New Fluorene Derivatives for Blue Electroluminescent Devices

Jan 18, 2008 - and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,. Beijing 100080, People's Republi...
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J. Phys. Chem. C 2008, 112, 2165-2169

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New Fluorene Derivatives for Blue Electroluminescent Devices: Influence of Substituents on Thermal Properties, Photoluminescence, and Electroluminescence Zhaokuai Peng, Silu Tao, and Xiaohong Zhang* Nano-organic Photoelectronic Laboratory and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China

Jianxin Tang, Chun Sing Lee, and Shuit-Tong Lee* Center of Super-Diamond and AdVanced Films and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, People’s Republic of China ReceiVed: June 21, 2007; In Final Form: October 29, 2007

A new series of fluorene derivatives with different substituents at the C-2,7- and C-9 positions has been synthesized and characterized with respect to their luminescence and thermal properties. It was found that substitutions at the C-2,7 positions significantly affected the thermal properties and optical properties, whereas substitutions at the C-9 position mainly affected the thermal properties of the fluorene derivatives. Organic light-emitting diodes (OLEDs) with a structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (15 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) (50 nm)/fluorene derivative (30 nm)/tris(8quinolinolato)aluminum (Alq3) (50 nm)/Mg/Ag (200 nm) were fabricated. OLEDs based on the naphthylsubstituted fluorenes gave saturated blue emission centered at 452 nm, while those based on anthryl-substituted fluorenes showed a blue-green emission with a wider spectrum. OLEDs based on 2,7-dipyrenyl-9′9dimethylfluorene (DPF) exhibited blue emission with a maximum efficiency of 4.8 cd/A and CIE coordinates of x ) 0.17 and y ) 0.24. A similar DPF-based device with an additional hole block layer exhibited better blue emission with a maximum efficiency of 5.2 cd/A and CIE coordinates of x ) 0.15 and y ) 0.21, which are among the best values ever reported for blue OLEDs without a dopant based on fluorescence molecules. The results show that fluorenes with suitable substitutents can act as blue-emitting materials with a high thermal stability and high electroluminescence efficiency.

Introduction Since the seminal reports of Kodak’s team and Cambridge’s group on small-molecule and polymer-based organic lightemitting diodes (OLEDs), respectively,1,2 OLEDs have attracted a great deal of attention because of their practical applications in full-color flat-panel displays. During recent years, considerable progress has been made in this field.3-6 Complementary to the optimization of device structure, another key approach to produce marketable OLEDs is the development of highperformance materials with desirable properties. Indeed, many new materials with RGB (red, green, blue)-emitting colors have been developed to address the requirements of full-color displays.7-14 Although the three primary colors have been demonstrated in OLEDs, only red and green OLEDs have sufficient efficiencies and lifetimes to be of marketable value. To achieve full-color flat-panel displays, there is still a need for stable blue-emitting materials with a high photoluminescence efficiency and thermal stability. Among the various blue-emitting materials reported, fluorenebased polymers, oligomers, and small molecules have attracted wide interest in recent years as efficient blue emitters in OLEDs due to their high photoluminescence efficiency.15-29 However, blue OLEDs based on polyfluorenes usually suffer from poor * Corresponding authors. (X.Z.) Fax: +86-10-62554670; e-mail: [email protected]. (S.-T.L.) Fax: +852-27844696; e-mail: [email protected].

stability with a drastic loss in efficiency and the appearance of an undesirable green emission after a short time of operation.18 Conjugated oligomers based on fluorenes have also been reported.19-22 As compared to polymers, small-molecule fluorenes have a number of advantages, such as ease of purification and characterization, well-defined structures, and easily tunable luminescence and thermal properties. However, small-molecule fluorene derivatives used as practical blue emitters in OLEDs are still rare.23-29 In this paper, we report the synthesis of a new series of fluorene derivatives and their tunable luminescence and thermal properties. We found that substituents at the C-2,7 positions of fluorenes affect the thermal properties and optical properties, whereas those at the C-9 position mainly affect the thermal properties of the fluorene derivatives. Electroluminescent devices based on the fluorenes were fabricated. OLEDs based on 2,7dipyrenyl-9′9-dimethylfluorene (DPF) exhibited blue emission with a maximum efficiency of 4.8 cd/A and CIE coordinates of x ) 0.17 and y ) 0.24. Such a device optimized by adding a hole blocking layer exhibited better blue emission with a maximum efficiency of 5.2 cd/A and CIE coordinates of x ) 0.15 and y ) 0.21. The performance of the non-doped fluorenebased devices is among the best blue OLEDs based on fluorescent molecules. Significantly, by suitable adjustment of the substitutent group in fluorenes, blue-emitting materials with

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a high thermal stability and high electroluminescence efficiency can be achieved. Experimental Procedures Material Synthesis. All solvents were purified by routine procedures. Other reagents were used as received from commercial sources, unless otherwise stated. Suzuki Coupling Reaction. 2,7-Dibromofluorene derivatives (1,3,4) (0.5 mmol) and the corresponding aryl boronic acids (1.5 mmol), Pd(PPh3)4 (0.05 mmol), aqueous Na2CO3 (2.0 M, 1 mL), ethanol (2 mL), and toluene (4 mL) were mixed in a flask. The mixture was degassed, and the reaction was refluxed for 24 h under nitrogen. After the solution was cooled, the solvent was evaporated under vacuum, and the product was extracted with CH2Cl2. The CH2Cl2 solution was washed with water and dried with MgSO4. Evaporation of the solvent, followed by column chromatography on silica gel (petroleum ether/CH2Cl2) yielded the product. 2,7-Diphenyl-9,9′-dimethylfluorene (DPhF). mp 170172 °C; 1HNMR (CDCl3, 300 MHz): δ: 7.82 (d, J ) 7.9 Hz, 2H), 7.70-7.67 (m, 6H), 7.61 (d, J ) 7.9 Hz, 2H), 7.51-7.46 (m, 4H), 7.40-7.37 (m, 2H); 1.85 (s, 6H), MS m/z: 346 (M+); Anal. calcd for C27H16: C, 95.29; H, 4.71. Found: C, 95.20; H, 4.69. 2,7-Dinaphthyl-9,9′-dimethylfluorene (DNF). mp 256258 °C; 1H NMR (CDCl3, 300 MHz) δ: 8.1492 (s, 2H), 7.997.70 (m, 10H), 7.54-7.52 (m, 8H), 1.67 (s, 6H), MS (m/z): 446 (M+); Anal. calcd for C35H26: C, 94.17; H, 5.83. Found: C, 94.11; H, 5.82. 2,7-Dianthryl-9,9′-dimethylfluorene (DAF). mp 332333 °C; 1H NMR (CDCl3, 300 MHz) δ: 8.55 (s, 2H), 8.118.04 (m, 6H), 7.81 (d, J ) 8.7 Hz, 4H), 7.59 (s, 2H), 7.537.38 (m, 10H), 1.62 (s, 6H); MS (m/z): 546 (M+); Anal. calcd for C43H30: C, 94.50; H, 5.50. Found: C, 94.36; H, 5.48. 2,7-Dipyrenyl-9,9′-dimethylfluorene (DPF). mp 323325 °C; 1H NMR (CDCl3, 300 MHz) δ: 8.01-8.35 (m, 20H), 7.78 (s, 2H), 7.71 (d, J ) 7.6 Hz, 2H), 1.70 (s, 6H); MS (m/z): 594 (M+); Anal. calcd for C47H30: C, 94.95; H, 5.05%. Found: C, 94.90; H, 5.04%. 2,7-Dinaphthyl-9,9′-diphenylfluorene (DPhDNF). mp 278280 °C; 1H NMR (CDCl3, 300 MHz) δ: 8.03 (s, 2H), 7.957.84 (m, 10H), 7.80-7.65 (m, 6H), 7.56-7.45 (m, 4H),7.417.37 (m, 4H), 7.32-7.30 (m, 4H); MS (m/z): 570 (M+); Anal. calcd for C45H30: C, 94.74; H, 5.26. Found: C, 94.66; H, 5.24. 2,7-Dinaphthyl-9,9′-spirobifluorene (SDNF). mp 278279 °C; 1H NMR (CDCl3, 300 MHz) δ: 8.01 (d, J ) 7.9 Hz, 2H), 7.93 (d, 8.5, 2H), 7.90 (s, 2H), 7.81-7.78 (m, 8H), 7.60 (d, J ) 7.5.5 Hz, 1.4 Hz, 2H), 7.45-7.39 (m, 6H), 7.19-7.14 (t, 2H), 7.10 (d, J ) 1.1 Hz, 2H), 6.89 (d, J ) 7.5 Hz, 2H); MS (m/z): 568 (M+); Anal. calcd for C45H28: C, 95.07; H, 4.93. Found: C, 95.15; H, 4.92. Measurements and OLED Fabrication. The absorption and fluorescence spectra were recorded using a Hitachi U-3010 UV-vis spectrophotometer and a Hitachi F-4500 fluorescence spectrophotometer, respectively. The highest occupied molecular orbital (HOMO) values were measured directly by using ultraviolet photoelectron spectroscopy (UPS), while the lowest unoccupied molecular orbital (LUMO) values were determined from the HOMO and the lowest energy absorption edge of the UV absorption spectra. TGA measurements were performed on a TA Instrument TGA2050 with a heating rate of 20 °C/min. Differential scanning calorimetry (DSC) was performed on a TA Instrument DSC2910, the sample was first heated at a heating rate of 20 °C/min to melt and then was quenched with

Figure 1. Chemical structure of the fluorene derivatives.

liquid nitrogen, and the glass transition temperature was recorded by heating the quenched sample at a heating rate of 10 °C/min. TGA measurements were performed on a TA Instrument TGA2050 at a heating rate of 20 °C/min. Blue OLEDs were fabricated by vacuum deposition on ITO glass substrates with a sheet resistance of 50 Ω/°C. Before deposition, the ITO substrate was carefully cleaned with isopropyl alcohol and deionized water, dried in an oven at 120 °C for 1 h, and finally treated with UV-ozone. The ITO substrate was then loaded into a deposition chamber with a base pressure greater than 10-7 Torr. The devices were fabricated by evaporating organic layers onto the ITO substrate sequentially with an evaporation rate of 2-4 Å/s. The Mg/Ag alloy cathode was prepared by coevaporation of Mg and Ag at a volume ratio of 10:1. EL spectra and CIE color coordinates were measured with a spectrascan PR650 photometer, and the current-voltageluminescence characteristics were measured with a computercontrolled Keithley 2400 SourceMeter under ambient atmosphere. Results and Discussion The molecular structures of the new fluorene derivatives used in this study are illustrated in Figure 1. Scheme 1 outlines the synthetic routes for the preparation of these compounds. Aryl boronic acids and compounds 1-3 were synthesized as described in a literature procedure.27 All of the fluorene derivatives were synthesized according to the standard Suzuki coupling reaction with a moderate yield and characterized by 1H nuclear magnetic resonance (1H NMR), mass spectrometry, and elemental analysis.31 Thermal Properties. The thermal properties of the fluorene derivatives were determined by TGA and DSC, and the results are summarized in Table 1. DPhF bearing phenyl at the 2,7 position of fluorene has the lowest melting point of 171 °C, among all the fluorene derivatives. All the compounds except DPhF are thermally stable up to 350 °C in air as shown by their decomposition temperatures (Td) in Table 1. With the change of the substituents at the C-2,7 position in fluorene, the glass transition temperature (Tg) of the fluorene derivatives changes from 85 °C (DNF), to 164 °C (DAF), to 145 °C (DPF). Different substituents in fluorene change the rigidity and planarity of the molecule. This change should be responsible for the change of the thermal properties of these compounds. As shown in Table 1, the Tg of the derivatives DNF, DPhDNF, and SDNF increases from 85 to 136 °C by varying the

Fluorene Derivatives for Electroluminescent Devices

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SCHEME 1: Synthetic Route of Fluorene Derivatives

TABLE 1: Key Physical Data of the Fluorene Derivatives compd

Td (°C)

Tg (°C)

λmax,abs (nm)a

λmax,PL (nm)b

φf

λmax,PL (nm)c

DPhF DNF DPhDNF SDNF DAF DPF

Nme

Nm 85 136 136 164 145

326 340 342 342 368 360

357, 374 381, 399 381, 400 382, 401 414 418

0.89 0.57 0.66 0.68 0.45 0.68

370, 384 392, 413 395, 414 394, 414 450 460

357 405 421 367 457

HOMO/LUMO (eV)d Nm 5.8/2.6 5.9/2.7 5.8/2.6 5.9/2.9 5.7/2.7

a Measured in CH2Cl2 solution. b Measured in CH2Cl2 solution. c Film samples. φf: Fluorescence quantum efficiency in CH2Cl2 solution. d HOMO level obtained from UPS experiment. LUMO estimated from the relationship Eg ) HOMO - LUMO, where Eg was obtained from the edge of the electronic absorption band. e Nm, not measured.

substitution group at the C9 position of the fluorene molecule. The C9-aryl substituents and spiro-type group render the molecular structure rather bulky as compared to the C9-methyl substituents. The former structures not only hinder close packing and intermolecular interaction but also increase molecular rigidity. The introduction of a spiro-type linkage leads to the reduction of crystallization tendency and an increase in the glass transition temperature.10 The thermal properties of the fluorene derivatives are changed by different replacement at the C-2,7 position and C-9 position of the fluorene backbone. The results indicate that suitable substitution on the fluorene can lead to a good thermal stability with a high Tg value. Optical Properties. The absorption and fluorescence spectra of the fluorene derivatives were measured in dilute dichloromethane solution. Both the photophysical data in dilute solution and the PL spectra data of the compounds in thin films are listed in Table 1. As compared to the PL spectra of fluorene derivatives in solution, the PL spectra of similar compounds in films are red-shifted, which is probably due to the dielectric environment of the solid.32 Fluorescence quantum yields of the fluorene derivatives in dilute dichloromethane solution were measured by using 9,10-diphenylanthracene (øf ) ∼0.90) as a calibration standard.14 All of these fluorene derivatives were

Figure 2. Emission spectra of DPhF, DNF, DAF, and DPF.

strongly fluorescent with emission wavelengths ranging from 367 to 418 nm. The fluorescent spectra in the solution of DPhF, DNF, DAF, and DPF are shown in Figure 2. As the substitution at the C-2,7 position of fluorene changes from the phenyl to the pyrenyl group, the peaks of the fluorescent spectra shift from the nearUV region to the blue region. Comparing the absorption and fluorescence spectra of DNF, DPhDNF, and SDNF with that of naphthalene and fluorene

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Figure 3. Emission spectra of DNF, DPhDNF, and SDNF.

reveals large differences in both the peak wavelengths and the shape of the spectra. This indicates that absorption and fluorescence are determined by the whole conjugation system along the naphthalene and fluorene axis. On the other hand, substitutions at the C-9 position have little influence on the optical properties of the molecule. The shape of the absorption spectrum of DAF is almost identical to that of anthracene except for a small red-shift, while the PL spectrum of DAF is quite different from that of anthracence. This result indicates that the compound shows that the absorption of the individual functional group in the molecule is due to poor conjugation in the ground state but exhibits PL characteristics of the whole molecule because of favorable conjugation in the excited state. The fluorescence spectra of DNF, DPhDNF, and SDNF, as shown in Figure 3, are almost identical. This indicates that C-9 substitution on fluorene has a negligible effect on the optical properties of the compounds due to the non-conjugate structure in the ground and excited states between the moieties at the C-9 position and the moieties at the fluorene backbone, which is consistent with the results reported previously.21 Electroluminescent Properties. OLEDs incorporating the fluorene derivatives, except for DPhF with a poor thermal property, were fabricated with a configuration of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (15 nm)/4,4′-bis[N(1-naphthyl)-N-phenylamino]biphenyl (NPB) (50 nm)/fluorene derivative (30 nm)/tris(8-quinolinolato)aluminum (Alq3) (50 nm)/Mg/Ag (200 nm). In these devices, ITO (indium tin oxide) and Mg/Ag are the anode and cathode, respectively, CuPc is the hole injection layer, NPB is the hole transporting layer (HTL), and Alq3 is the electron transporting layer (ETL). The electroluminescent device using DNF shows a poor performance probably due to the poor thermal and morphological instability of DNF. The lifetime of the DNF-based device is very short, and the luminescence of the device can only be sustained for several seconds when 8 V is applied to the device. Figure 4 shows the EL spectra of the DPhDNF- and SDNFbased devices and the PL spectra of their films. The EL spectra of the two devices peaked at 452 nm. As compared to the PL spectra of the DPhDNF and SDNF films, the EL spectra show a large difference in both the peak wavelengths and the shape of the spectra. It is unreasonable to attribute the EL of the devices to the radiation decay of the singlet excited states of DPhDNF and SDNF. Moreover, the EL spectra are also different from those of the NPB film. The PL spectra of the co-deposited blend film and the two layer film of NPB/DPhDNF(SDNF) on quartz substrates do not give any exciplex emission, and this excludes the possibility that the emission comes from the exciplex between the NPB layer and the DPhDNF(SDNF) layer. The electroluminescence of the device perhaps comes from

Figure 4. PL of DPhDNF, SDNF, and NPB and EL of DPhDNF- and SDNF-based devices.

TABLE 2: EL Performance of the Fluorene-Based Devicesa compd DPhDNF SDNF DAF DPF DPF

luminance device (cd/m2) (20 mA/cm2) structure 1 1 1 1 2

306 371 970 780 985

max current efficiency (cd/A) 1.6 2.0 5.0 4.8 5.2

CIE λ (nm) (20 mA/cm2) 452 452 490 464 472

0.15, 0.14 0.16, 0.16 0.19, 0.35 0.17, 0.24 0.15, 0.21

a Structure 1: ITO/CuPc (15 nm)/NPB (50 nm)/blue host (30 nm)/ Alq3 (50 nm)/Mg/Ag. Structure 2: ITO/NPB (50 nm)/blue host (30 nm)/BCP (10 nm)/Alq3 (50 nm)/Mg/Ag.

Figure 5. EL of DAF-based device at different applied voltages and PL of DAF film.

combining the emission of both NPB and DPhDNF(SDNF), which in the induced EL spectrum is different from both NPB and DPhDNF(SDNF) emission. Important EL data of the two devices are listed in Table 2. Figure 5 shows the EL spectra of the DAF-based device at 10, 12, 14, and 16 V, respectively, and the PL spectrum of the DAF film as well. Two peaks, 450 and 490 nm, were present in the EL spectra. Emission at 450 nm increases with increasing applied voltages. As compared to the PL spectrum of the DAF film, the peak at 450 nm in the EL spectrum may be reasonably attributed to the radiation decay of singlet excited state of DAF, while the EL peak at 490 nm perhaps comes from the excimer emission. The existence of excimer emission is also observed in the PL spectra both in DAF solution and in its film, in which there have been shoulder peaks at 432 and 490 nm, respectively. As described in our previous paper, the device based on DPF with structure 1 exhibits a bright blue emission with a maximum current efficiency of 4.8 cd/A and CIE coordinates of x ) 0.17 and y ) 0.24.27 To limit hole transport to ETL and to enhance hole-electron recombination in the EML and thus to increase

Fluorene Derivatives for Electroluminescent Devices

J. Phys. Chem. C, Vol. 112, No. 6, 2008 2169 efficiency of 5.2 cd/A and CIE coordinates of x ) 0.15 and y ) 0.21. The performance of the non-doped fluorene-based device is among the best of blue-emitting devices based on fluorescent molecules. The present findings show that by proper adjustment of the substitutent group in fluorene, blue-emitting materials with a high thermal stability and high electroluminescence efficiency can be achieved.

Figure 6. EL spectra of the DPF-based device with structure 2.

Acknowledgment. We thank National Natural Science Foundation of China (Grant 50773090) and Beijing Natural Science Foundation (Grant No. 2072017 and KZ200711417017) for financial support. The work in Hong Kong was supported by the Innovation and Technology Commission (Project GHP/ 023/05), China. Dr. Jack Chang is kindly acknowledged for help with manuscript revision. References and Notes

Figure 7. I-V-L curves of the DPF-based device with the hole blocking layer.

device efficiency, a hole blocking layer of 2,9-dimethyl-4,7diphenyl-1,10-phenanthroline (BCP) having a low-lying HOMO level is commonly employed between the EML and the ETL. Therefore, we fabricated an optimized device with a configuration of ITO/NPB (50 nm)/DPF (30 nm)/BCP (10 nm)/Alq3 (50 nm)/Mg/Ag. The EL spectrum of the device is shown in Figure 6. The similarity of the PL and EL spectra indicates that the EL can be attributed to emission from the radiation decay of the singlet excited state of DPF. The device with such a hole blocking layer indeed showed a higher efficiency of 5.2 cd/A and better CIE coordinates of x ) 0.15 and y ) 0.21. These values are among the best ever reported for blue-emitting OLEDs without a dopant. The device gives a peak centering at 472 nm and a smaller full-width at half-maximum (fwhm) value than the device without the hole blocking layer. This is mainly attributed to the reduction of the emission from the Alq3 layer as a result of the addition of the hole blocking layer. The device has better CIE coordinates due to the reduction of the longwavelength region of the spectrum as compared to the device without the hole blocking layer. The I-V-L characteristics of the device are shown in Figure 7. The device has a turn-on voltage of 5.3 V and a maximum brightness of 7680 cd/m2 at a voltage of 12.5 V and a current density of 160 mA/cm2. The key performance parameters of the device are listed in Table 2. The present results show that fluorene derivatives can serve as excellent blue host emitters in OLEDs by proper adjustment of substituents at the C-2,7 position on the fluorene backbone. Suitable adjustment can yield fluorene-based blue-emitting materials with a high thermal stability and high electroluminescence efficiency. In summary, we synthesized and characterized a new series of fluorene derivatives with different substituents at the C-2,7 and C-9 positions of fluorene. Luminescence and thermal properties of the fluorene derivatives were investigated. Different substituents at the C-2,7 position of fluorene affect the thermal properties and optical properties of the fluorene derivatives. The performance of the fluorene-based OLEDs is very much dependent on the molecular structure of the fluorene. OLEDs incorporating DPF exhibit bright blue emission with a maximum

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