(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(19) World Intellectual Property Organization
`International Bureau
`
`(43) International Publication Date
`21 February 2002 (21.02.2002)
`
`
`
`(10) International Publication Number
`WO 02/15645 Al
`
`G1)
`
`HO05B 33/14,
`International Patent Classification’:
`CO9K 11/06, CO7D 213/02, 231/10, 241/10, 333/52
`
`@))
`
`International Application Number:
`
`=PCT/USO1/25108
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`(22)
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`International Filing Date: 10 August 2001 (10.08.2001)
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`(25)
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`Filing Language:
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`(26)
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`Publication Language:
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`English
`
`English
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`(30)
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`Priority Data:
`09/637,766
`60/283 814
`
`11 August 2000 (11.08.2000)
`13 April 2001 (13.04.2001)
`
`US
`US
`
`(71)
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`(72)
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`Applicants: THE TRUSTEES OF PRINCETON UNI-
`VERSITY [US/US], P.O. Box 36, Princeton, NJ 08544-
`0036 (US). THE UNIVERSITY OF SOUTHERNCAL-
`IFORNIA [US/US]; 3716 South Hope Street, Suite 313,
`Los Angeles, CA 90007-4344 (US). UNIVERSALDIS-
`PLAY CORPORATION [US/US]; 375 Phillips Boule-
`vard, Ewing, NJ 08618 (US).
`
`Inventors; LAMANSKY,Sergey; 112 South Greenwood
`Boulevard, #2, Pasadena, CA 91102 (US). THOMPSON,
`Mark, E.; 4447 Pepper Creek Way, Anaheim, CA 92807
`(US). ADAMOVICH, Vadim; Apartment 6, 1103 W.
`30th Street, Los Angeles, CA 90007 (US). DJUROVICH,
`Peter, L.; 1723 1/2 E. Second Street, Long Beach, CA
`90802 (US). ADACHI, Chihaya; 8-2-3 eirzu, Chitose,
`
`Hokkaido 066-0081 GP). BALDO, Mare, A.; 7V Magie
`Apt., Faculty Road, Princeton, NJ 08540 (US). FOR-
`REST,Stephen, R.; 148 Hunt Drive, Princeton, NJ 08540
`(US). KWONG, Raymond, C.; 5109 Quail Ridge Drive,
`Plainsboro, NJ 08536 (US).
`
`(74)
`
`Agents: MEAGHER, Thomas, F. et al.; Kenyon &
`Kenyon, One Broadway, New York, NY 10004 (US).
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`(81)
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`(84)
`
`Designated States (national): AE, AG, AL, AM, AT, AU,
`AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CO, CR, CU,
`CZ, DE, DK, DM, DZ, EC, EE, ES, FI, GB, GD, GE, GH,
`GM,HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC,
`LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, Mw,
`MX, MZ, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK,
`SL, TJ, TM, TR, TT, TZ, UA, UG, UZ, VN, YU, ZA, ZW.
`
`Designated States (regional): ARIPO patent (GH, GM,
`KE, LS, MW, MZ, SD, SL, SZ, TZ, UG, ZW), Eurasian
`patent (AM,AZ, BY, KG, KZ, MD, RU, TJ, TM), European
`patent (AT, BE, CH, CY, DE, DK, KS, FI, FR, GB, GR, TF,
`IT, LU, MC, NL, PT, SE, TR), OAPI patent (BF, BJ, CF,
`CG, CI, CM, GA, GN, GQ, GW, ML, MR, NE, SN, TD,
`TG).
`
`Published:
`with international search report
`
`For two-letter codes and other abbreviations, refer to the “Guid-
`ance Notes on Codes andAbbreviations" appearing at the begin-
`ning ofeach regular issue of the PCT Gazette.
`
`(54) Titles ORGANOMETALLIC COMPOUNDS AND EMISSION-SHIFTING ORGANIC ELECTROPHOSPHORESCENCE
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`5645Al
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`(57) Abstract: Emissive phosphorescent organometallic compounds are described that produce improved electroluminescence,
`=
`“~~ particularly in the blue regions of the visible spectrum. Organic light emitting devices empolying such emissive phosphorescent
`organometallic compoundsare also described. Also described is an organic light emitting layer including a host material having a
`lowest triplet excited state having a decay rate of less than about | per second; a guest matcrial dispersed in the host matcrial, the
`guest material having a lowest triplet excited state having a radiative decay rate of greater than about 1x10° or about 1x10° per second
`and wherein the energy level of the lowest triplet excited state of the host material is lower than the energy level of the lowest triplet
`excited state of the guest material.
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`S
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`ORGANOMETALLIC COMPOUNDS AND EMISSION-SHIFTING ORGANIC
`ELECTROPHOSPHORESCENCE
`
`FIELD OF THE INVENTION
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`The present invention is directed to phosphorescence based organic light
`
`emitting devices that have improved electroluminescent characteristics.
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`10
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`BACKGROUND OF THE INVENTION
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`The technology of organic light emitting diodes (OLEDs) is undergoing
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`rapid development. OLEDsoriginally utilized the electroluminescence produced from
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`15
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`electrically excited molecules that emitted light from their singlet states. Such radiative
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`emission from a singlet excited state is referred to as fluorescence. More recent work has
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`demonstrated that higher powerefficiency OLEDs can be made using molecules that emit
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`light fromtheir triplet state, defined as phosphorescence.
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`20
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`Such electrophosphorescence makesit possible for phosphorescent OLEDs
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`to have substantially higher quantum efficiencies than are possible for OLEDsthat only
`
`produce fluorescence. This is based on the understanding that the excitons created in an
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`OLEDare produced, according to simple statistical arguments as well as experimental
`
`measurements, approximately 75% astriplet excitons and 25% as singlet excitons. The
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`triplet excitons more readily transfer their energy to triplet excited states that can produce
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`phosphorescence whereas the singlet excitons typically transfer their energy to singlet
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`excited states that can produce fluorescence. Since the lowest emissive singlet excited
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`state of an organic moleculeis typically at a slightly higher energy than the lowesttriplet
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`excited state, the singlet excited state may relax, by an intersystem crossing process, to the
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`emissive triplet excited state. This meansthatall the exciton excitation energy may be
`
`converted into triplet state excitation energy, which then becomes available as
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`phosphorescent emission. Thus, electrophosphorescent OLEDs have a theoretical
`
`quantum efficiency of 100%, since all the exciton excitation energy can become available
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`10
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`as electrophosphorescence.
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`As a consequence, since the discovery that phosphorescent materials could
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`be used in an OLED,Baldoet al., "Highly Efficient Phosphorescent Emission from
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`Organic Electroluminescent Devices", Nature, vol. 395, 151-154, 1998, there is now much
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`15
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`interest in finding more efficient electrophosphorescent materials.
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`Typically phosphorescent emission: from organic molecules is less common
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`than fluorescent emission. However, phosphorescence can be observed from organic
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`molecules under an appropriate set of conditions. Organic molecules coordinated to
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`20
`
`lanthanide elements often phosphoresce from excitedstates localized on the lanthanide
`
`metal. The europium diketonate complexes illustrate one group of these types of species.
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`Organic phosphorescenceis also often observed in molecules containing heteroatoms with
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`unshared pairs of electrons at very low temperatures. Benzophenone and 2,2'-bipyridine
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`are such molecules. Phosphorescence can be enhancedover fluorescence by confining,
`preferably through bonding, the organic molecule in close proximity to an atom ofhigh
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`atomic number. This phenomenon, called the heavy atom effect, is created by a
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`mechanism knownasspin-orbit coupling. A related phosphorescenttransition is a metal-
`
`to-ligand charge transfer (MLCT) that is observed in molecules such astris(2-
`
`phenylpyridine)iridium(IID.
`
`However, molecules that phosphoresce from MLCTstates typically emit
`
`light that is of lower energy than that observed from the unbound organic ligand. This
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`10
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`lowering of emission energy makes it difficult to develop organic molecules that
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`phosphoresce in the technologically useful blue and green colors of the visible spectrum
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`where the unperturbed phosphorescence typically occurs.
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`It would be desirable if more efficient electrophosphorescent materials
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`15
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`could be found, particularly materials that produce their emission in the blue region of the
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`spectrum.
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`The realization of highly efficient blue, green and red
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`electrophosphorescence is a requirement for portable full color displays and white lighting
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`20
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`applications with low power consumption. Recently, high-efficiency green and red
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`organic electrophosphorescent devices have been demonstrated which harvest both singlet
`
`and triplet excitons, leading to internal quantum efficiencies (nj,,) approaching 100%. See
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`Baldo, M. A., O’Brien, D. F., You, Y., Shoustikov, A., Sibley, S., Thompson, M. E., and
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`Forrest, S. R., Nature (London), 395, 151-154 (1998); Baldo, M. A., Lamansky,S.,
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`Burrows, P. E., Thompson, M. E., and Forrest, S. R., Appl. Phys. Lett., 75, 4-6 (1999);
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`Adachi, C., Baldo, M. A., and Forrest, 8. R., App. Phys. Lett., 77, 904-906, (2000);
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`Adachi, C., Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S.
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`R., App. Phys. Lett., 78, 1622-1624 (2001); and Adachi, C., Baldo, M. A., Thompson, M.
`
`E., and Forrest, S. R., Bull. Am. Phys. Soc., 46, 863 (2001). Using a green phosphor, fac
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`tris(2-phenylpyridine)iridium (Ir(ppy),), in particular, an external quantum efficiency (Nex)
`
`of (17.6+£0.5)%corresponding to an internal quantum efficiency of >85%,wasrealized
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`using a wide energy gap host material, 3-phenyl-4-(1'-naphthyl)-5-phenyl-1,2,4-triazole
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`10
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`(TAZ). See Adachi, C., Baldo, M. A., Thompson, M. E., and Forrest, S. R., Bull. Am.
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`Phys. Soc., 46, 863 (2001). Most recently, high- efficiency (He. = (7-0+0.5)%) red
`
`electrophosphorescence was demonstrated employing bis(2-(2'-benzo[4,5-a]
`
`thienyl)pyridinato-N, C*) iridium (acetylacetonate) [Btp,Ir(acac)]. See Adachi,C.,
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`Lamansky, S., Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, 8. R., App.
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`15
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`Phys. Lett., 78, 1622-1624 (2001).
`
`In each ofthese latter cases, high efficiencies are obtained by energy
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`transfer from both the host singlet and triplet states to the phosphortriplet, or via direct
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`trapping of charge on the phosphor, thereby harvesting up to 100% ofthe excited states.
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`20
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`This is a significant improvement over what can be expected using fluorescence in either
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`small molecule or polymerorganic light emitting devices (OLEDs). See Baldo, M.A.,
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`O'Brien, D. F., Thompson, M. E., and Forrest, 8. R., Phys. Rev., B 60, 14422-14428
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`(1999); Friend, R. H., Gymer, R. W., Holmes, A. B., Burroughes, J. H., Marks, R. N.,
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`Taliani, C., Bradley, D. D. C., Dos Santos, D. A., Bredas, J. L., Logdlund, M., Salaneck,
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`W. R., Nature (London), 397, 121-128 (1999); and Cao, Y, Parker, I. D., Yu, G., Zhang,
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`C., and Heeger, A. J., Nature (London), 397, 414-417 (1999). In either case, these
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`transfers entail a resonant, exothermic process. Asthe triplet energy of the phosphor
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`increases, it becomesless likely to find an appropriate host with a suitably high energy
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`triplet state. See Baldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000).
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`The very large excitonic energies required of the host also suggest that this material layer
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`may not have appropriate energy level alignments with other materials used in an OLED
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`structure, hence resulting in a further reduction in efficiency. To eliminate this
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`competition between the conductive and energy transfer properties of the host, a route to
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`efficient blue electrophosphorescence may involve the endothermic energy transfer from a
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`near resonant excited state of the host to the higher triplet energy of the phosphor. See
`Baldo, M. A., and Forrest, S. R., Phys. Rev. B 62,10958-10966 (2000); Ford, W. E.,
`
`Rodgers, M. A. J., J. Phys. Chem., 96, 2917-2920 (1992); and Harriman, A.; Hissler, M.;
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`15
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`Khatyr, A.; Ziessel, R. Chem. Commun. , 735-736 (1999). Provided that the energy
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`required in the transfer is not significantly greater than the thermal energy, this process can
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`be very efficient.
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`Organic light emitting devices (OLEDs), which make use ofthin film
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`20
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`materials that emit light when excited by electric current, are expected to become an
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`increasingly popular form of flat panel display technology. This is because OLEDs have a
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`wide variety of potential applications, including cellphones, personal digital assistants
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`(PDAs), computer displays, informational displays in vehicles, television monitors, as well
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`as light sources for general illumination. Dueto their bright colors, wide viewing angle,
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`compatibility with full motion video, broad temperature ranges, thin and conformable
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`form factor, low power requirements and the potential for low cost manufacturing
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`processes, OLEDsare seen as a future replacement technology for cathode ray tubes
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`(CRTs) andliquid crystal displays (LCDs), which currently dominate the growing $40
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`billion annual electronic display market. Dueto their high luminousefficiencies,
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`electrophosphorescent OLEDsare seen as having the potential to replace incandescent,
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`and perhaps even fluorescent, lamps for certain types of applications.
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`Light emission from OLEDsis typically via fluorescence or
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`phosphorescence. Asused herein, the term “phosphorescence”refers to emission from a
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`triplet excited state of an organic molecule and the term fluorescence refers to emission
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`from a singlet excited state of an organic molecule.
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`15
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`Successful utilization of phosphorescence holds enormous promise for
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`organic electroluminescent devices. For example, an advantage of phosphorescenceis that
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`all excitons (formed by the recombination of holes and electrons in an EL), which are
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`formed either as a singletor triplet excited state, may participate in luminescence. Thisis
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`because the lowest singlet excited state of an organic molecule is typically at a slightly
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`20
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`higher energy than the lowest triplet excited state. This meansthat, for typical
`
`phosphorescent organometallic compounds, the lowest singlet excited state may rapidly
`
`decay to the lowest triplet excited state from which the phosphorescence is produced. In
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`contrast, only a small percentage (about 25%) of excitons in fluorescent devices are
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`capable of producing the fluorescent luminescence that is obtained from a singlet excited
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`state. The remaining excitons in a fluorescent device, which are producedin the lowest
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`triplet excited state of an organic molecule, are typically not capable of being converted
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`into the energetically unfavorable higher singlet excited states from whichthe fluorescence
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`is produced. This energy, thus, becomeslost to radiationless decay processes that heat-up
`
`the device.
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`10
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`SUMMARY OF THE INVENTION
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`The present invention is directed to emissive phosphorescent
`
`organometallic compoundsthat produce improved electroluminescence, organic light
`
`emitting devices employing such emissive phosphorescent organometallic compounds,
`
`and methods of fabricating such organic light emitting devices.
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`Specific embodiments of the present invention are directed to OLEDsusing
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`emissive phosphorescent organometallic compoundsthat produce improved
`
`electrophosphorescencein the blue region of the visible spectrum.
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`The present invention is directed, in addition, to a methodof selecting
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`20
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`organometallic compoundsthat have improved electroluminescent properties, for example,
`
`in the blue region ofthe visible spectrum.
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`The present invention is also directed to an organic light emitting layer
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`including a host material and a guest material dispersed in the host material, the guest
`
`material having a lowest triplet excited state having a radiative decay rate of greater than
`
`about 1x10° or about 1x10° per second and wherein the energy level of the lowesttriplet
`excited state ofthe host material is lower than the energy level ofthe lowesttriplet excited
`
`state of the guest material. The sum ofthe radiative and non-radiative decay rates of the
`
`hosttriplet is preferably not greater than about 5 x 10?/sec, and morepreferably, not
`
`greater than about 1 x 107/sec.
`
`The present invention is also directed to an organic light emitting layer
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`10
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`including a host material having a lowest triplet excited state having a decay rate of less
`
`than about 1 per second; a guest material dispersed in the host material, the guest material
`
`having a lowesttriplet excited state having a radiative decay rate of greater than about
`
`1x10° or about 1x10° per second and wherein the energy level of the lowesttriplet excited
`
`state of the host material is lower than the energy level of the lowesttriplet excited state of
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`the guest material.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`For the purpose offurtherillustrating the invention, representative
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`20
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`embodiments are shown in the accompanyingfigures, it being understood that the
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`inventionis not intended to be limited to the precise arrangements and instrumentalities
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`shown.
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`Figure 1a shows photoluminescent (PL) spectra in a dilute (10°M)
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`chloroform solution of three different iridium-based phosphors: Iridium(IIDbis(4,6-di-
`
`fluorophenyl)-pyridinato-N, C”) picolinate (FIrpic) (curve a); bis (4,6-di-fluorophenyl)-
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`pyridinato-N, C*)iridium(acetylacetonate) [Fir(acac)] (curve b); and bis(2-
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`phenylpyridinato-N,C?jiridium(acetylacetonate) [ppy,Ir(acac)] (curve c); as well as the
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`molecular structures of these iridium complexes: FIrpic (structure a); FIr(acac) (structure
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`b); and ppy-Ir(acac) (structurec).
`
`Figure 1b shows showsthe electroluminescence spectra of the following
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`OLEDstructure: ITO/ CuPc (10nm)/ a-NPD(30nm)/ CBP host doped with 6% FIrpic
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`(30nm)/ BAIg (30nm)/ LiF (1nm)/ Al (100nm).
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`Figure 2 showsexternal electroluminescent quantum (1..: filled squares) and
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`power(np: open circles) efficiencies as functions of current density for the following OLED
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`15
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`structure: [TO/ CuPc (10nm)/ w-NPD(30nm)/ CBP host doped with 6% FIrpic (30nm)/
`
`BAlq GOnm)/ LiF (1nm)/ Al 100nm). The inset to Figure 2 shows an energy level
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`diagram oftriplet levels of a CBP host and a FIrpic guest.
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`Figure 3 shows a streak image of the transient decay of a 6%-FlIrpic:CBP
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`20
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`film (100nm thick) on a Si substrate under nitrogen pulse excitation (~500ps) at T=100K.
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`Also shown is the CBP phosphorescence spectrum obtained at 10K.
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`Figure 4 showsthe transient photoluminescence decay characteristics of a
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`100nm thick 6%-FIrpic:CBP film on a Si substrate under nitrogen pulse excitation
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`(~500ps) at T=50K, 100K, 200K and 300K. Theinset to Figure 4 shows the temperature
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`dependence of the relative photoluminescence (PL)efficiency (Np) of FIrpic doped into
`
`CBP.
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`Figures 5a, 5b and Sc show generic representative examples ofthe atleast
`
`one mono-anionic, bidentate, carbon-coordination ligand of the present invention.
`
`Figure 5d showsthree specific examples of the at least one mono-anionic,
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`10
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`bidentate, carbon-coordination ligand of the present invention with specific substituents.
`
`Figures 6a and 6b show generic representative examples of the at least one
`
`non-mono-anionic, bidentate, carbon-coordination ligand of the present invention.
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`15
`
`Figure 6c shows specific examples ofthe at least one non-mono-anionic,
`
`bidentate, carbon-coordination ligand of the present invention with specific substituents.
`
`Figures 7a through 7r show representative examples of the phosphorescent
`
`organometallic compoundsof the present invention, along with their emission spectra.
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`20
`
`Figures 8a through 8d show the chemical structures ofthe phosphorescent
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`organometallic compounds from Figures 7a through 7r, along with some of the ligands
`
`comprising these compounds.
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`Figures 9a through 9g show the chemical structures of representative
`
`organometallic compoundsdescribed herein.
`
`Figure 10 shows the emission spectrum of both Pt(ppy), and Pt(ppy),Br).
`
`5
`
`The former gives green emission, partly from MLCTtransitions, and the latter gives blue
`
`emission, predominantly fromatriplet 1- = * transition. The structure observed for the
`
`Pt(ppy),Br, spectrum is consistent with ligand-centered emission. The luminescent
`
`lifetimes for the two complexes are 4 and 150 microseconds.
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`10
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`Figure 11 is a plot showing the emission spectra of (ppy)AuCl, and
`
`(ppy)Au(2,2"biphenylene). Both emit from ligandtriplet 1 - 1 * transitions.
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`Figure 12 is a CIE diagram providing the coordinates of (C-N)Pt(acac)
`
`complexes. All coordinates are based on solution photoluminescent measurements except
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`15
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`for 4,5-F,ppy-EL, which correspondsto the electroluminescent spectrum. The Ir(ppy); is
`
`an electroluminescent spectrum as well.
`
`Figure 13 is a plot depicting the photoluminescent emission spectra of (4,6-
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`F,ppy)Pt(acac) at room temperature (RT) and at 77 K. Also shown are the excitation
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`20
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`spectra taken at 77 K and the absorbancespectra taken at room temperature for the same
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`complex.
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`Figure 14 illustrates the normalized photoluminescent emission spectra of
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`(ppy)Pt(acac), (4,5 dfppy)Pt(acac), and (4,5 dfppy)Pt(pico).
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`Figure 15 illustrates the normalized photoluminescent emission spectra of
`
`typPy(acac), bzqPt(acac), and btpPt(acac).
`
`Figure 16 illustrates the normalized electroluminescent emission spectra for
`
`OLEDsprepared with (2-(4,5-F,phenyl)pyridinato)platinum(acetyl acetonate). The
`
`OLEDshad a ITO/PVK-PBD-dopant/Alq;/Mg-Ag layer structure. The PVK layer was
`
`deposited as a single, homogeneouslayer by spin coating. PVK = polyvinylcarbaozole
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`10
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`and PBD = (4-biphenyl)(4-tertbutyloxidiazole. The Alq, and Mg-Ag layers were
`
`deposited by thermal evaporation. The OLED had an external efficiency of 1.3 % anda
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`turn on voltage of 5 Volts. The spectra of the EL output as well as the PL signal are
`
`shown.
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`15
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`Figure 17 shows the molecular structures of some materials studied for the
`
`present invention and a view ofthe triplet dynamics in a guest-host system of the present
`
`invention.
`
`Figure 18 showsthe structure of the electroluminescent devices used to
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`20
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`observe the transient responseoftriplet diffusion in organic host materials.
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`Figure 19 shows the phosphorescent spectra of TPD, BCP, CBP and
`
`Ir(ppy)3 with PtOEP according to the present invention.
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`Figure 20 showsthe transient response of four phosphorescent guest-host
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`systems according to the present invention.
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`Figure 21 showsthe electroluminescent response of 8% Ir(ppy); in TPD.
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`Figure 22 showsthe normalized phosphorescenttransients for PtOEP in
`
`Alq; recorded at 650 nm for diffusion distances of (a) 200 A, (b) 400 A, (c) 600 A and (d)
`
`800 A.
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`10
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`Figure 23 showsthe exciton current incident in the phosphorescent zone for
`
`diffusion distances (a) 200 A, (b) 400 A,(c) 600 A and (d) 800 A.
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`DETAILED DESCRIPTION
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`15
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`The present invention will now be described in detail for specific preferred
`
`embodiments of the invention. These embodiments are intended only asillustrative
`
`examples and the invention is not to be limited thereto.
`
`The phosphorescent organometallic compoundsof the present invention are
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`20
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`comprised of: (a) a heavy transition metal such as Ir, but not limited to Ir, which produces
`
`efficient phosphorescent emission at room temperature from a mixture of MLCT and 7 - 1*
`
`ligand states; (b) wherein the metal is bound to at least one mono-anionic, bidentate,
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`carbon-coordination ligand substituted with electron donating and/or electron withdrawing
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`substituents that shift the emission,relative to the un-substituted ligand,to either the blue,
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`green or red region of the visible spectrum; and (c) wherein the metal is boundto at least
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`one non-mono-anionic, bidentate, carbon-coordination ligand, which may be substituted or
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`un-substituted, that causes the emission to have a well defined vibronic structure.
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`A carbon-coordination ligand is a ligand that is boundto the metal atom via
`
`a carbon-metal bond. In view of what one skilled in the art might view asa strict
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`definition of organometallic compounds, such as described in Inorganic Chemistry, by
`Gary L. Miessler and Donald A. Tarr, 2nd edition, Prentice Hall, 1999, the compounds of
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`the present invention are referred to herein as organometallic compoundssince these
`
`compoundsinclude a metal-carbon bond.
`
`The phosphorescent organometallic compounds ofthe present invention
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`haveat least one carbon-coordination ligand wherein the at least one carbon-coordination
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`ligand is a mono-anionic ligand. That is, the metal atom is bound to only one carbon atom
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`of the at least one carbon-coordination ligand. Furthermore, the at least one mono-anionic,
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`carbon-coordination ligand of the present invention is a bidentate ligand. A bidentate
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`ligand has two points at whichit attaches to a central atom, in this case, the metal atom.
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`Thus, the phosphorescent organometallic compoundsof the present invention have at least
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`one mono-anionic, bidentate, carbon-coordination ligand.
`
`The at least one mono-anionic, bidentate, carbon-coordination ligand of the
`
`present invention is substituted with electron donating and/or electron withdrawing
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`substituents that shift the emission, relative to the un-substituted ligand,to either the biue,
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`green or red region of the visible spectrum. The particular substituents used on particular
`
`ligands will depend uponthe desired shift in emission. Generic representative examples of
`the at least one mono-anionic, bidentate, carbon-coordination ligand ofthe present
`
`invention are listed in Figures 5a, 5b and 5c. In addition, two specific examples of the at
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`least one mono-anionic, bidentate, carbon-coordination ligand of the present invention
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`with specific substituents are listed in Figure 5d. As can be seen in Figures 5a, 5b and 5c,
`
`the at least one mono-anionic, bidentate, carbon-coordination ligand of the present
`
`invention can form a cyclometallated ring that includes the organometallic carbon-metal
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`bond and a dative bond between the metal atom and a nitrogen, sulfur or oxygen group.
`
`The carbon atom that is bound to the metal may be presentas part of a substituted or
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`unsubstituted, saturated hydrocarbon; a substituted or unsubstituted, aromatic system, for
`
`example, phenylene or naphthalene compounds;or a substituted or unsubstituted
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`heterocyclic system, which might include, for example, substituted or unsubstituted
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`thiophenes, furans, pyridines and pyrroles. The group in the cyciometallated ring that
`
`forms a dative bond with the metal atom may be independently selected also to include a
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`substituted or unsubstituted, saturated hydrocarbon; a substituted or unsubstituted,
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`aromatic system, for example, phenylene or naphthalene compounds;or a substituted or
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`unsubstituted heterocyclic system, which might include, for example, thiophenes, furans,
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`pyridines and pyrroles. Oneof these aforementioned groups must be substituted, however,
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`becausethe at least one mono-anionic, bidentate, carbon-coordination ligand of the present
`
`invention is substituted with electron donating and/or electron withdrawing substituents
`
`that shift the emission relative to the un-substituted ligand.
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`The preferred metals of the present invention are metals that can provide
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`strong spin-orbit coupling of the metal atomwith the at least one mono-anionic, bidentate,
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`carbon-coordination ligand. Such metals include,in particular, the heavy metals having an
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`atomic numberofat least 72, such as Os, Ir, Pt and Au, with Ir and Pt being particularly
`
`preferred metals.
`
`In addition to being bound to at least one mono-anionic, bidentate, carbon-
`
`coordination ligand, the metal atom of the organometallic compounds ofthe present
`
`invention is also bound to at least one non-mono-anionic, bidentate, carbon-coordination
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`ligand. The at least one non-mono-anionic, bidentate, carbon-coordination ligand of the
`
`present invention is either not mono-anionic, not bidentate, not a carbon-coordination
`
`ligand, or some combination thereof. The at least one non-mono-anionic, bidentate,
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`carbon-coordination ligand causes the emission to have a well defined vibronic structure,
`
`and generic representative examples thereof are listed in Figures 6a and 6b. In addition,
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`specific examples of the at least one non-mono-anionic, bidentate, carbon-coordination
`
`ligand of the present invention with specific substituents are listed in Figure 6c.
`
`In one embodiment of the organometallic compoundsofthe present
`
`invention, the organometallic compound includes, in particular, a metal atom boundto a
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`20
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`single carbon-coordination ligand, wherein the carbon-coordination ligand is a mono-
`anionic carbon-coordination ligand. In particular, the metal atom is bound to only one
`
`carbon atom of the carbon-coordination ligand. Thus, while the organometallic
`
`compoundsthat are used in the OLEDsofthe present invention include more than one
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`ligand, in this embodimentof the present invention only one ligand is a carbon-
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`coordination ligand. Thus, in this embodiment of the present invention the organometallic
`
`compoundsinclude only one carbon-metal bond.
`
`In this same embodiment of the present invention, the carbon-coordination
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`ligandis preferably selected from those ligands that exhibit strong charge transfer
`
`absorption characteristics, for example, a molar absorptivity of at least 1,000 L/mole-cm,
`
`preferably, at least about 2,000-4,000 L/mole-cm. Such absorption bands involvetransfer
`
`of electrons from molecular orbitals that are primarily ligand in character to orbitals that
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`are primarily metal in character or, alternatively, from orbitals that are primarily metal in
`
`character to molecular orbitals that are primarily ligand in character. Miessler and Tarr.
`Such an excitation mechanism results in a charge transfer transition that may be
`
`designated as a ligand-to-metal charge transfer (LMCT)or as a metal-to-ligand charge
`
`transfer (MLCT), respectively. The former may be characterized as a partial reduction of
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`the metal atom andthelatter as a partial oxidation of the metal atom.
`
`Selection of a carbon-coordination ligand to give a high molar absorptivity
`
`of the organometallic compoundresults in an organometallic compoundthat is capable of
`
`providing highly efficient electroluminescence when used in an OLED. However, rather
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`20
`
`than functioning as strongly absorbing species in the OLED, such organometallic
`
`compounds have highly emissive excited states that are produced when a voltage is
`
`applied across the OLED. The high molar absorptivities of such ligands may be used to
`
`select ligands that produce highly efficient electroluminescence in an OLED. Such
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`ligands may be selected to have empty pi-symmetry orbitals on the ligands that become
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`acceptor orbitals upon absorption oflight.
`
`In this same embodiment of the present invention, the ligand is preferably
`
`selected, in particular, so as to give a strong metal-to-ligand charge transfer (MLCT)
`
`absorption band. Such ligandsare selected to have empty anti-bonding 1* orbitals on the
`
`ligands that becomeacceptor orbitals upon absorption of light. As representative
`
`embodiments of the present invention, the carbon-coordination ligand may be selected
`
`from the class of materials such as described, for example, in Comprehensive
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`10
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`Coordination Chemistry, Vols. 1-7, G. Wilkinson, Ed., Pergamon Press, 1987.
`
`In this same embodimentof the present invention, in addition to being
`
`boundto a single mono-anionic carbon-coordination ligand, the metal atom of the
`
`organometallic compoundis also bound to one or more additional ligands, each of which
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`are all non-carbon-coordination ligands. A non-carbon-coordination ligand is one that
`
`does not form any metal-carbon bonds with the metal atom of the organometallic
`
`compound. Preferably, in this same embodiment of the present invention, a metal to
`
`ligand charge transfer complex (MLCT) is employed, where the non-carbon-coordination
`
`ligands are preferably ligands having a strong electrophilic character such that the ligands
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`draw electrons away from the metal atom. Representative non-carbon-coordination
`
`ligands mayalso be selected, for example, from Comprehensive Coordination Chemistry,
`
`Vols. 1-7, G. Wilkinson, Ed., Pergamon Press, 1987.
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`Without intending to be limited to the theory of how the present invention
`
`works,it is believed that the improved electroluminescent properties that are observed for
`
`the OLEDsofthe present invention may be attributed to a combination of factors. For
`
`example, it is believed that selection of heavy metals that are capable of forming metal-to-
`
`ligand charge transfer (MLCT) states with carbon-coordination ligands that have empty «*
`
`orbitals, such phosphorescent materials produce highly efficient electrophosphorescent
`
`OLEDs. The electroluminescence from representative organometallic compoundsof the
`
`present invention showsa vibronic fine structure that indicates that the emission is from an
`
`excited state that has a