(12) INTERNATIONALAPPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`G9) World Intellectual Property Organization
`International Burean
`
`(43) International Publication Date
`31 May 2001 (31.05.2001)
`
` HOE EEEEE
`
`{10} International Publication Number
`WO 01/39234 A2
`
`(}
`
`international Patent Classification’:
`
`Hou
`
`international Application Number:
`
` PCT/US00/31456
`
`}
`
`International Filing Date
`13 Noventber 21100 (15.11.2000)
`
`Filing Language:
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`Publication Language:
`
`Bngtish
`
`Enetish
`
`(34)
`
`{Rij Desiguated States rational}: AB, AG, AL, AM, AT, AU,
`AZ, BA, BB, BG BR, BY BZ, CA, CH, ON, CR, CU, Oe,
`DE, DE, DM, DZ, EE, ES. FILGE, GD, GE, GR, GM, HR,
`HU IDLE. EN, IS JP, RE, EG, RRR, RCL ERR,
`ES.UT LU, LM MA, MD, MG, MIR, MIN, MW, MEX. Mz,
`NONZ, PL. PP RG, RO. SD, SE, SG. S188, SL, TY, TM,
`TR, TY, TH. UA, UG. UZ, VN, WO, ZA, AW,
`
`Besignated States pregienali:s ARIPO patent (GH, GM,
`RE, LS. MW, M2, SE, SL. 82, T2, OG, 2), Eurasian
`patent (AM, AZ, BY. AG, RZ, MD RU, ES, TM), European
`patent (AT, BE CH CY) DE, DEES, FL PR, GB, GREE,
`iT, LU, MOC. NL. PY SE. TR}, OAPT pater UGE, BI, CR,
`OG, CY CM GA LGN, GW, MIL, MR, NE, SN TD, TG).
`
`Published:
`Widead guiermational search report and to be repultished
`spon receipt of hat report
`
`For pwo-letter codes and other abbreviations. refer to the “Cuid-
`ance Notes on Codes and dhhreviations” appearing at the begin-
`ning ofeach regular issue afthe PCYGazelte.
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`Prioritv Bata:
`09/449,363
`
`24 Novertber 1999 (24.177,1990}
`
`US
`
`Applicant: THE TRUSTEES OF PRINCETON UNI-
`VERSITY (USAIS]: P.O). Box 36, Princeton, NJ O8544
`CUS,
`
`(72)
`
`tnventers: FORREST, Stephen, BR. 148 Hunt Drive,
`Princeton, NJ G8540 (DS) ADACHI, Chihava; 11 Hop-
`kinson Court, East Windsor, NP O8520 (0S).
`
`Fo et al; Kenyon &
`Agents; MEAGHER, Thomas,
`Kenyon, One Broadway, New York, N'Y10004 (5).
`
`ee
`WO01/39234AZ
`
`
`
`(54) Title: ORGANTC LIGHT EMITTING DIODE HAVING
`
`(47) Abstract: Improved electroluminescent efficiency ia organic Hight emitting diodes is obtained with an emitter laver comprising
`organic complenes of transition metals with benzoxazols derivatives. A dimethylated benzonazole derivative with zinc shows blue
`fhuorescence and phosphorescence.
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`

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`WO 01/39234
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`PCT/USO0/91 456
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`ORGANIC LIGHT EMITTING DIODE
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`HAVING A BLUE PHOSPHORESCENT MOLECULE AS AN EMITTER
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`i. FIELD OF INVENTION
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`The present invention is directed to the use of organometallic compounds,
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`especially of certain benzoxazoles with transition metals, as dopants in certain hosts
`
`10
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`as emitters in organic light emitting diodes.
`
`iL. BACKGROUNDOF THE INVENTION
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`HA. General Background
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`Organic hght emitting devices (OLEDs) are comprised of several organic
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`layers in which one of the layers is comprised of an organic material that can be made
`
`to clectrolumunesce by applying a voltage across the device, C.W. Tang et al., Appl
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`Phys. Lett. 1987, 51, 913. Certain OLEDs have been shownto have sufficient
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`brightness, range of color and operating lifetimes for use as a practical alternative
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`technology to LCD-based full color flat-panel displays (S.R. Forrest, PLE, Burrows
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`and M.E. Thompson, Laser Focus World, Feb. 1995). Since many of the thin organic
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`films used in such devices are transparent in the visible spectral region, they allow for
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`the realization of a completely newtype of display pixel in which red (R), green (0).
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`and blue (3) emitting OLE-Dsare placed in a vertically stacked peometry to provide a
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`simple fabrication process, a small R~G-B pixel size, and a large fill factor,
`
`fo tty
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`Intemational Patent Application No. PCT/UIS95/15790.
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`4 transparent OLED (TOLED), which represents a significant step toward
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`realizing high resolution, independently addressable stacked R-G-B pixels, was
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`reported in International Patent Application No. PCTYUS97/02681 in whichthe
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`JOLED had greater than 71% transparency when turned off and emitted light from
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`both tep and bottom device surfaces with high efficiency (approaching 10 quantum
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`efficiency) when the device was tumed on. The TOLED ased transparent indium tin
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`oxide (ITO) as the hole-injecting electrode and a Mg-Ag-ITOelectrode layer for
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`WO 6199234
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`electron-injection. A device was disclosed in which the ITOside of the Mg-Ag-fTO
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`laver was used as a hole-injecting contact for a second, different color-emitting OLED
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`stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was
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`independently addressable and emitted its own characteristic color, This colored
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`tt
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`emission could be transmitted through the adjacentlystacked, transparent,
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`independently addressable, organic layer or layers, the transparent contacts and the
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`glass substrate, thus allowing the device to emit any color that could be produced by
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`varying the relative output of the red and blue color-emitting layers.
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`PCTAUS9S/15796 application disclosed an integrated SOLED for which both
`
`10
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`intensity and color could be independently varied and controlled with external power
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`supplies in a color tunable display device. The PCT/US95/15790 application, thus,
`
`illastrates a principle for achieving integrated, fall color pixels that provide high
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`image resolution, which is made possible by the cornpact pixel size. Furthermore,
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`relatively lowcost fabrication techniques, as compared with prior art methods, may be
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`utilized for making such devices.
`
`ILB. Background of emission
`
`TEB.1. Basics
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`H.B.l.a. Singlet and Triplet Excitons
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`Because light is generated in organic materials from the decay of molecular
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`excited states ar excitons, understanding their properties andinteractions is crucial to
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`the design of efficient light emitting devices currently of significant interest due to
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`their potential uses in displays, lasers, and other Uhimimation applications. For
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`example, if the symmetry of an exciton is different from that of the ground state, then
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`the radiative relaxation of the exciton is disallowed and luminescence will be slow
`
`and inefficient. Because the ground state is usually anti-synumetric under exchange of
`
`spins of electrons comprising the exciton, the decay of a symmetric exciton breaks
`
`symmetry, Such excitons are known as triplets, the term reflecting the degeneracy of
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`the state. For every three triplet excitons that are formed byelectrical excitation in an
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`OLED, only one symmetric state (or singlet) exciton is created. (M.A. Baldo, D. F.
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`O'Brien, M.E. Thompson and S. R. Forrest, Very high-efficienoy green organic light-
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`WO O1/49244
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`PCTAS00/31 486
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`emitting devices based on electrophasphorescence, Applied Physics Letters, 1999, 75,
`
`4-6.) Luminescence froma symmetry-disallowed process is known as
`
`phosphorescence. Characteristically, phosphorescence may persist for up to several
`
`seconds after excitation due ta the low probability of the transition. In contrast,
`
`tat
`
`fluorescence originates in the rapid decay of a singlet exciton. Since this process
`
`eceurs between states of like symmetry, it may be very efficient.
`
`Many organic materials exhibit fluorescence from singlet excitons. However,
`
`cmly a very fewhave been identified which are also capable of efficient room
`
`ternmperature phosphorescence fromtriplets. Thas, in most fluorescent dyes, the
`
`io
`
`energy contained in the triplet states is wasted. However, if the triplet excited state is
`
`perturbed, for example,
`
`through spin-orbit coupling (typically introduced by the
`
`presence of a heavy metal atom), then efficient phosphoresence is more likely. In this
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`case, the triplet exciton assumes somesinglet character and it has a higher probability
`
`of radiative decayio the ground state. Indeed, phospherescent dyes with these
`
`properties have demonstrated high efficiency electroluminescence.
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`Only a feworganic materials have been identified which showefficient room
`
`temperature phosphorescence from triplets.
`
`In contrast, many Auorescent dyes are
`
`known (CH. Chen, J. Shi, and C.W. Tang, "Recent developments in molecular
`
`organic clectroluminescent materials,” Macromolecular Symposia, 1997, 124, 1-48:
`
`20
`
`U. Brackmann, Lambdachrome Laser Dyes (Lambda Physik, Gottingen. 1997) and
`
`fuorescent efficiencies in salution approaching [00%are net uncommon. (C.H. Chen,
`
`1997, op. cit.} Fluorescence is also not affected hytriplet-triplet annihilation, which
`
`degrades phosphorescent emission at high excitation densities. (M.A. Baldo, et al.
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`“High efficiency phosphorescent emission from organic clectroluminescent devices,”
`
`be in
`
`Nature, 1998, 395, 151-154; M. A. Baldo. M. E. Thompson, and $.R. Forrest, "An
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`analvtic model of viplet-triplet annihilation in electrophosphorescent devices,” 1999},
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`Consequently, fhiorescent materials are suited to many electroluminescent
`
`applications, particularly passive matrix displays.
`
`ILB.iob. Overviewof inventionrelative to basics
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`This invention is directed to the use of complexes of metals, including zinc, as
`
`dopants in a hast layer comprising ta function as a emitter layer in organic light
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`fod
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`WO 0139234
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`POCTADSON3 14356
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`emitting diodes.
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`‘There are a few papers in the literature on Zn benzoxazoles. [N. Nakamura, §.
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`Wakabayashi, RK. Mivairi and T. Pupil, Chem. Lett., 1741 (1994); Y. Hamada, T. Sano,
`
`T. Fujii, H. Takahashi, and K. Shibata, Jpn. J. Appl. Phys.. 38, L339 (1996)}] These
`
`previous reports de not mention the phosphorescence emission at all. Further, the
`
`previous reports teach only the use of neat films as an emitter layer, There are no
`
`reports on the doped films. We examined the use of neat films of Zn(BOX)2;
`
`however, the OLED device using these neat films showed broad ernission
`
`ranging fram blue to yellowregions. Thus, the present invention on doped forms is the
`
`only report of phosphorescent blue enussion suitable for use in OLEDs.
`
`Generally, the metal compounds ofthe invention have the formula
`
`Re
`By
`R,
`ae
`KN
`S,
`3, ene
`pet Sr
`Rag ON
`sel
`ips
`oy
`R,
`Mm
`
`Ry
`
`y
`
`Re
`
`X, ¥ indenendently O, 8
`
`Mrepresents a metal
`
`nis a integer for ET to 3
`
`R, to Ry independently hydrogen atom, an ary! group or
`
`an alkyl group.
`
`In this specification, we will use the term "BOX" to encompass any benzoxazole
`
`derivative as depicted above with X, Y. and R, to Ry as defined above. Alkyl groups
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`of length one to four carbons are preferred alkyl chain lengths.
`
`in one embodiment of the present invention, a 2n(BOX) compound is usedas
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`a dopant at a level of 2.8 mol®s in a CBP host (CBP is a carbazole "dimer", in which a
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`bo it
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`carbazole molecule is attached to the 4 and 4 positions of bipheny!) and provides a
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`quantum ¢efficiency of 0.6%. The formula of 4,4'-N,N’-dicarbazole-bipheny! (CBP)
`
`18
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`WO 6199234
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`PCTUSO0/31 486
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`iS
`Co
`seo é oNafOe
`AN.
`of Nee
`NX wo :
`A Pa
`Sgae
`
`The compound 2n{BOX)}, can function as a blue emitter, yielding blue light
`
`emission fom an OLED; its photoluminescence shows both a fast decay and a slow
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`decay both of color blue, wherein the fast decay correspondsto afluorescence
`
`$
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`process and the slaw decay corresponds to a phosphorescence process. Emitters
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`showing both fluorescence and phosphorescence are of value in OLEDs.
`
`These embadiments are discussed im more detail im the examples below.
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`However the embodiments may operate by diferent mechanisms. Without limiting
`
`the scope of the invention, we discuss the different mechanisms.
`
`id
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`H.B.i.c. Dexter and Frster mechanisms
`
`To understand the different embodiments ofthis irvention it is useful to
`
`discuss the underlying mechanistic theory of energytransfer. There are two
`
`mechanisms commonly discussed for the transfer of energyto an acceptor molecule.
`
`1S
`
`in the first mechanism of Dexter transport (D.L. Dexter. "A theory of sensitized
`
`luminescence in solids,“ J. Chem. Phys., 1953, 21, 836-850), the exciton may hop
`
`directly from one molecule to the next. This is a short-range process dependent on the
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`overlap af molecular orbitals of neighboring molecules.
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`It alse preserves the
`
`symmetry of the donor and acceptor pair (E. Wigner and E,W. Wittmer, Uber die
`
`20
`
`Struktur der zweiatomigen Molekelspektren nach der Quantenmechanik, Zeitschrift
`
`fur Physik, 1928, 51, $89-886, M. Klessinger and J. Michl, Excited states and
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`photochemistry of organic molecules (VCH Publishers, New York, 1995). Thus, the
`
`enercy transfer of Eq. (1) is not possible via Dexter mechanism. In the second
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`Lift
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`WO 0139244
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`PCTAISOM3 1456
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`mechanisrn of Férster transfer ( T. Forster, Zwischenmolekulare Energiewanderang
`
`and Flooreszenz, Annalen der Physik, 1948, 2, 55-74: T. Farster, Flaereszenz
`
`organischer Verbindugen (Vandenhock and Ruprecht, Gottinghen, 1951 3, the energy
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`transfer of Eq. (1) is possible.
`
`In FGrster transfer, similar to a transmitter and an
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`antenna, dipoles on the donor and acceptor molecules couple and energy may be
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`transferred. Dipoles are generated from allowed transitions in both donor and
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`accepior molecules. This typically restricts the Forster mechanism to transfers
`
`between singlet states,
`
`Nevertheless, as long as the phosphor can emit light due to some perturbation
`
`10
`
`of the state such as due to spin-orbit counling introduced by a heavy metal atom, it
`
`may participate as the donor in Forster transfer. The efficiencyof the process is
`
`determined by the luminescentefficiency of the phasphor (F Wilkinson, in Advances
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`in Photochemustry (eds. W.A. Noyes, G. Nammond, and J.N. Pitts, pp. 241-268, John
`
`Wiley & Sons, New York, 1964), Le. if a radiative transition is more probable than a
`
`is
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`non-radiative decay, then energy transfer will be efficient. Such triplet-singlet
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`transfers were predicted by Forster (T. Férster,” Transfer mechanisms of electronic
`
`excitation,” Discussions of the Faraday Society, 1959, 27, 7-17) and confirmed by
`
`Ermolaev and Sveshnikova (V.L. Ermolaev and E. B. Sveshnikovwa, *Inductive-
`
`resonance transfer of energy from aromatic molecules in the triplet state.” Doklady
`
`hoa
`
`Akademii Nauk SSSR, 1963, 149, 1295-1298), who detected the energytransfer using
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`arange of phosphorescent donors and fluorescent acceptors in rigid media at 77Kar
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`90K. Large transfer distances are observed: for example, with triphenylamine as the
`donor and chrysoidine as the acceptor. the interaction range is 52A.
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`The remaiming condition for Forster transfer is that the absorption spectrum
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`ba tit
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`should overlap the emussion spectrum of the donor assuming the energy levels
`
`between the excited and ground state molecular pair are in resonance.
`
`In Example |
`
`of this application, we ase the green phosphor fac tris(2-phenylpyridine} iridium
`
`(irfppy},. M.A. Baldo, et al, Appl. Phys. Leit., 1999, 7S, 4-6) and the red fluorescent
`
`dye [2-methy!-6-[2-(2,3.4,7-tetrahydro- | HSH-benzofij]quinolizin-9-y]) etheny]-4H-
`
`pyrain-ylidene] propane-dimtrile] OSDCM2", C. W. Tang, S. A. VanSiyke, and C. H.
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`Chen, “Electraluminescence of doped organic films,” J. Appl. Phys., 1989, 65, 3610-
`
`o
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`WO Q)39734
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`PCT/US00/31456
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`3616). DCM? absorbs in the green, and, depending on the local polarizationfleld (V.
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`Bulovic, et al., “Bright, saturated, red-to-yellow organic light-ernitting devices based
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`on polarization-induced spectral shifts,” Chem. Phys. Lett. 19@8, 287, 455-460), it
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`emits at wavelengths between A=570 nm and A650 nm.
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`tis
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`itis possible to implement Forster energy transfer fromatriplet state by
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`doping a fluorescent guest into a phosphorescent host material. Unfortunately, such
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`systems are affected by campetitive energy transfer mechanisms that degrade the
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`overall efficiency. In particular, the close proximity of the host and guest increase the
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`itkelihood of Dexter transfer between the host to the guest triplets. Once excitons
`
`reach the guest triplet state, they are effectively lost since these fluorescent dves
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`typically exhibit extremely mefficiem phosphorescence.
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`To maximize the transfer of hosttriplets to fluorescent dye singlets, it is
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`desirable to maximize Dexter transfer into the triplet state of the phosphor while also
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`minimizing transfer into the triplet state of the fluorescent dye. Since the Dexter
`
`i
`
`S
`
`mechanism transfers energy between neighboring molecules, reducing the
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`concentration of the fluorescent dye decreases the probability oftriplet-triplet transfer
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`to the dye. On the other hand, long range Firster transfer to the singlet state is
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`unaffected. In contrast, transfer inte the triplet state of the phosphoris necessaryto
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`harness host triplets. and may be improved by increasing the concentration ofthe
`
`20
`
`phosphor.
`
`H.B.2.
`
`interrelation of device structure and emission
`
`Devices whose structure is based apon the use of layers of organic
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`optoelectronic materials generally rely on a common mechanism leading to optical
`
`b> Hs
`
`emission. Typically. this mechanismis based upon the radiative recombination ofa
`
`trapped charge. Specifically. OLEDs are comprised ofat least two thin organic layers
`
`separating the anode and cathode of the device. The material of ong ofthese laversis
`
`specifically chosen based on the material's ability to transport holes, a “hole
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`transporting Jayer” (HTL), anc! the material of the other layer is specifically selected
`
`according to its ability to transport electrons, an “electron transporting layer” CETL).
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`With such a construction, the device can be viewed as a diode with a forward bias
`
`J
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`WO 02/99234
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`POCT/US00/31456
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`when the potential applied to the anode ts higher than the potential applied to the
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`cathode. Linder these bias conditions, the anode injects holes (positive charge
`
`carriers} into the hole transporting layer, while the cathode injects electrons inte the
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`electron transporting layer. The portion ofthe luminescent mediumadjacentto the
`
`3
`
`anode thus forms a hole injecting and transporting zane while the portion of the
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`luminescent medium adjacent ta the cathode forms an electron injecting and
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`transporting zone. The injected holes and electrons each migrate toward the
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`oppositely charged electrode. When an electron and hole localize on the same
`
`molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be
`
`IQ
`
`visualized as an electron dropping from its conduction petential to a valence band,
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`with relaxation eccurring, under certain conditions, preferentially via a phoicemissive
`
`mechanism, Under this viewof the mechanismof operation oftypical thin-layer
`
`organic devices, the electroluminescent layer comprises a luminescence zone
`
`receiving mobile charge carriers (clectrons and holes) from each electrode.
`
`13
`
`As noted above, light emission from OLEDs ts typically via fluorescence or
`
`phosphorescence. There are issues with the use of phosphorescence.
`
`It has been
`
`noted that phosphorescent efficiency can decrease rapidly at high current densities. It
`
`may be that long phosphorescentlifetimes cause saturation of emissive sites, and
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`triplet-triplet annihilation may alsa produce efficiencylosses. Another difference
`
`20
`
`between fluorescence and phosphorescence is that energytransfer oftriplets from a
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`conductive host io a luminescent guest molecule is typically slower than that of
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`singlets; the long range dipole-dipole coupling (P&rster transfer} which dominates
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`energy transfer of singlets is (theoretically) forbidden for tripiets bythe principle of
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`spin symmetry conservation. Thus, for triplets, energy transfer typically occurs by
`
`Feud MH
`
`diffusion of excitons to neighboring molecules (Dexter wansfer}, significant overlap
`
`of donor and acceptor excitonic wavefunctions is critical to energy transfer. Another
`issue is that uiplet diffusion lengths are typically long (e.g., >1400A) compared with
`ipical singlet diffasion lengihs of about 700A. Thus, if phosphorescent devices are
`
`io achieve their potential. device structures need to be optimized for wiplet properties.
`
`39
`
`In this invention, we exploit the property of long triplet diffusion lengths to improve
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`external queutium effimency.
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`WO 01/39234
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`PCT/US00/31 456
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`Successful utilization of phosphorescence holds enormous promise for organic
`
`electroluminescent devices. For example, an advantage of phosphorescenceis that all
`
`excitons (formed by the recombination of holes and electrons in an EL}, which are (in
`
`part} triplet-based in phosphorescent de'ices, may participate in energy transfer and
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`luminescence in certain electroluminescent materials.
`
`In contrast, only a small
`
`percentage of excitons in fluorescent devices, which are singlet-based, result in
`
`fluorescent luminescence.
`
`An alternative is to use phosphorescence processes to improve the efficiency
`
`of fluorescence processes. . Fluorescence is in principle 73% less efficient due the
`
`three times higher number of symmetric excited states.
`
`Inc. Background of materials
`
`ILC 1. Basic heterostructures
`
`Because one typically has at least one electron transporting layer and at least
`
`one hole transporting layer, one has layers of different materials, forming a
`
`heterostructure. The materials that produce the electraluminescent emission may be
`
`the same materials that function either as the electron transporting layer or as the hole
`
`transporting layer. Such devices in which the electron transporting layer or the hale
`
`transporting layer also functions as the emissive layer are referred to as having a
`
`single heterastructure. Alternatively, the electroluminescent material may be present
`
`in & Separate emissive layer between the hole transporting layer and the electron
`
`transporting layer in what is referred to as a double heterostructure. The separate
`
`emissive layer may contain the emussive molecule daped into a host or the emissive
`
`layer may consist essentially of the emissive molecule.
`
`tea Ca
`
`That is, in addition to emissive materials that are present as the predominant
`
`component in the charge carrer layer, that is, either in the hole transporting layer or
`
`in the electron transporting layer, and that function bothas the charge carrier material
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`as well as the emussive material, the emissive material may be present in relatively
`
`low concentrations as a dopant in the charge carrier layer. Whenever a dopant is
`
`Spat
`
`present, the predominant material in the charge carrier layer maybe referredto as a
`
`host compoundor as a receiving compound. Materials that are present as host and
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`WO 0139234
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`PCTUSO1456
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`dopant are selected so as to have a high level of energy transfer from the host to the
`
`depant material.
`
`In addition, these materials need to be capable ofproducing
`
`acceptable electrical properties for the OLED. Purthermore, such hast and dopant
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`materials are preferably capable of being mcorporated into the OLED using
`
`materials that can be readily incorporated into the OLED by using convenient
`
`fabrication techniques, in particular, by using vacuum-deposition techniques.
`
`11.0.2. Exeiten blocking layer
`
`One can have an exciton blocking layer in OLED devices to substantially
`
`id
`
`block the diffusion of excitons, thus substantially keeping the excitons within the
`
`emission layer to enhance device efficiency. The material of blocking layer is
`
`characterized by an energydifference (“band gap") between its lowest unoccupied
`
`mofecular orbital (LUMO)and its highest occupied molecular orbital (HOMO) This
`
`band gap substantially prevents the diffusion of excitons through the blocking layer,
`
`15
`
`vet has only a minimal effect on the turn-on voltage of a completed
`
`electroluminescent device. The band gap is thus preferably greater than the energy
`
`level of excitons produced in an emission layer, such that such excitons are not able to
`
`exist in the blocking layer. Specifically, the band gap ofthe blocking layer is at least
`
`as great as the difference in energy between the triplet state and the groundstate ofthe
`
`host.
`
`Fora situation with a blocking layer between a hole-conducting host and the
`
`electron transporting layer, one seeks the following characteristics, which are listed in
`
`order of relative importance.
`
`BR tay
`
`1. The difference in energy between the LUMO and HOMOofthe blocking laver is
`
`greater than the difference in energy between the triplet and ground state singlet of the
`
`host material,
`
`2. Triplets in the host material are not quenched by the blocking layer.
`
`Spd >
`
`3. The ionization potential (IP} of the blocking layer is greater than the ionization
`
`id
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`WO 61/39244
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`POCTAUSOWS1S56
`
`potential of the host. (Meaning that holes are held in the host)
`
`4. The energy level of the LUMO of the blocking layer and the energy level of the
`
`LUMO ofthe host are sufficiently close in energy such that there is less than 50%
`
`5
`
`change in the overall conductivity of the device.
`
`5. The blockinglayer is as thin as possible subject to having a thickness of the laver
`
`thai is sufficient to effectively hlock the transport of excitons from the emissive layer
`
`into the adjacent layer.
`
`10
`
`Thatis, to block excitons and holes, the ionization potential of the blocking layer
`
`should be greater than that of the HTL, while the electron affinity of the blocking
`
`layer should be approximately equal to that of the ETL. to allowfor facile transport of
`
`electrons.
`
`ana 4
`
`{For a situation in which the emissive (“emitting”) molecule is used without a hale
`
`transporting host, the above rules for selection of the blocking layer are modified by
`
`replacement of the word "host" by “emitting molecule."
`
`For the complementary situation with a blocking laver between a electron-
`
`20
`
`conducting host and the hole-transporting layer one seeks characteristics (listed in
`
`order of importance}:
`
`1. The difference in energy between the LUMO and HOMO ofthe blocking laver is
`
`greater than the difference in energy betweenthe triplet and ground state singlet of the
`
`h3 Late
`
`host maierial.
`
`be . Triplets in the hast material are not quenched by the blocking layer.
`
`3. The energy of the LUMOof the blocking layer is greater than the energy of the
`
`30
`
`LUMOofthe (electron-transporting) host. (Meaning that electrons are heldin the
`
`host}
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`i
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`

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`WO 0199234
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`POCT/AUS60/31456
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`4. The ionization potential of the blocking layer and the ionization potemtial of the
`
`host are such that holes are readily injected from the blocker into the host and there is
`
`less than a 50% change in the overall conductivity of the device.
`
`habit
`
`§. The blocking laver is as thin as possible subject to having a thickness of the layer
`
`that is sufficient to effectively block the transport of excitons from the emissive layer
`
`into the adjacent layer.
`
`[For a situation in which the emissive (‘emitting’) molecule is used without an
`
`i0
`
`electron transporting host, the above rules for selection of the blocking layer are
`
`modified by replacement of the word “host” by “emitting molecule."]
`
`YW.D. Color
`
`As to colors, it is desirable for OLEDs to be fabricated using materials that
`
`re Nat
`
`provide electroluminescent emission in a relatively narrow band centered near
`
`selected spectral regions, which correspond to one of the three primarycolors, red,
`
`green and blue so that they may be used as a colored layer in an OLED or SOLED.
`
`Ie
`
`is also desirable that such compounds be capable of being readily deposited as a thin
`
`layer using vacuum deposition techniques so that they may be readily incorporated
`
`20
`
`ume an OLED that is prepared entirely from vacuum-deposited organic materials.
`
`LLS. O8/774,333, filed December 23, 1996 (allowed), is directed to OQLEDs
`
`containing emitting compounds that produce a saturated red emission.
`
`
`
`LE. SUMMARYOF THE INVENTION
`
`to A
`
`Ata general level, the present inventionis directed to the use of complexes of
`
`transition metal species with organic benzoxazole ligands as dopants in the eminer
`
`layer of organic light emitting diodes. Using a heavy metal atom along with certain
`
`bhae emitting ligands (typically benzoxazole derivatives), one obtains blue
`
`phosphorescence. Efficient electroluminescence is observed using CBPas a host and
`
`30
`
`BCP as a hole blocking layer.
`
`

`

`WO 0139234
`
`PETAISOG/(32456
`
`The general formula of the organometallic compound is as follows.
`
`Bs
`
`Re
`
`Ry
`
`wy.SohRo
`ern pe }8
`
`K,
`
`i
`
`Men
`
`5
`
`X, Y independently O, 8
`
`Mrepresents a metal
`
`nis aimeger for ] to 3
`
`R, to R, independently hydrogen atom, an aryl group or
`
`an alkyl group.
`
`16
`
`Preferred embodiments are within the following class:
`
`In this representation, the ine segments to CH, (methyl) denote substitution at any
`
`ine att
`
`allowed ring position by each methyl independently
`
`— Saal
`
`

`

`WO 0139234
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`PCT/US80/31 456
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`Specific preferred embodiments are the following compounds:
`
`
`
`This is one species of the previous genus.
`
`ew ’
`
`‘
`
`OF ye re So
`
`‘5
`EE
`
`Sarg’
`re Amen
`:
`wf
`\
`N
`3
`Bre
`CHy
`
`4
`
`This is another species of the previous genus.
`
`These are bis(2-(2-hydroxypheny])-benzoxazolatejzine derivatives.
`
`Preferred embodiments are also within the following class:
`
`
`
`13
`
`in this representation, the line segments fo CH, (methy)) denote substitution at any
`
`allowed ring pesition by each methyl independently.
`
`ig
`
`

`

`WO 01/39234
`
`PCTAUSOO/31456
`
`Another preferred embodimentis the following compound.
`
`Lar
`
`iD
`
`This is a bis(2-(2-hydroxypheny)-henzthiazolatejzine derivative.
`
`The inventionis exemplified for the metal Zn, and it is expected useful results
`
`are obtained for Os, Tr, and Pt compounds with BOX and analogous derivatives.
`
`Another advantage of the OLED devices of this invention is based on the use
`
`1S
`
`of both fhuorescence and phasphorescence. The theoretical treatment of carrier
`
`recombination revealed that bole and electron recombination creates 25%of singlet
`
`exciton (which is fhiorescence} and 73%of triplet exciton (which is
`
`phosphorescence). Thas, ff we use fluorescent molecules as an emitter, we can use
`
`only 24%of fluorescence for ELand the EL efficiencyis limited. However, when we
`
`20
`
`use the molecules which possess both of fluorescence and phospharescence
`
`characters, we could obtain 100% luminescence in principle. In our
`
`experiment, we observed that Zn(BOX), a5 an emitting material shows both
`
`lumimescences. While the ohserved EL effictencyis around
`
`0.3.0.6% at room temperature, we can expect higher efficiency with the routine
`
`245
`
`optimization af the molecular structures and device structures according to the
`
`teachings of this invention.
`
`In addition. 2n(BOX). is a promising material for
`
`particularly blue emission. In thefirst device the CLE was true
`
`blued XO.16.¥=0.0S9). Figures 1-8 describe working exantples. Figures 9 and 1}
`
`rene LA
`
`

`

`WO (1/99234
`
`PCTAJSOO3 1456
`
`prove that the electroluminescence is composed of both fluorescence and
`
`phosphorescence.
`
`IV. BRIEF DESCRIPTION OF THE DRAWINGS
`
`iat
`
`Figure 1. Quantum efficiency as a function of current density for the device of
`
`Example 1. (Device: ITOPTPD(S50nm ¥2n(BOR} Smal%-
`
`CBPCOnmYBCP( 10am)Alqg3(20nm)/MgAg/Ag)
`
`10
`
`Figure 2. Quantum efficiency as a function of current density and doping level (box)
`
`for the device of Example |,
`
`Figure 3. Current density as a function of voltage for the device of Example 1.
`
`iS
`
`Figure 4, Electroluminescence spectrumofthe device of Example 1.
`
`Figure 5. Quantum efficiency as a funetion of current density for the device of
`
`Example 2. (Device: 1fO/TPDS0nmyZnBOR 4<mola-
`
`TPD20nmyYBCPi Gom)Alg3(20nm)\/MgAg/Ag}
`
`20
`
`Figure 6. Output power as a function of current density for the device af Example 2.
`
`Figure 7. Current density as a finction of voltage for the device of Example 2.
`
`25
`
`Figure 8. Electroluminescence spectrum ofthe device of Example 2.
`
`Figure 9. Luminescent output as a function of time.
`
`Figure 10. EL intensity as a function of wavelength for fasi and slow component.
`
`30
`
`shawing both have blue aspect.
`
`16
`
`

`

`WO 0139234
`
`PCT/US00/31456
`
`
`
`V.A. Chemustry
`
`This invention is directed to the use of certain organometallic molecules doped
`
`into a host phase in an emitter layer of an organic light emitting diode.
`
`VAAL. Dopants
`
`The general chemical formula for the molecules which are deped inte the host
`
`phase is as follows.
`
`KS As
`Rs
`Y »
`R Sm a at
`Cr «
`ood
`fs am S
`OT a
`Ry
`a aoe
`Re
`
`x
`
`5
`
`X, Y independently 0, §
`
`Mrepresents a metal
`
`his a integer for | to 3
`
`R, to R, independently hydrogen atom, an aryl group or
`
`an alky! group.
`
`a preferred embodiment class is as follows
`
`CH}
`\
`:
`
`o
`
`CH
`eye
`
`i a / I
`¢ OR N sms
`j
`s ,
`é
`SO20
`
`5
`
`1Q
`
`15
`
`20
`
`

`

`WO @1/397234
`
`PCTAUS00/31 4356
`
`and a specific preferred embodiment speciesis
`
`
`
`Ne
`
`N
`
`ae
`
`eee
`
`i\
`Nea
`
`us
`
`Another preferred embodimentis
`
`10
`
`gr} \
`i ay_2
`OO NS
`t
`(
`t
`t
`4
`,
`Lo ARLEN
`wom
`fd
`FC
`STOay’ “Hy
`
`
`These are within the genus of bis(2-(2-hydroxypheny!}-benzoxazolate)zine
`
`derivatives.
`
`An important aspect of the present invention is the discovery that when the above-
`
`noted compound,
`
`
`
`18
`
`

`

`WO 01/39234
`
`PCT/USN0S31456
`
`with the methy] groups on the aromatic rings, is used in an OLED, the
`
`electroluminescence shows both