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VIVID-RED AND HIGHLY EFFICIENT PHOSPHORS BEARINGDIPHENYLQUINOXALINE UNITS AND THEIR APPLICATION TOORGANIC LIGHT-EMITTING DEVICES

Hirovuki FUJIItt, Hidehiro SAKURAIt, Kazuyasu TANIt, Lisheng MAOt,Kenichiro WAKISAKAtt, and Toshikazu HIRAOtt) Handai Frontier Research Center and Department of Applied Chemistry,Graduate School of Engineering, Osaka Universitytt Material and Devices Development Center, SANYO Electric Co., Ltd.

The characterization of the iridium compounds bearing 2,3-diphenylquinoxalineunits shown in Fig. 1, is presented here to afford vivid-red emitting phosphors withexceedingly high quantum yields and enhanced radiative transition performance.

Table I summarizes photophysical properties of the bis(2,3-diphenyl-quinoxalinato)iridium(acetylacetonate) derivatives, [Q(R)21r(acac)]. The absorption ataround 480 nm clearly suggests substantial allowance for triplet metal-to-ligandcharge-transfer (3MLCT) transition due to strong spin-orbit coupling on iridium atom.Therefore, their 3MLCT excited states are considered to be dominantly the lowestexcited ones. All four derivatives exhibited saturated red luminescence. The emissionpeak wavelengths ,em for three derivatives (R=H, Me, MeO) were present at around670 nm in solutions and neat films. In contrast, the fluorinated derivative,[Q(F)21r(acac)] solely showed a substantial blue shift of Xem to 647 nm in solution.Only slight spectral shift (I or 6 nm) was observed between neat film and solution ofeach derivative. The most important feature of the Q(R)21r(acac) derivatives is thattheir efficient photoluminescence (PL) quantum yields cDPL are greater than 50% inCH2Cl2 solutions. Substitution with methyl, methoxy, or fluoro group on the benzenering greatly enhanced (IDPL. The methyl-substituted derivative, [Q(Me)21r(acac)]exhibited the more efficient PL quantum yield of 79% and preferable emission peakwavelength at around 670 nm.

The EL (electroluminescence) spectra for tris(2,3-diphenylquinoxalinato)iridium(Q3Ir) or (btp)21r(acac) doped device are shown in Fig. 2. The EL of the Q3Ir deviceoriginates from the 3MLCT excited state because it is almost coincident with PLspectra of Q3Ir itself and the non-substituted Q(H)21r(acac) derivative in solution.Another reason may be explained by the single broad emission of Q3Ir, suggesting the3MLCT excited state as the lowest excited one. On the contrary, the EL of(btp)21r(acac) device was reported to originate dominantly from the 3%-7r* excited statesas observed in the vibronic peaks. For the purpose of vivid-red luminescence, the ELpeak wavelength XEL of 675 nm for Q3Ir is clearly superior to the one of 616 nm for(btp)21r(acac). To the best of our knowledge, Q3Ir possesses the most vivid-red ELwith the 1931 CIE chromaticity coordinates of (0.70,0.28).

Figure 3 illustrates transient PL decay curves for Q3Ir (left) and (btp)21r(acac)(right) in CBP coevaporated films at room temperature. Although the PL decaylifetime TPL for (btp)21r(acac) is 5.3 [tsec, the CPL for Q3Ir is greatly innovated to 1.1,tsec. Radiative transition performance of the lowest excited state for Q3Ir is 5 timesmore efficient than that for (btp)21r(acac) in solid state. Such a short PL decay lifetimeminimizes quenching effect due to triplet-triplet annihilation. Therefore, the series of2,3-diphenylquinoxalinato-iridium phosphors are proposed to realize an efficient,bright and vivid-red emission.

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Q(R)21r(acac)R = H, Me, MeO, F

N N

Q3Ir

Fig. I Molecular structures of (a) Q(R)21r(acac) and (b) Q31r.

Table 1 Photophysical properties of Q(R)21r(acac) derivatives for CH2CI2 solutions and evaporated neat films.

CH2CI2 solution a Neat film

R Abs./nm Xem/nm DPL Abs./nm Xem/1l1n

F 471 647 0.71 473 653MeO 477 659 0.67 481 664Me 477 669 0.79 481 670H 488 670 0.50 483 671

a 1.0 x 10-5 mol/l, room temperature.

C6*1-0.5

': 0.5

O0

380 480 580 680 780Wavelength (nm)

Fig. 2 Electroluminescence spectra of (btp)21r(acac) (square) and Q3Ir (circle) OLEDs at room temperature.

1

0.1 (btp)21r(acac)

0.010 10 20 30

Time (ps)

Fig. 3 Transient PL decay curves for Q3Ir and (btp)21r(acac) in CBP coevaporated films at room temperature.

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I(0