Highly efficient orange-emitting OLEDs based on
phosphorescent platinum(II) complexes
Inamur R. Laskar, Shih-Feng Hsu, Teng-Ming Chen
*Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan Received 9 November 2004; accepted 4 February 2005
Available online 25 April 2005
Abstract
The syntheses, characterisations, photophysical properties and their applications in organic light emitting devices of a series of 2-phenylbenzothiazolato (bt)/substituted 2-phenylbenzothiazolato (X-bt) platinum(II) based square-planar complexes [(X-bt)Pt(acac); acac = acetylacetonate; X = unsubstituted (1), F (2), OMe (3) and CF3(4)] are discussed. Reaction of K2PtCl4with btH/X-btH in
glacial acetic acid for a few days results in the dinuclear chloro-bridged Pt(II) complex, (bt/X-bt)Pt(l-Cl)Pt(bt/X-bt) which is cleaved with acetylacetone to give the corresponding mononuclear (bt/X-bt)Pt(acac) complex. The (MeO-bt)Pt(acac) complex has been characterized by X-ray single crystal structure analysis. The packing diagram shows the Pt–Pt distances and the intermolecular spac-ings are 3.369 and 3.360 A˚ , respectively, which is consistent with excimer formation. It has also been supported by time-resolved photoluminescence (PL) measurements. The nature of the lowest emitting states are the triplet MLCT as well as triplet p–p* states and it has been tuned according to the electronic properties of the substituents. The electroluminescent (EL) devices were fabricated by doping platinum complexes 1 and 2 (the corresponding devices denoted as D-1 and D-2, respectively) into the host CBP (4,40
-(N,N0-dicarbazole)biphenyl), in the emitting zone with a doping content of 5%, 7% and 9%. The EL performances for these devices
are exceptionally high (14.3 and 16.0 cd A1@2 mA cm2and luminances 10 550 and 11 320 cd m2@100 mA cm2for D-1 and D-2, respectively).
2005 Elsevier Ltd. All rights reserved.
Keywords: 2-Phenylbenzothiazole; Platinum(II)-complex; Photoluminescence; Eximer; Electroluminescence; Dopant
1. Introduction
The development of organometallic based optoelec-tronics, such as light-emitting devices, chemosensors
and photovoltaic dye-sensitized devices [1] is one of
the exciting applications of transition metal complexes. Extensive work on cyclometallating or oligopyridine complexes of Ru(II)[2], Os(II)[3], Re(I)[4]and Ir(III)
[5]have been studied for their long triplet excited-state
lifetimes, high emission quantum yields and tunable emission wavelengths in the above mentioned areas.
Re-cently, significant research has been focused on the syn-theses of cyclometallated complexes of iridium(III) and
particularly in light emitting diodes [5]. Platinum(II)
complexes have also been used as luminescent centres
in organic light-emitting diodes (OLEDs) [6]. Only few
of the reported platinum(II) complexes are emissive at room temperature in solution. Pt(II) species chelated with aromatic ligands, such as bipyridine, phenanthro-line, 2-phenylpyridine or similar derivatives, emit in solu-tion from excited states localized on the aromatic systems. These Pt(II) complexes benefit from having rel-atively high metal-centred (MC) states when compared
to their palladium analogues [7]. If the emitting states
(MLCT or intraligand) and MC states lie too close in en-ergy, they can thermally equilibrate, thereby quenching 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.02.009
* Corresponding author. Tel.: +88635712121x56526; fax: +88635723764.
E-mail address:[email protected](T.-M. Chen).
the emission through fast radiationless decay through
the MC states[8]. Moreover, the open coordination
cen-tres of the square planar platinum(II) complex can allow for other deactivating pathways to occur through metal
interactions with the environment [9]. These facts are
responsible for not so encouraging photoluminescence (PL) and electroluminescence (EL) performances of platinum based complexes. Recently, Che et al. reported new orange-yellow emitting schiff-base complexes of
platinum(II) [10] showing modest results. The present
work reports the syntheses and characterization of some strong orange-emitting cyclometallated platinum(II) complexes, (bt)Pt(acac) (1)/(X-bt)Pt(acac) (X = F (2),
OMe (3), CF3(4); bt = 2-phenylbenzothiazolato; acac =
acetylacetonate), study of their photophysical properties, X-ray single crystal structure analysis of (MeO-bt)Pt-(acac) and their applications in OLEDs.
2. Experimental 2.1. Materials
Potassium tetrachloroplatinate(II) was purchased from Alfa Aesar, USA, 2,4-pentanedione from Lans-caster, 2-ethoxyethanol and glacial acetic acid from Te-dia Company, Ins. and the rest of the chemicals and solvents (AR grade) from Tokyo Kasei Kogyo Co. Ltd., Japan, and they were used as received.
2.2. Instrumentation
The ultraviolet–visible (UV–Vis) spectra of the phos-phorescent Ir(III) complexes were measured on an UV–Vis spectrophotometer (Agilent model 8453) and corrected for background due to solvent absorption. Photoluminescence (PL) spectra were carried out with a spectrofluorometer (Jobin-Yvon Spex, model Fluoro-log-3). The photo-physical measurements of these complexes have been carried out in aerated dichloro-methane. NMR spectra were recorded on a Varian 300 MHz. MS spectrometer (EI and FAB) were taken by micromass TRIO–2000. Elemental analyses have been carried out by using a Heraeus CHN-O-RAPID analyzer. TG–DTA analysis was carried out by using a thermal analyzer (SEIKO 1TG/DTA 200). Emission lifetimes were obtained by exponential fitting of emis-sion decay curves recorded on a Continuum NY61 spectrofluorometer.
2.3. Crystallography
Single crystal diffraction data for Pt(MeO-bt)(acac) were collected on a Bruker Smart-CCD diffractometer equipped with a normal focus, 3kW sealed-tube X-ray
source (k = 0.71073 A˚ ). The intensity data were collected in the x scan mode (width of 0.3/ frame) and corrected
for Lp and absorption effects using the SAINT[11]
pro-gram. Cell refinement and data reduction were carried
out using the program Bruker SHELXTL [12]. The
struc-ture was solved by direct methods using the SHELXTL
[12] version 5.1 software packages. The structure was
further refined by full-matrix least-squares methods
based on F2usingSHELXTLversion 5.1[12].
Non-hydro-gen atom positions were refined anisotropically, whereas the hydrogen positions were not refined.
2.4. Syntheses
bt/X-bt (X = –F, –OMe, –CF3): The detailed
syn-thetic procedure of these ligands has been described in the literature [13].
(bt/X-bt)Pt(l-Cl)2Pt(bt/X-bt) [14]: The ligands
(2 mmol) were added to a suspension of potassium tetra-chloroplatinate (1 mmol) in glacial acetic acid (300 ml) and the reaction mixture was stirred for several days (3–5 days) whilst maintaining the temperature at 80 C, which resulted in a yellow coloured product that was distinctly discernable from the red platinum(II) salt. The product was isolated through filtration and washed with water, acetone and ether 5–6 times to re-move almost all the excess starting materials.
(bt)Pt(l-Cl)2Pt(bt). Yield: 45%. Anal. Calc. for
C26H16N2Cl2S2Pt2: C, 35.4; H, 1.8; N, 3.2. Found: C,
35.2; H, 1.8; N, 3.1%.
(F-bt)Pt(l-Cl)2Pt(F-bt). Yield: 48%. Anal. Calc. for
C26H14N2F2Cl2S2Pt2: C, 34.0; H, 1.5; N, 3.1. Found:
C, 33.7; H, 1.7; N, 3.1%.
(MeO-bt)Pt(l-Cl)2Pt(MeO-bt). Yield: 50%.1H NMR
(300 MHz, CDCl3): 9.92 (d, 1H, J = 8.7 Hz), 9.93 (d, 1H, J = 7.5 Hz), 8.40 (d, 2H, J = 8.6 Hz), 7.92 (d, 1H, J = 6.3 Hz), 7.78 (d, 1H, J = 6.7 Hz), 7.52 (m, 2H), 7.35 (m, 2H), 6.96 (d, 2H, J = 8.2 Hz), 6.53 (d, 1H, J = 7.2 Hz), 5.56 (s, 1H), 3.81 (s, 3H), 3.51 (s, 3H). Anal. Calc. for C28H20N2O2Cl2S2Pt2: C, 35.4; H, 1.8; N, 3.2. Found: C, 35.2; H, 1.8; N, 3.1%.
(CF3-bt)Pt(l-Cl)2Pt(CF3-bt). Yield: 44%. Anal. Calc.
for C28H14N2F6Cl2S2Pt2: C, 33.0; H, 1.4; N, 2.8. Found:
C, 32.9; H, 1.4; N, 3.0%.
(bt/X-bt)Pt(acac) [15]: The dinuclear complexes
(1 mmol) were dissociated into the mononuclear plati-num(II) complexes by reacting with acetylacetone (2.5 mmol) in the presence of sodium carbonate (8 mmol) and 2-ethoxyethanol (10 ml) as solvent under refluxing conditions for 12 h. These complexes show bet-ter solubility in common organic solvents with respect to their corresponding dinuclear complexes. These com-plexes were recrystallised from a mixture of dichloro-methane and methanol (1:1).
(bt)Pt(acac): 1H NMR (300 MHz, CDCl3): 9.20 (d,
J = 7.5 Hz), 7.47 (m, 3H), 7.19 (d, 1H, J = 7.5 Hz), 7.10 (t, 1H, J = 7.2 Hz), 5.56 (s, 1H), 2.09 (s, 3H), 2.03 (s, 3H). FABMS: m/z: 504, Calc. 504.1. Anal. Calc. for
C18H15NO2SPt: C, 42.8; H, 3.0; N, 2.8. Found: C,
42.5; H, 3.0; N, 2.7%.
(F-bt)Pt(acac):1H NMR (300 MHz, CDCl3): 9.17 (d,
1H, J = 8.1 Hz), 7.79 (d, 1H, J = 12 Hz), 7.44 (m, 4H), 6.81 (d, 1H, J = 6.0 Hz), 5.78 (s, 1H), 2.10 (s, 3H), 2.04 (s, 3H). FABMS: m/z: 522, Calc. 522.1. Anal. Calc.
for C18H14NO2SPt: C, 41.4; H, 2.7; N, 2.7. Found: C, 42.5; H, 2.6; N, 2.7%. (MeO-bt)Pt(acac):1H NMR (300 MHz, CDCl3): 9.11 (d, 1H, J = 8.4 Hz), 7.75 (d, 1H, J = 8.4 Hz), 7.51 (t, 1H, J = 6.7 Hz), 7.39 (m, 2H), 7.19 (d, 1H, J = 2.4 Hz), 6.67 (dd, 1H, J = 2.4, 8.7 Hz), 5.56 (s, 1H), 3.89 (s, 1H), 2.08 (s, 3H), 2.02 (s, 3H). FABMS: m/z: 534, Calc. 534.1. Anal. Calc. for C19H17NO3SPt: C, 41.4; H, 3.2; N, 2.6.
Found: C, 41.2; H, 3.2; N, 2.5%.
(CF3-bt)Pt(acac): 1H NMR (300 MHz, CDCl3): 9.24
(d, 1H, J = 8.7 Hz), 7.93 (s, 1H), 7.85 (d, 1H, J = 7.2 Hz), 7.52 (m, 3H), 7.34 (d, 1H, J = 7.5 Hz), 5.59 (s, 1H), 2.35 (s, 3H), 2.26 (s, 3H). FABMS: m/z: 572, Calc. 572.1. Anal. Calc. for C19H14NO2F3SPt: C,
39.9; H, 2.4; N, 2.4. Found: C, 39.8; H, 2.4; N, 2.4%. 2.5. OLED fabrication and testing
Organic layers were fabricated by high-vacuum ther-mal evaporation onto a glass substrate precoated with an indium–tin-oxide (ITO) layer with a sheet resistance of 20 X. Prior to use, the ITO surface was ultrasonicated in a detergent solution followed by rinsing with deion-ized (DI) water, dipped into acetone, trichloroethylene and propanol, and then degreased with a vapor of 2-propanol. After degreasing, the substrate was oxidized and cleaned in a UV-ozone chamber before it was
loaded into an evaporator. In a vacuum chamber at a
pressure of 106torr, 500 A˚ of NPB (N,N0
-di-1-nap-thyl-N,N0-diphenylbenzidine) as the hole transporting
layer; 200 A˚ the complex doped (7%) CBP as the
emit-ting layer; 100 A˚ of BCP
(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) as a hole and exciton blocking
layer (HBL); 650 A˚ of Alq3
(tris(8-quinolinato)alumin-ium) as electron transport layer; and a cathode
com-posed of 10 A˚ lithium fluoride and 2000 A˚ aluminum
were sequentially deposited onto the substrate to give
the device structure. Fig. 1(a) and (b) show the
multi-layer structure of the device and the chemical structures of the molecules used in fabricating the EL device, respectively. The current–voltage (I–V) profiles and light intensity characteristics for the above-fabricated devices
were measured in a vacuum chamber of 106torr at
ambient temperature using a Keithley 2400 Source Me-ter/2000 Multimeter coupled to a PR 650 Optical Meter.
3. Results and discussion
3.1. Syntheses and characterizations
The synthetic routes for the platinum(II) com-plexes described here involves two steps. In the first step, K2PtCl4is reacted with an excess of bt/substituted
bt to produce a chloride-bridged dinuclear complex (Fig. 2, inset), i.e. (bt/X-bt)Pt(l-Cl)2Pt(bt/X-bt). All these
complexes, except (MeO-bt)Pt(l-Cl)2Pt(MeO-bt), are
al-most insoluble in common organic solvents which makes them impossible for solution phase characteriza-tion. The polar methoxy substituent is assumed to be responsible for making the corresponding dinuclear platinum(II) complex have a better solubility in more
or less polar organic solvents. The 1H NMR spectrum
N N NPB CBP N N N N BCP N O Al N O N O Alq Pt O O N N S X N X = 0; for D-1 X = F; for D-2 Dopant Glass ITO CHF3 NPB (500 Å) CBP+ dopant(200 Å) BCP (100 Å) Alq3(650 Å) LiF(10 Å) / Al (2000 Å) (a) (b)
of (MeO-bt)Pt(l-Cl)2Pt(MeO-bt) (deposited) shows the
splitting of the methyl proton in the methoxy substituent of the dinuclear complex, suggesting that the two non-equivalent methoxy protons exist in the two separate li-gands of that complex. It is the result from different interactions with the solvent molecules. The chloride-bridged dinuclear complexes can be readily dissociated to emissive, mononuclear complexes by replacing the bridging chlorides with monoanionic
2,4-pentanedio-nate (Fig. 2, inset). These mononuclear complexes were
characterised by1H NMR spectra and elemental
analy-ses, but13C NMR spectra were not recorded due to not
having the required level of solubility in deuterated sol-vents. Thermogravitometric analysis (TGA) shows the
stability of these complexes in the range of 245–375C
(Fig. 2) and they can be easily sublimed at low pressure
(103mmHg).
3.2. Crystal structures
X-ray single crystal structure analysis was carried out for one of the platinum complexes 3, which authenti-cates the square planar geometry (discrepancy factor 5%;Fig. 3(a)). X-ray quality single crystals of the com-plex were grown by slow diffusion of methanol into a dichloromethane solution of 3. The bond length of
Pt1–N1 (2.02 A˚ ) is slightly larger compared to that of
common Pt–N bonds, as this N is opposite to the acet-ylacetonate oxygen atom, having a weak trans influence.
The bond length of Pt1–O2 (2.085 A˚ ) is higher
com-pared to that of Pt1–O1 (1.994 A˚ ), as O2 is trans to car-bon which has a strong trans-influence. The car-bond length
of Pt1–C9 (1.967 A˚ ) is consistent with other reported
cyclometallated complexes [6d,16]. The Pt(1)–O(1)
(1.994 A˚ ) and Pt(1)–O(2) (2.085 A˚) bond lengths are
also consistent with other reported cyclometallated–
diketonato derivatives [17]. The C(9)–Pt(1)–O(1),
90.65(16); C(9)–Pt(1)–N(1), 80.48(17); O(1)–Pt(1)– N(1), 171.11(15); C(9)–Pt(1)–O(2), 176.82(15); O(1)–
Pt(1)–O(2), 91.23(13); N(1)–Pt(1)–O(2), 97.62(14)
bond angles are typical for cyclometallates and –diketo-nate derivatives of Pt[16,17]. There is very little distor-tion away from the square plane. The molecular packing
diagram of Pt(MeObt)(acac) (Fig. 3(b)) shows the
mol-ecules pack as head-to-tail dimers; each molecule of the dimer is related to the other by a center of inversion. The short inter plane and metal–metal distances (Pt1 Pt1 = 3.369; Pt2 Pt2 = 3.369; Plane–Plane = 3.360 A˚ ; 0 100 200 300 400 500 600 1.0 1.5 2.0 4.5 5.0 O O S N X Pt X = unsubstituted (1); F (2); OMe(3); CF3 (4) S X Pt N S X Pt N Cl Cl Dinuclear complex Mononuclear complex 4 3 1 Weight Loss Temperature (oC) TOdC 1 245 3 255 4 375
Fig. 2. TGA thermograms of the platinum(II) complexes 1, 3 and 4, Tdrepresents decomposition temperature (inset: the chemical struc-tures of the complexes investigated).
centre of symmetry is formed between the acac and the five membered ring) suggest that there exist extensive
p–p and metal–metal interactions [6d] which support
the excimer formation. 3.3. Photophysical behavior
The absorption spectra of all the complexes in dichlo-romethane were recorded at room temperature, and
show intense higher energy transitions 1p–p* (280–
350 nm; e 5 · 103dm3mol1cm1) as well as compar-atively lower energy, broad triplet state transitions (350– 475 nm; e 2 · 103dm3mol1cm1), as shown inFig. 4. These broad MLCT states exhibit solvatochromic ef-fects. The complexes emit orange light in the solid state as well as in solution. Highly structured PL emission and
sharp vibronic progressions of 2185 cm1 were
ob-served (Fig. 5). These observations helped us to identify the nature of the lowest-emitting states of both triplet p– p* and MLCT character. In general, the electron-donat-ing substituent raises the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) and the reverse is truth for an
elec-tron-withdrawing substituent [18,19]. The complex 4
which contains electron-withdrawing substituted
(–CF3) benzothiazolate shows a bathochromic shifted
emission, while those contain electron-releasing substi-tuted (–F (2), –MeO (3)) benzothiazolates undergo ipso-chromic shifted emission with respect to the
unsub-stituted complex, 1, as shown inFig. 5. Hence it can be
inferred that the position occupied by the substituents in (2-phenyl)-benzothiazolate of the irdium(III) complexes are dominated by LUMO character. The maximum absorbance and emission of all the complexes are listed in Table 1. The excimer formation was evidenced through the time resolved photoluminescence
measure-ments [20]. The emission kinetic trace for one of the
complexes 4 produced by excitation at 378 nm and mon-itored near the lower energy shoulder emission (e.g.,
656 nm;Fig. 5) shows monoexponential decay behavior.
The reciprocal of the lifetimes (known as emission decay
rate constant, kobs) obtained are shown in Fig. 6 as a
function of concentration. The lifetime was found to de-crease steadily with increasing concentration, indicative of excimer formation. The emission lifetime of the
monomer of complex 4 at infinite dilution, s0(0.58 ls),
and the apparent rate constant of self-quenching, kQ,
were determined by using Eq. (1) (0.58 ls;
7· 108M1s1) from the linear variation of the
ob-served emission decay constant, kobs, as a function of the
kobs ¼ 1=s0þ kQ½Pt [20] ð1Þ
concentration of the complex, [Pt]. The high value of kQ
indicates extensive excimer formation. It should also be true for the other platinum(II) complexes.
300 350 400 450 500 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 3 2 1 4 1 Absorbance (a.u.) Wavelength (nm) 4 Pt(CF 3bt)(acac) 1 Pt(bt)(acac) 2 Pt(Fbt)(acac) 3 Pt(MeObt)(acac)
Fig. 4. UV–Vis absorption spectra of the complexes 1, 2, 3 and 4 measured at 298 K showing the tuning of the lowest excited state.
400 500 600 700 800 3 2 4 1 PL Intensity (a.u.) Wavelength (nm) 10-3 M in DCM 1 Pt(bt)(acac) 2 Pt(F-bt)(acac) 3 Pt(MeO-bt)(acac) 4 Pt(CF3-bt)(acac)
Fig. 5. PL spectra showing tuning of emission wavelength with respect to the electronic properties of the substituents.
Table 1
Absorption and emission properties of the platinum(II) complexes 1–4 Complexes abs, kmax, nm (loge)
a Emission at 298 K, kmax, nm a 1 228 (6.0); 254 (5.9); 262 (5.7); 316 (5.8); 330 (5.7); 387 (5.4); 431 (5.0) 539; 580 2 228 (5.8); 252 (5.7); 313 (5.6)sh; 327 (5.6); 377 (5.3)sh; 416 (5.0) 531; 571 3 228 (6.2); 242 (6.1)sh; 279 (5.9); 320 (6.1); 334 (6.1); 373 (5.7); 402 (5.6)sh; 423 (5.5)sh 534; 575 4 228 (6.2); 259 (6.2); 318 (6.1); 333 (6.0)sh; 398 (5.7)b 551; 594 a
3.4. Electroluminescence
All these complexes are thermally stable up to 245–
375C and they can be easily sublimed at low pressure
(103mmHg) and emit tunable bright yellow-orange
light at room temperature. All these features make them good emitters in highly efficient EL devices. The plati-num complexes, 1 and 2 (the corresponding devices de-noted as D-1 and D-2, respectively) were doped into the host material, CBP (4,40-(N,N0-dicarbazole)biphenyl), in
the emitting zone with doping contents of 5%, 7% and 9%. The observed device performances were found to vary marginally throughout the doping concentration
range (Table 2). Current-density, voltage and luminance
characteristics (I–V–L) for devices D-1 and D-2 with 5% doping level are shown inFig. 7(a) and (b), respectively. Comparison of device performances for the devices D-1
and D-2 at 5% doping level are also summarized in
Ta-ble 3, which show the maximum luminance yields of 14.3
and 16.0 cd A1@2 mA cm2for D-1 and D-2,
respec-tively. The maximum emission wavelength ðkmaxem Þ for
D-2 shows apparent ipso-chromic shift with respect to
that of device D-1 (Fig. 8), as observed in PL emission
spectra. The maximum emission wavelengths in the EL spectra are found to be independent of the applied
volt-age (for current density up to 100 mA cm2), but show
bathochromically shifted emission with increasing dop-ing concentration. The EL intensity of the shoulder peak
0 100 200 300 400 500 0.20 0.22 0.24 0.26 0.28 0.30 k / 10 7 s -1 Concentration / 10-6 M
Fig. 6. Plot of the measured luminescence decay constants versus concentration of platinum complex 4.
Table 2
EL performance of D-1 and D-2 at different dopant concentrations [at 20 mA cm2current density in parentheses]
Device wt% Vd(V) Lmax(cd m2) CIE gext. max(%) gL, max(cd A1) gP, max(lm W1)
x y D-1 5 10.7 2573 0.48 0.50 4.9 12.9 3.8 7 11.2 2638 0.49 0.48 5.6 13.2 3.7 9 10.6 2197 0.52 0.47 5.3 11.0 3.3 D-2 5 11.2 2820 0.47 0.50 5.7 14.1 4.0 7 11.5 2769 0.48 0.58 6.9 13.8 3.8 9 11.7 2442 0.47 0.58 6.7 12.2 3.3 6 8 10 12 14 16 0.1 1 10 100 Voltage (V)
Current Density (mA/cm
2 ) 0 200 400 600 800 100 120 0 20 40 60 80 100 -2 0 2 4 6 8 10 12 14 16 18 Luminance Yield(cd A -1)
Current Density (mA cm-2)
D-1-5% D-2-5% 6 8 10 12 14 16 0.1 1 10 100 Voltage (V)
Current Density (mA/cm
2 ) 0 2000 4000 6000 8000 1000 1200 Luminance (cd/m 2 ) (a) (b)
Fig. 7. Current density, voltage and luminance characteristics for OLED using complexes 1 (a) and 2 (b) with 5% doping level (b: inset: plot of luminescence efficiency vs. current density).
(625–630 nm) has been observed to increase with increasing doping concentration, whereas that of the other two major peaks was found to decrease (540
and 584 nm). These observations support that the
emission from the shoulder peak is generated from the excimer and it is also consistent with the results obtained
from the molecular packing diagram (Fig. 3(b)) and
concentration dependent luminescence decay measure-ments discussed before. It is to be noted that at
100 mA cm2current density, the luminances are found
to be 10 550 and 11 320 cd m2, obtained for D-1 and
D-2, respectively, and under similar conditions there have not been significant changes of luminance yields (i.e., 10.6 and 11.3 cd A1, for D-1 and D-2, respectively).
4. Conclusion
The syntheses and characterizations of a series of platinum(II) complex dopants have been performed. The EL performances of OLEDs employing these plati-num(II) dopants are found to be substantially superior to those previously reported for OLEDs based on
cyclo-metallated platinum(II) complexes[6]. These EL results
were even far better than those previously reported[21]
where a similar type of cyclometallated iridium(III)
do-pants were used in the same device structure. It should be mentioned that the device containing fluoro-substi-tuted dopant shows relatively better EL performances with respect to that using the unsubstituted ben-zothiazolato Pt(II) complex.
Acknowledgements
The work has been supported in part by the Program for Promoting University Academic Excellence from Ministry of Education, Taiwan, ROC under the Con-tract No. 91-E-FA04-2-4 (B).
Appendix A. Supplementary data
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 247375 for compound Pt(MeO-bt)(acac). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44
1223 336033; email: [email protected] or http://
www.ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version at
doi:10.1016/j.poly.2005.02.009.
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Comparison of EL performances for D-1 and D-2 with 5% doping level operated at different current densities
Current density (mA cm2) V (V) L (cd m2) CIE gext(%) gL(cd A1) gP(lm W1)
x y x y D-1 D-2 D-1 D-2 D-1 D-2 D-1 D-2 D-1 D-2 D-1 D-2 0.5 6.7 7.1 70 77 0.40 0.50 0.49 0.50 5.4 6.5 13.9 15.3 6.5 6.5 2.0 7.9 8.4 286 319 0.49 0.50 0.48 0.51 5.5 6.6 14.3 16.0 5.7 6.6 6.0 9.0 9.6 834 924 0.49 0.50 0.48 0.51 5.3 6.3 13.9 14.8 4.8 6.3 20.0 10.7 11.2 2573 2820 0.48 0.50 0.48 0.51 4.9 5.7 12.9 14.1 3.8 5.7 40.0 12.1 12.5 4787 5197 0.48 0.50 0.48 0.51 4.6 5.3 12.0 13.0 3.1 5.3 100.0 14.5 14.7 10 550 11 320 0.48 0.50 0.47 0.51 4.0 4.6 10.6 11.3 2.3 4.6 300 400 500 600 700 800 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Glass ITO CHF3 NPB (500Å) CBP+ dopant (200 Å) BCP (100 Å) Alq3(650 Å) LiF(10 Å) / Al (2000 Å) EL Intensity (a.u.) Wavelength (nm) D- 1 D- 2
Fig. 8. EL spectra for the platinum(II) complexes 1 and 2 (inset: multilayer EL device configuration).
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