyclooctadiene)platinum(II), and (ethylenediamine) palladium(II) chloride. Examples of solvents include but are not limited to glacial acetic acid, dichloromethane, chloroform, THF, DMF and DMSO, and mixtures thereof—refer to Reaction 201, for example. The product can then be optionally purified by column chromatography using alumina or silica as a stationary phase. Further purification by sublimation can be preformed if desired. 
     
       
                 
         
             
             
         
      
     
     Some examples of protonated ligands are shown below, though claimed subject matter is not so limited: 
     
       
                 
         
             
             
         
      
       
                 
         
             
             
         
       
     
     The following examples illustrate the subject invention. Unless otherwise indicated in the following examples and elsewhere in the specification and claims, all parts and percentages are by weight, all temperatures are in degrees Centigrade, and pressure is at or near atmospheric pressure. 
     Example 401 
     Synthesis of Complex 101 
     In one implementation, Ligand 301 (0.30 g, 0.59 mmol) was mixed with palladium acetate (0.14 g, 0.65 mmol) in glacial acetic acid (50 ml), and the reaction mixture was refluxed for 12 hr. The mixture was extracted with CHCl 3 , and the organic layer was dried with MgSO 4 . Purification was performed by alumina chromatography using chloroform as eluent to give Complex 101 as a yellow solid. Yield: 0.22 g (60%).  1 H NMR (500 MHz, CDCl 3 , 25° C., TMS): δ=1.44 (s, 18H), 6.67 (t,  3 J (H, H)=8.0 Hz, 1H), 7.18-7.24 (m, 2H), 7.30-7.34 (m, 2H), 7.47 (d,  3 J (H, H)=7.6 Hz, 1H), 7.56-7.57 (m, 3H), 7.61 (s, 1H), 7.70-7.72 (m, 2H), 7.90 (t,  3 J (H, H)=7.8 Hz, 1H), 7.95 (d,  3 J (H, H)=8.1 Hz, 1H), 8.18 (s, 1H), 8.86 (d,  3 J (H, H)=4.7 Hz, 1H). FAB-MS (+ve, m/z): 616 [M] + . An example, of the X-ray crystal structure of Complex 101 is depicted in  FIG. 1 , for example. 
     Example 402 
     Synthesis of Complex 102 
     Ligand 302 (0.30 g, 0.53 mmol) was mixed with palladium acetate (0.13 g, 0.59 mmol) in glacial acetic acid (50 ml), and the reaction mixture was refluxed for 12 hr. The mixture was extracted with CHCl 3 , and the organic layer was dried with MgSO 4 . Purification was performed by alumina chromatography using dichloromethane as eluent to give Complex 102 as yellow solid. Yield: 0.21 g (58%).  1 H NMR (500 MHz, CD 2 Cl 2 , 25° C., TMS): δ=1.46 (s, 18H), 6.62 (ddd,  4 J (H, H)=1.4 Hz,  3 J (H, H)=6.6 Hz,  3 J (H, H)=8.1 Hz, 1H), 7.15 (dd,  4 J (H, H)=1.3 Hz,  3 J (H, H)=8.4 Hz, 1H), 7.21 (t,  3 J (H, H)=7.6 Hz, 1H), 7.29 (ddd,  4 J (H, H)=1.7 Hz,  3 J (H, H)=6.6 Hz,  3 J (H, H)=8.4 Hz, 1H), 7.48 (d,  3 J (H, H)=7.5 Hz, 1H), 7.52 (d,  3 J (H, H)=7.5 Hz, 1H), 7.59-7.62 (m, 2H), 7.64 (t,  4 J (H, H)=1.7 Hz, 1H), 7.67 (d,  3 J (H, H)=1.6 Hz, 1H), 7.74 (dt,  4 J (H, H)=1.1 Hz,  3 J (H, H)=8.1 Hz, 1H), 7.81 (d,  3 J (H, H)=8.1 Hz, 1H), 7.87 (s, 1H), 7.93 (dd,  4 J (H, H)=1.5 Hz,  3 J (H, H)=8.4 Hz, 1H), 8.04 (d,  3 J (H, H)=8.1 Hz, 1H), 8.13 (d,  4 J (H, H)=1.3 Hz, 1H), 9.35 (s, 1H). FAB-MS (+ve, m/z): 666 [M] + . 
     Example 403 
     Synthesis of Complex 103 
     Ligand 303 (0.30 g, 0.52 mmol) was mixed with palladium acetate (0.13 g, 0.57 mmol) in glacial acetic acid (50 ml), and the reaction mixture was refluxed for 12 hr. The mixture was extracted with CHCl 3 , and the organic layer was dried with MgSO 4 . Purification was performed by alumina chromatography using chloroform as eluent to give Complex 103 as pale yellow solid. Yield: 0.26 g (65%).  1 H NMR (500 MHz, CD 2 Cl 2 , 25° C.): δ=1.45 (s, 18H), 6.59 (ddd,  4 J (H, H)=1.4 Hz,  3 J (H, H)=6.7 Hz,  3 J (H, H) 8.2 Hz, 1H), 6.85 (dd,  3 J (H, H)=8.3 Hz,  3 J (F, H)=11.6 Hz, 1H), 7.11 (dd,  4 J (H, H)=1.4 Hz,  3 J (H, H)=8.4 Hz, 1H), 7.26 (ddd,  4 J (H, H)=1.7 Hz,  3 J (H, H)=6.7 Hz,  3 J (H, H)=8.4 Hz, 1H), 7.47 (dd,  4 J (F, H)=3.8 Hz,  3 J (H, H) 8.4 Hz, 1H), 7.57 (d,  4 J (H, H)=1.6 Hz, 1H), 7.60 (d,  4 J (H, H)=1.8 Hz, 1H), 7.63-7.68 (m, 2H), 7.80 (dt,  4 J (H, H)=1.1 Hz,  3 J (H, H)=8.1 Hz, 1H), 7.84 (d,  3 J (H, H)=8.0 Hz, 1H), 7.88 (dd,  4 J (H, H)=1.5 Hz,  3 J (H, H)=8.4 Hz, 1H), 8.06-8.08 (m, 2H), 8.12 (s, 1H), 9.38 (s, 1H). FAB-MS (+ve, m/z): 684 [M] + . 
     Example 404 
     Synthesis of Complex 104 
     Ligand 304 (0.30 g, 0.67 mmol) was mixed with palladium acetate (0.17 g, 0.74 mmol) in glacial acetic acid (50 ml), and the reaction mixture was refluxed for 12 hr. The mixture was extracted with CHCl 3 , and the organic layer may be dried with MgSO 4 . Purification was performed by alumina chromatography using dichloromethane as eluent to give Complex 104 as yellow solid. Yield: 0.22 g (60%).  1 H NMR (500 MHz, CD 2 Cl 2 , 25° C., TMS): δ=0.68-0.82 (m, 10H), 1.09-1.17 (m, 4H), 1.95-2.08 (m, 4H), 6.60 (d, J=7.1 Hz, 1H), 6.90 (d, J=8.4 Hz, 1H), 7.25 (t, J=7.7 Hz, 1H), 7.36-7.29 (m, 2H), 7.50-7.53 (m, 2H), 7.59 (d, J=7.6 Hz, 1H), 7.79-7.81 (m, 2H), 7.95 (dt, J 1,2 =1.6 Hz, J 1,3 =7.8 Hz, 1H), 8.84-8.85 (m, 1H). FAB-MS (+ve, m/z): 552 [M] + . An example, of the x-ray crystal structure of Complex 104 is depicted in  FIG. 2 . 
     Example 405 
     Synthesis of Complex 105 
     Ligand 305 (0.30 g, 0.59 mmol) was mixed with palladium acetate (0.15 g, 0.65 mmol) in glacial acetic acid (50 ml), and the reaction mixture was refluxed for 12 hr. The mixture was extracted with CHCl 3 , and the organic layer may be dried with MgSO 4 . Purification was performed by alumina chromatography using dichloromethane as eluent to give Complex 105 as pale yellow solid. Yield: 0.22 g (60%).  1 H NMR (500 MHz, CD 2 Cl 2 , 25° C., TMS): δ=0.68-0.79 (m, 10H), 1.09-1.16 (m, 4H), 1.95-2.08 (m, 4H), 6.58 (d,  3 J (H, H)=7.0 Hz, 1H), 6.88 (d,  3 J (H, H)=8.3 Hz, 1H), 7.34-7.39 (m, 2H), 7.54 (d,  3 J (H, H)=7.6 Hz, 1H), 7.62 (d,  4 J (H, H)=1.6 Hz, 1H), 7.68 (d,  4 J (H, H)=1.6 Hz, 1H), 7.80 (d,  3 J (H, H)=7.6 Hz, 1H), 7.85 (d,  3 J (H, H)=7.9 Hz, 1H), 7.96-7.99 (m, 1H), 8.82-8.84 (m, 1H). FAB-MS (+ve, m/z): 608 [M] + . 
     Example 406 
     Synthesis of Complex 106 
     Ligand 306 (0.25 g, 0.54 mmol) was mixed with palladium acetate (0.13 g, 0.59 mmol) in glacial acetic acid (50 ml), and the reaction mixture was refluxed for 12 hr. The mixture was extracted with CHCl 3 , and the organic layer may be dried with MgSO 4 . Purification was performed by alumina chromatography using dichloromethane as eluent to give Complex 106 as yellow solid. Yield: 0.17 g (55%).  1 H NMR (500 MHz, CD 2 Cl 2 , 25° C., TMS): δ=0.68-0.80 (m, 10H), 1.09-1.16 (m, 4H), 1.95-2.08 (m, 4H), 2.52 (s, 3H), 6.59 (d,  3 J (H, H)=6.8 Hz, 1H), 6.88 (d,  3 J (H, H)=8.2 Hz, 1H), 7.20-7.21 (m, 1H), 7.25 (t,  3 J (H, H)=7.7 Hz, 1H), 7.36 (dd,  3 J (H, H)=7.1 Hz,  3 J (H, H)=8.4 Hz, 1H), 7.49-7.53 (m, 2H), 7.59 (d,  3 J (H, H)=6.9 Hz, 1H), 7.64 (s, 1H), 7.80 (d,  3 J (H, H)=7.6 Hz, 1H), 8.67 (d,  3 J (H, H)=5.6 Hz, 1H). FAB-MS (+ve, m/z): 567 [M] + . 
     Example 407 
     Photophysical Data for Complexes 101-106 
     The absorption spectra of complexes 101-103 (an example of which is depicted in  FIG. 3 ) show relatively intense transitions with λ max  ranging from 250-340 nm, which can be assigned to be intraligand transitions with mainly ligand character, while the transitions ranging from 340-400 nm can be assigned to be metal perturbed intraligand transitions (with considerable metal character). Relatively broad absorption at about 400-470 nm (ε≈5000-11000 dm 3  mol −1  cm −1 ) for complexes 101-103 can be attributed to a  1 MLCT (5d)Pt→π*(L) transition, although mixing with IL may not be excluded. These assignments can be supported by a solvent effect experiment on complex 101. Upon or after forming UV spectra, with an increase in solvent polarity, blue shifts of ˜8 nm and ˜13 nm in regions of 340-400 nm and 400-470 nm, respectively, can be recorded to show metal involvement in excited states, for example. 
     Absorption spectra of complexes 104-106 (an example of which is depicted in  FIG. 4 ) show relatively intense transitions with λ max  ranging from 250-320 nm, which can be assigned to comprise intraligand transitions having substantially ligand character, while transitions ranging from 320-400 nm can be assigned to comprise metal perturbed intraligand transitions having considerable metal character. Broad absorption at 400-470 nm (ε≈7200-7700 dm 3  mol −1  cm −1 ) for complexes 104-106 can be attributed to  1 MLCT (5d)Pt→π*(L) transition, although mixing with IL need not be excluded. These assignments can be supported by a solvent effect experiment on complex 105. Upon or after forming UV spectra, with an increase in solvent polarity, blue shifts of ˜6 nm in the regions of 320-400 nm and 400-470 nm can be recorded to show metal involvement in excited states, for example. 
     Complexes 101-103 can be emissive in degassed CH 2 Cl 2 . Emission energy listed in descending order: Complex 103&gt;Complex 101&gt;Complex 102, which can be due to an effect of fluorine substituent (e.g., lowers the HOMO energy) and extension of pyridine to isoquinoline (e.g., lowers the LUMO energy). For complexes 101-103, their emission spectra may not be vibronically resolved (an example of which is depicted in  FIG. 5 ), and emission energy need not be affected by a concentration ranging from 1×10 −4  M to 1×10 −5  M. Thus their emissions can originate from an excited state of  3 ILCT (e.g., lone pair on oxygen atom to other parts of the ligand) mixed with  3 MLCT, tentatively. These assignments can be supported by a solvent effect experiment on complex 101. Upon or after forming emission spectra, with an increase in solvent polarity, red shifts of ˜20 nm can be recorded, which can be a characteristic behaviour of  3 ILCT. 
     Complexes 104-106 can be highly emissive in degassed CH 2 Cl 2 . Their emission energy, which can be similar, can mean that alkyl substituents may not affect emission energy. And their emission spectra can be vibronically resolved (an example of which is depicted in  FIG. 6 ), which can possess vibronic spacings of 1200 cm −1  and emission energy may not be affected from the concentration ranging from 1×10 −  M to 1×10 −5  M, for example. Thus their emissions can be assigned to come from an excited state of  3 IL, and the mixing of  3 MLCT may not be excluded. These assignments can be supported by the solvent effect experiment on Complex 105. Upon or after forming emission spectra, with an increase in solvent polarity, emission energy can be similar, which can be a characteristic behaviour for an emission from  3 IL. 
     Emission quantum yields of complexes 101-103 (Φ≈0.0018-0.0030) can be lower than that of complexes 104-106 (Φ≈0.11-0.20), which can reveal that complexes 104-106 can possess more rigid structures and can reduce excited state distortions. For solid state emission at room temperature, all complexes except complex 104, can show excimeric emissions due, at least in part, to serious aggregations. Complex 104 can show a structured emission with λ max  of 500 nm. Such emission can be attributed to originate from  3 IL excited state. On cooling to 77 K, emission spectra of complexes 101-104 can show vibronic structures with vibronic spacings of 1300-1400 cm −1  (an example of which is depict in  FIG. 7-FIG .  10 ), which may correspond to vibration frequency of C═C and C═N, and the λ max  may be in a range from 529 to 557 nm. These emissions can be attributed to come from  3 IL excited states, for example. Emission spectra of complex 105 and complex 106 narrowed down upon cooling may still be affected by serious aggregations. Such emission specta can comprise mainly excimeric emissions. Glassy solution (2-MeTHF) of complexes 101-106 can show vibronic structured emissions with vibronic spacings of 1300-1400 cm −1 , which can correspond to a vibrational frequency of C═C and C═N (an example of which is depicted in  FIG. 11-FIG .  12 ). The λ max  of complexes 101-103 can vary from 477 to 505 nm, while that of complexes 104-106 can be almost the same at ˜488 nm. 
     In one embodiment, the term “relatively narrow color emission spectrum” refers to a spectrum that is 30 or fewer nanometers wide, such as that of a “single” color spectrum, for example. In another embodiment, the term “relatively narrow color emission spectrum” refers to a spectrum that is 20 or fewer nanometers wide. In yet another embodiment, the term “relatively narrow color emission spectrum” refers to a spectrum that is 10 or fewer nanometers wide. Of course, a relatively narrow color emission spectrum can be wider or narrower, and claimed subject matter is not limited in this respect. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 501 
               
             
             
               
                   
               
               
                 Physical properties of illustrative Complexes 101-106. 
               
             
          
           
               
                   
                 Medium 
                 λabs/nm a,b   
                   
                   
               
               
                 Complexes 
                 (T/K) 
                 (ε/x 10 4  dm 3  mol −1  cm −1 ) 
                 λem/nm c  (τ/μs); 
                 Φem d   
               
               
                   
               
             
          
           
               
                 101 
                 CH 2 Cl 2   
                 246 (7.15), 283 (8.54), 358 
                 538 
                 0.002 
               
               
                   
                 (298) 
                 (2.53), 415 (1.14) 
                   
                   
               
               
                   
                 Solid 
                 — 
                 434 (0.31), 571 
                 — 
               
               
                   
                 (298) 
                   
                 (0.32), 650 (0.57) 
                   
               
               
                   
                 Solid 
                 — 
                 539 (62.3), 549 
                 — 
               
               
                   
                 (77) 
                   
                 (66.9), 576 (61.5) 
                   
               
               
                   
                 2- 
                 — 
                 477 (274), 502 
                 — 
               
               
                   
                 MeTHF 
                   
                 (287), 513 (305), 
                   
               
               
                   
                 (77) 
                   
                 545 (292), 554 
                   
               
               
                   
                   
                   
                 (296), 585 (296) 
                   
               
               
                 102 
                 CH 2 Cl 2   
                 267 (5.92), 280 (6.46), 306 
                 543 (0.18) 
                 0.003 
               
               
                   
                 (298) 
                 (4.17), 357 (2.06), 386 
                   
                   
               
               
                   
                   
                 (1.13), 407 (0.63) 
                   
                   
               
               
                   
                 Solid 
                 — 
                 426 (0.16), 526 
                 — 
               
               
                   
                 (298) 
                   
                 (0.24), 575 (0.27) 
                   
               
               
                   
                 Solid 
                 — 
                 552 (153), 578 (148) 
                 — 
               
               
                   
                 (77) 
                   
                   
                   
               
               
                   
                 2- 
                 — 
                 504 (2761), 
                 — 
               
               
                   
                 MeTHF 
                   
                 519 (2883), 
                   
               
               
                   
                 (77) 
                   
                 545 (2816), 
                   
               
               
                   
                   
                   
                 587 (2659), 
                   
               
               
                   
                   
                   
                 638 (3008) 
                   
               
               
                 103 
                 CH 2 Cl 2   
                 240 (5.01), 281 (6.33), 300 
                 535 (0.22) 
                 0.0018 
               
               
                   
                 (298) 
                 (4.16), 354 (1.74), 367 
                   
                   
               
               
                   
                   
                 (1.34), 410 (0.50) 
                   
                   
               
               
                   
                 Solid 
                 — 
                 525 (0.28), 615 
                 — 
               
               
                   
                 (298) 
                   
                 (0.35), 663 (0.24) 
                   
               
               
                   
                 Solid 
                 — 
                 491 (3.48), 556 
                 — 
               
               
                   
                 (77) 
                   
                 (101), 594 (65.79) 
                   
               
               
                   
                 2- 
                 — 
                 493 (3309), 
                 — 
               
               
                   
                 MeTHF 
                   
                 509 (3313, 
                   
               
               
                   
                 (77) 
                   
                 534 (3351), 
                   
               
               
                   
                   
                   
                 576 (3593), 
                   
               
               
                   
                   
                   
                 624 (3679) 
                   
               
               
                 104 
                 CH 2 Cl 2   
                 252 (4.23), 270 (3.06), 345 
                 527 (82) 
                 0.110 
               
               
                   
                 (298) 
                 (1.44), 372 (1.20), 409 
                   
                   
               
               
                   
                   
                 (0.64) 
                   
                   
               
               
                   
                 Solid 
                 — 
                 483 (0.55), 500 
                 — 
               
               
                   
                 (298) 
                   
                 (0.68), 536 (0.43) 
                   
               
               
                   
                 Solid 
                 — 
                 458 (0.22), 500 
                 — 
               
               
                   
                 (77) 
                   
                 (6.11), 530 (591), 
                   
               
               
                   
                   
                   
                 558 (553) 
                   
               
               
                   
                 2- 
                 — 
                 487 (775), 517 (809), 
                 — 
               
               
                   
                 MeTHF 
                   
                 526 (846), 563 (828), 
                   
               
               
                   
                 (77) 
                   
                 601 (832) 
                   
               
               
                 105 
                 CH 2 Cl 2   
                 253 (4.82), 272 (3.36), 295 
                 498 (121) 
                 0.200 
               
               
                   
                 (298) 
                 (2.01), 347 (1.46), 379 
                   
                   
               
               
                   
                   
                 (1.33), 407 (0.79) 
                   
                   
               
               
                   
                 Solid 
                 — 
                 497 (1.39), 537 
                 — 
               
               
                   
                 (298) 
                   
                 (4.03), 554 (4.08), 
                   
               
               
                   
                   
                   
                 576 (4.10) 
                   
               
               
                   
                 Solid 
                 — 
                 490 (4.93), 535 
                 — 
               
               
                   
                 (77) 
                   
                 (643), 549 (693) 
                   
               
               
                   
                 2- 
                 — 
                 486 (749), 524 (742), 
                 — 
               
               
                   
                 MeTHF 
                   
                 564 (735), 604 (747) 
                   
               
               
                   
                 (77) 
                   
                   
                   
               
               
                 106 
                 CH 2 Cl 2   
                 252 (4.85), 269 (3.69), 347 
                 527 (122) 
                 0.170 
               
               
                   
                 (298) 
                 (1.64), 372 (1.34), 405 
                   
                   
               
               
                   
                   
                 (0.81) 
                   
                   
               
               
                   
                 Solid 
                 — 
                 498 (0.60), 542 
                 — 
               
               
                   
                 (298) 
                   
                 (0.68), 568 (0.62) 
                   
               
               
                   
                 Solid 
                 — 
                 451 (0.52), 500 
                 — 
               
               
                   
                 (77) 
                   
                 (4.60), 515 (5.10), 
                   
               
               
                   
                   
                   
                 541 (10.16), 554 
                   
               
               
                   
                   
                   
                 (8.55), 567 (7.12), 
                   
               
               
                   
                   
                   
                 585 (6.18), 
                   
               
               
                   
                   
                   
                 732 (1.11) 
                   
               
               
                   
                 2- 
                 — 
                 487 (771), 506 (837), 
                 — 
               
               
                   
                 MeTHF 
                   
                 515 (832), 526 (832), 
                   
               
               
                   
                 (77) 
                   
                 561 (835), 571 (845), 
                   
               
               
                   
                   
                   
                 601 (841) 
                   
               
               
                   
               
               
                   a Absorption maxima. 
               
               
                   b at 2 × 10 −5  M. 
               
               
                   c Emission maxima. 
               
               
                   d Emission Quantum Yield. 
               
             
          
         
       
     
     Example 408 
     Thermal Behaviour for Complexes 101-106 
     Thermal behaviour of some illustrative materials can be measured using thermogravimetric analyses (TGA) at a heating rate of 40° C. min −1 , for example. Some examples of thermograms are depicted in  FIG. 13 . TGA can measure weight changes in a material as a function of temperature (or time) under a controlled atmosphere. Complexes 101-106 may possess relatively high thermal stability and can be stable to air and moisture. Decomposition temperature (T d ) of complexes 101-106 can range from 414 to 468° C., as shown in Table 502. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 502 
               
             
             
               
                   
               
               
                 Decomposition temperature for 101-106. 
               
             
          
           
               
                   
                   
                 Decomposition 
               
               
                   
                 Complex  
                 Temperature/° C. 
               
               
                   
               
             
          
           
               
                   
                 101 
                 450 
               
               
                   
                 102 
                 468 
               
               
                   
                 103 
                 444 
               
               
                   
                 104 
                 421 
               
               
                   
                 105 
                 418 
               
               
                   
                 106 
                 414 
               
               
                   
               
             
          
         
       
     
     Example 409 
     OLED Fabrication 
     In some embodiments, OLEDs can be prepared on patterned indium tin oxide (ITO). Pre-coated glass slides with a sheet resistance of 10 Ω/m 2  can be used as anodic substrates, for example. The glass slides can be cleaned with Decon 90 detergent, rinsed in de-ionized water, and dried in an oven before successive film deposition. Glass slides can then be treated in an ultraviolet-ozone chamber before loading into an evaporation chamber. Layers of organic material and metal can be thermally deposited sequentially in a high vacuum evaporator (such as that manufactured by Trovato Mfg., Inc., Fairport, N.Y., for example) with a base pressure of 10 −6  Torr. Films can be sequentially deposited at a rate of 0.1-0.2 nm/s without vacuum break. Film thicknesses can be determined in-situ by calibrated oscillating quartz-crystal sensors. Shadow masks can be used to define organic layers and a cathode may be used to make, for example, four 0.1 cm 2  devices on each substrate. The Commission Internationale de L&#39;Eclairage (CIE) coordinates, current density-voltage-luminance characteristics (J-V-L), and electroluminescence (EL) spectra were measured (at different times or at the same time) with a programmable Keithley model 2400 source-meter measurement unit and a Photoresearch PR-655 spectrascan spectroradiometer. All experiments and measurements can be performed at room temperature under ambient environment without device encapsulation, though claimed subject matter is not so limited. 
     OLEDs 601-604 were prepared in the following configuration: ITO/NPB (40 nm)/mCP: Complex 5, X %, 30 nm)/BAlq 3  (40 nm)/LiF (0.5 nm)/Al (80 nm), wherein OLED 601 (X=2%), OLED 602 (X=4%), OLED 603 (X=6%) and OLED 604 (X=8%). These devices were CIE coordinates of: OLED 601: 0.22, 0.32; OLED 602: 0.25, 0.40; OLED 603: 0.27, 0.44; OLED 604: 0.28, 0.47. The EL λ max  (500, 530 nm with a shoulder at ˜580 nm) can be independent of doping concentrations for complex 105. Turn on voltages of OLED 601-604 are 5.3 V, 4.9 V, 4.6 V, and 4.4 V, respectively. For OLED 601, an upper current efficiency of 9.2 cd A −1  was obtained at 0.006 mA cm −2 . An upper power efficiency (PE) and an upper external quantum efficiency (EQE) were 5.7 lmW 1  and 4.0%, respectively. For OLED 602, an upper current efficiency of 12.5 cd A −1  was obtained at 0.029 mA cm −2 . An upper power efficiency (PE) and an upper external quantum efficiency (EQE) were 7.8 lmW 1  and 4.7%, respectively. For OLED 603, an upper current efficiency of 20.0 cd A −1  was obtained at 0.013 mA cm −2 . An upper power efficiency (PE) and an upper external quantum efficiency (EQE) were 13.6 lmW 1  and 7.4%, respectively. For OLED 604, an upper current efficiency of 18.0 cd A −1  was obtained at 0.008 mA cm −2 . An upper power efficiency (PE) and an upper external quantum efficiency (EQE) were 13.1 lmW −1  and 6.4%, respectively.  FIG. 16 ,  FIG. 17 , and  FIG. 18  show examples of an EL spectrum, J-V-B curves, external quantum efficiency, current efficiency, and power efficiency as a function of drive current density for OLEDs 601-604, respectively. 
     Some examples of EL spectra, J-V-B relationships, and efficiency curves for OLEDs 601-604 are depicted in  FIG. 13 ,  FIG. 14 , and  FIG. 15 , respectively. 
     With respect to any figure or numerical range for a given characteristic, a figure or a parameter from one range may be combined with another figure or a parameter from a different range for the same characteristic to generate a numerical range. 
     Other than in the operating examples, or where otherwise indicated, all numbers, values and/or expressions referring to quantities of ingredients, reaction conditions, etc., used in the specification and claims are to be understood as modified in all instances by the term “about.” 
     While there has been illustrated and described what are presently considered to be example features, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from claimed subject matter. Additionally, many modifications may be made to adapt a particular situation to the teachings of claimed subject matter without departing from the central concept described herein. Therefore, it is intended that claimed subject matter not be limited to the particular examples disclosed, but that such claimed subject matter may also include all aspects falling within the scope of appended claims, and equivalents thereof.