New Organic Light-Emitting Materials: Synthesis, Thermal

J. Phys. Chem. C 2007, 111, 1029-1034

1029

New Organic Light-Emitting Materials: Synthesis, Thermal, Photophysical, Electrochemical, and Electroluminescent Properties Shiyan Chen,†,‡ Xinjun Xu,† Yunqi Liu,*,† Wenfeng Qiu,† Gui Yu,† Huaping Wang,‡ and Daoben Zhu*,† Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China, and State Key Laboratory for Modification of Chemical Fiber and Polymer Materials, School of Materials Science and Engineering, Donghua UniVersity, Shanghai 200051, P. R. China ReceiVed: September 7, 2006; In Final Form: October 27, 2007

A new series of organic-light-emitting materials, 6,7-dimethyl-2,3-di-(4′-diphenylamino-biphenyl-4-yl)quinoxaline (MAPQ), 6,7-dimethyl-2,3-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyl]-quinoxaline (MFPQ), 2,3dicyano-5,6-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyl]-pyrazine (CFPP), and 6,7-dicyano-2,3-di-[4-(9,9dibutyl-9H-fluoren-2-yl)-phenyl]-quinoxaline (CFPQ), have been synthesized in high yields and fully characterized. These compounds have high thermal stability and show bright-light-emission varying from blue to green owing to the different strengths of the donor and acceptor. Moreover, good reversible oxidation or reduction waves were observed except for compound MFPQ due to the potential limitation of the solvent we used, which suggests these compounds have potential applications for hole/electron transportation. Organic light-emitting diodes were fabricated in a facile nondoped configuration based on these materials. Compared to MFPQ, CFPP, and CFPQ, the higher lying HOMO level of MAPQ facilitates more efficient hole injection/ transport and a higher charge-recombination rate; thus, the device based on MAPQ shows the highest luminous efficiency. For compounds CFPP and CFPQ, the LUMO levels are obviously decreased because of the incorporation of electron-accepting cyano group, so the devices based on these two compounds display better electron transportation/injection properties and better performances than those of MFPQ. These results demonstrate that high-performance light-emitting devices can be achieved from intramolecular charge-transfer emission.

Introduction π-Conjugated organic materials with a donor-acceptor (DA) system usually have unique optical and electrical properties, which might endow their potential applications in many fields such as light-emitting diodes (OLEDs),1 field-effect transistors (FETs),2 nonlinear optics (NLO),3 and photovoltaic devices.4 This kind of material applied in OLEDs would make the hole and electron efficiently recombined in the active layers, subsequently improving the performance of the devices. Moreover, the electronic structure (HOMO/LUMO level) can be manipulated by designing and synthesizing different donor and acceptor, accordingly tuning the electroluminescent properties of the materials and improving the ability of hole/electron injection/transportation. A few reports on this kind of material applied in OLEDs have been published in recent years.4a,5 Triarylamine derivatives have been intensively investigated as electron donors in hole transport or emissive materials in OLEDs because the triarylamine group exhibits good reversible redox potential and has low ionization potential. In addition, its nonplanarity and bulky structure characters can effectively diminish the intermolecular π-stacking and thus reduce the concentration quenching in the solid state. It has been demonstrated to enhance the amorphous properties and thus the solidstate fluorescence intensity.6 As an important class of π-conjugated organic materials, oligofluorenes or polyfluorenes have

been extensively explored as optoelectronic molecular materials because of their excellent chemical and thermal stabilities, high fluorescence quantum yields in the solid state, acceptable hole-transporting properties, and the ease of structural modification to adjust their electronic properties.7 Although these compounds display excellent hole transport/injection properties for application as OLED materials, they were far from approving due to the imbalance of electron-hole recombination in the active layer, which diminishes the quantum efficiency of the OLED device. Quinoxaline or pyrazine derivatives applied in OLEDs as electron-transporting materials have been explored in many reports due to their high thermal stability, outstanding mechanical properties, and good film-forming ability.1d,8 Herein, we reported a new series of organic light-emitting materials used in nondopant OLEDs whose structures are shown in Scheme 1. For the molecule design, we incorporated the donor and acceptor moieties, which endow them with efficiency for electron/hole injection/transport and recombination in OLEDs. In addition, the emission color can be tuned by adjusting the donor and acceptor ability in these molecules. These new compounds can be prepared by a facile process in high yields. We investigated their thermal, photophysical, electrochemical, and electroluminescent properties. The best device performances for MAPQ were achieved with a maximum luminance of 1818 cd/m2 and a luminance efficiency of 2.97 cd/A.

* Authors to whom correspondence should be addressed. E-mail: [email protected] (Y.Q. Liu). † Chinese Academy of Sciences. ‡ Donghua University.

Experimental Section Materials. N,N′-dicarbazolyl-4,4′-biphenyl (CBP) and tetrabutylammonium hexafluorophosphate (Bu4NPF6) were pur-

10.1021/jp065822i CCC: $37.00 © 2007 American Chemical Society Published on Web 12/10/2006

1030 J. Phys. Chem. C, Vol. 111, No. 2, 2007 SCHEME 1: Chemical Structures of the Compounds

chased from Acros and Aldrich, respectively. 4,4′-Di-(4′-diphenylamino)-phenylbenzil (APB),1j tris(8-hydroxyquinolinolato)aluminum (Alq3),9 N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB),10 and tetra-naphthalen-2-yl-silane (TNS)11 were prepared by published methods. Alq3, NPB, CBP, and TNS were purified by sublimation prior to use. Tetrahydrofuran (THF) was purified by distillation over Na/benzophenone and dichloromethane (CH2Cl2) was distilled over CaH2. All reactions were carried out under a nitrogen atmosphere. Instrumentations. 1H NMR and 13C NMR spectra were recorded on a Bruker DMX 400 NMR spectrometer. Electronic absorption and fluorescence spectra were obtained on an Hitachi U-3010 and Hitachi F-4500 spectrometer, respectively. MS spectra (MALDI-TOF-MS) were determined on a Bruker BIFLEX III mass spectrometer. Elemental analyses were carried out on a Carlo-Erba 1160 elemental analyzer. Thermogravimetric analyses (TGA) were carried out using a Perkin-Elmer thermogravimeter (Model TGA7) under a dry nitrogen gas flow at a heating rate of 10 °C/min. Glass transition temperatures (Tg) were determined by differential scanning calorimetry (DSC) at a heating rate of 10 °C/min using a Perkin-Elmer differential scanning calorimeter (DSC7). 6,7-Dimethyl-2,3-di-(4′-diphenylamino-biphenyl-4-yl)-quinoxaline (MAPQ). A suspension of 4,4′-di-(4′-diphenylamino)phenylbenzil (140 mg, 0.2 mmol) and 4,5-dimethyl-benzene1,2-diamine (33 mg, 0.24 mmol) in acetic acid (AcOH) (10 mL) was heated to reflux for 6 h, during which time a yellow precipitate formed. After filtration, the resulting solid was purified by recrystallization using a mixture of CH2Cl2/ petroleum ether, giving the product 150 mg as a pale yellow solid in 94% yield. MALDI-TOF-MS: m/z 796.9 (M+). 1H NMR (CDCl3, 400 MHz): δ 7.94 (s, 2H), 7.61 (d, 4H), 7.56 (d, 4H), 7.51 (d, 4H), 7.25-7.29 (m, 8H), 7.13 (d, 12H), 7.04 (t, 4H), 2.53 (s, 6H). 13C NMR (CDCl3, 100 MHz): δ 152.2, 147.6, 147.5, 140.6, 140.5, 140.3, 137.9, 134.2, 130.3, 129.3, 128.2, 127.7, 126.3, 124.5, 123.8, 123.0, 20.4. Elemental analysis (%). Calcd for C58H44N4: C, 87.41; H, 5.56; N, 7.03. Found: C, 87.14; H, 5.69; N, 7.18. 4,4′-Di-(9,9-dibutyl-9H-fluoren-2-yl)benzyl (DFB). A mixture of 2-(9,9-dibutyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (2.42 g, 6.0 mmol), 4,4′-dibromobenzil

Chen et al. (920 mg, 2.5 mmol), trimethylbenzylammonium chloride (20 mg), and Pd(PPh3)4 catalyst in 60 mL of 1 M aqueous K2CO3 solution and toluene (v/v, 1:1) was heated to reflux for 24 h. After being cooled to room temperature, the mixture was extracted with 100 mL of diethyl ether three times. The combined organic phase was washed with brine, dried over anhydrous MgSO4, and concentrated. The residue was purified by column chromatography using a mixture of dichloromethane/ petroleum ether (1:1) as the eluent, affording the product as a yellow solid (1.72 g). Yield: 90%. 1H NMR (CDCl3, 400 MHz): δ ) 8.12 (d, 4H), 7.83 (d, 4H), 7.80 (d, 2H), 7.75 (d, 2H), 7.64 (d, 2H), 7.60 (s, 4H), 7.34-7.38 (m, 6H), 2.00-2.04 (m, 8H), 1.05-1.12 (m, 8H), 0.60-0.70 (m, 10H). 6,7-Dimethyl-2,3-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyl]-quinoxaline (MFPQ). A suspension of 4,4′-di-(9,9-dibutyl9H-fluoren-2-yl)benzil (150 mg, 0.2 mmol) and 4,5-dimethylbenzene-1,2-diamine (32 mg, 0.2 mmol) in acetic acid (AcOH) (10 mL) was heated to reflux for 6 h, during which time a pale yellow precipitate formed. After filtration, the resulting solid was purified by recrystallization using a mixture of CH2Cl2/ petroleum ether giving the product 158 mg as a pale yellow solid in 92% yield. MALDI-TOF-MS: m/z 863.8 (M+). 1H NMR (CDCl3, 300 MHz): δ 7.95 (s, 2H), 7.68-7.78 (m, 12H), 7.59-7.64 (m, 4H), 7.28-7.36 (m, 6H), 2.54 (s, 6H), 1.972.03 (m, 8H), 1.02-1.34 (m, 8H), 0.67 (t, 20H). 13C NMR (CDCl3, 75 MHz): δ 150.8, 150.3, 149.8, 140.5, 139.6, 139.4, 139.3, 139.1, 138.0, 137.1, 129.2, 127.0, 125.9, 125.8, 125.6, 124.7, 121.7, 120.1, 118.8, 118.6, 53.9, 39.0, 24.8, 21.9, 19.3, 12.6. Elemental analysis (%). Calcd for C64H66N2: C, 89.05; H, 7.71; N, 3.25. Found: C, 88.72; H, 7.74; N, 3.19. 2,3-Dicyano-5,6-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyl]-pyrazine (CFPP). The synthesis was carried out as described for MFPQ. Yield: 95%. MALDI-TOF-MS: m/z 836.8 (M+ + 2). 1H NMR (CDCl3, 300 MHz): δ 7.72-7.79 (m, 12H), 7.597.64 (m, 4H), 7.30-7.35 (m, 6H), 1.97-2.03 (m, 8H), 1.011.13 (m, 8H), 0.66 (t, 20H). 13C NMR (CDCl3, 75 MHz): δ 153.6, 150.5, 149.8, 143.2, 140.4, 139.1, 136.9, 132.7, 129.2, 128.2, 126.4, 126.3, 125.7, 124.8, 121.8, 120.1, 118.9, 118.7, 112.1, 53.9, 39.0, 24.8, 21.8, 12.6. Elemental analysis (%). Calcd for C60H58N4: C, 86.29; H, 7.00; N, 6.71. Found: C, 86.02; H, 7.10; N, 6.57. 6,7-Dicyano-2,3-di-[4-(9,9-dibutyl-9H-fluoren-2-yl)-phenyl]-quinoxaline (CFPQ). The synthesis was carried out as described for MFPQ. Yield: 88%. MALDI-TOF-MS: m/z 885.9 (M+ + 1). 1H NMR (CDCl3, 300 MHz): δ 8.64 (s, 2H), 7.72-7.81 (m, 12H), 7.61-7.66 (m, 4H), 7.30-7.35 (m, 6H), 1.98-2.04 (m, 8H), 1.02-1.14 (m, 8H), 0.67 (t, 20H). 13C NMR (CDCl3, 75 MHz): δ 155.8, 150.4, 149.8, 142.4, 140.4, 140.2, 139.2, 137.2, 135.6, 134.8, 129.3, 126.2, 126.1, 125.7, 124.8, 121.8, 120.1, 118.9, 118.7, 113.9, 112.8, 53.9, 39.0, 24.8, 21.9, 12.6; Elemental analysis (%). Calcd for C64H60N4: C, 86.84; H, 6.83; N, 6.33. Found: C, 86.38; H, 6.88; N, 6.37. Cyclic Voltammetry (CV) Measurements. Cyclic voltammetric measurements were carried out in a conventional threeelectrode cell using a Pt button working electrode of 2 mm in diameter, a platinum wire counter electrode, and a Ag/AgCl reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Reduction CV of MAPQ was performed in THF containing Bu4NPF6 (0.1 M) as supporting electrolyte, in which the internal standard, ferrocene/ferrocenium ion (Fc/Fc+) redox couple, shows E1/2 ) 0.59 V. Oxidation CV of MAPQ and reduction CV of CFPP and CFPQ were performed in CH2Cl2, in which Fc/Fc+ redox couple gives E1/2 ) 0.45 V. The platinum button electrode was

Properties of Organic Light-Emitting Materials SCHEME 2: Synthetic Route to the Compoundsa

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1031 TABLE 1: Thermal and Electrochemical Data of the Compoundsa compd Tg (°C) Td (°C) E1/2re (V) E1/2ox (V) EHOMO (eV) ELUMO (eV) MAPQ MFPQ CFPP CFPQ

133 116 113 128

490 422 419 430

-1.72 -1.05 -1.00

0.97

-5.32

-2.57

-6.20 -6.11

-3.30 -3.35

a E1/2 ) 1/2(Epa + Epc), where Epa and Epc are the anodic and cathodic peak potentials.

a (i) 4,5-Dimethyl-benzene-1,2-diamine, AcOH, reflux, 6 h. (ii) KOtBu, n-BuBr, THF, rt, 2 h. (iii) (a) n-BuLi, THF, -78 °C, 1h, (b) 4,4,5,5tetramethyl-[1,3,2]dioxaborolane, -78 °C to rt, 1h. (iv) Pd(PPh3)4, toluene/1 M K2CO3 aq. (v/v, 1:1), reflux, 24 h. (v) 2,3-Diamino-but2-enedinitrile, AcOH, reflux, 6 h. (vi) 1,2-Diamino-4,5-dicyanobenzene, AcOH, reflux, 6 h.

polished with a 0.05 µm alumina paste before each experiment, and N2 bubbling was used to remove oxygen from the electrolyte solutions in the electrochemical cell. LEDs Fabrication and Measurements. The substrates were indium tin oxide (ITO) coated glass with a sheet resistance of 20 Ω/square. The ITO-coated glass substrates were etched, patterned, and washed with detergent, deionized water, acetone, and ethanol in turn. The multilayered devices ITO/NPB(20 nm)/ CBP(20 nm)/MAPQ, MFPQ, CFPP, CFPQ(20 nm)/TNS(20 nm)/Alq3(10 nm)/LiF(1 nm)/Al(100 nm) were fabricated, where NPB and TNS works as the hole-transporting and the holeblocking layer, respectively. CBP is used to decrease the holeinjection barrier especially for the materials MFPQ, CFPP, and CFPQ (the HOMO level is very high) and it is important to improve the devices’ efficiency. The materials were used as the emission layers and all organic layers were successively deposited onto the ITO/glass substrates at a pressure of 3 × 10-4 Pa. The active area of devices was about 5 mm2. All device tests were carried out under an ambient atmosphere at room temperature. EL spectra of LEDs were recorded on an Hitachi F-4500 spectrophotometer. Current-voltage characteristics for the LEDs were measured with a pA meter/dc voltage source (HP4140B).

[1,3,2]dioxaborolane. Suzuki coupling between 2-(9,9-dibutyl9H-fluoren-2-yl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane and 4,4′-dibromobenzil using Pd(PPh3)4 as a catalyst afforded the precursor DFB. After purification by silica gel column chromatography, DFB was obtained as a pale yellow solid in a good yield of 90%. All compounds were synthesized by a simple route in high yields and the approach of purification is easy. These advantages make them available in a mass batch. These compounds exhibit excellent solubility in common organic solvents such as CH2Cl2, toluene, and THF. Thus, they were fully characterized with MALDI-TOF mass spectroscopy, elemental analyses, and NMR spectroscopy, especially 13C NMR, and found to be in good agreement with the structures. The thermal properties of these new compounds were determined by DSC and TGA measurements. All compounds exhibit relatively high glass transition temperatures (Tg) above 113 °C and high thermal decomposition temperature above 419 °C. The data are listed in Table 1. Comparing the data in Table 1, we can find that the thermal stability of compound MAPQ is much better than those of MFPQ, CFPP, and CFPQ. Such an outcome may be attributed to the presence of four flexible n-butyl substituents in the latter three compounds. This is consistent with the previous observation.12 In addition, facile thermal sublimation of the molecules allowed the deposition of uniform thin films by vacuum evaporation for the fabrication of OLEDs. Photophysical Properties. The photophysical properties of the molecules were examined by UV-vis and fluorescence spectroscopy in CH2Cl2 solution (Figure 1). They exhibit two main absorption bands: one absorption band at about λmax ) 300 nm can be assigned to π-π* transition and the other absorption band at about λmax ) 400 nm can be assigned to charge-transfer (CT) transition. Moreover, the absorption maxima of the compounds undergo a red shift with different donor and acceptor which could be explained by the fact that the groups have different electron-donating and electron-accepting abilities. Similarly, the phenomenon is also observed in the emission band, for example, the emission bands for MAPQ, MFPQ, CFPP, and CFPQ at about 470, 437, 532, and 555 nm in CH2Cl2, respectively (Figure 2).

Results and Discussion Synthesis and Characterization of the Materials and Thermal Properties. The compounds were synthesized following the synthetic route outlined in Scheme 2. MAPQ can be prepared by reaction of APB1j with 1,2-diamino-4,5methylbenzene in a high yield of 94%. MFPQ, CFPP, and CFPQ were prepared by reaction of 4,4′-di-(9,9-dibutyl-9H-)benzyl (DFB) with different diamine, respectively. DFB was prepared by the following route: 2-Bromofluorene reacted with n-butylbromide under potassium tert-butoxide to give 9,9dibutyl-2-bromofluorene, and then 9,9-dibutyl-2-bromofluorene was converted into 2-(9,9-dibutyl-9H-fluoren-2-yl)-4,4,5,5tetramethyl-[1,3,2]dioxaborolane in a high yield by lithiumbromide exchange, followed by reaction with 4,4,5,5-tetramethyl-

Figure 1. Normalized electronic absorption in CH2Cl2.

1032 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Chen et al.

Figure 2. PL emission of compounds (a, MAPQ; b, MFPQ; c, CFPP; d, CFPQ) in solvents of varying polarity.

In solvents with different polarity (hexane, toluene, THF, CH2Cl2, and MeCN), weak solvatochromism was observed in the charge-transfer absorption band, so they are not shown here. A more remarkable positive solvatochromism and band broadening with the increase of the solvent polarity were observed in the photoluminescence (PL) emission as is shown in Figure 2. In the representative MAPQ, both the emission maximum (λmax em) and the full-width at half-maximum (fwhm) increase with solvent polarity, varying from (λmax em) 423 nm (fwhm ) 48 nm) in nonpolar hexane to 552 nm (fwhm ) 112 nm) in polar CH2Cl2. Similar large intramolecular charge-transfer (ICT) effects were observed in the PL emission of MFPQ, CFPP, and CFPQ (Figure 2b-d). Moreover, the PL intensity gradually decreases with the increase of the polarity of the solvent. The result implied that the excited-state energy levels are more easily influenced than those in the electronic ground state1h,4a,13 and the excited state is highly polarized and stabilized by solvation with the polar solvent after photoexcition.14 In high polar MeCN, an unusual dual fluorescence in compound MAPQ with a blue emission band at 470 nm and a red emission at 621 nm was observed. The higher energy band is similar to the emission band in dilute toluene solution, so it can be assigned to the singlet emission of compound MAPQ. The lower energy emission band centered at 621 nm, which was possibly related to intermolecular excimers.4a,13 Similarly, in the emission spectra of CFPP and CFPQ, shoulder emission bands attributed to the singlet emission and the lower emission bands assigned to intermolecular excimers were also observed. Electrochemical Properties. All newly synthesized compounds were characterized by cyclic voltammograms as shown in Figure 3, and the data are listed in Table 1. For compound MAPQ, the anodic scan was performed in CH2Cl2. A good reversible oxidation wave was observed at lower potential (E1/2 ) 0.97 V), which might be attributed to the strong donor ability of the triphenylamine group. The estimated ionization potential (IP ) Eoxonset + 4.35 eV, HOMO levels) of MAPQ is about 5.25 eV. This value is lower than that of NPB (5.46 eV),15 which is one of the most widely used hole-transport materials. Thus,

Figure 3. Cyclic voltammograms of MAPQ, CFPP, and CFPQ (1 mM) in 0.1 M Bu4NPF6, scan rate 50 mV/s.

they would be beneficial for hole injection and transportation. The cathodic scan for compound MAPQ was performed in THF due to the potential limitation of CH2Cl2. A good reversible reduction potential at E1/2 ) -1.72 V was observed. Thus, the EA derived from the onset reduction potentials were about 2.55 eV (EA ) Eredonset + 4.21 eV). The observed reversible reductive process suggests that compound MAPQ also has a potential for electron-transporting properties. Compounds CFPP and CFPQ exhibit good reversible cathodic redox couples in CH2Cl2 observed at -1.05 and -1.00 V (E1/2), respectively. It can be explained that the cyano group with stronger accepting electron ability obviously reduced the reduction potential. So the EA (EA ) Eredonset + 4.35 eV) derived from the onset reduction potentials were about 3.38 and 3.42 eV for CFPP and CFPQ, respectively. The relatively high electron affinity and the observed reversible reductive process suggest that these

Properties of Organic Light-Emitting Materials

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1033

Figure 4. PL and EL spectra of thin films of the compounds.

TABLE 2: EL Data of the Devicesa

compd

turn-on voltage (V)

peak position (nm)

MAPQ MFPQ CFPP CFPQ

11.5 7.9 10.4 9.8

471 440 512 528

Lmax (cd/m2), voltage (V) 1818, 21.5 425, 21.5 3079, 23.0 3373, 22.5

ηc.max (cd/A)

ηext,max (%)

2.97 0.62 2.95 2.64

1.79 0.51 1.03 0.85

Lmax ) maximum luminance, ηc.max ) maximum current efficiency, ηext,max ) maximum external quantum efficiency. a

two compounds are in favor of electron injection and have a potential for electron-transporting properties.16,7c,7d Electroluminescent Properties. We explored the performance characteristics of the devices using these compounds as light-emitting layers and the data are listed in Table 2. Figure 4 shows the PL/EL spectra of thin films of the compounds. Compared with the PL spectra of the solid film, the EL spectra were almost unchanged except for a slight red shift of the emission maximum. Subsequently, it can be elucidated that the EL emission of the three type devices comes from the host material layer. The current-voltage-luminance (I-V-L) characteristics of the devices are shown in Figure 5. The best performance was achieved for the device using MAPQ as lightemitting layer with a maximum luminance of 1818 cd/m2 and a luminous efficiency of 2.97 cd/A. Relatively, the device performance with a MFPQ layer is much lower, and its maximum luminance is 425 cd/m2 and the maximum luminous efficiency is 0.62 cd/A. This result can be explained by the fact that the donor ability of the triphenylamine group is much stronger than that of the fluorenyl group, which leads to the increase of HOMO level and the decrease of the LUMO level for MAPQ compared with those of compound MFPQ verified by the cyclic votammograms. Therefore, the device with MAPQ possesses better ability of the hole/electron injection/recombination and better devices performance. The luminance efficiencies of the devices fabricated with compounds CFPP and CFPQ are 2.95 and 2.64 cd/A, respectively, which are a little bit lower than that of MAPQ, however, and are much higher than that of MFPQ. This can be attributed to the obvious decrease of the LUMO levels of compounds CFPP and CFPQ resulting from the introduction of the cyano group, which will be more favorable for electron injection/transport compared to that of compound MFPQ. However, the HOMO level was not improved compared to that of compound MAPQ. Although the primary devices efficiencies are not as high as those of the best previously reported devices,17 it is really good among the nondoped blue OLEDs. With further studies into device optimization, these materials with proper D-A structure may

Figure 5. Current density and luminance versus applied electric field characteristics of the devices.

prove to be excellent light emitters for future OLED display applications. Conclusions In summary, we have developed a new series of organic lightemitting materials via an easy synthetic approach in high yields. Their photophysical, thermal, electrochemical, and electroluminescent properties were investigated. Larger positive solvatochromism was observed in the emission spectra than in the absorption spectra. From the observed reversible redox process, these materials possess good electron/hole injection/transport properties. The best device performance was achieved with MAPQ due to the relatively good combination of hole/electron for the appropriate acceptor and the donor accepting ability. These results demonstrated that changing the groups with different electron-donating and electron-accepting ability can adjust the recombination of hole/electron and the ability of hole/ electron injection, and in this way, the device performance can be optimized. Subsequently, except for the optimization of the

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