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Multifunctional Diarylamine-Substituted Benzo[k]fluoranthene Derivatives as Green Electroluminescent Emitters and Nonlinear Optical Materials Zhen-Yuan Xia,† Jian-Hua Su,*,† Hai-Hua Fan,‡ Kok-Wai Cheah,‡ He Tian,*,† and Chin H. Chen§ Key Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science & Technology, Shanghai 200237, People’s Republic of China, Centre for AdVanced Luminescence Materials, Department of Physics, Hong Kong Baptist UniVersity, Hong Kong, People’s Republic of China, and Displays & Lighting Center, National Engineering Lab of TFT-LCD Materials and Technologies, Shanghai Jiao Tong UniVersity, Shanghai 200240, People’s Republic of China ReceiVed: March 25, 2010; ReVised Manuscript ReceiVed: May 27, 2010
A new series of benzo[k]fluoranthene derivatives incorporating a diarylamine group have been synthesized and characterized. These compounds show high thermal stability with glass transition temperatures in the range of 162-205 °C. Two-layer electroluminescent devices employing them as the hole-transporting layer and light-emitting layer achieved efficient green emission under low driving voltage. A further optimized three-layer device based on one of these compounds, 3-(1-naphthylphenylamino)-7,12-diphenylbenzo[k]fluoranthene (PNDPBF), exhibited a maximum current efficiency of 10.2 cd/A (6.7 lm/W at 4.70 V). Nonlinear optical properties of these molecules were also investigated by measuring their two-photon excited fluorescence. Introduction Polycyclic aromatic hydrocarbons (PAH) with π-conjugated length have attracted intense attention in the area of optoelectronics due to their unique properties of high thermal stability and excellent fluorescent emission.1 To date, numerous PAHtype materials, such as anthracene, fluorene, pyrene, perylene, and fluoranthene, have been extensively investigated for their potential application as fluorescent sensors, organic nonlinear optical materials (NLO), and electroluminescent emitters (EL).2–8 However, these planar π-conjugated PAH molecules will induce strong intermolecular π-π stacking and then result in luminance concentration quenching and tendency of crystalline formation in the solid state.9 Therefore, it is critical to develop amorphous molecules with nonplanar structures and high quantum fluorescence yield. The noncoplanar molecular structures can be realized by the combination of some bulky functional groups with PAH fragments. Arylamine moieties or their analogues have been successfully employed to combine with PAH backbone to suppress the molecular packing as well as improve the hole injection and transport properties, with which the relative organic light-emitting devices (OLEDs),10,11 especially the green-color OLEDs,12–14 show enhanced efficiency and stability. For example, Yu et al. employed one of the diarylamine end-capped anthracene derivatives, 9,10-bis(1-naphthylphenylamino)anthracene (R-NPA), as host emitter and hole transporter for an EL device that gives green light with maximum current efficiency of 14.79 cd/A and power efficiency of 7.76 lm/W.12 Lee et al. reported a nondoped green OLED device using phenyl-9-[8-(7,10-diphenylfluoranthenyl)]phenylcarbazole (TDPFPC) as the emitter that displays a maximum current efficiency of 10.1 cd/A and a power efficiency of 12.1 lm/W.13 Wong et * To whom correspondence should be addressed. Fax: (+86) 2164252288. E-mail:
[email protected]. † East China University of Science & Technology. ‡ Hong Kong Baptist University. § Shanghai Jiao Tong University.
al.useddiphenylamino-terminatedindeno[1,2-b]fluorene(DPAInF) with another triaryldiamine as two-layer hole transport materials, and employed 9,9-diarylfluorene-terminated 2,1,3-benzothiadiazole (DFBTA) as a green emitter to achieve an EL efficiency of 12.9 cd/A and a maximum external quantum efficiency (EQE) of 3.7%.14 In this work, we incorporated benzo[k]fluoranthene, a representative member of the PAH family, with diarylamine moiety by palladium-catalyzed C-N coupling reactions and obtained a series of benzo[k] fluoranthene derivatives, namely, 3-diphenylamino-7,12-diphenylbenzo[k]fluoranthene (DPDPBF), 3-(1naphthylphenylamino)-7,12-diphenylbenzo[k]fluoranthene (PNDPBF), 3-diphenylamino-7,12-di(1-naphthyl)benzo[k]fluoranthene (DPDNBF), and 3-(1-naphthylphenylamino)-7,12-di(1-naphthyl)benzo[k] fluoranthene (PNDNBF). The unsymmetry inherent in these benzo[k]fluoranthenes can prevent close-packing of the molecules in the solid state and thus facilitate the formation of amorphous films. Two types of organic light-emitting devices based on these compounds as both green host emitters and hole transporters have been fabricated. In addition, the unsymmetrical structure of these materials with a benzo[k]fluoranthene π-conjugated bridge is favorable for two-photon response. Experimental Section Materials Synthesis. The synthetic routes for the benzo[k]fluoranthene derivatives are shown in Scheme 1. The precursor 3-bromo-7,12-diphenylbenzo[k]fluoranthene and 3-bromo-7,12di(1-naphthyl)benzo[k]fluoranthene were synthesized according to the literature procedures.15 The solvents in the reactions were dried by using routine procedures. All other reagents in the scheme were used as received from commercial sources. 3-Diphenylamino-7,12-diphenylbenzo[k]fluoranthene (DPDPBF). 3-Bromo-7,12-diphenylbenzo[k]fluoranthene (1.0 g, 2.1 mmol), diphenylamine (370 mg, 2 mmol), potassium tertbutoxide (300 mg, 2.7 mmol), tri-tert-butylphosphine (0.015 mL, 0.06 mmol), and palladium(II) acetate (6 mg, 0.027 mmol) were dissolved in dry toluene (20 mL) under argon atmosphere. The
10.1021/jp102725p 2010 American Chemical Society Published on Web 06/10/2010
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SCHEME 1: Synthesis of Compounds DPDPBF, PNDPBF, DPDNBF, and PNDNBF
reaction mixture was heated under reflux for 20 h. After being cooled, the organic layer was dried with anhydrous Na2SO4 and filtered. Then the solvent was evaporated under reduced pressure, and the residue was purified through a silica gel column (CH2Cl2:petroleum ether, 1:10) to give a yellow solid product DPDPBF with 75% yield. 1H NMR (400 MHz, CDCl3): δ 7.49-7.66 (m, 14H), 7.37-7.39 (m, 2H), 7.14-7.18 (m, 4H), 7.08-7.12 (t, 1H), 6.99-7.02 (m, 4H), 6.91-6.95 (t, 2H), 6.48-6.53 (m, 2H). 13C NMR (400 MHz, CDCl3): δ 122.09, 122.29, 122.84, 123.08, 123.56, 125.60, 125.77, 126.66, 126.80, 127.28, 127.56, 127.88, 129.08, 129.22, 129.92, 129.97, 132.63, 133.85, 134.31, 134.33, 134.89, 134.91, 136.90, 137.35, 138.79, 143.69, 148.72. MS-EI m/z calcd for C44H29N 571.2300, found [M+] 571.2302. 3-(1-Naphthylphenylamino)-7,12-diphenylbenzo[k]fluoranthene (PNDPBF). The synthetic procedure was similar to that of DPDPBF, producing a yellow solid product with 70% yield. 1 H NMR (400 MHz, CDCl3): δ 7.97-7.99 (d, 1H), 7.82-7.84 (d, 1H), 7.49-7.67 (m, 14H), 7.30-7.42 (m, 4H), 7.23-7.26 (d, 1H), 7.18-7.19 (d, 1H), 7.07-7.10 (t, 3H), 6.79-6.88 (m, 4H), 6.53-6.55 (d, 1H), 6.38-6.40 (d, 1H). 13C NMR (400 MHz, CDCl3): δ 121.25, 121.35, 122.29, 123.02, 123.68, 124.25, 125.02, 125.56, 125.77, 125.85, 125.86, 126.07, 126.11, 126.37, 126.65, 126.83, 127.50, 127.85, 127.97, 128.40, 128.97, 129.20, 129.26, 129.98, 130.01, 130.30, 132.61, 132.95, 133.16, 134.16, 134.42, 134.93, 135.19, 136.98, 137.40, 138.88, 144.79, 144.92, 150.48. EI-MS m/z calcd for C48H31N 621.2457, found [M+] 621.2462. 3-Diphenylamino-7,12-di(1-naphthyl)benzo[k]fluoranthene (DPDNBF). The synthetic procedure was similar to that of DPDPBF, producing a yellow solid product with 72% yield. 1 H NMR (400 MHz, CDCl3): δ 8.14-8.16 (d, 1H), 8.09-8.11 (d, 1H), 8.02-8.05 (t, 2H), 7.70-7.78 (m, 4H), 7.57-7.64 (m, 2H), 7.50-7.56 (m, 3H), 7.44-7.47 (m, 2H), 7.38-7.40 (d, 1H), 7.26-7.36 (m, 3H), 7.08-7.12 (t, 4H), 6.86-6.94 (m, 7H), 6.80-6.82 (d, 1H), 6.09-6.9415 (m, 2H). 13C NMR (400 MHz, CDCl3): δ 122.08, 122.29, 122.85, 123.11, 123.54, 125.85, 126.06, 126.19, 126.33, 126.50, 126.89, 127.02, 127.27, 127.42, 127.60, 127.78, 127.84, 128.36, 128.39, 128.44, 129.02, 132.48, 133.07, 133.16, 133.45, 133.50, 133.97, 135.50, 136.08, 136.39, 136.57, 137.33, 143.65, 148.64. EI-MS m/z calcd for C52H33N 671.2613, found [M+] 671.2614.
3-(1-Naphthylphenylamino)-7,12-di(1-naphthyl)benzo[k]fluoranthene (PNDNBF). The synthetic procedure was similar to that of DPDPBF, producing a yellow solid product with 70% yield. 1H NMR (400 MHz, CDCl3): δ 8.13-8.15 (d, 1H), 8.04-8.06 (t, 2H), 7.97-7.99 (d, 1H), 7.90-7.92 (d, 1H), 7.66-7.80 (m, 5H), 7.42-7.64 (m, 9H), 7.35-7.39 (t, 1H), 7.27-7.33 (m, 4H), 7.21-7.23 (d, 1H), 7.09-7.11 (d, 1H), 7.01-7.05 (t, 2H), 6.87-6.91 (t, 1H), 6.79-6.83 (t, 1H), 6.67-6.72 (m, 3H), 6.14-6.16 (d, 1H), 6.00-6.02 (d, 1H). 13C NMR (400 MHz, CDCl3): δ 121.15, 121.19, 122.26, 123.03, 123.64, 124.17, 125.04, 125.77, 125.84, 125.93, 126.02, 126.11, 126.15, 126.21, 126.34, 126.44, 126.51, 126.86, 127.03, 127.52, 127.84, 128.34, 128.46, 128.89, 130.27, 132.30, 132.56, 132.84, 133.11, 133.48, 133.96, 134.00, 135.11, 135.58, 136.10, 136.46, 136.63, 137.36, 144.69, 144.73, 150.37. MS-EI m/z calcd for C56H35N 721.2770, found [M+] 721.2769. Measurements. 1H and 13C NMR spectra were recorded on a Bru¨cker AM 400 spectrometer. Mass spectrometric measurements were recorded by a HP5989 mass spectrometer. UV-vis spectra were obtained on a Varian Cary 200 spectrophotometer. Fluorescence spectra were obtained on a Perkin-Elmer LS55 luminescence spectrometer. The differential scanning calorimetry (DSC) analysis was performed under a nitrogen atmosphere on a TA Instruments DSC 2920. Thermogravimetric analysis was undertaken using a TGA instrument. To measure the fluorescence quantum yields (ΦF), degassed solutions of the compounds in chloroform were prepared. The concentration was adjusted so that the absorbance of the solution would be lower than 0.1. The excitation was performed at 440 nm, and courmarin 120 in methanol (Φf ) 0.77) was used as a standard.16 Cyclic voltammetric (CV) measurements were carried out in a conventional three-electrode cell, using a Pt button working electrode 2 mm in diameter, a platinum wire counter electrode, and a SCE reference electrode on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room temperature. Reduction CV of all compounds was performed in dichloromethane containing Bu4NPF6 (0.1 M) as the supporting electrolyte. For the femtosecond-pulse experiment, a self-mode-locked Tisapphire laser (pulsed width 80-100 fs, repetition rate 82 MHz in quasi-continuous wave) was used as the excitation source to measure the two-photon cross sections. These respond signals
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TABLE 1: Photophysical, Electrochemical, and Thermal Properties of These Compounds compd DPDPBF PNDPBF DPDNBF PNDNBF
λmaxUV a(nm) 259, 259, 260, 258,
312, 313, 305, 306,
439 438 439 436
λmaxFL a,b(nm)
Φ fc
λmaxTPF a(nm)
σ750a,d(GM)
Eg(eV)
Tg, Td(°C)
HOMO/LUMO(eV)
520, 525 516, 521 526, 531 518, 522
0.25 0.24 0.24 0.22
518 516 518 516
11.3 11.7 10.1 12.2
2.58 2.61 2.58 2.61
162, 468 185, 489 191, 478 205, 514
5.26/2.68 5.27/2.66 5.28/2.70 5.32/2.71
a As measured in CHCl3. b Excited at 440 nm. c With courmarin 120 in methanol (Φf ) 0.77) as a standard.16 d Determined by the two-photon-induced fluorescence method, using 750 nm femtosecond laser pulses with rhodamine 6G as a standard (σ750 ) 25 GM).19
Figure 1. The absorption and PL spectra of compounds DPDPBF, PNDPBF, DPDNBF, and PNDNBF.
are collected by PMT and transferred to a Lock-in-Amplifier for recording the results by computer. Device Fabrication. Prior to the deposition of organic materials, indium-tin oxide (ITO)/glass was cleaned with a routine cleaning procedure and pretreated with oxygen plasma. Devices were fabricated under about 10-6 Torr base vacuum in a thin-film evaporation coater following a published protocol.17 The current-voltage-luminance characteristics were measured with a diode array rapid scan system, using a Photo Research PR650 spectrophotometer and a computer-controlled, programmable, direct-current (DC) source. All measurements were carried out in air at room temperature without encapsulation. Results and Discussion Scheme 1 illustrates the synthetic routes used to prepare these benzo[k]fluoranthene derivatives end-capped with diarylamine moieties. These compounds were easily prepared by a palladium-catalyzed coupling reaction. All of the target compounds were characterized by NMR spectroscopy and high-resolution mass spectrometry. Their thermal properties were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements (Table 1). These benzo[k]fluoranthene derivatives exhibit high thermal stabilities with decomposition temperatures (Td) over 460 °C. High glass transition temperature (Tg) in the range of 162-205 °C were observed for these four compounds and the Tg increases in the order DPDPBF < PNDPBF < DPDNBF < PNDNBF, which is consistent with the increasing order of their molecular weight. Additionally, the substituent moiety change from phenyl group to naphthyl group may also enhance the molecule rigidity and then raise the glass transition temperature. The good Tg and Td values of these compounds suggest stable morphological properties, which is desirable for OLEDs with high stability.
Figure 2. TPE spectra of DPDPBF, PNDPBF, DPDNBF, and PNDNBF in chloroform. Inset: Logarithmic plots of the power dependence of relative two-photon induced fluorescence on pulse intensity, using a 750 nm femtosecond laser as an excitation source for PNDPBF (5 × 10-4 mol/L).
Figure 1 shows the absorption and fluorescence spectra of all compounds in dilute chloroform solution (Table 1). The lowest absorption maxima for these compounds all occur in the narrow region of 436-439 nm, and these vibronic bands are assigned to the π-π* transitions of the conjugated molecular backbone. Additionally, two strong absorption bands in the region from 250 to 350 nm are attributed to the combination of the π-π* transitions of the peripheral aryl groups and the n-π* transitions of the diarylamino groups. Upon irradiation at 440 nm, the PL spectra of these four compounds in solution were found to exhibit similar green emission with the peak maxima varying from 516 to 531 nm. The PL spectra of PNDPBF (λmax ) 516, 521 nm) and PNDNBF (λmax ) 518, 522 nm) exhibit a slight hypsochromic shift with respect to that of DPDPBF (λmax ) 520, 525 nm) and DPDNBF (λmax ) 526, 531 nm), respectively. This result could be explained by the pronounced steric crowding between the bulky 1-naphthylphenylamino group and benzo[k]fluoranthene backbone in both PNDPBF and PNDNBF, which lead to less coplanarity and less π-electron conjugation of the molecules compared to that of DPDPBF and DPDNBF with the diphenylamino group. The quantum yields of these compounds in chloroform solution have been measured and summarized in Table 1. The two-photon absorption (TPA) cross sections (d) of the diarylamino-substituted benzo[k]fluoranthenes were determined by the two-photon induced fluorescence method with use of a femtosecond (fs) pulsed laser as an excitation source in chloroform solution.18 A commercial TPA chromophore, rhodamine 6G (with a TPA cross section of 25 GM),19 was used as the reference. The results of two-photon absorption cross sections measured at 750 nm are shown in Table 1. As shown in Figure 2, the two-photon excited fluorescence (TPF) spectra
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SCHEME 2: Energy Level Diagram of the Two Types of Devices
were found to be similar to the one-photon emission spectra (SPF). It suggests that both TPF and SPF must relax quickly to the same fluorescence emission state, even if the primary excited states obtained by single-photon absorption and two-photon absorption may be different due to the dissimilar parity selection rules. The power-squared dependence of the up-converted fluorescence for PNDPBF was followed with the slope value of 2.2 (the inset of Figure 2), which gives the direct experimental evidence of a two-photon excitation process. Cyclic voltammetry (CV) analyses were carried out to identify the HOMO values of these materials. All of the compounds show a reversible one-electron oxidation event. Their HOMO values are all near 5.3 eV, which is much higher than that of the traditional fluoranthene derivatives.2,7,13,20 It is obvious that the electron-donating diarylamino groups greatly lower the oxidation potential and then elevate the HOMO energy levels. The LUMO values were calculated from the optical band gap (estimated from the absorption edge) and the HOMO values. The intensive green emission, good thermal stability, and relatively high HOMO levels of the four diarylamino-substituted benzo[k]fluoranthene derivatives imply that these compounds can be employed as potential green host materials with hole transport ability in OLED devices. To investigate the EL performance of these new compounds, two types of nondoped electroluminescent devices have been fabricated. Devices A-D were the two-layer devices with the same configuration of ITO/CFx/green emitter (80 nm)/1,3,5tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (20 nm)/LiF (1 nm)/Al, where CFx was the hole injection layer (HIL), TPBI was the electron transporting layer (ETL) as well as hole blocking layer (HBL), and various green dyes (DPDPBF, PNDPBF, DPDNBF, and PNDNBF) were used as the emitting layer (EML) as well as the hole transport layer (HTL) in devices A-D, respectively. For the purposes of comparison and optimization, an extra hole injection as well as electron blocking layer (EBL) was added in device E with the configuration of ITO/CFx/4,4′,4′′-tris[(N-(2-naphthyl)-N-phenylamino)]triphenylamine (2-TNATA) (40 nm)/PNDNBF (40 nm)/TPBI (20 nm)/ LiF (1 nm)/Al, where 2-TNATA was used as the HIL and EBL. A schematic energy level diagram for the two types of devices is shown in Scheme 2. Figure 3 depicts the normalized electroluminescence (EL) spectra of all five devices. The EL maxima of these devices were in the narrow range of 512-516 nm with the full-width at half-maximum (fwhm) of about 72 nm. The EL spectra of the devices were nearly identical to the PL spectra of these fluorescent dyes, indicating that all of the EL emissions came from the singlet-excited state of the green emitters. Moreover, there is almost no EL color shift of all devices with varying
drive current. For example, the CIEx,y color coordinates of device A only shifted from (0.277, 0.615) at 0.1 mA/cm2 to (0.281, 0.607) at 500 mA/cm2 with ∆CIEx,y ) (0.004, 0.008). Figure 4 shows the dependence of current density-voltageluminance (J-V-L) characteristics of the devices. The extremely low turn-on voltage (at a brightness of 1 cd/m2) ranging from 2.8 to 3.0 V was observed in two-layer devices A-D, while for device E with an additional 2-TNATA layer, the turn-on voltage was increased to 4.7 V. It was partially attributed to the better hole mobility in these diarylamino-substituted benzo[k]fluoranthenes compared to that of 2-TNATA, resulting in relatively lower driving voltage in devices A-D. Meanwhile, from the energy level diagram in Scheme 2, the higher LUMO energy level of 2-TNATA (2.2 eV) than these green dyes (2.66-2.71 eV) would induce more electron blocking, which also caused the higher turn-on voltage in device E.
Figure 3. EL spectra of devices A-E.
Figure 4. Current density-voltage-luminance characteristics of devices A-E.
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TABLE 2: EL Performance of Devices A-E device
green emitters
Vona (V)
V20b (V)
ηl,maxc (cd/A)
ηp,maxd (lm/W)
Lmaxe (cd/m2)
EQEmaxf (%)
λEL (fwhm)g (nm)
A B C D E
DPDPBF PNDPBF DPDNBF PNDNBF PNDPBF
2.8 2.8 2.9 3.0 4.7
4.04 4.04 4.54 4.76 6.92
4.1 5.0 4.5 2.1 10.2
4.7 5.7 4.8 1.8 6.7
10190 12370 7957 5703 17840
1.6 2.0 1.7 0.7 3.1
512 (72) 512 (72) 516 (76) 516 (72) 512 (72)
CIEx,y 0.28, 0.26, 0.30, 0.29, 0.28,
0.61 0.61 0.61 0.61 0.61
a Turn-on voltage at which emission starts to be detectable. b Voltage taken at a current density of 20 mA/cm2. c Maximum current efficiency. d Maximum power efficiency. e Maximum luminance. f Maximum external quantum efficiency. g Full-width at half-maximum.
to realize tailor-made chemical or physical properties by attaching multifunctional groups. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China, National Basic Research 973 Program, and Hong Kong Baptist University. References and Notes
Figure 5. EL efficiency-current density characteristics of devices A-E.
Detailed EL performances of all devices were summarized in Table 2. PNDPBF-based device B showed the highest efficiency among the four two-layer devices. The maximum current and power efficiencies of device B were 5.0 cd/A (EQE ) 2.0%) and 5.7 lm/W with a maximum luminance of 12370 cd/m2 at 10 V, respectively (see Figure 5). Based on the structure of device B, the device performance was further improved by inserting a HIL/EBL in device E. Although the driving voltage of device E was higher than that of the other devices, the maximum EL efficiencies of device E still arrived 10.2 cd/A (ηp ) 6.7 lm/W and EQE ) 3.1%) with a maximum luminance of 17840 cd/m2 at 13.9 V, whose current efficiency was even two times as high as that of device B. This phenomenon could be explained by the retarded hole-transporting ability of 2-TNATA in device E, in which more balanced hole/electron currents were realized in emitting layer. On the other hand, the high energy barrier of EBL/HBL efficiently confined these charge carriers in the narrow recombination zone and enhanced the device performance as well. Conclusion In summary, we have synthesized a homologous series of diarylamino-end-capped benzo[k]fluoranthene derivatives. All the benzo[k]fluoranthenes process low oxidation potential, good single-photon and two-photon excited fluorescence emission, as well as high thermal and amorphous morphological stabilities. Nondoped EL devices employing them as the hole-transporting layer and light-emitting layer achieved efficient green emission under low driving voltage. Our findings show that the benzo[k]fluoranthene molecule platform is an interesting building block
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